Displays

The processes driving Andean mountain-building and crustal thickening have been largely questioned since the ~1970's but have remained relatively unclear. However, the discovery of an active fold-and-thrust belt along its western flank at the latitude of Santiago (Chili, ~33.5 °S) has launched a recent vigorous debate on the relative contribution of these structures to Andean mountain-building. Based on an original approach for structural mapping, we have quantitatively investigated the structure of this fold-and-thrust belt, as well as that of the other structural units of the range at this latitude. By combining these data to published structural geometries of the eastern mountain flank, together with constraints on the timing of faulting and exhumation, we were able to revise the overall structure of the range and to quantify the kinematics of Andean orogenic growth at ~33°S-33.5°S. We find that crustal shortening has first primarily been sustained along the western mountain flank by west-vergent structures, synthetic to the subduction zone, with the subsequent increasing contribution of out-of-sequence thrusting, followed by late east-vergent thrusting along the eastern mountain flank. This pattern seems not to be specific to the Andes at this latitude, as similar observations can be made to the first-order by ~20°S, ie ~1300 km further north. There, the kinematics of the fold-and-thrust belt forming the western flank of the Andes cannot be as precisely documented because most structures are hidden beneath the later Cenozoic Atacama gravels. However, first-order quantitative results indicate similar kinematics, where Andean mountain building initiated on west-vergent structures synthetic to the subduction zone and where the later significant cumulated take-over by east-vergent structures towards the South American continent has led to the building of the Altiplano-Puna Plateau.

We propose that such kinematics - ie deformation initially on west-vergent structures along the western mountain flank, with significant later back-arc antithetic deformation - are first-order characteristics of Andean mountain-building, and result from the limited mechanical flexure of the underthrusting forearc, eventually locally modulated by climate-driven erosion.

How to cite: Simoes, M., Riesner, M., Habel, T., Lacassin, R., Carrizo, D., and Armijo, R.: Cordilleran-type orogens and plateaus: new views from a quantitative re-evaluation of mountain-building in the western Central Andes., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1626, https://doi.org/10.5194/egusphere-egu2020-1626, 2020.

The Andes are the case example of an active Cordilleran-type orogen. It is generally admitted that, in the Central Andes (~20°S), mountain-building started ~50-60 Myr ago, close to the subduction margin, and then propagated eastward. Though suggested by some early geological cross-sections, the structures sustaining the uplift of the western flank of the Altiplano have been largely dismissed, and the most common view theorizes that the Andes grow only by east-vergent deformation along its eastern margin. However, recent studies emphasize the significant contribution of the West Andean front to mountain-building and crustal thickening, in particular at the latitude of Santiago de Chile (~33.5°S). The contribution of similar structures elsewhere along the Andes to the kinematics of the orogen is still poorly solved, because not yet well synthesized nor quantified. Here, we focus on the western margin of the Altiplano at 20°S, in the Atacama desert of northern Chile. We focus our work on two sites where structures are well exposed.
Our results confirm two main structures: (1) a major west-vergent thrust placing Andean Paleozoic basement over Mesozoic strata, and (2) a west-vergent fold-and-thrust-belt involving Mesozoic units. Once restored, we calculate a minimum of ~4 km of shortening across the sole ~10 km-wide outcropping fold-and-thrust-belt. Further west, structures of this fold-and-thrust-belt are unconformably buried under slightly deformed Cenozoic units, as revealed from seismic profiles. By comparing the scale of these buried structures to those investigated previously, we propose that the whole fold-and-thrust-belt has most probably absorbed ~15-20 km of shortening, sometime between ~68 Ma (youngest folded Mesozoic layers) and ~29 Ma (oldest unconformable Cenozoic layer). Preliminary (U-Th)/He thermochronological data suggest that basement exhumation by thrusting happened at the beginning of this ~40 Ma time span. Minor shortening affecting the mid-late Cenozoic deposits indicates that deformation continued after 29 Ma along the western Andean fold-and-thrust-belt, but remained limited compared to the more intense deformation during the Paleogene. Altogether, the data presented here will provide a quantitative evaluation of the contribution of the western margin of the Altiplano plateau to mountain-building at this latitude.

How to cite: Habel, T., Lacassin, R., Simoes, M., and Carrizo, D.: Unraveling the contribution of the western margin of the Altiplano plateau in North Chile (20°S) to Andean mountain-building, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3012, https://doi.org/10.5194/egusphere-egu2020-3012, 2020.

The Andes are the modern active example of a Cordilleran-type orogen, with mountain-building
 and crustal thickening within the upper plate of a subduction zone. Despite numerous studies of
 this emblematic mountain range, several primary traits of this orogeny remain unresolved or poorly documented. The timing of uplift and deformation of the Frontal Cordillera basement culmination of
 the Southern Central Andes is such an example, even though this structural unit appears as a first-order topographic and geological feature. Constraining this timing and in particular the onset of uplift is a key point in the ongoing debate about the initial vergence of the crustal-scale thrusts at the start of the Cenozoic Andean orogeny. To solve for this, new apatite and zircon (U-Th)/He ages from granitoids of the Frontal Cordillera at ~33.5°S are provided here. These data, interpreted as an age-elevation thermochronological profile, imply continuous exhumation initiating well before ~12–14 Ma, and at most by ~22 Ma when considering the youngest zircon grain from the lowermost sample (Riesner et al. 2019). The inverse modeling of the thermochronological data using QTQt software confirms these conclusions and point to a continuous cooling rate since onset of cooling. The minimum age of exhumation onset is then refined to ~20 Ma by combining these results with data on sedimentary provenance from the nearby basins. Such continuous exhumation since ~20 Ma needs to have been sustained by tectonic uplift on an underlying crustal-scale thrust ramp. Such early exhumation and associated uplift of the Frontal Cordillera question the classically proposed east-vergent models of the Andes at this latitude. Additionally, this study provides further support to recent views on Andean mountain-building proposing that the Andes-Altiplano orogenic system grew firstly over west-vergent basement structures before shifting to dominantly east-vergent thrusts. 
Riesner M. et al. 2019, Scientific Reports, DOI: 10.1038/s41598-019-44320-1

How to cite: Lacassin, R., Riesner, M., Simoes, M., Habel, T., Margirier, A., and Carrizo, D.: Early exhumation of the Frontal Cordillera (southern Central Andes at ~33.5°S) and implications for Andean mountain-building, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3346, https://doi.org/10.5194/egusphere-egu2020-3346, 2020.

With an average elevation of about 3.7 km the semi-arid to arid Central Andean Plateau (Altiplano-Puna) constitutes the world’s second largest orogenic plateau. The internally drained region is characterized by compressional basin-and-range topography. Many of the basins in the Argentine sector of the plateau (Puna) are presently evaporitic salt pans, but during the Pleistocene the basins have repeatedly experienced high lake-level phases during pluvial periods. Due to protracted sedimentary infilling and sustained internal drainage conditions the basins have thick sedimentary sequences that have partially coalesced. The basins are bordered by reverse-fault bounded ranges, reaching 5 to 6 km elevation, but the history and extent of tectonic deformation in this region is not very well known. Global Navigation Satellite System (GNSS) data have been used to estimate decadal-scale tectonic shortening rates but the spatiotemporal pattern of surface deformation is complex and includes the compounding effects of subduction zone megathrust earthquake transients.

Here, we use a combination of field observations, cosmogenic nuclide dating of deformed alluvial-fan surfaces, Interferometric Synthetic Aperture Radar (InSAR), and GNSS data time series to quantify Quaternary to decadal-scale tectonic deformation. The arid mountain ranges provide ideal conditions to observe deformation from multiple sensors, including TerraSAR-X, Sentinel-1, ALOS2, and ENVISAT. Furthermore, we rely on 12 m TanDEM-X topographic data to characterize 10³-10⁶ yr surface deformation using cosmogenic nuclide exposure dating and digital elevation model analysis.

The Puna has been previously characterized as a region with little tectonic activity including very low levels of seismicity despite evidence for strike-slip and extensional faulting accompanied by mafic volcanism. The eastern plateau margins in particular record this type of kinematic regime, while the adjacent foreland is characterized by a higher level of seismicity and ongoing contraction. Here, we present evidence of ongoing contraction during the past two decades compatible with tectono-geomorphic phenomena that support the notion of tectonic shortening in the central Puna Plateau. For example, tilted shorelines associated with former lake-highstands along the flanks of an anticline and Neogene-Pleistocene growth strata associated with this structure indicate that shortening in this region has been sustained since the Neogene. InSAR and GNSS time series analysis permit the identification and characterization of previously unrecognized tectonic activity in adjacent sectors of the intermontane basins, thus helping to improve our understanding of crustal dynamics in the Central Andes.

How to cite: Bookhagen, B., Strecker, M. R., Weiss, J. R., and Alonso, R. N.: Active surface deformation in the south-central Andes revealed by multiple-sensor InSAR, GNSS and field observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13209, https://doi.org/10.5194/egusphere-egu2020-13209, 2020.

Quantifying the interactions between tectonics and Earth surface processes on orogenic plateaus requires the acquisition of a multitude of field observations and geological proxies. Here, we reconstruct the topographic development of the Central Anatolian Plateau (Turkey), identify the geodynamic drivers of plateau formation, and constrain the climatic boundary conditions that shaped the fluvio-lacustrine basins, drainage integration, and vegetation and biodiversity dynamics.

Our comprehensive dataset includes sedimentological and field observations, ⁴⁰Ar/³⁹Ar ages, magnetostratigraphy, lacustrine carbonate δ¹⁸O and δ¹³C data (n=665) from thirteen sections in upper Oligocene to Pliocene continental basins of the CAP interior, and ¹⁰Be erosion rates. We also analyze existing fossil faunal (mammal) and floral databases to assess biodiversity dynamics through time and we model isostatic rebound to understand drainage integration.

The CAP and its steep, southern Tauride margin emerged from the Mediterranean Sea ~12-11 Ma and ~8-7 Ma ago, respectively. Contemporaneously to surface uplift, a fluvio-lacustrine system covered extensive parts of the rising CAP. Today, the semi-arid CAP interior − except for the Konya Closed Catchment (KCC) − drains towards the Black Sea, Mediterranean Sea and Persian Gulf.

Our stable isotope paleoaltimetry data show similar-to-present elevations (~2 km) of the southern CAP margin by 5 Ma. Surface uplift affected the diversity of plants and large mammals, and was coeval with ignimbritic magmatism, forearc shortening and distributed compression. We suggest that removal of lithospheric mantle below Anatolia led to surface uplift of the CAP interior, followed by surface uplift of the southern CAP margin as a result of subduction-related crustal thickening. Persistently (>1 Myr) stable paleoenvironmental and hydrological conditions recorded by the former fluvio-lacustrine Anatolian depocenters suggest that a low-relief environment characterized the CAP during plateau uplift. Throughout the late Miocene, various open and closed lakes of the southern CAP drained into closed, terminal lakes within the plateau interior. Sedimentation east of the Tuz Gölü Fault ceased rapidly during the early Pliocene (from 5.3-3.6 Ma), when river incision led to a connection with marine base level. Analysis of incision patterns reveals that drainage integration was not driven by capture of the interior drainage by aggressive rivers draining the plateau margin, but rather by top-down avulsion or overflow due to the establishment of a more positive water balance in some of the closed catchments of the plateau interior. Drainage integration occurred shortly after the switch from regional compression to extension and the onset of escape tectonics of the new Anatolian microplate, when fault partitioning of the existing low-relief plateau interior may have led to drainage integration.

In a next step to reconstruct the paleoenvironmental conditions of the CAP, we obtain δ¹⁸O and δ¹³C values from fossil mammal tooth enamel, which allows for the reconstruction of mammalian diet, and in turn reflects paleovegetation, as well as seasonality for the Mio-Pliocene climate.

References

Meijers et al., 2018a: Palaeo3, doi: 10.1016/j.palaeo.2018.03.001

Meijers et al., 2018b: EPSL, doi: 10.1016/j.epsl.2018.05.040

Huang, Meijers et al., 2019: J of Biogeography, doi: 10.1111/jbi.13622

Meijers et al., 2020: Geosphere, doi: 10.1130/GES02135.1

How to cite: Meijers, M. J. M., Brocard, G. Y., Kaya, F., Pehlevan, C., Başoğlu, O., Cosca, M. A., Huang, S., Fritz, S. A., Teyssier, C., Langereis, C. G., Whitney, D. L., and Mulch, A.: Interactions between tectonics and Earth surface processes of the Central Anatolian Plateau and its southern margin during Mio-Pliocene surface uplift, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9900, https://doi.org/10.5194/egusphere-egu2020-9900, 2020.

Orogenic plateaus are extensive, elevated, arid, generally internally drained, morphotectonic provinces of low internal topographic relief that represent a striking and enigmatic feature of Earth’s continental landscapes. They are located along convergent plate boundaries and have a profound impact on regional and global climate, erosional processes, local- to far-field deformation mechanisms and the long-term distribution of biomes and biodiversity. Although the paramount role of large orogenic plateaus in shaping our planet is widely appreciated, the question of why, where, and how some orogenic systems develop large plateaus remains a first-order problem in our understanding of lithospheric evolution and orogenic processes.

Here, we present a clumped isotope paleoaltimetry study to document the elevation history of the Iranian Plateau, with the goal of understanding the rates and mechanisms of orogenic plateau rise. This plateau is in the Arabia-Eurasia collision zone, has a mean elevation of ~ 1.8 km, steep margins with mountain peaks higher than 4 km, and experienced surface uplift sometime after the middle Miocene as documented by the occurrence of ca. 17-My-old marine deposits in the plateau interior.

Preliminary results from Early Miocene to Quaternary pedogenic carbonates on the plateau interior and the adjacent, less elevated, intermontane Tarom basin suggest that surface uplift must have occurred sometime between 12-11 and 8 Ma. The lack of significant crustal shortening and thickening during this time interval and the occurrence of a renewed phase of adakitic volcanism by ca. 11 Ma suggests that surface uplift may have been driven by deep-seated processes associated with asthenospheric flow.

How to cite: Ballato, P., Licht, A., Huntington, K., Schauer, A., Mulch, A., Heidarzadeh, G., Paknia, M., Hassanzadeh, J., Mattei, M., Ghassemi, M., and Strecker, M.: Constraints on the Timing of Surface Uplift of the Iranian Plateau (Arabia-Eurasian Collision Zone) from Clumped Isotope Thermometry on Pedogenic Carbonates, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13280, https://doi.org/10.5194/egusphere-egu2020-13280, 2020.

The Qilian Shan, located in the northeastern margin of the Tibetan Plateau, is characterized by intensive Cenozoic structural deformation with rapid lateral growth due to the continuous Indo-Asian continental collision. Both low-temperature thermochronological dating and geological mapping suggest that the major emergence of Cenozoic Qilian Shan occurred in the Miocene. The central and northern Qilian Shan uplift successively, and deformation has passed away from the adjacent Hexi Corridor Basin into the Gobi-Alashan. The regional landform shows a low-relief surface in the Qilian Shan hinterland and high steep relief in the northern range front.

The rivers rising in the hinterland of the Qilian Shan, i.e., the Shule River (SL), Beda River (BD), and Hei River (HE), are flowing across the northern range front. It is noteworthy that the development of these rivers is within the context of the in-sequence fault propagation pattern with the lifespan of ~3 Ma. When combined with the differential topographies between hinterland and range front, this kind of river drainage pattern inevitably has abundant geodynamical significances, mainly in terms of the long-term coupling between tectonic and surficial processes. To date, the dynamic conditions in shaping the aforementioned tectonic landscape features remain unknown and are critical in revealing the lateral growth of the NE Tibetan Plateau. A series of landscape evolution models are conducted based on thick-skinned Qilian Shan structural wedge. The wavelength of mountains is constrained by the critical wedge theory.

Our results show that the in-sequence fault propagation together with the arid climate since the Miocene contributes to the low-relief topography in the hinterland of Qilian Shan. The front regions with rapid uplifting rates cut off rivers. Thus, sediments from the hinterlands cannot be directly carried out by rivers. The intermountain areas receive sediments from the adjacent uplift regions, resulting in an increased elevation. Because of the long-term average arid climate, the river incision is limited. For most areas, it is difficult to form transversal rivers immediately that cut through mountains and carry sediment out of the plateau. With the northeastward in-sequence fault propagation, the transversal rivers finally formed with headwaters within the hinterland of Qilian Shan, such as the SL, BD and HE. The broad consistency of landforms, in turn, strongly favors the geological conclusion that faults in the central and northern Qilian Shan were activated sequentially. The rapid uplift rate in the active range front is tested in the range of 0.6-1.0 mm/a. It is found that this rate is insensitivity to the drainage and landscape evolution pattern. However, the background uplift rate has a great influence on the elevation of the plateau and is positively correlated. The current topography of >4000 m in the hinterland of Qilian Shan is controlled by a background uplift rate of ~0.2mm /a.

How to cite: Li, Y., Zhang, H., and Zhang, Z.: The interaction between uplift and landscape evolution in central and northern Qilian Shan: Insights from numerical modeling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12130, https://doi.org/10.5194/egusphere-egu2020-12130, 2020.

During the Late Miocene, the climate patterns and ecosystems of continental land masses experienced crucial transitions, but whether the principal driver was regional tectonic forcing or a decline in CO₂ concentrations remains debated. Terrestrial paleotemperature records from tectonically active regions can conserve both paleoaltitudinal and global temperature changes which have occurred as a result of fluctuations in the levels of CO₂. However, high-quality quantitative data remain scarce, due to the lack of terrestrial paleotemperature reconstruction tools and well-dated continuous stratigraphic sequences. Based on a continuous sedimentary sequence with high precision dating from ~54-4.8 Ma in Xining Basin, northeastern Tibetan Plateau established, and evaluation of the potentiality of the branched glycerol dialkyl glycerol tetraethers (brGDGTs) in paleotemperature/paleoelevation reconstruction in Tibetan Plateau by our group, we present a terrestrial paleotemperature record spanning ~12.7-5.2 Ma based on tetraether lipids extracted from the northeastern Tibetan Plateau. Our results reveal a sharp cooling (~8°C) during ~10.5-8 Ma, asynchronous with minor fluctuations in global sea temperatures, suggesting a rapid tectonic uplift of ~1 km in extent. This event appears consistent with the simultaneous aridification and transitions of ecosystems experienced in adjacent regions. Moreover, the amplitude of the cooling over land is less than that which occurred over the ocean during the CO₂-dominated Late Miocene cooling event (~7-5.4 Ma). We therefore concluded that tectonic forcing, rather than a decline in CO₂ levels, most likely dominated continental climate patterns and ecosystem transitions during the Late Miocene.

How to cite: Bai, Y., Chen, C., Fang, X., Guo, H., Meng, Q., and Zhang, W.: A Late Miocene terrestrial temperature history for the northeastern Tibetan Plateau’s period of tectonic expansion, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22396, https://doi.org/10.5194/egusphere-egu2020-22396, 2020.

Long‐term (million year time scale) fault‐slip history is crucial for understanding the processes and mechanisms of mountain building in active orogens. Such information remains elusive in the Longmen Shan, the eastern Tibetan Plateau margin affected by the devastating 2008 Wenchuan earthquake. While this event drew attention to fault deformation on the foreland side (the Yingxiu‐Beichuan fault), little is known about the deformation history of the hinterland Wenchuan‐Maoxian fault. To address this gap, thermochronological data were obtained from two vertical transects from the Xuelongbao massif, located in the hanging wall of the Wenchuan‐Maoxian fault. The data record late Miocene rapid cooling and rock exhumation at a rate of 0.9–1.2 km/m.y. from ~13 Ma to present. The exhumation rate is significantly higher than that in the footwall (~0.3–0.5 km/m.y.), indicating a differential exhumation of ~0.6 km/m.y. across the fault. This differential exhumation provides the first and minimum constraint on the long‐term throw rate (~0.6 km/m.y) of the Wenchuan‐Maoxian fault since the late Miocene. This new result implies continuous crustal shortening along the hinterland fault of Longmen Shan, even though it has not been ruptured by major historic earthquakes. Our study lends support to geodynamic models that highlight crustal shortening as dominating deformation along the eastern Tibetan Plateau.

How to cite: Shen, X., Tian, Y., Zhang, S., Carter, A., Kohn, B., Vermeesch, P., Liu, R., and Li, W.: Late Miocene hinterland crustal shortening in the Longmen Shan thrust belt, the eastern margin of the Tibetan Plateau, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9479, https://doi.org/10.5194/egusphere-egu2020-9479, 2020.

Geophysical studies have shown that middle-lower crustal flow started from central Tibetan Plateau may exist in the eastern margin of the Tibetan Plateau, which controls the mountain building, crustal thickening and deformation (Schoenbohm et al., 2006; Bai et al., 2010; Bao et al., 2015; Zhu et al., 2017). However, no geological and petrological evidence have been presented. We carried out detailed studies on the geochemical and isotopic compositions of the Mesozoic-Cenozoic Zheduo-Gongga granitic intrusive complex on the eastern margin of the Tibet Plateau. Geochronology studies show that these granitoid rocks are formed during Mesozoic to Cenozoic, including ~220-200 Ma Gongga granodiorite to biotite granite with mafic enclaves, ~40 Ma Zheduo gneissic granite, ~28 Ma Zheduo monzogranite, and ~20-4 Ma Zheduo biotite granite and monzogranite. Two groups of geochemical features are obtained: Group 1 (gnessic granite, granodiorite, monzogranite, and leucogranite) has relatively low K2O, Th/La, La/Yb and Rb/Sr ratios, but high Sr/Y ratio with no Eu negative anomalies; Group 2 (biotite granite) has relatively high K2O, Th/La, La/Yb and Rb/Sr ratios, but low Sr/Y with strong negative Eu anomalies. The Sr-Nd-Hf-O isotopic studies on plagioclase, apatite and zircon show that their sources are primarily the basement of the western margin of Yangtze Craton and Songpan-Ganzi sediments. These features indicate that they have different petrogenesis processes. Group 1 is mainly derived from partial melting of mafic rocks in the lower crust, whereas the Group 2 is primarily derived from partial melting of metasedimentary rocks experiencing fractionation of plagioclase. Magma derived from different sources mixing with each other are observed as well. Therefore, from geochemical aspects, no exotic materials are involved in the formation of granitoid rocks during Mesozoic to present. The flow of crustal material in the middle-lower crust may be not existed. The low velocity and high conductivity layer in the middle-lower crust may represent a regional partial melting zone, which could be related to the upwelling of asthenosphere. Both crustal deformation and upwelling of asthenosphere may contribute to the crustal thicknening and uplift.

How to cite: Hu, F., Wu, F., Ducea, M., and Chapman, J.: Geochemical and isotopic data of Zheduo-Gongga granitic intrusive complex, eastern margin of the Tibetan Plateau: no evidence for middle-lower crustal flow, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1918, https://doi.org/10.5194/egusphere-egu2020-1918, 2020.

The Himalaya is the highest and steepest mountain range on Earth and an efficient north-south barrier for moisture-bearing winds. The close coupling of changes in topography, erosion rates, and uplift has previously been interpreted as an expression of a climatic control on tectonic deformation. Here, we present 17 new zircon U/Th-He (ZHe) bedrock-cooling ages from the Sutlej Valley that – together with >100 previously published mica ⁴⁰Ar/³⁹Ar, zircon and apatite fission track ages – allow us to constrain the crustal cooling and exhumation history over the last ~20 Myr. Using 1D-thermal modeling, we observe a rapid decrease in exhumation rates from >1 km/Myr to <0.4 km/Myr that initiated at 15-13 Ma across the entire Greater Himalaya and the north-Himalayan Leo Pargil gneiss dome, both in the hanging and footwall of major Miocene shear zones, suggesting that cooling is associated to surface erosion and not due to tectonic unroofing. We explain the middle Miocene deceleration of exhumation by the onset of the growth of the Lesser Himalayan duplex, which resulted in accelerated uplift of the Greater Himalaya above a mid-crustal ramp and the establishment of an efficient orographic barrier. The period of slow exhumation in the upper Sutlej Valley coincides with a period of internal drainage in the south-Tibetan Zada Basin farther upstream, which we interpret to be a consequence of tectonic damming of the upper Sutlej River. External drainage of the Zada Basin was re-established at ~1 Ma, when we observe exhumation rates in the upper Sutlej Valley to accelerate again.

How to cite: Scherler, D., Thiede, R., and Glotzbach, C.: Middle Miocene rise of the High Himalaya and the disruption of transverse drainage due to basal accretion, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19241, https://doi.org/10.5194/egusphere-egu2020-19241, 2020.

Knickpoints in longitudinal river profiles provide proxies for the climatic and tectonic history of active mountains. The analysis of river profiles commonly relies on the assumption that drainage network configurations are stable. Here we show that this assumption must made cautiously if changes in contributing area are fast relative to knickpoint migration rates. We study the Parachute Creek basin in the Roan Plateau, Colorado, United States. Low spatial variations in climate and erosional efficiency permit us to reveal and quantify drainage-area loss that occurred in one of the subbasins where observed knickpoint locations are farther upstream than predicted by a model that takes present-day drainage areas into account. We developed a Lagrangian model of knickpoint migration which enables us to study the kinematic links between drainage area loss and knickpoint migration and that provides us with constraints on the temporal aspects of area loss. Modelled onset and amount of area loss are consistent with cliff retreat rates along the margin of the Roan Plateau inferred from the incisional history of the upper Colorado River.

How to cite: Schwanghart, W. and Scherler, D.: Divide mobility controls knickpoint migration on the Roan Plateau, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11177, https://doi.org/10.5194/egusphere-egu2020-11177, 2020.

High-plateaus are relatively flat areas at high elevations. The stream-power river-incision law predicts that surface water incises the landscape proportionally to local river slope, and therefore the margins of high-plateaus are prone to a river erosion that should terminate the low relief of the highlands that characterizes the plateau. This means that long-lived high-plateaus need an additional mechanism to compete with river incision.

In absence of tectonic deformation, river networks propagate into the plateau via a retrogressive wave of river incision. A well-constrained non-tectonic scenario is provided by the Neogene Duero and Ebro sedimentary basins in N Iberia, where ongoing incision rates presently range from .02 (Duero) to .5 m/kyr (Ebro) and have propagated upstream at similar rates of up to 0.2 km/kyr, based on cosmogenic dating studies combined with numerical modeling. These rates started with the transition from internal (endorheic) to external (exorheic) drainage of both basins sometime between 8 and 12 million years ago. Interestingly, while the pre-exorheic Ebro Basin sedimentary plateau has been mostly obliterated by erosion, the Duero Basin still preserves large areas of low relief, in spite of the very similar geological setting. The causes will be discussed using landscape evolution numerical modeling.

In contrast, tectonically active regions can counteract river incision and preserve high plateaus by longer time periods. Recent studies based on sedimentary stratigraphy of endorheic basins suggest that large areas of the Tibetan high plateau remain internally drained since ca 35 Ma. In the Altiplano/Puna plateau region internal drainage dates to ~15 Ma and the majority of the topographic uplift has taken place after 10 Ma. Computer models have shown that tectonic deformation is sensitive to internal drainage, because endorheism implies a nearly perfect sediment trap that effectively reduces the output of orogenic erosion to zero. The cancellation of orogen-scale erosion can severely modify tectonic deformation patterns, increase topography and propagate deformation further into the undeformed forelands of the orogenic system. Symmetrically, internal drainage is also promoted by the orographic rain shadow due to the growth of topography in the early stages of tectonism.

Numerical models coupling the aforementioned mechanisms have shown that, as sediment transport and accumulation within the endorheic region progresses, the propagation of deformation to areas more distal to the tectonic plate boundary can lead to a lower‐relief landscape. A recent reassessment of the ages of the Tibetan plateau sedimentary record in the Lunpola Basin seems consistent with an early onset of low relief and internal drainage. Finally, as topography and crustal thickness increase, lower crust flow is facilitated by the lower viscosity implied by higher pressure, favoring a further reduction of local relief within the highlands.

How to cite: Garcia-Castellanos, D., Liu, W., Lai, Z., Jiménez-Munt, I., Struth, L., Rodríguez-Rodríguez, L., Hu, G., Wang, P., and Llorens, G.: Feedbacks between internal fluvial drainage and high-plateau tectonic growth. A mechanistic perspective. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17481, https://doi.org/10.5194/egusphere-egu2020-17481, 2020.

Many passive continental margins around the world are characterised by elevated plateaus at 1 to 2 km or more above sea level cut by deeply incised valleys and commonly separated from an adjacent coastal plain by one or more escarpments. Mesozoic–Cenozoic rift systems parallel to the coast are commonly present offshore with a transition from continental to oceanic crust further offshore. These landscapes occur in arctic, temperate and tropical climate and in different geological settings independent of the time span since break-up (e.g. along the Atlantic from south to north).

The plateaux are typically more than 100 km wide, much larger in some cases, and extend hundreds of kilometres along the margin, cutting across bedrock of different ages and resistances. The key to understanding the formation of regional, low-relief erosion surfaces is the base-level, as this is the level to which fluvial systems grade the landscape. The most likely base level is sea level, particularly for locations along continental margins during the post-rift development of passive margins.

It is commonly assumed that the characteristic, large-scale morphology of elevated, passive continental margins with  high-level plateaux and deeply incised valleys persisted since rifting and crustal separation Further, it is assumed that the absence of post-rift sediments is evidence of non-deposition, despite continental-stretching theory predicting deposition of a thick post-rift sequence overlying both the rift and its margins.

However, our studies of the passive continental margins of West and East Greenland, Norway, NE Brazil and southern Africa provide evidence of km-scale, post-rift subsidence and that the plateau surfaces were graded to sea level long after break-up and subsequently lifted to their present elevations. In some of these cases, the presence of post-rift marine sediments at high elevation provide direct proof of this interpretation. Since elevated plateaux cut by deeply incised valleys are a characteristic feature of these and other margins, this similarity suggests that such topography elsewhere in the world may also be unrelated to the processes of rifting and continental separation. We present a wide range of evidence from passive margins around the world in support of this hypothesis,

Bonow et al. 2014: High-level landscapes along the margin of East Greenland – a record of tectonic uplift and incision after breakup in the NE Atlantic. Global and Planetary Change.

Green et al. 2018: Post-breakup burial and exhumation of passive continental margins: Seven propositions to inform geodynamic models. Gondwana Research.

Japsen et al. 2019: Elevated passive continental margins: Numerical modeling vs observations. A comment on Braun (2018). Gondwana Research.

How to cite: Bonow, J. M., Japsen, P., Green, P. F., and Chalmers, J. A.: The origin of elevated plateaus along passive continental margins, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19532, https://doi.org/10.5194/egusphere-egu2020-19532, 2020.

Mature orogenic plateaux grow in response to the lateral redistribution of plateau material, driven by gravitational potential energy, from the thick plateau crust toward the thinner foreland crust. Folding and thrusting in the shallow crust as well as flow of weak deep crust toward the foreland result in plateau growth. The balance between plateau growth processes, including gravitational collapse of the orogenic crust, and the resistance to plateau propagation controls the position of plateau margins. Toward the end of orogenic plateau development, plateau margins are the loci of steep topographic gradients, where erosional processes can be aggressive. The margins also represent the transition between thick crust and thin/weak lithosphere beneath the plateau, and thinner crust and strong/thick lithosphere below the foreland.

The juxtaposition of thick and thin lithosphere favors strain localization along plateau margins, where thick lithosphere may partially subduct, or where strike-slip systems, such as the Altyn Tagh region of northern Tibet, develop. In either case, it is likely that the deep, flowing, partially molten crust will sample and entrain high-P (HP) metamorphic rocks such as granulite and eclogite. In the case of lithospheric strike-slip systems, crustal thickening in transpressional domains may lead to HP metamorphism, and crustal thinning in transtensional domains may lead to rapid exhumation of the deep crust, particularly where pull-apart structures in the shallow-crust allow the upward flow and emplacement of migmatite domes. For example, the Montagne Noire dome (French Massif Central) formed at the southern margin of the Variscan orogen in the late Carboniferous (315-295 Ma). This dome is filled with Variscan migmatite containing eclogite fragments that were sampled near Moho depths and entrained in the flowing partially molten crust; eclogitization and early crystallization of melt were coeval. In this example, the redistribution of mass and heat across the plateau margin, including the exhumation of near-Moho rocks, stabilized the crust and marked the end of orogeny.

How to cite: Teyssier, C., Whitney, D. L., Rey, P. F., and Roger, F.: Orogenic plateau margin and the coexistence of HP and HT metamorphic rocks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6798, https://doi.org/10.5194/egusphere-egu2020-6798, 2020.