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GM9.1

The coupling between tectonics, climate and surface processes governs the dynamics of mountain belts and basins. First order constraints on this coupling are provided by geomorphic and sedimentary records, including longitudinal river profiles, fluvial terraces, downstream fining trends, growth strata, sediment provenance, sequence stratigraphy, and changing depositional environments. Moreover, the increasing integration of geochronological methods for quantifying erosion rates and source-to-sink sediment transfer with landscape evolution, stratigraphic, climatic, and tectonic models allows to advance our understanding of the interactions between surface processes, climate and tectonic deformation.

We invite contributions that use geomorphic and/or sedimentary records to understand tectonic deformation, climate histories, and surface processes, and welcome studies that address their interactions and couplings at a range of spatial and temporal scales. In particular, we encourage coupled catchment-basin studies that take advantage of numerical/physical modelling, geochemical tools for quantifying rates of surface processes (cosmogenic nuclides, low-temperature thermochronology, luminescence dating) and high resolution digital topographic and subsurface data. We also encourage field or subsurface structural and geomorphic studies of landscape evolution, sedimentary patterns and provenance in deformed settings, and invite contributions that address the role of surface processes in modulating rates of deformation and tectonic style, or of tectonics modulating the response of landscapes to climate change.

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Co-organized by TS9
Convener: Dirk Scherler | Co-conveners: Anneleen Geurts, Duna Roda-BoludaECSECS, Alex Whittaker
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| Attendance Tue, 05 May, 10:45–12:30 (CEST), Attendance Tue, 05 May, 14:00–15:45 (CEST)

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Chat time: Tuesday, 5 May 2020, 10:45–12:30

Chairperson: Duna Roda-Boluda and Anneleen Geurts
D1118 |
EGU2020-6508
| solicited
| Highlight
Whitney Behr, Adam Holt, Thorsten Becker, and Claudio Faccenna

Tectonic plate velocities predominantly result from a balance between the potential energy change of the subducting slab and viscous dissipation in the mantle, bending lithosphere, and slab–upper plate interface. A range of observations suggest that slabs may be weak, implying a more prominent role for plate interface dissipation than previously thought. Behr & Becker (2018) suggested that the deep interface viscosity in subduction zones should be strongly affected by the relative proportions of sedimentary to mafic rocks that are subducted to depth, and that sediment subduction should thus facilitate faster subduction plate speeds. Here we use fully dynamic 2D subduction models built with the code ASPECT to quantitatively explore how subduction interface viscosity influences: a) subducting plate sinking velocities, b) trench migration rates, c) convergence velocities, d) upper plate strain regimes, e) dynamic topography, and f) interactions with the 660 km mantle transition zone.  We implement two main types of models, including 1) uniform interface models where interface viscosity and slab strength are systematically varied, and 2) varying interface models where a low viscosity sediment strip of finite width is embedded within a higher viscosity interface. Uniform interface models indicate that low viscosity (sediment-lubricated) slabs have substantially faster sinking velocities prior to reaching the 660, especially for weak slabs, and also that they achieve faster ‘steady state’ velocities after 660 penetration. Even models where sediments are limited to a strip on the seafloor show accelerations in convergence rates of up to ~5 mm/y per my, with convergence initially accommodated by trench rollback and later by slab sinking. We discuss these results in the context of well-documented plate accelerations in Earth’s history such as India-Asia convergence and convergence rate oscillations along the Andean margin.

References: Behr, W. M., & Becker, T. W. (2018). Sediment control on subduction plate speeds. Earth and Planetary Science Letters502, 166-173.

How to cite: Behr, W., Holt, A., Becker, T., and Faccenna, C.: Plate speeds modulated by sediment subduction: insights from numerical models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6508, https://doi.org/10.5194/egusphere-egu2020-6508, 2020

D1119 |
EGU2020-4317
Sebastian G. Wolf, Ritske S. Huismans, Jean Braun, and Xiaoping Yuan

It has been a long-standing problem how mountain belts gain and loose topography during their tectonically active growth and inactive decay phase. It is widely recognized that mountain belt topography is generated by crustal shortening, and lowered by river bedrock erosion, linking climate to tectonics. However, it remains enigmatic how to reconcile high erosion rates in active orogens as observed in Taiwan or New Zealand, with long term survival of topography for 100s of Myrs as observed for example in the Uralides and Appalachians. Here we use for the first time a tight coupling between a landscape evolution model (FastScape) with an upper mantle scale tectonic (thermo-mechanical) model to investigate the different stages of mountain belt growth and decay. Using two end-member models, we demonstrate that growing orogens with high erosive power remain small (<200 km), reach steady state between tectonic in- and erosional material eff-flux, and are characterized by transverse valleys. Contrarily, mountain belts with medium to low erosive power will not reach growth steady state, grow wide, and are characterized by longitudinal rivers deflected by active thrusting. However, during growth both types of orogens reach the same height, controlled by rheology and independent of surface process efficiency. Erosional efficiency controls orogenic decay, which is counteracted by regional isostatic rebound. Rheological control of mountain height implies that there is a natural upper limit for the steepness index of rivers on Earth. To compare model results to various natural examples, we quantify the degree of longitudinal flow of modeled rivers with river “longitudinality” in several active or recently active orogens on Earth. Application of the river “longitudinality index” gives information whether (parts of) an orogen is or was at steady state during orogenic growth.

How to cite: Wolf, S. G., Huismans, R. S., Braun, J., and Yuan, X.: Topographic evolution of mountain belts controlled by rheology and surface process efficiency, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4317, https://doi.org/10.5194/egusphere-egu2020-4317, 2020

D1120 |
EGU2020-9056
Lorenz Michel, Ritske S. Huismans, and Sebastian G. Wolf

Disentangling the interactions and possible feedbacks between tectonic and earth surface processes has been a focus of geoscientific research for the past decades. Recent work has highlighted the importance of erosional processes at the Earth’s surface for impacting e.g. the evolution of topography, deformation or the sedimentary yield. Many of these studies were conducted in convergent settings and fewer studies focused on extensional settings. Here, we present the results from forward numerical models, using a geodynamic model (Fantom) coupled with a landscape evolution model (Fastscape) in order to explore the coupling and interactions between tectonics and earth surface processes in extensional continental rift settings.

We model the formation of continental extensional rift systems and compare, how the structure of the subsequent rifts, topography and sedimentary yield evolve over time depending on the combination of five key parameters. For this, we run and compare a series of model experiments, varying crustal rheology (weak to strong crust), duration of extension (5 – 20 Myr), distribution of inherited strain (single vs. distributed weakness), efficiency of erosion (through different rock erodibilities) and the base level for erosion. The modeling results show that structure and topography of the intra-continental rift strongly depend on crustal strength, on the distribution of inherited strain and on the duration of extension. Formation of a major rift basin followed by considerable uplift of the rift shoulders and generation of topography is facilitated by models with a strong crust. The distribution of inherited strain controls the distribution of deformation, such that models with a distributed area of inherited strain yield wider rift zones with partly several, smaller basins. Additional to the respective base level of erosion, the build-up of topography plays a key role in driving the efficiency of erosion, such that high erosion rates are observed for models with significant topography. Hence, models that produce high topography (i.e. models with high crustal strength) display a significant sediment flux during the syn-rift phase. Furthermore, the activity of single faults is impacted by sediment loading, resulting in different styles of deformation, depending on the amount of delivered sediment. For all simulations, topography is erased rapidly (i.e. <5 Myr) following the cessation of rifting activity and rock uplift, if the erosional efficiency is high.

Taken together, our results suggest a strong dependency of the formation of topography, sediment flux and erosion on the respective tectonic circumstances. However, given that surface processes are efficient, the style of rifting can be impacted. Hence, our simulations suggest significant feedbacks between tectonic and surface processes.

How to cite: Michel, L., Huismans, R. S., and Wolf, S. G.: Evolution of topography, sediment yield and efficiency of erosion in intra-continental rift settings: A perspective from numerical modeling using coupled surface processes and tectonic models., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9056, https://doi.org/10.5194/egusphere-egu2020-9056, 2020

D1121 |
EGU2020-9079
Thomas Bernard, Hugh Sinclair, Mark Naylor, Elliot Weir, Frédéric Christophoul, and Mary Ford

The transition from syn- to post-orogenesis is generally identified in foreland basins by a switch from active subsidence and deposition to isostatic rebound and erosion. However, the nature of the interplay between isostatic rebound and sediment supply, and their impact on the topographic evolution of a range and foreland basin during this transition has not been fully explored.

Here, we use a box model to explore the syn- to post-orogenic evolution of foreland basin/thrust wedge systems. Using a set of parameter values that approximate the northern Pyrenees and the neighbouring Aquitaine foreland basin, we evaluate the controls on: 1) the sediment drape over the frontal parts of the retro-wedge and 2) the sediment accumulation into surrounding continental margins following cessation of crustal thickening. Conglomerate and sandstone sediments preserved at approximately 600 m elevation, which is ~300 m above the present mountain front in the northern Pyrenees record an age of ca. 12 Ma, approximately 8 Myrs younger than the last evidence of crustal thickening in the wedge. These sediments formed a regional drape that reached up to approximately 800 m elevation, but are now preserved in low gradient patches, and are associated with more regional surfaces across the northern Pyrenees. Using the model, this post-orogenic sediment drape can be explained by the combination of a sustained, high sediment influx from the range into the basin relative to the efflux out of the basin, combined with cessation of basin subsidence. The model also predicts higher sediment flux out of the system during the post-orogenic phase involving an increase of sediment accumulation as observed in the Bay of Biscay during this interval.

Post-orogenic sediment drape and increased sediment flux out the mountain range-foreland basin system are proposed as generic processes of these systems.

How to cite: Bernard, T., Sinclair, H., Naylor, M., Weir, E., Christophoul, F., and Ford, M.: Post-orogenic sediment drape and flux of mountain range-foreland basin systems: An example from the Northern Pyrenees, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9079, https://doi.org/10.5194/egusphere-egu2020-9079, 2020

D1122 |
EGU2020-9516
Nicolas Perez-Consuegra, Edward R Sobel, Andres Mora, Jose R Sandoval, Paul G Fitzgerald, Sebastian Zapata, Mauricio Parra, Johannes Glodny, and Gregory D Hoke

The relative controls of rock uplift (tectonics) and precipitation (climate) on the exhumation of earth’s rocks in tectonically active mountain ranges are still debated. In low latitude tropical regions where rates of precipitation and the amount of vegetation cover are higher, more data is required to test the relative contribution of these factors to the evolution of orogenic topography. To contribute to this debate, cooling ages were derived for 25 bedrock and four detrital samples using the apatite (U-Th-Sm)/He (AHe) low temperature thermochronometer. AHe ages are reported along a ~450-km-wide swath on the eastern flank of the Northern Andes in Colombia (South America). The AHe cooling ages, that range from 2.5 Ma to 17 Ma, are compared to precipitation rates and geomorphic parameters in order to discern the relative importance of climate and/or tectonics on exhumation. Along the transect, AHe cooling ages are poorly correlated with the rates of precipitation but show a good correlation with landscape parameters such as average hillslope and average channel steepness. Moreover, young AHe cooling ages coincide with areas where deformation is mainly compressional; older AHe cooling ages are found in the middle part of the study area where strike-slip deformation dominates. The spatial distribution of the new AHe cooling ages suggests that in mountainous regions, in this case with high precipitation rates (> 1500 mm/yr), denudation is mainly controlled by the rate of vertical advection of material via tectonic processes. The spatial variations in precipitation may only have a second-order role in modulating exhumation rates.

How to cite: Perez-Consuegra, N., Sobel, E. R., Mora, A., Sandoval, J. R., Fitzgerald, P. G., Zapata, S., Parra, M., Glodny, J., and Hoke, G. D.: What controls erosion (exhumation) along the humid eastern margin of the Northern Andes? Insights from U-Th/He thermochronology, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9516, https://doi.org/10.5194/egusphere-egu2020-9516, 2020

D1123 |
EGU2020-949
Jorien L.N. van der Wal, Veit C. Nottebaum, Georg Stauch, Frank Lehmkuhl, and Klaus Reicherter

The tectonically active northern margin of the Gobi Altai in southern Mongolia is best known for the 1957 Mw 8.1 Bogd earthquake. Cumulative offsets along the Bogd fault indicate that the area was subject to repeated earthquakes in the past. North of the Bogd fault, the Valley of Lakes characterises a seismically quiescent zone between the Gobi Altai and the central Mongolian Hangay dome, with little to no instrumentally recorded earthquakes. However, Quaternary alluvial fans of rivers that drain toward the endorheic lakes in this basin are crosscut by multiple fault scarps with displacements up to 15 m. Additionally, river channel morphology is significantly altered by tectonic lineaments indicating that, despite the lack of recorded seismicity, this area may indeed have been seismically active in the recent past. By applying remote sensing techniques, UAV photogrammetry, and morphometric studies, we aim to understand i) the effect these faults had on the landscape evolution of the Valley of Lakes, ii) their relationship to deformation along the Bogd fault and iii) whether these faults accommodate a significant amount of strain related to the India-Eurasia collision.

The lack of available material for dating requires palaeoseismological studies to make use of morphotectonic observations as an alternative, relative dating method. At the Bogd fault, such studies were combined with sparsely available cosmogenic nuclide age data to determine that vertical slip rates vary between 0.1 and 1 mm/yr on individual faults and at the scale of the entire mountain front, respectively. In the Valley of Lakes, a total lack of age data complicates the extrapolation of slip rates, however scarp degradation indicates that slip rates are likely lower than at the Bogd fault. Fluvial terraces of the Tuyn Gol river are crosscut by at least three major fault scarps, which contribute to valley width variations of the river from ±3500 m to ±20 m at the current fan apex, and which are reflected in steepness index variations along minor drainages. Additionally, a large paleochannel suggests that major drainage reorganisation events took place in Quaternary times, either reflecting periods of high tectonic activity or as a result of significant climate variations. The transtensional nature of some faults in the Valley of Lakes is unique; however fault mechanisms in the area are generally in line with the active deformation in the Gobi Altai. Our results stress the earthquake potential of regions with low instrumental seismicity and demonstrate that deformation in the Gobi Altai may reach further north than previously expected.

How to cite: van der Wal, J. L. N., Nottebaum, V. C., Stauch, G., Lehmkuhl, F., and Reicherter, K.: Geomorphological evidence of active faulting in low seismicity regions - examples from the Valley of Lakes, southern Mongolia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-949, https://doi.org/10.5194/egusphere-egu2020-949, 2019

D1124 |
EGU2020-9518
Sebastian G. Mutz and Todd A. Ehlers

The interpretation of Earth surface archives often requires consideration of distant off-site events. One such event is the surface uplift of Earth’s major mountain ranges, which affects climate and the Earth’s surface globally. In this study, the individual and synergistic climatic effects of topographic changes in major mountain ranges are explored with a series of General Circulation Model (GCM) experiments and analyses of atmospheric teleconnections. The GCM experiments are forced with different topographic scenarios for Himalaya-Tibet (TBT) and the Andes (ADS), while environmental boundary conditions are kept constant. The topographic scenarios are constructed by successively lowering modern topography to 0% of its modern height in increments of 25%. This results in a total of 5 topographic scenarios for TBT (tbt100, tbt075, tbt050, tbt025, tbt000) and ADS (ads100, ads075, ads050, ads025, ads000). TBT scenarios are then nested in ADS scenarios, resulting in a total of 25 experiments with unique topographic settings. The climate for each of those 25 scenarios is simulated with the GCM ECHAM5-wiso. We then explore possible synergies and distant impacts of topographic changes by testing the hypothesis that varying ADS has no effect on simulated climate conditions in the TBT region (c_tbt) and vice versa. This can be expressed as the null hypothesis c_tbt(ads100) = c_tbt(ads075) = c_tbt(ads050) = c_tbt(ads025) = c_tbt(ads000) for each of the 5 TBT scenarios, and vice versa. We conduct Kruskal-Wallis tests for a total of 10 treatment sets to address these hypotheses. The results suggest that ADS climate is mostly independent of TBT topography changes, whereas TBT climate is sensitive to ADS topography changes when TBT topography is high, but insensitive when TBT topography is strongly reduced. Analyses of atmospheric pressure fields suggest that TBT height acts as a control on cross-equatorial atmospheric transport and modifies the impact of ADS topography on northern hemisphere climate. These results dictate a more careful consideration of global (off-site) conditions in the interpretation of Earth surface records.

How to cite: Mutz, S. G. and Ehlers, T. A.: On the Synergistic Climatic Effects of Covarying Major Mountain Range Topographies, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9518, https://doi.org/10.5194/egusphere-egu2020-9518, 2020

D1125 |
EGU2020-13336
| solicited
Paolo Ballato, Sascha Brune, and Manfred Strecker

The removal, redistribution, and transient storage of sediments in tectonically active mountain belts is thought to exert a first-order control on shallow crustal stresses, fault activity, and hence on the spatiotemporal pattern of regional deformation processes. Accordingly, sediment loading and unloading cycles in intermontane sedimentary basins may inhibit or promote intrabasinal faulting, respectively, but unambiguous evidence for this potential link has been elusive so far.

Here we combine 2D numerical experiments that simulate contractional deformation in a broken-foreland setting (i.e., a foreland where shortening is diachronously absorbed by spatially disparate, reverse faults uplifting basement blocks) with field data from intermontane basins in the NW Argentine Andes. Our modelling results suggest that thicker sedimentary fills (> 0.7-1.0 km) may suppress basinal faulting processes, while thinner fills (< 0.7 km) tend to delay faulting. Conversely, the removal of sedimentary loads via fluvial incision and basin excavation promotes renewed intrabasinal faulting.

These results help to better understand the tectono-sedimentary history of intermontane basins that straddle the eastern border of the Andean Plateau in northwestern Argentina. For example, the Santa María and the Humahuaca basins record intrabasinal deformation during or after sediment unloading, while the Quebrada del Toro Basin reflects the suppression of intrabasinal faulting due to loading by coarse conglomerates. We conclude that sedimentary loading and unloading cycles may exert a fundamental control on spatiotemporal deformation patterns in intermontane basins of tectonically active broken forelands.

How to cite: Ballato, P., Brune, S., and Strecker, M.: Sedimentary loading-unloading cycles and faulting in intermontane basins: insights from numerical modeling and field observations from the broken foreland basin of NW Argentine Andes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13336, https://doi.org/10.5194/egusphere-egu2020-13336, 2020

D1126 |
EGU2020-10246
Sarah Boulton, Alexander Whittaker, Emiko Kent, M. Cihat Alcicek, and Derek Fabel

The Gediz (Alaşehir) Graben is located in the highly tectonically active and seismogenic region of Western Turkey, which has been experiencing high-angle normal faulting since ~ 2 Ma.  Rivers upstream of the normal fault-bounded graben each contain a lithologic knickpoint related to the change in bedrock geology (from soft sediments to hard metamorphic rocks) and a non-lithologic knickpoint, caused by an increase in fault slip rate at ~ 0.8 Ma.  Therefore, this system represents an ideal natural laboratory to investigate the relative roles of bedrock lithology / rock strength and rates of faulting on the behaviour and evolution of bedrock river systems. Our results show that metamorphic rocks in the catchments are 2-3 times harder than the sedimentary rocks. Stream power increases downstream reaching local maxima upstream of the fault within the metamorphic bedrock but declines rapidly once softer sedimentary rocks are encountered. We also demonstrate a positive correlation between throw rate and stream power in the metamorphic rocks characteristic of rivers obeying a detachment-limited model of erosion. In sedimentary rocks stream powers are invariant with throw rate but do scale with the river’s sediment transport capacity. We also present new Be10 denudation rates that show correlations with calculated stream power and fault throw rates. This study demonstrates that the strength of underlying bedrock is a major influence on river evolution and that the nature of the underlying lithology profoundly influences the way in which the river behaves.

How to cite: Boulton, S., Whittaker, A., Kent, E., Alcicek, M. C., and Fabel, D.: The importance of lithology and throw rate on bedrock river behaviour and evolution in the Gediz (Alaşehir) Graben, Turkey. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10246, https://doi.org/10.5194/egusphere-egu2020-10246, 2020

D1127 |
EGU2020-790
| Highlight
Richard Ott, Sean F Gallen, and David Helman

In the Mediterranean, carbonate massifs occupy many of the highest mountains, whereas quartzofeldspathic units are often associated with more subdued topographic relief. In contrast, carbonates occupy valley bottoms, and quartzofeldspathic bedrock forms ridgelines in more humid and tectonically quiescent regions, such as the eastern United States and Ireland. This observation implies changes in the pace and style of denudation associated with climate and tectonic regime. Denudation in carbonates is traditionally thought to be controlled by dissolution; however, this paradigm has not been quantitatively vetted. Here we present new results of cosmogenic basin-average denudation rate measurements from both 10Be and 36Cl in meta-clastic and carbonate bedrock catchments on Crete, Greece, compile all existing 36Cl denudation measurements globally, and calculate dissolution rates from water chemistry and satellite-derived water flux data to improve understanding of landscape evolution and the partitioning between physical and chemical denudation in carbonates in the Mediterranean and elsewhere. In Crete, basin average erosion rates in meta-clastic and carbonate catchments are similar, with mean values of 0.10 and 0.13 mm/a, respectively, but the total relief is almost double in carbonates relative to meta-clastic bedrock. Results show that both carbonates and meta-clastic units on Crete are dominated by physical denudation with < 10% and ~40% of total denudation attributed to dissolution, respectively. Water mass-balance analysis shows that 40-90% of surface runoff is lost to groundwater infiltration in carbonates due to the development of mature karst hydrology. We incorporate chemical weathering and infiltration into a simple one-dimensional landscape evolution model based on the widely used stream power model and show that relief production in carbonates in Crete is largely due to reduced erosive power associated with water lost to infiltration into karst. Relief production in carbonates results in enhanced slope-dependent erosion, allowing carbonate denudation rates to keep pace with those in the meta-clastic catchments. From a global perspective, we observed a strong relationship between total denudation rate and physical erosion rate, but a weak scaling with dissolution rate. This observation implies that slope-dependent erosion becomes progressively more important as erosion rates increase, whereas rates of dissolution are limited by other effects, such as water flux. These findings lead to a new conceptual model where there is a dissolution speed limit in carbonates due to available water and acid such that areas of high local uplift require substantial mechanical erosion to balance uplift and form steep slopes. In contrast, areas experiencing low uplift rates with sufficient water availability (e.g. humid climate) can balance uplift entirely with dissolution resulting in subdued carbonate landscapes.

How to cite: Ott, R., Gallen, S. F., and Helman, D.: Why are Mediterranean carbonate mountains high and steep? Climatic and tectonic controls on carbonate landscape evolution, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-790, https://doi.org/10.5194/egusphere-egu2020-790, 2019

D1128 |
EGU2020-12680
Claire Masteller, Niels Hovius, Catriona Thomspon, Emma Vann-Jones, Han Byul Woo, Pete Adams, Mark Dickson, Adam Young, and Nick Rosser

The integration of wave energy imparted to sea cliffs and its conversion into erosion and mechanical work drives the evolution of rocky coasts. However, this near-shore transformation of wave energy remains poorly constrained.

We compare 4 cliff-top seismic records (Orkney Islands, UK; La Jolla, USA; Santa Cruz, USA; Boulby Cliffs, UK) to characterize the response of sea cliffs to the prevailing wave climate. Across all sites, ground displacement scales with wave height and decays with distance from the cliff, but with varying degrees of sensitivity. 3 of 4 sites behave in a mechanically consistent manner - only showing modest increases in ground shaking. Further, decay in displacement at these 3 sites is consistent with energy loss due entirely to geometric spreading of seismic waves. Near-shore wave modeling suggests that shore platform morphology at these sites has evolved to an equilibrium state, such that delivered cliff-face wave energy is roughly constant across the full range of wave conditions.

Ground displacement on Orkney is significantly more sensitive to changes in wave height. Landward energy loss at Orkney is also more pronounced, potentially a signature of active rock damage processes. This increased sensitivity suggests that the near-shore has not yet evolved to reflect the incident wave climate. Indeed, wave breaking on Orkney is concentrated at the cliff face. As such, the transfer of wave energy is more efficient, resulting in wave energy flux orders of magnitude larger, and more variable, than all other sites.

Vertical land motion on Orkney is 2x more rapid than all other sites. This more rapid vertical motion is likely to outpace cliff retreat and beveling of the shore platform. As such, the near near-shore cannot adjust to the incoming wave climate, and does not reach an equilibrium state. Instead, wave breaking remains pinned at the cliff face, enhancing wave energy transfer.

We compile vertical land motion rates across the United Kingdom with coincident wave buoy data and bathymetry. We find that for more rapid vertical land motion, wave breaking is concentrated at the coast in comparison with more distributed wave breaking at sites with more gradual vertical motion. We suggest that these differences in vertical land motion exert a first order control on the transfer of wave energy to rocky coasts, such that areas with rapid vertical land motion rates are (1) more susceptible to changes in wave climate and (2) remain in a prolonged transient state relative to the dominant wave climate.

These results have implications both for the processes and timescales governing the long-term evolution of rocky coasts, as well as for determining the susceptibility of modern coastlines to a changing wave climate.

How to cite: Masteller, C., Hovius, N., Thomspon, C., Vann-Jones, E., Woo, H. B., Adams, P., Dickson, M., Young, A., and Rosser, N.: Exploring the interplay of wave climate, vertical land motion, and rocky coast evolution , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12680, https://doi.org/10.5194/egusphere-egu2020-12680, 2020

D1129 |
EGU2020-13508
Xiaoping Yuan, Kimberly Huppert, Jean Braun, and Laure Guerit

The SE Tibetan Plateau has extensive broad, low-relief, high-elevation surfaces perched above deep valleys, as well as in the headwaters of the three rivers (the Salween, the Mekong, and the Yangtze). However, understanding the presence of these low-relief surfaces is a long-standing challenge because their formation process remains highly debated. While alternate mechanisms have been proposed to explain the low-relief surface formation in this setting (e.g., drainage-area loss mechanism due to horizontal advection; Yang et al., 2015, Nature), a long-standing hypothesis for the formation of low-relief surfaces is by a step change in uplift and incision into a pre-existing, low-relief surface (Clark et al., 2006, JGR; Whipple et al., 2017, Geology).

The morphology of low-relief surfaces in the SE Tibetan Plateau is largely consistent with formation by a step change in uplift, but one problem with this model is that low-relief surfaces formed by a step change in uplift are relatively short-lived, since they are incised and steepened by erosion, which sweeps upstream at the response time of mountain ranges (in the order of several million years). Using a landscape evolution model that combines erosion, sediment transport and deposition processes (Yuan et al., 2019, JGR), we demonstrate that propagating uplift form large parallel rivers, with broad low-relief, high-elevation interfluves that persist for tens to hundreds of million years, consistent with various dated ages. These low-relief surfaces can be long-lived because the drainage areas in these interfluves are insufficient to keep up with rapid incision of the large parallel mainstem rivers. Our simulated features match various observations in the SE Tibetan Plateau: (i) low-relief surfaces are approximately co-planar in headwaters, and decrease in elevation smoothly from northwest to southeast across the plateau margin; (ii) χ-elevation plots of the mainstem rivers are convex; (iii) low-relief surfaces have low erosion rates; and (iv) erosion rates are high in the mainstem rivers at the propagating margin.

How to cite: Yuan, X., Huppert, K., Braun, J., and Guerit, L.: Propagating uplift controls on formation of low-relief, high-elevation surfaces in the SE Tibetan Plateau, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13508, https://doi.org/10.5194/egusphere-egu2020-13508, 2020

D1130 |
EGU2020-18761
Kurt Stüwe, Gerit Gradwohl, Thorsten Bertosch, Konstantin Hohmann, Jörg Robl, and Moritz Liebl

The eastern end of the Alps features a series of low relief surfaces at elevations up to 2500 m. These surfaces have long been known to reflect uplifted planation surfaces that have not yet been dissected by fluvial processes and thus preserve a strong geomorphic disequilibrium. While their age would present a good handle on the age of surface uplift in the Eastern Alps, these surfaces are barely dated and their age is only indirectly inferred to reflect the Miocene and Pliocene uplift history. Recent geomorphological cosmogenic nucleide-based studies have shown that these surfaces may record up to 1000 m of surface uplift in the last 5 Ma. Such a distinct uplift event in the recent past is surprising and needs to be interpreted. Interestingly, this time frame appears not to be accompanied by crustal shortening and the standard hypothesis about the inversion of the Pannonian Basin as the underlying cause needs to be questioned. In order to get a better handle on the nature of this young uplift event and its overriding driver it is crucial to understand its spatial extent. However, much of the Eastern Alps was glaciated in the Pleistocene and currently several studies suggest that elevated low-relief landscapes were shaped by the glacial buzz-saw, instead of interpreting them in terms of fluvial prematurity of recently uplifted planation surfaces. The models of glacial erosion versus fluvial prematurity as the formation agent of the low-relief surfaces can be discerned if it can be shown that the surfaces formed prior to the Pleistocene. Here we report of a currently operating research project in which we employ cosmogenic nucleide burial dating on a substantial part of the entire Eastern Alps to derive the age of these surfaces. We use the burial age of siliceous sediments in caves formed at the phreatic-vadose transition as a proxy. Correlation of cave levels with low-relief surfaces and their mapping in the field is an integral part of the project.

 

How to cite: Stüwe, K., Gradwohl, G., Bertosch, T., Hohmann, K., Robl, J., and Liebl, M.: Young Uplift at the Eastern End of the Alps. Evidence for Uplift Unrelated to the Inversion of the Pannonian Basin?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18761, https://doi.org/10.5194/egusphere-egu2020-18761, 2020

D1131 |
EGU2020-20252
John Jansen, Mike Sandiford, Toshiyuki Fujioka, Timothy Cohen, Martin Struck, Suzanne Anderson, Robert Anderson, and David Egholm

The mantle convection accompanying plate motion causes vertical movements of up to a few hundred metres at Earth’s surface over wavelengths of 102–103 km. This dynamic topography appears to come and go at ~ 1–10 Myr timescales in areas that are often well away from plate margins, although its spatial and temporal characteristics are subject to ongoing debate. Since such motions are small and transient, discriminating convective signals from other drivers of relief generation and/or sediment dispersal remains tricky. An outstanding challenge is to detect these elusive, transient undulations from a tell-tale geomorphic imprint preserved in either drainage patterns or the stratigraphic record.

In the intra-plate setting of central Australia, a 30 km long sinuous gorge is developed where the major regional drainage, Finke River, dissects a band of low hills. Remarkably, this gorge is intertwined with an abandoned and less deeply incised gorge that forms hanging junctions and shares similar width and sinuosity. This unusual overprinting of the two gorges remains unexplained.

With an aim to investigate the history of the intertwined gorges, we measured cosmogenic 10Be and 26Al in fluvial gravels stored in the palaeovalley cutoffs. The gravels are remnants of major alluviation episodes that we surmise result from ongoing vertical motions associated with dynamic topography. We use a Markov chain Monte Carlo-based inversion model to test two hypotheses to explain the nuclide inventory contained within the stored fluvial gravels. In the first case, rapid alluviation and erosion since 1 Ma preserves the nuclide memory of the source area; in the second, the nuclide memory is erased during long-term fluvial storage (> 5 Myr) and is restored during exhumation of the palaeovalley gravel-pile. The two hypotheses are therefore limiting-case scenarios that constrain overall fast versus slow landscape evolution, respectively. Our model results suggest that long-term burial decouples the source-area signal from nuclide abundances measured in the palaeovalley gravels. This casts events into a Miocene timescale.

How to cite: Jansen, J., Sandiford, M., Fujioka, T., Cohen, T., Struck, M., Anderson, S., Anderson, R., and Egholm, D.: Geomorphic imprint of dynamic topography and intraplate tectonism in central Australia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20252, https://doi.org/10.5194/egusphere-egu2020-20252, 2020

D1132 |
EGU2020-5609
Jens Turowski, Wolfgang Schwanghart, Kim Huppert, and Claire Masteller

In recent years, Chi analysis has become an important tool for tectonic and geomorphic analyses of longitudinal and planform patterns of river networks. Predicated on the commonly observed inverse scaling between drainage area and slope in rivers and integrating drainage area, the metric Chi has several advantages over other topographic metrics used to describe river long profiles. For a steady state river, Chi scales linearly with elevation, simplifying visual interpretation and further analysis. As an integral property, it also reduces scatter in noisy topographic data. In addition, comparison of computed Chi values to the steady state assumption are a popular tool to determine the stability of river networks and mobility of drainage divides. In this application it is thought that the drainage divide is mobile when Chi values are unequal at adjacent channel heads when integrated from a common base level. These differences in Chi are now frequently used to map mobile and stationary divides and to interpret their spatial patterns in terms of tectonic forcing.

 

As the interpretation of divide mobility relies on a difference in Chi values across the divide, the question arises: how magnitude of cross-divide differences in Chi is necessary for a statistically significant result, given inherent uncertainty in calculations of Chi and the topographic data from which they are derived? Currently, uncertainties in Chi have not been formally evaluated. As such, it remains unclear how robust measurements of differential cross-divide Chi are as a proxy for interpreting drainage divide mobility. Here, we argue that uncertainties in differential cross-divide Chi depend on the location and length of the drainage divide. In a discrete representation of topography, we identify two sources of error. The first source of error can arise if a pixel is incorrectly assigned to a catchment on one side of the divide due either to error in the topographic data or uncertainty in the delineation of drainage area from a digital elevation model (DEM). The second source of error arises because the divide is a linear feature, which cuts across individual pixels in a gridded DEM. Thus, a pixel at the boundary of one designated catchment typically contains area that should drain to its neighboring catchment. We develop an analytical description of these sources of error and show that uncertainties in differential cross-divide Chi can be of the same order as the cross-divide difference in Chi itself. The results from the analytical solution are consistent with a numerical assessment of Chi uncertainties from flow routing on DEMs using multiple flow directions. We discuss scaling with drainage area, and the implications for drainage network mobility using type examples.

How to cite: Turowski, J., Schwanghart, W., Huppert, K., and Masteller, C.: Uncertainties in Chi analysis: implications for drainage network and divide stability, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5609, https://doi.org/10.5194/egusphere-egu2020-5609, 2020

D1133 |
EGU2020-11809
Léopold de Lavaissière, Stéphane Bonnet, Vincent Regard, Pierre Voinchet, Sébastien Carretier, Anne Guyez, and Jean-Jacques Bahain

The topography of continents is a dynamic interface that evolves in response to several external (tectonics, mantle dynamics, climate) or internal factors to the geomorphic system. If these systems tend naturally toward a steady-state, they often show transient regimes as evidenced by retreating knickpoints, fluvial captures or migrating divides. These two last phenomena are indicative of drainage reorganization and imply the growth of a network at the expense of another. Yet, the rate of drainage network growth is very poorly known. To our knowledge for example, only one study (Craddock et al., 2010) has attempted to constrain the rate of growth of a natural drainage network, in the specific case of a growth by sequential captures of endorheic systems.

Our aim here is to constrain the mechanism, timing and rates of network growth and drainage reorganization in a natural setting located in southwest France (Aude river catchment), in a current anorogenic setting in the northern foreland of the Pyrenees.

Geomorphic evidence indicate that this catchment is enlarging with about 40 km of displacement of its main divide in the last few hundreds of thousand years (precise timing under investigation). The Aude river and main tributaries show flight of strath terraces that converge downward over ~150 km long distance. This specific fan-shape of paleo-longitudinal profiles implies an upward increase of fluvial incision that we interpret as the consequence of a long-term growth of the Aude drainage network. The dating of these terrace system using cosmogenic isotopes (in-situ 10Be depth-profiles and 10Be-26Al burial isochrones) and Electron Spin Resonance (ESR) is under progress and will allow us to quantify the longitudinal trend of differential incision through time, which we will use to estimate the rate of drainage network growth and divide migration. To support these results, an analysis of the catchment-wide erosion rates on both sides of the migrating divide is also performed. First preliminary results indicate catchment-wide erosion rates of 0.06-0.08 mm.yr-1 in the Aude river catchment.

In complement to this natural case study, the main question of network growth dynamic is also addressed through laboratory-scale experiments performed at the Géosciences Environnement Toulouse (GET) laboratory. The first results show that the divide migration rate related to drainage network growth depends positively on the uplift/base level fall rate. In the detail, the divide migration rate is however non-linear, it evolves step by step with periods of acceleration when cyclic retreating knickpoints hit the divide.

How to cite: de Lavaissière, L., Bonnet, S., Regard, V., Voinchet, P., Carretier, S., Guyez, A., and Bahain, J.-J.: Rate and pattern of drainage network growth and induced drainage divide migration in natural (Aude river catchment, France) and laboratory-scale landscapes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11809, https://doi.org/10.5194/egusphere-egu2020-11809, 2020

D1134 |
EGU2020-12519
Liran Goren, Elhanan Harel, Eitan Shelef, and Hanan Ginat

Observations from around the globe show that drainage reversal toward cliffs (and at a larger scale, toward escarpments) is a common phenomenon.  Drainage reversal occurs when a channel that used to grade in one direction reverses its gradient while exploiting its antecedent valley, forming barbed tributaries with junction angle >90°. Drainage reversal is an important end-member of fluvial reorganization that drastically shifts the hydrologic and geomorphic functionality of the landscape.  The processes that induce drainage reversals, however, remain largely enigmatic. In many cases, tectonic or structural tilt of the surface is invoked to explain reversal toward the tilt direction, but independent evidence for tilting is rare. Moreover, in great escarpments, geodynamic models predict tilting away from the escarpment, opposite to the sense of reversal discussed here.

We study drainage reversals toward the southern Arava Valley escarpment in Israel, along the Sinai-Arabia transtentional plate boundary. In this area, we establish reversals by observations of barbed tributaries, valley-confined windgaps, and terraces and interfluves that grade opposite to the grading direction of the active channel. Detailed morphological and geological analysis of the field area gives rise to a new, tilting independent mechanism for drainage reversal toward cliffs. The initial condition for this mechanism is a cliff that truncates fluvial channels that flow over the highland and away from the cliff, and a water divide that coincides with the cliff. The truncated channels appear as saddles along the cliff and are commonly filled with alluvial and colluvial sediments. Such initial conditions characterize shoulder-type great escarpments and cliffs that form following river capture events. Importantly, in these settings, the sediments that fill the truncated channels are more erodible than the bedrock that builds the interfluves.

According to the mechanism we propose, the erodible valley fill near the steep cliff is initially transported down the cliff via hillslope processes, which results in a gradual migration of the divide along the antecedent valley and away from the cliff. A reversed channel segment forms between the receding divide and the cliff, such that along the channel, the divide and the cliff are not coincident anymore. The faster fluvial incision in the reversed segment with respect to the antecedent channel further pushes the divide away from the cliff. When the receding divide traverses a tributary confluence, a barbed tributary forms. The increased discharge of the reversed segment facilitates cliff embayment that eventually affects cliff retreat and morphology.

This new mechanism indicates that a relatively thin layer of erodible valley fill could be a tipping point that completely changes the trajectory of landscape evolution via drainage reversal. Importantly, however, flow reversal towards cliffs does not necessitate such a layer but instead could be triggered by other hydrological and geological conditions that promote faster erosion toward the cliff within the antecedent channel with respect to the interfluves. 

How to cite: Goren, L., Harel, E., Shelef, E., and Ginat, H.: Migrating divides induce drainage reversal toward cliffs and escarpments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12519, https://doi.org/10.5194/egusphere-egu2020-12519, 2020

D1135 |
EGU2020-7098
Lukas Plan, Stephanie Neuhuber, Susanne Gier, Esther Hintersberger, Christopher Lüthgens, Denis Scholz, Johannes Lachner, Sandra Braumann, and Markus Fiebig

The Hainburg Hills form an elevated range at the south of the Male Karpaty mountains and separate the Vienna Basin from the Danube Basin. They consist of Variscian magmatic and metamorphic rocks covered with anchimetamorphic Mesozoic carbonates. The area west of the Hainburg Hills is well-known for its thermal sulfuric spa since Roman times. About 30 karst caves have been mapped in the area that show signs of hydrothermal or sulphuric acid speleogenesis.

Two of these caves vertically separated by 92 m were numerically dated using terrestrial cosmogenic 26Al and 10Be in quartz washed into a cave and 230Th/U of calcite rafts. In addition, aeolian cover sediments were investigated using luminescence age dating.

The upper c. 15 m wide and c. 20 m high cave chamber was completely filled with large, well-rounded quartz cobbles in a red matrix. The matrix contains over 30% clay and consists of quartz, K-feldspar, muscovite, chlorite, hematite, kaolinite, illite, and smectite. The occurrence of smectite in combination with the small grain size indicates soil forming processes in the B-horizon. We conclude that fluvial gravels –similar to modern ones of the Danube river - were transported into the cave together with a matrix originating from a soil cover. In-situ produced cosmogenic 10Be and 26Al in five quartz cobbles was used to calculate the time of sediment emplacement into the cave. Results indicate a depositional age of c. 4.5 Ma using the isochron technique.

The lower cave was investigated using calcite rafts that form at the surface of cave pools using the 230Th/U dating method. One sample of thin, sharp-edged, and uncoated cave rafts gave the youngest age of c.0.32 Ma. Two other samples were more overgrown and gave older ages between 0.38 and 0.44 Ma. The pristine sample is best suited to reflect the time when the base level was close to the cave.

Rates of vertical displacement vary between 30 and 35 m/Ma for the last 4 Ma and between 150 and 160 m/Ma for the last 0.32 Ma and document an increase of uplift/incision for the region. These numbers compare well to published rates from the unglaciated surroundings that also range from a maximum of 140 m/Ma to a minimum of 20-25 m/Ma and are generally much lower compared to formerly glaciated areas in the Alps and GPS measured uplift (c. 1000 m/Ma).

The luminescence age of 14.6 ± 0.1 ka recorded in cover sands show that sediments they overly much older gravels. This implies sediments were repeatedly eroded from the top of the karstified bedrock surface. The aeolian sediments are primarily preserved in depressions within the bedrock surface. Therefore, the age may represent the end of a phase of intense aeolian activity when wind velocities decreased sufficiently to cause sand accumulation. This period is the peak in Western and Central Europe periglacial activity and accompanied by formation of aeolian deposits. The ages are comparable to aeolian deposits in the Vienna Basin area and cover sediments from the Transdanubian Range.

How to cite: Plan, L., Neuhuber, S., Gier, S., Hintersberger, E., Lüthgens, C., Scholz, D., Lachner, J., Braumann, S., and Fiebig, M.: Vertical movement at the Alpine-Carpathian border (Hainburg Hills) calculated from numerical ages of cave sediments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7098, https://doi.org/10.5194/egusphere-egu2020-7098, 2020

D1136 |
EGU2020-4114
Katrina Gelwick, Frank Pazzaglia, Kenneth Kodama, Lee Corbett, Paul Bierman, and Marc Caffee

Cyclical patterns in the lithology of terrestrial Pleistocene sedimentary deposits are traditionally interpreted as the result of exogenic interglacial-glacial cycles, with deposition accommodated by constant basin subsidence. Recent challenges to this model propose that autogenic surface processes inherent to hillslope, fluvial, and marine systems can both obscure exogenic signals in the sedimentological record and encode their own quasi-periodic signal that mimics exogenic cyclicity. We used rock-magnetic cyclostratigraphy to test the canonical climate-driven sedimentation model for terrestrial Pleistocene sedimentary cycles against competing tectonic- and autogenic process-modulated sedimentation models with a continuous 60 m exposure of middle Pleistocene fluvial sedimentary cycles located at the edge of the actively subsiding Po foreland basin in the Northern Apennines of Italy. We correlated magnetic susceptibility, sampled at 40 cm intervals, to orbital cyclicity to generate a high-resultion age model anchored by terrestrial cosmogenic nuclide (TCN) burial ages, optically stimulated luminescence (OSL), and magnetostratigraphy. Two new 26Al-10Be burial ages are 160±320 ka and 680±310 ka (2σSE); the age of a third buried sample is consistent with continuous exposure and thus recent burial. We mapped the age model into section lithostratigraphy and then compared to the global benthic δ18O stack to determine whether sedimentary cyclicity coincides with glacial-interglacial cycles. In addition, we calculated paleo-erosion rates based on the 10Be concentration of six samples distributed through the age model and find that they range from 244±23 to 444±52 m/Ma, which bracket the modern TCN-determined erosion rate of the Enza River of 351±40 m/Ma. Results show no clear correlation between lithostratigraphy, glacial-interglacial climate cycles, or paleo-erosion rates, indicating that the stratigraphy is probably not driven by exogenic climate forcing. Rather, based on the decoupling of lithology and paleo-erosion rates and the little variation in paleo-erosion and modern erosion rates (<20%), the cyclicity is best explained by periodic autogenic delta processes in a system where accommodation space in the depositional basin is limited. These findings exemplify the complex interplay of tectonics, climate, and autogenic processes in the generation, transport, and deposition of sediments. Results of this study contribute to the ongoing debate over whether signals generated by large scale, exogenic forcing can survive transport to be preserved in the sedimentary record and help define the temporal and spatial scales at which these processes operate.

How to cite: Gelwick, K., Pazzaglia, F., Kodama, K., Corbett, L., Bierman, P., and Caffee, M.: Decoupled lithostratigraphy, orbitally-driven climate, and tectonics for a middle Pleistocene stratigraphic section in the Northern Apennines, Italy , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4114, https://doi.org/10.5194/egusphere-egu2020-4114, 2020

D1137 |
EGU2020-11201
Stephane Bonnet, Valeria Zavala Ortiz, and Sébastien Carretier

Alluvial fans are cone-shaped bodies of alluvial deposits accumulated along mountain range fronts at the outlet of catchments. They represent valuable archives of mass transfer in their feeding catchment and can potentially be used to infer the impact of tectonic and climatic variations on erosion and landscapes, because of the influence of these factors on the sediment and water fluxes coming from the upstream catchment. Although a transition from aggradation to incision is observed in many natural alluvial fans, the conditions driving such change remain unclear. We investigate this problem here through a laboratory-scale approach where eroded materials from an uplifting mountain may deposit on a plateau, erosion being driven by the surface runoff of water from an artificial rainfall device. We consider here results from 8 experiments, 700 to 900 minutes-long, performed with the same uplift rate but with different sequences of variations of the rainfall rate (10 to 40 minutes-long) between two extreme values. The topography was digitized every 10 minutes thanks to a high-resolution laser sheet.

We observe that the mean slope of the alluvial fans is inversely proportional to the mean rainfall rate on the mountain and that the denudation rate of the uplifting landscape varies in phase with the cyclic variations of rainfall. Because catchments are out of equilibrium (denudation equals uplift) during most of the time, the sediment (Qs) and water (Qw) fluxes at their outlet continuously vary with time: Qs varying depending on the balance between erosion and uplift, Qs and Qw varying depending on whether the catchments enlarge or shrink. Depending on these conditions, catchments show a variety of trends of Qs vs Qw for a given value of rainfall, Qs increasing or decreasing with Qw, or being independent of Qw. Then for each catchment, oscillations of rainfall drive alternations between two individual Qs vs Qw trends, the slope of these trends being indicative of the sediment concentration in the mini-rivers at the outlet of catchments that feed alluvial fans.

From the analyze of our whole dataset, we conclude that incision of alluvial fans occurs when rainfall increases and when it goes with a decrease of the Qs/Qw ratio, i.e. with a decrease of concentration at the outlet of the catchment. This control is modulated by the slope of the fan, incision only occurring for fans above a threshold slope. Then, the decrease in sediment concentration required to initiate the incision is weaker for steeper fans and decreases with increasing fan slope.

Several studies already demonstrated how a decrease of Qs or an increase of Qw drives incision. We show here that these two parameters are coupled and covariate following the dynamical state of catchments. We also demonstrate that the decrease of the Qs/Qw ratio required for initiating the incision of a fan is lower for steeper fans, that is for fans that develop under more arid condition.

How to cite: Bonnet, S., Zavala Ortiz, V., and Carretier, S.: Condition for incision of alluvial fan in an experimental coupled catchment-fan geomorphic system forced by oscillatory precipitation , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11201, https://doi.org/10.5194/egusphere-egu2020-11201, 2020

D1138 |
EGU2020-7461
Janek Walk, Melanie Bartz, Georg Stauch, Mark Reyers, Steven A. Binnie, Dominik Brill, Paulina Vásquez, Fernando A. Sepúlveda, Dirk Hoffmeister, Helmut Brückner, and Frank Lehmkuhl

In the hyperarid environment of the Atacama Desert, alluvial fans are the principle fluvial geo-archive reflecting variations in climate and tectonics in their architecture. While they have been studied in the core of the desert to reconstruct long-term palaeoenvironmental changes from the Oligocene to the Quaternary and, in particular, to constrain the onset of hyperaridity, alluvial fans along the coast (20.5°S – 25.5°S) are younger and show a much higher activity; thus, they can serve as archives during the Pleistocene to Holocene evolution. However, past and recent morphodynamics of the coastal alluvial fans (CAF) are yet poorly reconstructed so that the understanding of an interplay between climatic, geologic, and biotic controls is still challenging.

We related climatic, lithologic, and tectonic source-area characteristics to geomorphometric parameters of the CAF and their catchments. Geomorphometric analyses were conducted based on the 12.5 m TanDEM‑X WorldDEM™, lithological and tectonic data were extracted from regional geological maps, and the frequency of heavy rainfall events derived from a regional Weather Research and Forecasting (WRF) model was used as a climate parameter. We further combined luminescence dating, cosmogenic nuclide exposure dating, and existing chronological data to constrain the timing of Pleistocene alluvial fan deposition as well as the ages of interbedded marine terraces.

Results indicate a primary climatic control on CAF dynamics shown in the functional relationships with catchment hydromorphometrics that mirror a high susceptibility to debris-flows as well as in the temporal pattern of CAF activity. Distinct phases of CAF activity occurred during the late Pleistocene (95–80 ka, 60–45 ka, and 35-20 ka) and the Holocene – driven by atmospheric changes from the Pacific Ocean. The primary source of precipitation is reflected along the latitudinal gradient: frontal systems and cut-off lows mainly originating from the extratropics. Towards the south, an increasing density of Loma vegetation can be observed so that also possible feedback mechanisms of biota on sediment supply need to be considered. While source-area lithology is of negligible relevance for CAF morphodynamics, an important long-term influence of tectonics can be seen in the regional uplift, which in turn controls the catchment shape and relief. From the numerical dating of the marine terraces, uplift rates ranging between ~0.06 and ~0.57 m/ka were derived for the late Pleistocene period.

Acknowledgement: TanDEM-X WorldDEM™ data is provided by a DLR Science grant, 2017.

References:
Bartz, M., Walk, J., Binnie, S.A., Brill, D., Stauch, G., Lehmkuhl, F., Hoffmeister, D., Brückner, H., in press. Late Pleistocene alluvial fan evolution along the coastal Atacama Desert (N Chile). Global and Planetary Change, 103091. https://doi.org/10.1016/j.gloplacha.2019.103091

Walk, J., Stauch, G., Reyers, M., Vásquez, P., Sepúlveda, F.A., Bartz, M., Hoffmeister, D., Brückner, H., Lehmkuhl, F., 2020. Gradients in climate, geology, and topography affecting coastal alluvial fan morphodynamics in hyperarid regions – The Atacama perspective. Global and Planetary Change 185, 102994. https://doi.org/10.1016/j.gloplacha.2019.102994

How to cite: Walk, J., Bartz, M., Stauch, G., Reyers, M., Binnie, S. A., Brill, D., Vásquez, P., Sepúlveda, F. A., Hoffmeister, D., Brückner, H., and Lehmkuhl, F.: Coupled controls of climate, geology, and biota on late Pleistocene alluvial fan morphodynamics along the coast of the hyperarid Atacama Desert, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7461, https://doi.org/10.5194/egusphere-egu2020-7461, 2020

D1139 |
EGU2020-252
Ahmed Obaid

The style of deformation of the Himalaya has been proposed to be localized along the Main Himalayan Thrust (MHT), underlying the entire range and becoming emergent at the Main Frontal Thrust (MFT). An alternative model focuses on the significance of a physiographic boundary known as PT2, south of the Main Central Thrust (MCT), and proposes out-of-sequence deformation. It is interesting to test these models using variations in drainage basin scale hypsometry of the Himalayan range, and to understand the relation between these variations and the main tectonic structures of the Himalaya. This study utilises SRTM 30 m datasets to extract Hypsometric Index (HI) values for the 3rd order river basins of the Himalayan range and southern Tibetan Plateau.

A major change in HI values is coincident with the trace of the Main Frontal Thrust (MFT), with higher values north of this structure than in the foreland to the south. There is smaller magnitude increase in HI across PT2. Results also show a pronounced drop in HI on the northern side of the Himalaya, which is roughly coincident with the location of the South Tibetan Detachment Fault (STDF). The sharp rise in HI values across the MFT is consistent with slip along the MHT raising the entire crustal wedge above it, but the limited rise across PT2 offers no strong support for the out-of-sequence model. The drop in HI across the STDF could represent geomorphic control by the STDF, but this fault has been inactive for millions of years. An alternative explanation is that the decrease in HI values is controlled by underlying changes on the MHT, and the transition from locked to creeping behaviour on this structure.

How to cite: Obaid, A.: Basin scale hypsometry of the Himalayan fold-and-thrust belt and its tectonic implications, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-252, https://doi.org/10.5194/egusphere-egu2020-252, 2019

D1140 |
EGU2020-566
Marcio Cardoso Junior, Ariane Santos da Silveira, Mateus Rodrigues de Vargas, José Manuel Marques Teixeira de Oliveira, Vinicius Lôndero, Dante Vinicius Eloy Barbosa, Luiz Felipe Bertoldi de Oliveira, Lucas Gabriel Ferreira Alves, and Aline Cambri Fredere

The Earth’s surface is a result of tectonic and erosional processes shaping landscapes and preserving transient signs of different evolutionary stages. These transient signs are produced by a gradual adjustment of rivers to an equilibrium stage through channel incision and uplift. The processes effects have different magnitudes according to lithologic contrasts and base level changes that combined influence in disequilibrium phases of bedrock rivers. A integrate study of geomorphic indices in bedrock rivers of the southernmost Brazilian and Uruguayan Shields is developed to identify key signs of transience associated to those surface process and compared between the contrasting drainage basins results. These indices are combined to published thermochronology ages to build a landscape evolution model of these shields. The study area is essentially composed by igneous-metamorphic rocks of Precambrian ages of the Dom Feliciano Belt amalgamated during the Proterozoic-Phanerozoic boundary in the Brasiliano Orogeny. Digital elevation models are used to extract geomorphic indices through interactive MATLAB tools and compared the erosional stages and uplifted regions. This study reveals lineament structures signatures aligned with knickpoints as indicator of the suture zones of distinct terranes in the area. These terranes also feature different erosional stages according to hypsometric results. Thermochronological data support the tectonic framework of three uplift phases starting by the exhumation of western terranes during Devonian ages. A second stage is connected to an uplift preceding the Pangea breakup with the reactivation of Brasiliano Orogeny lineaments. And, the third phase is associated with plate flexural responses of the adjacent oceanic crust during the Cenozoic Era. Finally, the evolutionary model shows strong transient signs in the north region of the studied area indicating a locus of a possible stronger uplift process. In this part of the Dom Feliciano Belt all exhumation phase are evidenced by transient signs of disequilibrium. Differently, the southern region in the Uruguayan Shield shows a more denudated landscape with more mature stages of erosional process.

How to cite: Cardoso Junior, M., Santos da Silveira, A., Rodrigues de Vargas, M., Marques Teixeira de Oliveira, J. M., Lôndero, V., Eloy Barbosa, D. V., Bertoldi de Oliveira, L. F., Ferreira Alves, L. G., and Cambri Fredere, A.: Transient signs in bedrock rivers of the southernmost Brazilian and Uruguayan shields, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-566, https://doi.org/10.5194/egusphere-egu2020-566, 2019

D1141 |
EGU2020-2772
Marcos Pinheiro and Paola Cianfarra

Classical geomorphological and geological researches state that the Brazilian landscapes are mostly influenced by the old tectonics (Pre Cenozoic), considering that such region is currently far away from the South America plate border, where seismic activity is higher. On the other hand, recent studies are pointing out present-day tectonic activity in the southeast Brazil thus revealing that Neotectonics, the tectonic regime acting since Neogene, has an important role on the evolution of the Brazilian landforms, including the Continental Rift of Southeastern Brazil, which comprises a set of 900 km length of ENE-WSW tectonic lines and tertiary basins. However, information about Neotectonic activity in the nearby zones of the rift are still missing, such as in the Serra da Cantareira Ridge, Pico do Jaraguá Hill and Perus region, both of them characterized by outcrops of Pre Cambrian igneous and metamorphic rocks. In this way, the objective of this research is to study the brittle deformation of these areas in order to identify possible traces of Neotectonics. This young tectonic was explored through lineament domains, which were automatically detected by SID software and statistically analyzed through Daisy 3 software. In the field, 712 structural data were surveyed in 51 outcrops and cumulated into databases of the Daisy 3 software, in order to identify the main fault azimuthal trends, fault kinematics, and compute the paleostresses. The lineament analysis show the presence of a principal E-W lineament domain, coincident and possibly related to an old (Neoproterozoic), shear zone probably reactivated in the current tectonic regime.  The field data indicate the predominance of NW-SE, E-W, and NE-SW strike-slip faults, compatible with the left and right-lateral kinematics of the E-W shear corridor. The computed paleostresses are similar to the Neotectonic stress regime identified in the surrounding areas by other researches: NE-SW compression and NW-SE extension (Neogene); Nearly N-S compression and E-W extension (Holocene). On the other hand, only some of the studied faults present evidence of Neotectonic activity. In fact, most of the surveyed faults are closed or mineral-filled, suggesting they are old or were not recently reactivated. The preliminary results of this work suggest the important role of inherited (Pre-Cambrian) crustal weakens crustal zone probably reactivated in Cenozoic and also in the Neotectonic stress regime. Further detailed studies and field surveys are still necessary to highlight the role of the current Neotectonic regime on the present-day Brazilian landscape as well as to better define the geographic extent and location of the E-W shear corridor.

How to cite: Pinheiro, M. and Cianfarra, P.: Brittle Deformation and Neotectonics of the Serra da Cantareira Ridge, Pico do Jaraguá Hill, and Perus Region – Southeastern Brazil, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2772, https://doi.org/10.5194/egusphere-egu2020-2772, 2020

D1142 |
EGU2020-3211
Ke Zhang, Hao Liang, and Zhongyun Li

The Yellow River, as the one of the largest rivers in the world, is considered to be formed by connection of several gorges and basins in between triggered by uplift of Tibetan Plateau. The Junshan Gorge with 600km length is the longest one and it lower gorge, the Senmen Gorge, is the last one for the River feeds into the great north China fluvial plain. This two Gorges used to be the last obstacle for the river running into the sea. In order to better understand the river processes, the Hetao Basin-Jinshan Gorge-Fenwei Basin-Sanmen Gorge-fluvial plain is taken as a whole river-lake system. Under this idea, the unexpected but reasonable complex evolution history of the river-lake system has been reconstructed, and more general evolutional laws for the big river under the tectonic activity and climate change regimes are revealed. In the study area, the terraces can be classified into iso-chronological and meta-chronological ones. Tectonic uplift results in knickpoint headward migration and forms meta-chronological terrace covered by increasing younger paleosol-loess sequences upstream but in most chance by paleosol in Quaternary because of faster and stronger carving during interglacial than glacial periods. The connection between the paleo-lake and its lower gorge form iso-chronological terrace along the gorge but meta-chronological terrace ahead of the gorge. The drainage for the Fenwei paleo-lake into the Sanmen Gorge was earlier (ca. 200ka) than that of the Hetao paleo-lake into the Jinshan Gorge (ca. 100ka), leading to the iso-chronological terrace covered by the paleosol S2 along the Sanmen Gorge while iso-chronological terrace covered by the paleosol S1 from the Jinshan Gorge, Fenwei Basin to Sanmen Gorge. Drainage of the Fenwei Basin resulted in the base level lowering and affected all the rivers that fed into the basin, while drainage of the Hetao Basin only affected the main course of the Jinshan and Sanmen Gorges, resulting in many "suspended valleys" along the gorge where the tributaries fed into because they could not keep pace of the main course incision. The Yuncheng Salt Lake is a relic of Fenwei paleo-lake after the drainages. The Jinshan Gorge is superposed by the broad, V-shape and vertical valleys, respectively. The broad valley was formed by the ancient meandering channel shifting in Pliocene and initial incised in late Pliocene to early Pleistocene, leaving relic meta-chronological terraces covered by the late Pliocene red clay or early Pleistocene loess, and forming popular incised meanderings. The V-shape valley was formed by increasing down cutting initially in middle Early Pleistocene, leaving series of meta-chronological terraces covered by loess-paleosol sequence. The vertical valley was formed by the connection between the gorges and their upper paleo-lakes, leaving iso-chronological terraces covered by S2 or S1. Before river-lake connection, the Jinshan and Sanmen Gorges were affected by slowly tectonic uplift plus periodic climate changes, forming several levels of meta-chronological terraces while after the connection, they were cut down quickly since sharp discharge increased. Comparing with this down cutting, the tectonic uplifts and periodic climate changes could be neglected.

How to cite: Zhang, K., Liang, H., and Li, Z.: How did the Middle Reach of the Yellow River Connect and Form?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3211, https://doi.org/10.5194/egusphere-egu2020-3211, 2020

Chat time: Tuesday, 5 May 2020, 14:00–15:45

Chairperson: Dirk Scherler and Alexander Whittaker
D1143 |
EGU2020-5273
Milena Różycka and Piotr Migoń

The basement block of the Orlickie and Bystrzyckie Mountains represents one of the largest morphostructural units in the entire Sudetes, which are the NE marginal mountain range of the Bohemian Massif. Despite its morphological distinctiveness, the area as a whole has never been studied from the tectonic geomorphology point of view. Thus the morphostructural pattern of the Orlickie and Bystrzyckie Mountains Block has remained poorly recognized. Availability of new sources of elevation data, obtained from airborne laser scanning of the Earth surface (LiDAR), and new tools and techniques offered by GIS software, have opened new research opportunities directed towards the recognition of spatial pattern of tectonic deformations which affected the area.

Quantitative studies of geomorphic expression of tectonic processes presented here are focused on different components of geomorphic systems, including fault-generated mountain fronts, drainage basins of streams crossing the base of these fronts, longitudinal stream profiles, and river valleys. Analyses were also carried out for the entire study area, without differentiation into individual drainage basins or physiographic units of lower order. The results of quantitative analysis were each time confronted with lithological diversity of the area and hence, strength and erosional resistance of different bedrock units. This exercise aimed to isolate signals resulting from non-tectonic controls of landform evolution. It was demonstrated that the influence of lithological diversity on quantitative attributes of landforms and characteristics of fluvial systems is of secondary importance.

In respect to morphometric indices considered as indicators of increasing or decreasing intensities of uplift it is concluded that the information potential of particular measures is not unequivocal. In particular, statistical and spatial correlations between indices calculated for drainage basins and the other indices are imperfect. Similarly, there are ambiguities and inconsistencies concerning inferred intensity of tectonic activity of mountain fronts on the eastern side of the mountain block, although it was possible to distinguish two groups of fronts, of higher and lower relative activity.

Despite partially ambiguous information, but in view of the demonstrated secondary role of lithological diversity in explaining values of morphometric parameters and indices applied in this study, an attempt was made to identify belts of tectonic deformation of relief on the western side of the Orlickie and Bystrzyckie Mountains block. Identification criteria included spatial distribution of strong erosional signal recorded in morphometric attributes of the land surface, longitudinal stream profiles and valley morphology. Three such belts, elongated parallel to the morphological NNW–SSE axis of the mountain block, were recognized. The spatial pattern of variable intensity of endogenic processes is consistent with the geological situation of the region, especially with the distribution of remnants of sedimentary cover of Cretaceous age.

How to cite: Różycka, M. and Migoń, P.: Regional uplift, localized uplift, rock control and erosional response – deciphering gross landform pattern of the Orlickie-Bystrzyckie Mountains Block, Bohemian Massif, Central Europe, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5273, https://doi.org/10.5194/egusphere-egu2020-5273, 2020

D1144 |
EGU2020-5321
Piotr Migon, Kacper Jancewicz, Milena Różycka, and Mariusz Szymanowski

The Sudetes in Central Europe are part of an intraplate belt of highlands and mountains that extends north of the Alps, being also the highest (1602 m a.s.l.) and one of topographically most complex geomorphic units within this belt. The Sudetes consist of numerous semi-isolated mountain massifs, dissected uplands, intramontane troughs and basins, forming a seemingly disordered patchwork of high and low relief. This topographic pattern has developed upon lithologically diverse bedrock, suggesting at least some degree of superposition of different controls. Although specific areas within the Sudetes were subject to analysis focused on the recognition of tectonic imprint in the present-day topography, attempts to disentangle this landscape complexity across the entire Sudetes range were rare and largely inconclusive. Here we approach the problem from the perspective of multidimensional analysis of regional topography, using high-resolution digital elevation data as the primary background material. The building blocks used in the exercise are spatial distribution of altitude and relief, spatial pattern of erosional (dissection) hot spots, position of the main water divide and second-order divides, geometry of main mountain fronts, spatial distribution of surfaces of low relief, considered as inherited planation surfaces, selected features of the regional drainage pattern such as the position of gorges, dominant directions, geometric anomalies etc., and spatial pattern of intramontane basins. Topography is compared with lithology, following an assumption that high strength of a rock unit may also result in considerable elevation and relief, without the necessity to have active tectonics involved. An overlay of these various topographic features allows us to propose intra-regional differentiation of the Sudetes into units typified by different topographic signatures and to separate relief features, both linear and areal, primarily controlled by uplift and subsidence from those reflecting other controls. As an end-result, tectonic interpretation of the contemporary topography is offered.

How to cite: Migon, P., Jancewicz, K., Różycka, M., and Szymanowski, M.: The topographic pattern of the Sudetes and the tectonic message it conveys, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5321, https://doi.org/10.5194/egusphere-egu2020-5321, 2020

D1145 |
EGU2020-5743
Carole Petit, Rolland Yann, Braucher Régis, Bourlès Didier, Cardinal Thibaut, Mariotti Apolline, and Audin Laurence

In South-Western European Alps, although scarce, evidences of recent vertical motions suggest a slow (~0.1 mm/yr) uplift of the northern Ligurian margin, which increases towards to East from the Var river mouth to the gulf of Genova. Whether this uplift is due to active compressional tectonics, to isostatic rebound or to a combination of both is still unclear. In addition, because of the large topographic gradient, rivers have carved deep gorges in the bedrock of the SW subalpine chains. However, neither the role of vertical motion nor that of climatic changes since the LGM on river incision rates is well established.

Over the last 10 years, a dataset of 10Be and 36Cl based cosmic ray exposure (CRE) ages obtained on river and glacier polished surfaces in the SW French Alps has been gathered. This dataset covers several areas located in the Argentera crystalline massif, in the Nice and Castellane subalpine chains, and in the Provence domain.

We will present a compilation of these data in an attempt to answer the following questions: - what is the influence of the last glaciation on river incision rates? - Is there any evidence of a W-E gradient in incision rates that could reflect increasing uplift rates of the SW Alps and North Ligurian margin? First results tend to indicate that all river incision rates are remarkably similar since the Holocene glacial optimum, whereas two different tendencies arise before that time: catchments within the influence of Alpine glaciers tend to have larger incision rates during the last deglaciation, while at the same time catchments out of any glacial influence have slightly lower incision rates. This suggests that, at first order, the release of glacier meltwaters enhanced river incision rates downstream during the ~20-12 ka period.

How to cite: Petit, C., Yann, R., Régis, B., Didier, B., Thibaut, C., Apolline, M., and Laurence, A.: River incision, climate and vertical motions since the LGM in south-western Alps (France), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5743, https://doi.org/10.5194/egusphere-egu2020-5743, 2020

D1146 |
EGU2020-7587
Moritz Liebl, Jörg Robl, David Lundbek Egholm, Kurt Stüwe, and Gerit Gradwohl

The emerging Pleistocene glaciations have left a distinct topographic footprint in mountain ranges worldwide. However, it is still unclear how the formation of cirques above (including the potential destruction of peak relief) and the excavation of glacial troughs below the long-term snowline altered to the large-scale topographic pattern of mountain ranges originally conditioned by fluvial processes.

Some mountain ranges such as the Eastern Alps feature a bimodal topographic pattern characterized by a transition from increasing to decreasing slope with elevation. Bimodality might be an expression of glacial reshaping, as glacial troughs with steepened valley flanks have been formed at low elevations and low relief surfaces at high elevations. On the other hand, bimodality might represent the state of fluvial prematurity as expression of ongoing landscape adjustment to an uplift event in the recent past. Despite their completely different evolution, both hypotheses lead to a bimodal landscape with a similar slope-elevation distribution.

In this study, we explore the impact of cold climate erosional processes on the mountain range scale topographic pattern. For this, we use synthetically generated and natural mountain range landscapes conditioned by fluvial processes and apply a surface process model for cold climate conditions (iSOSIA). In regions with high glacial impact, we explore an upstream migrating glacial signature represented by two frequency maxima in the slope elevation distribution at lower elevations (i.e. below the snowline, where glacial troughs formed). This is accompanied with an increase in slope on average compared to the initial topography. Above the snow line, bimodality vanishes and mean slope is similar to the initial fluvial topography. Interestingly, in the Eastern Alps, we explore a similar pattern where the transition from increasing to decreasing slope with elevation is located at about 1800 m, which is roughly at the position of the last glacial maximum (LGM) snowline of this region.

How to cite: Liebl, M., Robl, J., Egholm, D. L., Stüwe, K., and Gradwohl, G.: The imprint of glacial and periglacial erosion processes on fluvial landscape metrics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7587, https://doi.org/10.5194/egusphere-egu2020-7587, 2020

D1147 |
EGU2020-7949
Juan Ricardo Diaz Munoz and Sheng Rong Song

The Colombian emeralds are  well-known green gems, which are very famous in its unique characteristics and quality of color and sizes in the world. It is dominantly distributed in the Eastern Cordillera of Colombia with underground mining in various locations of about 3km2 and a total extent of 500km2, separated by approximately 130Km are located the Eastern Emerald Belt ( EEB) and the Western Emerald Belts (WEB), in a general context they share chemical and tectonic similarities, but, with a complex tectonic evolution.

The geology of emerald and its tectonic configuration is believed to be composed of a series of disharmonic structures, e.g. thrusted and folded areas. Current and past exploitations created many mines, more than few tens in WEB, which are predominantly distributed in three areas, Muzo, Cunas and Coscuez. Based on field surveys into those mines, we observed paths that suggest the location of mines in debris flow deposits or slumped areas, which are characterized by matrix-supported structures with  block sizes ranging from few cm to  hundreds of meters. Rock types of blocks include black shale, calcite-rich veins with emeralds, stratiform-pyrite shale, hydrothermal hydraulic breccia, albitite. Most of the emeralds occur in calcite veins, but those cannot be traced along the veins in the mines and suddenly crosscut with no common factors involved (faults, discontinuities). The lines of evidence suggest that  the current mining of the emeralds in some places takes place on  slumped blocks or matrix of debris flow deposits. These observations attached with remote sensing techniques ( DEM, DTM, LANDSAT, AERIAL IMAGES) on WEB show slumped areas are well correlated with emerald mines in those three exploited areas. These findings could be of great usefulness for further exploration, ongoing research projects about the Eastern cordillera uplifting and emeralds worldwide tectonostratigraphy.

How to cite: Diaz Munoz, J. R. and Rong Song, S.: Emerald Mining in Large Scale Debris and Slumped Blocks from the Eastern Cordillera of Colombia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7949, https://doi.org/10.5194/egusphere-egu2020-7949, 2020

D1148 |
EGU2020-8014
Riccardo Reitano, Claudio Faccenna, Francesca Funiciello, Fabio Corbi, and Sean Willett

Convergent orogens are the best places on Earth for studying the interaction between surface processes and tectonics. They display the highest surface uplift rates and in turn are more likely affected by erosion. The balance between tectonics and erosion is responsible for many aspects in the evolution of a mountain belt. Despite the growth of analysis techniques, our understanding is still limited by the impossibility to observe these processes through their entire evolution. In particular, understanding how single parameters affect the system is necessary to unravel the nature of these multiple-interrelated processes.

Here we propose a new series of analogue models reproducing a simplified and scaled natural convergent orogenic system, to investigate the evolution of landscapes in which both tectonics and erosion/sedimentation are present. The growth of the orogenic wedge is driven by a rigid plate pushing the rear of the model. Deformed brittle granular material is a mixture of silica powder, glass microbeads and PVC powder. This mixture allows for the observation of both deforming structures and geomorphic features. Erosion is simulated by a water sprinkler system, providing a fine mist as precipitation which collects into simulated rivers, shaping the landscape. The model therefore allows observing the interaction between tectonics and surface processes. We analyze the model evolution monitoring oblique-view with cameras and top-view with a laser scanner. The latter is useful for measuring the mass balance between input fluxes (tectonics) and output fluxes (erosion) and in fulfilling a proper parametric study on the cause-effect relationship. The effect of different parameters on landscape evolution (e.g., precipitation rate, convergence velocity) is investigated systematically.

Our preliminary results analyze the relationship between single parameters and their effect on the models, allowing a proper definition of the role played in the landscape evolution. We also set up a benchmark with numerical models using DACI3ELVIS code in the same tectonic setting.

How to cite: Reitano, R., Faccenna, C., Funiciello, F., Corbi, F., and Willett, S.: Factors controlling the interaction between tectonics and surface processes in convergent orogens: insight from analogue and numerical models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8014, https://doi.org/10.5194/egusphere-egu2020-8014, 2020

D1149 |
EGU2020-8761
Gerold Zeilinger, Julius Jara-Muñoz, Jonathan R. Weiss, and Esther Lee

The Salar de Uyuni spans almost the entire width of the Bolivian Altiplano, thus providing a potential record of large wavelength deformation, which can be produced by various mechanism across the central Andean plateau interior. This study focuses on the mapping of paleo-lake terraces, which are geomorphic markers that represent past lake level positions and can be used to study differential vertical deformation. High-resolution TanDEM-X topography for the region, in combination with satellite imagery, reveal a wide range of well-preserved lake terraces in the Salar. Eleven prominent terraces have been identified in the study area at elevations ranging from 3701 m to 3815 m and with ages ranging from ~11.7-16.1 ka based on correlation with published ages. The elevation difference between the younger terraces (Level 1 to 5) is ~51 m in the west and ~46 m in the east, indicating an eastward tilting of about ~5 m across the Salar de Uyuni. The older terraces (Level 8 to 11), however, record an elevation difference of ~20 m in the west and ~ 24 m in the east, indicating a westward tilt of ~4 m. Thus a change in the polarity of tilting of the Uyuni paleo-lake basin occurred between the formation of terraces 5 to 8 from 13.1-14.8 ka.

We discuss different mechanisms that might drive this large wavelength deformation including 1) eastward tilting as a direct consequence of horizontal shortening in a compressional setting and “backtilting” by stress release during thrusting on a deep-seated structure, 2) addition of differentiated igneous bodies derived from the mantle perhaps associated with delamination processes, and 3) seismic coupling along the Chile subduction zone margin. Removal or delamination of mantle lithosphere is unlikely to produce 4 m of uplift in the relatively short, ~2 ka time span of our observations. Well-documented megathrust coupling and the subduction zone seismic cycle would explain the short time span but is unlikely to create significant vertical deformation ~200 km from the coast. We favour and explore the hypothesis that Andean shortening leads to large wavelength flexure (as the expression of an elastic deformation) as a result of strain accumulation that is eventually released by slip along structures beneath the Eastern Cordillera that are perhaps related to the active decollement and fold-and-thrust belt that comprise the Subandean ranges. The observed pattern of paleo-lake terraces may serve as a geologic archive recording a phase of major backarc seismic activity at ~14 ka.

How to cite: Zeilinger, G., Jara-Muñoz, J., Weiss, J. R., and Lee, E.: Large-wavelength deformation across the central Andean plateau interpreted from Salar de Uyuni (Bolivia) paleoshorelines , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8761, https://doi.org/10.5194/egusphere-egu2020-8761, 2020

D1150 |
EGU2020-8884
Timothee Sassolas-Serrayet, Martine Simoes, Rodolphe Cattin, Romain Le Roux-Mallouf, Matthieu Ferry, and Dawchu Drukpa

The quantification of active tectonics from geomorphological and morphometric approaches most often implies that erosion and tectonics have reached a certain balance. Such equilibrium conditions may however be seldom found in nature, as questioned and documented by recent theoretical studies, in particular because drainage basins may be quite dynamic even though tectonic and climatic conditions remain constant.

Here, we document this drainage dynamics from the particular case example of the Bhutan Himalayas. Evidence for out-of-equilibrium landscape features have for long been noticed in Bhutan, from major (> 1 km high) river knickpoints and from the existence of high-altitude low-relief surfaces within the mountain range. These geomorphologies were generally interpreted in the literature as representing a recent change in climatic and/or tectonic conditions, either related to the uplift of the Shillong Plateau (climate/tectonic change) or to the initiation of uplift over a blind ramp within the mountain range (tectonic change).

To further characterize these geomorphologies and discuss their origin and meaning in terms of regional tectonic or climatic evolution, we perform a detailed quantitative geomorphometric analysis using c plots and basin averaged aggressivity metrics, at various spatial scales, from large Himalayan rivers to local streams draining the low-relief surfaces. Our results first emphasize that the morphology of Bhutan does not result from a general wave of incision propagating upstream, as expected from most previous interpretations. Rather, we find that the river network is highly unstable and dynamic, in particular for the rivers draining the low-relief surfaces, hampering a proper quantification of tectonics from classical approaches based on denudation or incision rates. Finally, we discuss the origin and meaning of the observed dynamics, and from there draw some useful guidelines for future morpho-tectonic studies of active landscapes.

How to cite: Sassolas-Serrayet, T., Simoes, M., Cattin, R., Le Roux-Mallouf, R., Ferry, M., and Drukpa, D.: Quantifying active tectonics in the case of dynamic and instable landscape: an example from the Bhutan Himalayas, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8884, https://doi.org/10.5194/egusphere-egu2020-8884, 2020

D1151 |
EGU2020-9477
Silvia Crosetto, Sabrina Metzger, Dirk Scherler, and Onno Oncken

The Pamir and Hindu Kush are located at the western tip of the India-Asia collision zone. Approximately a third of the northward motion of India’s western syntax is mostly accommodated by continental-scale underthrusting of the Indian plate beneath Asia. On its way northwards the arcuate, convex Pamir mountain range acts as a rigid indenter penetrating the weaker Eurasian plate, while lateral extrusion occurs to the west in the Tajik Depression.

Intense present-day shallow seismicity indicates active deformation along the northern and north-western semi-arid margin of the Pamir, where over the last century several M>6 and three M>7 crustal earthquakes, including a recent M6.4 event in 2016, were recorded. Earthquakes are distributed in the proximity of three main fault systems: the Pamir thrust system to the north, and the Darvaz fault and Vakhsh thrust system to the north-west. The pronounced topographic expression of these lithospheric faults is associated to a deeply incised landscape, which was profoundly shaped by past widespread glaciations. The transient evolution of the landscape following deglaciation is observed in the dynamic river network, characterised by intense fluvial incision and changes in the fluvial connectivity of the drainage system.

At depth, recent seismic tomography studies suggest delamination, stretching and tearing of the Asian slab beneath SW Pamir, and slab break-off underneath Hindu Kush. Slab break-off episodes are known to result in stress surges in the overlying lithosphere, potentially causing deformation and uplift.

In this complex system characterised by an important interplay between tectonics, climate and surface processes, we use qualitative and quantitative analyses of the topography and of the drainage systems evolution, inclusive of numerical tools, in order to define what is –and has been- the role played by the main lithospheric active faults of this area. In addition, we aim at identifying how landscape and surface dynamics respond, temporally and spatially, to processes, such as slab tearing/break-off, occurring at depth.

How to cite: Crosetto, S., Metzger, S., Scherler, D., and Oncken, O.: From depth to surface: how deep-earth processes and active tectonics shape the landscape in Pamir and Hindu Kush, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9477, https://doi.org/10.5194/egusphere-egu2020-9477, 2020

D1152 |
EGU2020-10639
Kobi Havusha, Liran Goren, and Ron Nativ

Fluvial drainage systems are organized in drainage basins, whose boundaries are defined by water divides. The network of divides determines the geometry of the basins and the distribution of drainage area along flow. Drainage basins obey global geometric-geomorphic scaling relationships. These include Hack’s law that predicts the relation between channel length (L) and drainage area (A): L = c∗Ah  where c and h are referred to as Hack’s coefficient and exponent, respectively. These parameters have a relatively narrow range of 1.1 ≤ c ≤ 2.7 and 0.45 ≤ h ≤ 0.6. Additionally, the distance between basin outlets (S) has been shown to scale linearly with the distance between the main divide and the mountain front (W) and is expressed by the ratio: R = W ⁄ S , where R is within the range of 1.91 ≤ R ≤ 2.23. When the tectonic and climatic conditions change through time, drainage basins can change their geometry. It is not clear, however, if and how the global scaling relations evolve when basins change their shape and size. This gap in our understanding specifically relates to the links between geomorphic processes and surface forms. A promising approach to study fluvial landscape evolution is by using physical laboratory-scale models. These models provide a unique opportunity to study the details of drainage network evolution and geometrical changes by constraining climate and uplift and by maintaining the lithological parameters constant and uniform. In the current study, we utilize DULAB (Differential Uplift LAndscape-evolution Box), an experimental apparatus that simulates mountainous landscape evolution, to study the evolution of basin geometrical scaling relations. Our experimental scheme consists of two distinct settings: (1) uniform uplift, with basins that grow by incising backward towards an uplifting and shrinking plateau, and (2) differential uplift, where the main drainage divide migrates towards the higher uplift rate side, and the drainage basins adjust accordingly. During the experiments, precipitation is held constant, and we document the landscape geometry in predefined time intervals by applying a “Structure from Motion” algorithm on a series of photos. Experimental results show that while basins drastically change their size and shape, they tend to maintain the globally observed geometrical scaling relations. Hack’s parameters are computed to be c = 2.29 ± 0.08  and h = 0.51 ± 0.02 and the spacing ratio, R is R = 2.95 ± 0.4. This is achieved as only a subset of basins grow towards the migrating divide, while other basins maintain their former geometry or shrink. Additionally, processes of reorganization, such as basins merging close to their outlets and inter-basin divide migration, assist in maintaining the geometrical scaling relations.

How to cite: Havusha, K., Goren, L., and Nativ, R.: Geometrical Scaling Relations of Drainage Basins During Basin Evolution, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10639, https://doi.org/10.5194/egusphere-egu2020-10639, 2020

D1153 |
EGU2020-11034
Stephanie Neuhuber, Zsófia Ruszkiczay-Rüdiger, Christopher Lüthgens, Philip Martin, Bernhard Salcher, Esther Hintersberger, Régis Braucher, Johannes Lachner, Sandra Braumann, and Markus Fiebig

Fluvial terraces within the extensional structure of the Vienna Basin have been dissected by faults related to the sinistral movement of the Vienna Basin Transform Fault System (VBTF, Decker et al., 2005). Each fault block within the basin displays a slightly different succession of terraces regarding their number, elevation, and preservation. Generally, altitudes of terrace bases within the Vienna Basin vary between 5 and 130 m above the recent Danube river bed.

This study focuses on one clearly confined fault block, the Rauchenwarth Plateau, located south of the Danube. The plateau forms the western part of intra-basinal hills crossing the Vienna Basin and consists mainly of Miocene sediments that are in part covered by quaternary fluvial terrace deposits at different elevations. The entire succession is widely covered by loess or re-deposited aeolian sediments. To depict the formations below the loess cover we use 19 wells to construct three sections crossing the eastern part of the block in E-W and two parallel sections in N-S direction. The sections show that three levels of fluvial terraces at the northern eastern side of the block are preserved. The lowest and highest levels are accessible in gravel pits with well-defined Miocene bases. These two levels with terrace bases ~67 m and ~24 m above the recent Danube contain large quartz cobbles suitable for dating using in-situ produced 26Al and 10Be. Sample sets were taken at 11 m (higher terrace) and 14 m (lower terrace) below todays surface. Sandy sediments from the lower level were in addition dated by luminescence on feldspar using the pIRIR 225 signal. Age calculations using the isochron method (Balco and Rovey, 2008) as well as inverse modelling for the upper level suggest burial durations of ~1.2 Ma. Results of age calculations using cosmogenic nuclides as well as luminescence ages for the lower level will be presented at the conference.

Thanks to NKFIH 124807; OMAA 90öu17, the INSU/CNRS, the ANR through the program “EQUIPEX Investissement d’Avenir” and IRD

References

Balco, G., Rovey, C., 2008. Am. J. of Science 308, 1083-1114.

Decker, K., et al., 2005. Quat. Sci. Rev. 24, 305-320.

How to cite: Neuhuber, S., Ruszkiczay-Rüdiger, Z., Lüthgens, C., Martin, P., Salcher, B., Hintersberger, E., Braucher, R., Lachner, J., Braumann, S., and Fiebig, M.: Numerical age dating of Danube terraces from one fault block (Rauchenwarth) west of the Vienna Basin Transform Fault (Austria) , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11034, https://doi.org/10.5194/egusphere-egu2020-11034, 2020

D1154 |
EGU2020-12013
Victor Sacek, Tacio Cordeiro Bicudo, and Renato Paes de Almeida

The Andean orogeny had a profound impact on the evolution of the Amazon drainage system, modifying the climate in South America and the influx of sediments to the interior and marginal sedimentary basins. Additionally, the subduction of the Nazca plate under the continent produced dynamic topography that perturbed the landscape and the generation of accommodation space in the interior sedimentary basins mainly in western Amazonia. Therefore, the correct interpretation of the geological evolution of the northern South America during the Cenozoic depends on the coupling of different geodynamic processes with the erosion of the continents, deposition in the sedimentary basins and the interaction with the evolving climate. Due to the great complexity of the different processes involved in the geological evolution of Amazonia, the use of numerical models is a natural way to treat this problem. The aim of this work is to present numerical scenarios for the formation and evolution of the Amazon drainage system taking into account surface processes along with geodynamic processes like Andean uplift, flexure of the lithosphere, and dynamic topography induced by mantle convection. We conclude that the Amazon drainage system was formed essentially by the asymmetric influx of sediments from the Andes, while the dynamic topography modulated the timing for the transcontinental connection between western and eastern Amazonia and the stratigraphic evolution of interior basins.

How to cite: Sacek, V., Cordeiro Bicudo, T., and Paes de Almeida, R.: How the Andean tectonics and dynamic topography shaped the landscape evolution in Amazonia: a numerical approach, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12013, https://doi.org/10.5194/egusphere-egu2020-12013, 2020

D1155 |
EGU2020-12306
Shyh-Jeng Chyi, Jia-Hong Chen, Jiun-Yee Yen, Lih-Der Ho, Chia-Hung Jen, Christopher Lüthgens, Ting-Yu Wu, Ting-Yi Chang, I-Chin Yen, and Cheng-Hao Lu

Compare to rivers originated from western Taiwan flowing westward, rivers   originated from the southeastern side of the Central Range and the eastern side of the Coastal Range flow eastward directly into the Pacific Ocean and form very narrow alluvial plains or coastal plains immediately next to the mountain front. Based on the field evidences and mapping from field and high-res DTM, we classified these river basins into two types.

 

The geomorphic features of the first type are remarkably wide valley plain with flights of fill terrace and relatively narrow active channel in the downstream area. The radiocarbon dates of terrace sediments indicate that large-scale aggradation took place before 7ka, and formed fill terraces with the largest relative height of around 50 meters relative to the modern channel bed in the mid to late Holocene. We proposed the landscape evolutionary history for the first type of river basins is that significant river aggradation caused by rapid sea-level rise in estuary during the late Pleistocene to the early Holocene, followed by continuous and slow uplift or the relative sea-level falling that induced a long term basin-wide river incision.

 

The geomorphic features of the second type of the river basins are those that the knickpoint developed in the igneous rock gorge near the river mouth and often formed incised meander and unpaired rock terraces in its upstream area. The radiocarbon dates of terrace sediments indicate the average bedrock incision rate of upstream area is  noticeably lower than the rate near  the coast/river mouth area. For the second type river basins, we proposed that the climate turns warm and wet since the end of the last glacial period and the retreat of knickpoint in the igneous rock gorge exert the primary influence on terrace formation in the upper reaches, and the relative sea level falling is the main control on the terrace formation in the coastal area. In addition to those, the terraces of the main tributaries of the second type river basins which reveal the different cut-and-fill histories might be the results of complex response of sub-drainage systems to the multiple controls.

How to cite: Chyi, S.-J., Chen, J.-H., Yen, J.-Y., Ho, L.-D., Jen, C.-H., Lüthgens, C., Wu, T.-Y., Chang, T.-Y., Yen, I.-C., and Lu, C.-H.: Lithological control of drainage basins development post LGM and oscillating climate condition, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12306, https://doi.org/10.5194/egusphere-egu2020-12306, 2020

D1156 |
EGU2020-16537
Dylan Rood, Alex Hughes, Alex Whittaker, Rebecca Bell, Klaus Wilcken, Ashley Corbett, Paul Bierman, Duane DeVecchio, and Thomas Rockwell

Spatial and temporal variations in fault activity informs models of seismic hazards and can affect local patterns of relief generation and channel morphology. Therefore, the quantification of rates of fault activity has important applications for understanding natural hazards and landscape evolution. Here, we quantify the complex interplay among tectonic uplift, topographic development, and channel erosion recorded in the hanging walls of several seismically-active reverse faults in the Ventura basin, southern California, USA. We use cosmogenic 26Al/10Be isochron burial dating to construct a basin-wide geochronology for the Saugus Formation: an important, but poorly dated, regional Quaternary strain marker. Our geochronology of the Saugus Formation is used to calculate tectonically-driven rock uplift rates and reduce uncertainties in fault-slip rates. In addition, we calculate 10Be catchment-averaged erosion rates, characterise patterns of catchment relief and channel steepness indices, and analyse river long-profiles in fault hanging walls to compare with patterns of fault displacement rates averaged over various temporal scales.

 

The results of the burial dating confirm that the Saugus Formation is time-transgressive with ages for the top of the exposed Saugus Formation of ~0.4 Ma in the western Ventura basin and ~2.5 Ma in the eastern Ventura basin. The burial ages for the base of shallow marine sands, which underlie the Saugus Formation throughout the basin, are ~0.6 Ma in the western Ventura basin and ~3.3 Ma in the eastern Ventura basin. The results of the landscape analysis indicate that relief, channel steepness, and erosion rates are still adjusting to tectonic boundary conditions imposed by different tectonic perturbations that have occurred at various times since ~1.5 Ma, which include fault initiation and fault linkage. The data presented here suggest that, for transient landscapes in sedimentary basins up to 2500 km2, where climate can be considered uniform, fault activity is the primary control on patterns of relief generation and channel morphology over periods of 10to 10years.

How to cite: Rood, D., Hughes, A., Whittaker, A., Bell, R., Wilcken, K., Corbett, A., Bierman, P., DeVecchio, D., and Rockwell, T.: Tectonically-dominated Quaternary landscape evolution of the Ventura basin, southern California, quantified using cosmogenic isotopes and topographic analyses, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16537, https://doi.org/10.5194/egusphere-egu2020-16537, 2020

D1157 |
EGU2020-20549
David Whipp and Lars Kaislaniemi

Orogen fold-and-thrust belts (FTBs) often have a tapering wedge geometry in cross section, which develops as a result of the balance between stresses acting along the detachment fault beneath the wedge, its internal strength, and the average slope of the surface topography from the back of the wedge to its toe. The geometry of these critical wedges is thus sensitive to changes in factors that influence stress along the wedge base or the surface slope, including changes in the mechanical strength of the detachment fault or variations in surface erosional efficiency. The Andes of eastern Bolivia have differences in the basal detachment strength, resulting from a thinning of the weak Paleozoic sediments that host the basal detachment, and average annual rainfall north and south of the bend in the orogen at ~18°S. In addition, the orogen and active Subandean FTB are ~50% narrower in the north, where both the detachment layer strength may be higher and the average annual rainfall is around eight times that in the south. This raises the question: What controls orogen width in the Bolivian Andes?

We explore the effects of variations in the mechanical strength of the basal detachment and surface erosional efficiency on FTB width using 3D numerical geodynamic models with lateral variations in these parameters along strike. Our numerical experiments calculate the orogen geometry using the DOUAR geodynamic modelling software (Braun et al., 2008) coupled to the FastScape surface process model (Braun and Willett, 2013). The model design includes an elevated plateau region that is thrust over a weak frictional plastic detachment layer, resulting in growth of an orogenic wedge at the distal plateau margin. The plateau geometry is also bent, including a 40° change in margin orientation along strike; changes in the erosional efficiency and detachment strength are varied on either side of this bend. We find that changes in detachment strength result in significant differences in FTB width, while changes in erosional efficiency have little effect. Increasing the detachment strength by two results in limited forward propagation of the thrust front and a reduction in the FTB width by roughly 50% compared to the weaker side of the model. In contrast, increasing precipitation by a factor of three (as a proxy for enhanced erosional efficiency) does not significantly effect the FTB width. These results compare well with the observed variations in orogen width in the Bolivian Andes, suggesting the FTB width may be controlled by the detachment strength, while variations in erosional efficiency have a limited effect. Ongoing work is exploring how changes in detachment strength and erosional efficiency may affect thermochronometer ages predicted from the numerical experiments, and how the predicted ages compare to ages observed in the Bolivian Andes.

How to cite: Whipp, D. and Kaislaniemi, L.: Lithological and erosional controls on orogen width in the Bolivian Andes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20549, https://doi.org/10.5194/egusphere-egu2020-20549, 2020

D1158 |
EGU2020-444
Katharine Groves, Mark Allen, Christopher Saville, Martin Hurst, and Stuart Jones

The formation and uplift history of the Tibetan Plateau, driven by the India-Eurasia collision, is the subject of intense research. Geomorphic indices capture the landscape response to competition between climate and tectonics and reflect the spatial distribution of erosion. We analyse the link between climate and tectonics in the eastern part of the Tibetan Plateau using the mean annual precipitation, digital elevation data, and by calculating the geomorphic indices hypsometric integral (HI), surface roughness (SR) and elevation relief ratio (ZR). This is a region where competing tectonic models suggest either early Cenozoic plateau growth, or a late phase of crustal thickening, surface uplift and plateau growth driven by lower crustal flow (“channel flow”).

Swath profiles of rainfall, elevation and the geomorphic indices were constructed, orthogonal to the internal drainage boundary. Each profile was analysed to find the location of maximum change in trend. A broad transition zone is present in the landscape, where changes in landscape and precipitation are grouped and in alignment. The zone cuts across structural boundaries. It represents, from East to West, a sharp decline in precipitation below ~650 mm/yr (interpreted as the western extent of the East Asian monsoon), a change from a high relief landscape to smoother elevations at 4500-5000 m, a transition to low HI (< 0.05), a decrease in SR and an increase in ZR. This zone is not a drainage divide: the main rivers have their headwaters further West, in the interior of the plateau.

We argue that this geomorphic-climatic transition zone represents a change from incised to non-incised landscapes, the location of which is controlled by the western extent of the monsoon. Published low temperature thermochronology data suggest the plateau had reached its modern extent at the Eocene, but has been exhumed since ~15 Ma to the East of the transition zone, at least along major drainage networks. We therefore also suggest that the transition zone is the current position of a long-term wave of incision that has migrated from East to West, driven by late Cenozoic intensification of the monsoon climate. This work supports a model of early Cenozoic growth of the eastern Tibetan Plateau, superimposed by incision driven by climate change; it does not support the channel flow model.

How to cite: Groves, K., Allen, M., Saville, C., Hurst, M., and Jones, S.: Incision migration across Eastern Tibet controlled by monsoonal climate, not tectonics?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-444, https://doi.org/10.5194/egusphere-egu2020-444, 2019

D1159 |
EGU2020-5007
Clement de Sagazan and Jean-Arthur Olive

Tectonic models commonly predict that erosion and sedimentation enhance strain localization onto a few major faults at subaerial plate boundaries such as orogens and continental rifts. By contrast, the influence of “seafloor-shaping processes” on the tectonic makeup of submarine plate boundaries has received far less attention. Submarine plate boundaries are however subjected to a wide range of sedimentation rates, and as such constitute excellent natural laboratories to investigate the influence of sediment deposition on seafloor shaping tectonics. Here we assess the impact of sedimentation on fault development at the Andaman Sea spreading center (ASSC), by comparing it to unsedimented mid-ocean ridges (MORs) of commensurate spreading rate (38 mm/yr).

Seafloor spreading has been occurring for the last ~4 Myrs along the ASSC, which is located at the center of a pull-apart basin in the back-arc domain of the Sumatra subduction. Recent bathymetric and seismic reflection data show that fault-induced topography at the ASSC is buried under a sedimentary layer of thickness up to 1.5–2 km. This massive sedimentary input is largely provided by the Irawaddy river, and amounts to an average deposition rate of ~0.5 mm/yr over the last 4 Myrs. The structure of the ASSC is analogous to an intermediate- / slow-spreading MOR, with symmetric, evenly spaced axis-facing normal faults. The characteristic spacing of these faults is however unusually large (8.8 km) and their dips are unusually shallow (~30º) compared to typical MORs.

We use numerical modeling to assess whether sedimentation can explain the unusual longevity of ASSC normal faults. We use the FLAC method to model a spreading ridge subjected to a sedimentation rate ranging from 0 to 1 mm/yr. In our models, a fraction M of plate separation (between 0.6 and 0.8) is taken up by magma injection. This allows the sequential growth of regularly-spaced, axis-facing faults. In the absence of sedimentation, fault lifespan and spacing decrease with increasing M. We find that, for a given M of 0.7 or above, increasing the sedimentation rate increases fault lifespan by as much as ~50%, and the effect plateaus for rates > 0.5 mm/yr. By contrast, we cannot resolve any significant effect of sedimentation on fault lifespan for M < 0.7. The effect of sedimentation is more pronounced on fault spacing, with rates as fast as 1 mm/yr nearly suppressing the decrease in spacing with increasing M.

We propose that sedimentation prolongs slip on active faults by leveling seafloor relief and raising the threshold for breaking new faults. The effect is more pronounced for faults with a slower throw rate, which is favored by a greater M fraction. Our simulations show that enhancement of fault lifespan by sediment blanketing is a viable explanation for the anomalously high spacing of normal faults at the ASSC. This could therefore constitute the first field evidence of topographic reworking promoting strain localization at a major plate boundary, a mechanism predicted by over two decades of geodynamic modeling.

How to cite: de Sagazan, C. and Olive, J.-A.: Fault spacing enhanced by sedimentation at the Andaman Sea spreading center, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5007, https://doi.org/10.5194/egusphere-egu2020-5007, 2020

D1160 |
EGU2020-5239
Michele Delchiaro, Marta Della Seta, and Salvatore Martino

In the hillslope landscapes of tectonically active regions, the steep topography represents the most evident result of rock uplift, valley incision and landslide erosion. In response to rock uplift, relief and hillslope dip increase linearly in time mainly due to fluvial erosion processes in landscapes affected by low to moderate tectonic forcing. Nonetheless, such a linear increase in relief and hillslope dip is limited by the reaching of threshold slope conditions associated with the hillslope material strength, until the latter is exceeded by gravitational stress giving rise to bedrock landslides. In this regard, Mass Rock Creep (MRC) process may become a primary factor for damaging rock masses so leading to slope failures that generate huge rock avalanches. MRC acts on large time-space scale through a continuous and non-linear variation of stress-strain conditions of entire portions of slopes and the coupled role of tectonics and landscape evolution represents a predisposing factor for Deep Seated Gravitational Slope Deformations (DSGSD).

This research focused on the Loumar DSGSD that affects the NE slope of the Palganeh anticline in the Lorestan region (Zagros Mts., Iran), almost 90 km northwest of the Seymareh landslide which is more famous as it represents the largest landslide on Earth surface. The Loumar DSGSD evolution is strictly related to the vertical and lateral growth of the fold and to the evolution of the Seymareh river drainage system that kinematically released the slope at the bottom likely causing the initiation of the deformational process. We combined an inverse modelling of the river profiles linked to the fold uplift history and the analysis of a plano-altimetric distribution of geomorphic markers, correlated to the detectable knickpoints along the river longitudinal profiles, which allowed to constrain the main morpho-evolutionary stages of the valley. These data will be used to constrain a Landscape Evolution Model (LEM) and a stress-strain numerical model, to be performed under time-dependent creep conditions, that will be calibrated by a back analysing the slope evolution from the LEM. The final goal will be to discuss the possible role of impulsive triggers (earthquakes) in anticipating the time-to-failure of the MRC deformational process.

How to cite: Delchiaro, M., Della Seta, M., and Martino, S.: Evaluation of tectonics and landscape evolution as predisposing factor for a Mass Rock Creep deforming slope in the Zagros Belt (Iran), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5239, https://doi.org/10.5194/egusphere-egu2020-5239, 2020

D1161 |
EGU2020-6147
Jeni McDermott and Tim Redfield

The sharp, asymmetric ‘Great Escarpment’ of southwestern Norway mimics landforms commonly associated with fault-controlled ‘footwall uplift’ mountain ranges, bringing into question whether climate-driven erosion and consequent mass redistribution can generate kilometer scale topographic relief, or if tectonic forces are required instead.  Here we report on patterns of relief and fluvial incision in a region characterized by glacial sculpting, rapid isostatic uplift, and a well-established brittle template of normal faults.

The Surna valley (Surnadalen) of mid-southern Norway is a SW-NE striking wide, alluvial, U-shaped valley whose SW margin defines part of the Great Escarpment. Surnadalen displays clear morphometric asymmetry: its inland (SE) side is defined by high elevation (>1000 m) and well-developed drainage networks that display clear evidence of alpine glacial carving, while its seaward side is lower (~500 m) and has neither developed drainage networks nor evidence for valley glaciers. Inland drainages display a distinct set of aligned knickzones that maintain characteristics inconsistent with transient fluvial response to deglaciation. Incision occurs across fluvial process zones with no correlation to drainage area, suggesting regional forcing rather than catchment-scale drivers. Both lithology and structure are nearly identical across greater Surnadalen, and no change in rock type or erodibility correlate with the incision zones. Incision is axially asymmetric: All knickzones occur at the base of the ‘Great Escarpment,’ and the Tjellefonna Fault Zone (TFZ), a strand of a regionally important fault complex, projects into Surnadalen’s axis and aligns directly with the knickzone trace. The depth of incision decays from SW to NE in the direction of propagation of the TFZ tip at a mathematically predictable rate. We interpret the knickzone alignment to reflect active normal fault control over incision localization and depth. The depth and morphology of incision suggests Surnadal’s incision survived multiple glacial cycles. This interpretation implies that Norway’s ancestral structural template continues to impose a fundamental control over the creation and maintenance of the Great Escarpment. Although fault reactivation is not the result of regional tectonic extension, but rather is likely the product of erosion-induced shifting of loads, the pre-existing margin architecture appears to dominate the isostatic response to erosion.

How to cite: McDermott, J. and Redfield, T.: Landscape rejuvenation controlled by neotectonic fault reactivation on Norway’s post-glacial rifted margin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6147, https://doi.org/10.5194/egusphere-egu2020-6147, 2020

D1162 |
EGU2020-9824
Lorenzo Gemignani, Daniel Simon, Benedict Mittelbach, Kristina Hippe, and Mark, R. Handy

River terraces located in the Tropoja Basin in northern Albania are characterized by high elevation (c. 600 m) and by multiple incision events that highlight the interaction between tectonically induced surface uplift, glaciation, and erosion. The Tropoja Basin is located in the hanging wall of the Shkoder-Peja Normal Fault (SPNF) a crustal-scale normal fault trending orthogonal to the strike of the Dinarides Belt. Neo-tectonic activity along the SPNF is indicated by several recent earthquakes. Glaciated valleys drain into the upper reaches of the Valbona River draining the basin. The processes regulating the evolution of the basin infill and the incision of the river terraces remain unclear.

Our mapping reveals that the Pliocene basin fill is overlain by two sub-horizontal layers of Pleistocene-Holocene sediment: porous conglomerate below and red clay above. The conglomeratic layer contains components derived mostly from Mesozoic limestones in Dinaric nappes in the glacial Valbona and Gashi Valleys. The drainage direction in the basin inferred from paleo flow indicators in both layers was to the SW, i.e., in the direction of present-day flow into the Drini Gorge that cuts down to the Adriatic coast. In addition, we recognize three terrace levels in these sediments, the top two of which are carved by abandoned river channels. The terraces and channels cut across the Pleistocene-Holocene layer contacts and are therefore younger than the layers’ deposition. The layers themselves thicken away from the SPNF, and show no preferred dip toward or away from the SPNF. Work is underway to date these terraces with cosmogenic nuclides.

Initial relief leading to the Pliocene infill of the Tropoja Basin is interpreted to be a by-product of SPNF activity, which began already in mid-Miocene time. However, this activity is insufficient to explain post-Pliocene sedimentation in the Tropoja Basin because the thickness and dip of the Pleistocene-Holocene layers do not vary systematically with proximity to the SPNF. We, therefore, interpret the Pleistocene-Holocene basin fill to be glacial, with the basal porous conglomerates deposited during sudden out washing of the glacial valleys due to release of melting water behind moraine dams. The overlying red clay layer is interpreted to be a lacustrine deposit due to damming further down the Drini Valley. The preservation of abandoned stream channels in the terraces may reflect episodic uplift and fluvial down-cutting events. The down-cutting may be attributed either to isostatic uplift of the upper plate of the retreating Hellenic subduction and/or to interglacial unloading.

How to cite: Gemignani, L., Simon, D., Mittelbach, B., Hippe, K., and Handy, M. R.: Tectonic, erosional and climatic controls on sedimentary basin evolution a case study from the Tropoja Basin (Albania), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9824, https://doi.org/10.5194/egusphere-egu2020-9824, 2020

D1163 |
EGU2020-15856
Chiara Zuffetti and Riccardo Bersezio

The palimpsest landscape and stratigraphic architecture of the Quaternary Po Foreland Basin record the tectonic pulses of the N-Apennines fold-and-thrust belt (the southern basin floor and active structural margin) and the glacial dynamics on the Alps (the northern basin floor and margin). Climate-controlled sediment flux from the glaciated Alpine side of the basin accommodated in the mobile setting driven by Apennine N-wards thrusting. Deciphering the nature, hierarchy and timing of landscape-changing increments at the Po Basin-Apennines hinge helps to describe the Late Quaternary tectonic modulation of landscape response to glacial cycles.

The study integrates different-scale geological, sedimentological, stratigraphic, geo-pedological, geomorphological and structural field surveys constrained by C14 and OSL age determinations, and subsurface reconstructions obtained from borehole logs and geophysical images. Focus is on the culminations of Apennine ramp-folds, the San Colombano (SC hereafter) and Casale-Zorlesco (CZ) isolated reliefs, which elevate above the terrace orders of the latest Pleistocene-Holocene plain. These selected key-sectors expose unconformities, morphological surfaces and stratigraphic units otherwise buried in the adjacent plain sectors, and show the involvement of Quaternary, alpine-sourced littoral, alluvial and glacio-fluvial succession in Apennine folding and faulting.          

Evidences of syndepositional tectonics are the location of unconformable stratigraphic vs. conformable morphological boundaries, pinch-out and cross-cut relationships among glacio-fluvial and alluvial sedimentary bodies, uplifted paleovalley fills, cannibalism of pre-existing alluvial clastics, colluvial wedges and soft-sediment deformation structures. During Early-Middle Pleistocene, the SC-CZ ramp anticlines underwent thrusting, which uplifted and folded the Gelasian regional unconformity between deep-marine Miocene and littoral Calabrian formations. Late Pleistocene, distal alpine-sourced glacio-fluvial units terraced the deformed marine successions giving origin to the composite Late Pleistocene unconformity. These units, time-constrained by OSL data to MIS6-MIS5, progressively wedge-out and amalgamate S-wards, suggesting confinement by the uplifting ancestors of the present-day hills. MIS4 glacio-fluvial system, fed from the Verbano-Lario glacial amphitheatres, fringed-out above a western uplifted culmination, while a braided glacio-fluvial system flowing South from the central-eastern Lario amphitheatre, terraced the eastern subdued structural highs. Relics of the corresponding  planation surface are uplifted at the present-day eastern SC and CZ hilltops. On the uplifted proto-hills, Late Pleistocene climate cycles are registered by polycyclic loess-soil sequences. Relics of syn-tectonic paleovalley fills, valley diversions, polygonal facets, alignments of windgaps and hanging valleys, suggest that differential uplift and wrenching occurred, plausibly driven by slip along the eastern dextral lateral ramp of the SC structure. The LGM, glacio-fluvial systems prograded S-wards terracing the existing reliefs. Tilting and faulting of these LGM terraces in correspondence of the faceted SC hill fronts, drainage diversions and polyphasic soil reworking at the same sites, imply passive deformation and collapse of the SC structure and hill. Entrenchment and abrupt diversions of the river network which cross-cut the mentioned geological and geomorphological elements, suggest that the Holocene lowermost terraces of the Po Plain formed during concurrent post-glacial increase of fluviatile discharge and tectonic uplift.

How to cite: Zuffetti, C. and Bersezio, R.: The response of periglacial landscape to Late Pleistocene active thrusting: evidences at the Po Basin-Northern Apennines hinge (Lombardy, Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15856, https://doi.org/10.5194/egusphere-egu2020-15856, 2020

D1164 |
EGU2020-17488
Noémi Kósa and Balázs Székely

Freely-meandering rivers are sensitive indicators of neotectonic activity that is otherwise difficult to detect in low-relief areas. In this study sinuosity analysis has been carried out on 20 main rivers and tributaries of Central Amazonia Region as an aid for localization of river channel patterns influenced by on-going tectonic activity.

The main problem of such studies, however, the availability of accurate river channel data. For the Central Amazonia Region highly accurate dataset that has a good geographical coverage is hardly available: the datasets we found did not fulfill the accuracy criteria for our project.

Consequently, the first objective of this project was to develop a data processing method of high resolution satellite images which provides a quick and accurate way to digitize river sections of a large parts of the intracratonic sedimentary basin. Furthermore, this work aims to detect channel sinuosity changes that could indicate recent vertical crustal movements. To achieve this, the water courses were automatically digitized using Sentinel–2 data and classic sinuosity values were calculated using several window sizes. The distribution of sinuosity variations was analysed by classification and various representations of the calculated values like mapping, crossplots and sinuosity-spectrum.

As the visualization methods complement each other the variations in sinuosity values can be highlighted and verified in several aspects. The results compared to former neotectonic studies some significant sinuosity changes can be correlated to known faults. The mentioned sinuosity variations coincides with the location of NW–SE normal and thrust faults active since Pleistocene times and NE–SW Miocene normal faults supporting the idea that these structures may have been reactivated.

In conclusion, multi-window sinuosity index calculation applied to satellite data based digitized water courses is a useful tool for recognizing recent tectonic activity in large low-relief areas, such as Central Amazonia.

How to cite: Kósa, N. and Székely, B.: Development of a processing chain of multispectral Sentinel–2 data to extract meandering river courses for geomorphometric analysis in Central Amazonia Region, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17488, https://doi.org/10.5194/egusphere-egu2020-17488, 2020

D1165 |
EGU2020-20270
Gregory Hoke, Pedro Val, Gregory Ruetenik, and Robert Moucha

The geomorphic processes that control temporal and spatial patterns of erosion, sediment storage and evacuation in an active mountain range (source) have a direct impact on how the signal of tectonics and climate, are recorded in the adjacent sedimentary basins (sinks). Stream power based numerical models of landscape evolution predict strong time lags between rock uplift and waves of erosion in the foreland, but this is difficult to test without proper resolution between source and sink signals..  Confirmation of model results is typically gleaned through observations that are either snapshots of processes in modern systems, or inversion of the stratigraphic record to decipher what occurred in the uplands. While cosmogenic nuclide derived, catchment wide erosion rates in the modern rivers provide a snapshot of processes happening in the last thousands of years, thermochronmeters average over the ≥ millions of years it takes a rock to ascend from the closure isotherm to the Earth’s surface,making it difficult, if not impossible to capture a minimally time averaged signal of the geomorphic system in the stratigraphic record. Paleoerosion rates from the residual cosmogenic nuclide concentration of buried sediments offer a means to bridge the gap in resolution. 

 

This study combines numerical modeling and cosmogenic nuclide paleoerosion rates in the Argentine Precordillera to build a rich picture of how this foreland basin system, from the hinterland through the foreland basin evolves in time and space. Our modeling shows that the dynamics of wedge-top basin formation behind a rising, and then subsequently inactive range have profound and systematic effects on the geomorphic signals both upstream and downstream of the wedge-top basin. Downstream, it is clear that there are strong, million year time lags in the uplift-triggered erosive pulse and spatial controls on where the sediment delivered to the foreland is sourced. Upstream, aggradation in the wedge top leads to the development of a wave of low erosion into the hinterland that results in the creation of perched surfaces coeval to erosive pulses downstream. In the Argentine Precordillera at 30°S an 8 Ma record of paleoerosion rates from the wedge top and foreland basin deposits along with detrital zircons provenance in the foreland largely verifies the predictions of the numerical modeling. Similarly, upstream of the wedge-top basin, there are concordant knickpoints and large, broad planation surfaces perched some 1500 m above the floor of wedge top as predicted by the low erosion wave pulse. Our combination of numerical modeling and paleoerosion rates capture the dynamic evolution of mountain range at million to thousand year timescales. 

How to cite: Hoke, G., Val, P., Ruetenik, G., and Moucha, R.: From perched high elevation surfaces to sediment entering the foreland: the dynamics of erosion, deformation and landscape evolution in the Argentine Precordillera, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20270, https://doi.org/10.5194/egusphere-egu2020-20270, 2020

D1166 |
EGU2020-21038
Jianfeng Li, Zhicheng Zhang, and Yue Zhao

        The northern Tibetan Plateau, between the Kunlun and the Altyn Tagh faults, contains high relief topography, such as the Eastern Kunlun Range, the Altyn Tagh Range and the Qilian mountain belt, and plays an important role in researching the tectonic evolution and topographic growth of the Tibetan Plateau. We present new apatite fission track (AFT) and 40Ar/39Ar thermochronologic data from the Subei and Shibaocheng areas near the eastern Altyn Tagh fault. Two Cenozoic exhumation phases have been identified from our AFT thermochronology. The AFT cooling ages of ~ 60–40 Ma farther away from the faults represented a slow widespread denudation surface as response to the Indo-Eurasia collision and signified that the Subei and Shibaocheng areas denudated as a whole in the northern Tibetan Plateau. Another phase with AFT cooling ages between about 20.5 Ma to 13.6 Ma on the hanging walls near the faults, located in the Danghenanshan and Daxueshan Mountains, recorded widespread fault activities resulted from local uplift and exhumation in late Miocene (~ 8 Ma) acquired from AFT thermal history modeling. A Cretaceous exhumation (~ 120–70 Ma) acquired from AFT thermal history modeling may have made great contributions to the growth of the pre-Cenozoic northern Tibetan Plateau.

How to cite: Li, J., Zhang, Z., and Zhao, Y.: Low-temperature thermochronologic constraints on the tectonic evolution of the Subei and Shibaocheng areas, northern Tibetan Plateau, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21038, https://doi.org/10.5194/egusphere-egu2020-21038, 2020