GM9.14

EDI
Investigating deep-Earth dynamics through time and space using surface observations.

Geomorphic and geologic observations at the Earth's surface reflect the combined effects of erosional, tectonic, and deep-seated processes. Such surface observations therefore provide important constraints on mantle convection patterns and subduction dynamics through space and time, which compliment both studies of geophysical data and numerical simulation. However, using Earth's surface records to constrain deep-seated processes is complicated by (1) our as yet incomplete understanding of how mantle convection or subduction processes are manifest as surface forms and patterns, and (2) the effects of tectonic processes, spatio-temporal variations in climate, glacial isostatic adjustment, lithology, biota, and human alteration of landscapes and surface geology. We invite contributions that tackle these challenges and work toward identifying the surface expression of deep-Earth processes such as mantle convection, in different tectonic settings. We welcome studies that develop and apply a variety of approaches across temporal and spatial scales, including (but not limited to) geomorphic analysis, geophysics, thermochronometry, isotope and cosmogenic nuclide measurements, and numerical and analogue modeling of both surface and deep-Earth dynamics. We hope that this session will provide opportunities for presenters from all backgrounds, demographics, and all stages of their scientific career to engage in this exciting and emerging geological problem via a multidisciplinary approach.

Co-organized by GD1/TS5
Convener: Conor O'MalleyECSECS | Co-conveners: Claudio Faccenna, Audrey Margirier, Vivi Kathrine Pedersen, Simon StephensonECSECS
vPICO presentations
| Tue, 27 Apr, 15:30–17:00 (CEST)

vPICO presentations: Tue, 27 Apr

Chairpersons: Conor O'Malley, Audrey Margirier, Simon Stephenson
15:30–15:40
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EGU21-13996
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ECS
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solicited
Jessica Stanley and Kelsey Wetzel

Southwest Idaho has experienced substantial topographic changes over the Cenozoic that are reflections of complex tectonic and mantle processes. The western Snake River Plain (WSRP) in southwest Idaho has been characterized as an intracontinental rift basin but differs markedly in topography and style from other western Cordilleran extensional structures. It also differs in orientation and structural style from the down warped lava plain of the eastern Snake River Plain that follows the path of the Yellowstone hotspot (YHS). Potential magmatic drivers for WSRP formation include the ~12-10 Ma Bruneau-Jarbidge eruptive center of the YHS or the ~17-16 Ma Columbia River Basalt (CRB) large igneous province. To better constrain the timing and style of rifting in the region we sampled granitoid bedrock from Cretaceous and Eocene-aged plutons from the flanks of the WSRP to detail their exhumation history with apatite (U-Th)/He (AHe) thermochronometry. We present new AHe dates from seventeen samples, with cooling dates ranging range from 7 Ma to 55 Ma. The majority of cooling dates for the Cretaceous plutons are Eocene, and the Eocene intrusions yield Miocene dates. The AHe dates provide thermochronological evidence of rapid cooling and exhumation of the Idaho batholith during the Eocene. This supports the presence a high relief landscape in Idaho associated with regional uplift due to Farallon slab rollback and Challis magmatism. We also find evidence for a post-Eocene decrease in relief, seen in the negative slope on date-elevation relationships in the southwest flank of the WSRP. Our AHe dates indicate limited exhumation on the flanks of the WSRP during Miocene rift formation. We interpret this to be evidence of extension dominated by magmatic intrusions and intrabasin faults rather than basin-bounding faults. Miocene AHe dates show rapid exhumation along the Middle Fork Boise River that had begun by ~17 Ma. We take this to indicate focused incision along the river due to base level fall in the WSRP and the timing suggests that CRB activity was responsible for initiation of WSRP formation

How to cite: Stanley, J. and Wetzel, K.: Linking exhumation, paleo-relief, and rift formation to magmatic processes in the Western Snake River Plain, Idaho, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13996, https://doi.org/10.5194/egusphere-egu21-13996, 2021.

15:40–15:42
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EGU21-10257
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Highlight
Sven Fuchs, Graeme Beardsmore, Paolo Chiozzi, Orlando Miguel Espinoza-Ojeda, Gianluca Gola, Will Gosnold, Robert Harris, Sam Jennings, 'Shaowen Liu, Raquel Negrete-Aranda, Florian Neumann, Ben Norden, Jeffrey Poort, Dušan Rajver, Labani Ray, Maria Richards, Jared Smith, Akiko Tanaka, and Massimo Verdoya

The compilation of global heat-flow data is currently under major revision by the International Heat Flow Commission (IHFC) of the International Association of Seismology and Physics of the Earth's Interior (IASPEI). Heat flow represents a fundamental parameter in thermal studies, e.g., the evolution of hydrocarbons or mineral and geothermal resources. Comparable, comprehensible and reliable heat-flow data are of utmost interest also for geophysical and geological studies on the global scale. Here, we present the first results of a stepwise revision of the IHFC Global Heat Flow Database based on a researcher driven, collaborative approach. The first step comprises the review and revision of the most recent database structure established in 1976. The revised structure of the Global Heat Flow Database considers the demands and opportunities presented by the evolution of scientific work, digitization and the breakthroughs in database technologies over the past decades.  Based on the new structure, the existing dataset will be re-assessed and new data incorporated. By supporting the ideas of FAIR and open data principles, the new database facilitates interoperability with external data services, like DOI and IGSN numbers, and other data resources (e.g., world geological map, world stratigraphic system, and International Ocean Drilling Program data). We give an overview of the new database and introduce the community workflow of global heat-flow data revision.

How to cite: Fuchs, S., Beardsmore, G., Chiozzi, P., Espinoza-Ojeda, O. M., Gola, G., Gosnold, W., Harris, R., Jennings, S., Liu, '., Negrete-Aranda, R., Neumann, F., Norden, B., Poort, J., Rajver, D., Ray, L., Richards, M., Smith, J., Tanaka, A., and Verdoya, M.: The Global Heat Flow Database: a collaborative and fundamental revision process to ensure comprehensible and reliable heat-flow records, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10257, https://doi.org/10.5194/egusphere-egu21-10257, 2021.

15:42–15:44
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EGU21-591
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ECS
Veleda Astarte Paiva Muller, Pietro Sternai, and Christian Sue

The Southern Andes Volcanic zone (SVZ) is located between latitudes 33-46°S in the western margin of the South American continental plate, above the subducting Nazca oceanic plate. Although the slab dip angle is constant (~25°) along strike, the distance between the volcanic arc and the subduction trench decreases southward. In the northern segment (33-41°S) the volcanoes are co-located with the main orogenic water divide, whereas in the southern segment (41-46°S) the water divide is shifted eastward and the volcanic arc is shifted westward. The eastward water divide migration in the southern segment is explained by an orographic effect due to the westerlies winds, which causes high precipitation (>2 m/yr) and erosion rates (>1.5 mm/yr) on the upwind side of the orogen. Thermomechanical visco-elasto-plastic numerical models exploring the effects of the topographic shift on the magma upwelling path explain the westward migration of the volcanic arc. Results show that when the topographic barrier is shifted to the east with respect to a central magmatic source, asymmetric strain due to the magma emplacement into the crust drives preferential westward magma upwelling. The southern segment of the SVZ is proximal to an important strike slip fault system, the Liquiñe-Ofqui fault zone. We propose that the (re-)activation of these fracture zone on the western side of the Southern Andes is related to the orographic migration of the water divide during magma upwelling. This conclusion is further supported by the lack of structures accommodating magma emplacement/eruption and volcanoes east of the water divide. If correct, this is the first-recognized example of a climatic control on the location of a volcanic arc in convergent settings.

How to cite: Paiva Muller, V. A., Sternai, P., and Sue, C.: Climatic control on the Southern Andean volcanic arc location, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-591, https://doi.org/10.5194/egusphere-egu21-591, 2021.

15:44–15:46
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EGU21-6130
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ECS
Audrey Margirier, Manfred Strecker, Peter Reiners, Ismael Casado, Stuart Thomson, Sarah George, and Alexandra Alvarado

Cenozoic growth of the Andes has been strongly influenced by subduction dynamics, inherited crustal heterogeneities, and the superposed effects of climate. Subduction of the Carnegie Ridge in Ecuador has impacted late Cenozoic magmatism and tectonic activity, including the formation of a crustal sliver escaping northward. However, the relationship between ridge subduction and topographic growth has remained unclear. We present new thermochronological data from the Western Cordillera of Ecuador to (1) pinpoint the timing of ridge subduction, and (2) evaluate the role of ridge subduction in prompting growth of the Ecuadorian Andes. Time-temperature inverse modeling of our results shows two phases of cooling separated by tectonic quiescence. The first cooling phase immediately post-dates magmatism in the Western Cordillera, and hence we attribute it to magmatic cooling. The second cooling phase starts at ~6-5 Ma. This we associate with onset of enhanced exhumation at this time in the Western Cordillera, synchronous with the last cooling phase in the Eastern Cordillera. Based on our thermal modeling and thermochronological age patterns along geological cross-sections we propose that recent crustal shortening and rock uplift triggered exhumation of Western Ecuadorian Andes starting at ~6-5 Ma. We suggest that the onset of Carnegie Ridge subduction in the latest Miocene increased the coupling at the subduction interface and promoted shortening and regional rock uplift in the northern Andes. Overall, our new thermochronological results highlight the pivotal role of bathymetric anomalies in distinct upper-plate deformation processes at non-collisional convergent plate margins.

How to cite: Margirier, A., Strecker, M., Reiners, P., Casado, I., Thomson, S., George, S., and Alvarado, A.: Onset of Carnegie Ridge subduction from low-temperature thermochronology, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6130, https://doi.org/10.5194/egusphere-egu21-6130, 2021.

15:46–15:48
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EGU21-7172
Vincent Regard, Joseph Martinod, Marianne Saillard, Sébastien Carretier, Laetitia Leanni, Gérard Hérail, Laurence Audin, and Kevin Pedoja

We explore the coastal morphology along an uplifting 500 km-long coastal segment of   the Central Andes, between the cities of Chala (Peru) and Arica (Chile). We use accurate DEM and field surveys to extract sequences of uplifted shorelines along the study area. In addition, we consider continental pediment surfaces that limit both the geographical and vertical extent of the marine landforms. We establish a chronology based on published dates for marine landforms and pediment surfaces. We expand this corpus with new 10Be data on uplifted shore platforms. The last 12 Ma are marked by three periods of coastal stability or subsidence dated ~12-11 Ma, ~8-7 Ma and ~5-2.5 Ma ago. The uplift that accumulated between these stability periods has been ~1000 m since 11 Ma; its rate can reach 0.25 mm/a (m/ka). For the last period of uplift only, during the last 800 ka, the forearc uplift has been accurately recorded by the carving of numerous coastal sequences. Within these sequences, we correlated the marine terraces with the sea level highstands (interglacial stages and sub-stages) up to MIS 19 (790 ka), i.e., with a resolution of ~100 ka. The uplift rate for this last period of uplift increases westward from 0.18 mm/a at the Peru-Chile border to ~0.25 mm/a in the center of the study area. It further increases northwestward, up to 0.45 mm/a, due to the influence of the Nazca Ridge. In this study, we document an unusual forearc cyclic uplift with ~4 Ma-long cycles. This periodicity corresponds to the predictions made by Menant et al. (2020) based on numerical models, and could be related to episodic tectonic underplating (subducting slab stripping) beneath the coastal forearc area.

How to cite: Regard, V., Martinod, J., Saillard, M., Carretier, S., Leanni, L., Hérail, G., Audin, L., and Pedoja, K.: 4 Ma-long uplift cycles of the southern Peru forearc since Late Miocene, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7172, https://doi.org/10.5194/egusphere-egu21-7172, 2021.

15:48–15:50
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EGU21-13740
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ECS
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Highlight
Andrew Hollyday, Jacqueline Austermann, Andrew Lloyd, Mark Hoggard, Fred Richards, and Alessio Rovere

Bivalve and gastropod shell beds deposited during the Early Pliocene (4.69-5.23 Ma) occur in uplifted outcrops (36 – 180 m above sea level) along the east coast of Patagonian Argentina. These rock units provide a record of sea level during a geologic period when atmospheric CO2 and temperatures were higher than today. As such, reconstructing the elevation of global mean sea level (GMSL) during this time allows us to better understand how sensitive ice sheets are to increased past and future warming. However, reconstructing GMSL from local sea level indicators is hindered by effects such as mantle dynamic topography and glacial isostatic adjustment (GIA) that cause local sea level to deviate from the global mean. Here we use geodynamic modeling to better understand this complex dynamic setting and quantify the amount of uplift along this coastline.

Despite being located on a relatively stable passive margin, significant variations in the elevation of the paleo shoreline indicators imply that the underlying convecting mantle is deforming the coastline. In particular, the subduction of the Chile Rise beginning ~18 Ma beneath Patagonia has generated a slab window underneath this region through which hot asthenosphere ascends. However, the former slab is still present deeper in the mantle, which causes a complex interplay between the downwelling slab and the upwelling asthenosphere. To quantify the effects of dynamic topography change since the Pliocene, we run 3D mantle convection simulations using the code ASPECT. We initialize our global model with a composite temperature structure derived from recent tomographic studies and a calibrated parameterization of upper mantle anelasticity. Independent estimates of pressure and temperature from thermobarometric calculations of proximal Pali-Aike xenoliths agree with the thermal structure of the tomography-based Earth model. We back-advect temperature perturbations and extract the resulting change in dynamic topography. Pairing GIA models and a suite of convection simulations in which we vary the viscosity and buoyancy structure with the observed differential paleo shoreline elevations allows us to forward model the most likely scenario for uplift along this coast.

How to cite: Hollyday, A., Austermann, J., Lloyd, A., Hoggard, M., Richards, F., and Rovere, A.: Differential uplift of three Pliocene sea level indicator sites in southern Argentina driven by upwelling asthenosphere through the Patagonian slab window , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13740, https://doi.org/10.5194/egusphere-egu21-13740, 2021.

15:50–15:52
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EGU21-15505
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ECS
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Highlight
Simon Stephenson, Mark Hoggard, and Nicky White

Convective circulation in the mantle supports some fraction of Earth’s topography. It is challenging to observe this topographic expression of mantle convection because it is contaminated by crustal and lithospheric isostatic processes. In the oceanic realm, residual bathymetry has been calculated by removing the effects of sedimentary and crustal thickness variations and comparing to the well-known plate cooling trend, revealing sub-plate topographic support that is up to 1–2 km in amplitude and varies on wavelengths of ~1000 km. On the continents, crustal thickness and density variations support a much larger fraction of topography. Similarly, the density and thickness of the lithospheric mantle do not follow a simple predictive relationship. It is therefore significantly more challenging to calculate the component of sub-plate topographic support in the continental realm. Here, we assemble a database of > 20,000 continental crustal thickness estimates from published active and passive source seismic experiments with a goal of exploring the topographic expression of crustal, lithospheric, and sub-plate mantle structure. A subset of these studies provides constraints on crustal seismic velocity structure which can be used to estimate local density structure of the continental crust. We exploit a database of laboratory-determined density and velocity measurements of crustal rocks to develop a velocity to density conversion scheme. We use these constraints on crustal structure to remove the effects of crustal thickness and density variations and calculate an estimate of residual topography that arises from mantle processes. We compare these values of residual topography to an independent, tomographically derived map of lithospheric thickness. We find that residual topography is negatively correlated with lithospheric mantle thickness, which is expected and consistent with oceanic lithospheric observations. Next, we exploit a pressure and temperature-dependent density parametrisation to calculate and remove the expected topographic contribution of the lithospheric mantle. This approach demonstrates that up to 3 km of residual topography is positively correlated with lithospheric thickness even after removing the effects of cooling and thickening of the plate, indicating that compositional factors may lead to decreasing density with increasing lithospheric thickness. This relationship is in broad agreement with constraints on lithospheric mantle composition from xenoliths. Our analysis demonstrates the importance of accounting for lithospheric structure in the continental realm in constraining the sub-plate, dynamic contribution to Earth’s topography. An important corollary is that changes to the density and thickness of lithospheric mantle drive rapid vertical motions of the Earth’s surface which are increasingly recognised as causes of regional epeirogenic uplift.

How to cite: Stephenson, S., Hoggard, M., and White, N.: Global Continental Residual Topography, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15505, https://doi.org/10.5194/egusphere-egu21-15505, 2021.

15:52–15:54
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EGU21-8822
Victoria Milanez Fernandes and Gareth Roberts

The shape of the Earth's surface is the result of complex interactions between deep and surface processes operating on a range of spatial and temporal scales. However, generating sufficient geological observations at the spatial and temporal scales relevant to investigating deep-Earth processes (~100–1000 km, ~1–100 Ma) remains a challenge. To address this challenge, I exploit a paleobiological database to generate a new compilation of >24,000 spot measurements of net surface uplift across all continents. The present-day elevation of marine fossil assemblages that crop out at the Earth’s surface provides a direct constraint on the timing and amplitude of net surface uplift on geological timescales. I explore how these surface observations can be used to explore the evolution of sub-plate processes in three key regions: Western North America, Borborema Province in northeast Brazil, and Northern Africa. This new data compilation provides self-consistent, and in places high resolution measurements for Cretaceous to Recent net uplift. Geophysical observations (e.g., free-air gravity, shear-wave topography) and isostatic calculations are combined with net uplift measurements from these regions to explore how mantle thermal anomalies and lithospheric thinning might generate the observed uplift patterns. Uncertainties associated with paleo-bathymetry, post-deposition compaction and glacio-eustasy are assessed. The results emphasise the importance of large inventories of paleobiological data for understanding the history of tectonic and mantle convective processes as expressed at the Earth's surface.

How to cite: Milanez Fernandes, V. and Roberts, G.: Exploiting a paleobiological database to constrain sub-plate support: Examples from western North America, northeast Brazil and northern Africa, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8822, https://doi.org/10.5194/egusphere-egu21-8822, 2021.

15:54–15:56
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EGU21-9165
John Jansen, Mike Sandiford, Toshiyuki Fujioka, Timothy Cohen, Martin Struck, Suzanne Anderson, Robert Anderson, and David Egholm

The Finke River in central Australia is counted among the world’s oldest drainage systems, raising the prospect that it holds a geomorphic record relevant to testing ideas about the role of sub-lithospheric mantle flow in shaping the Australian landscape. The Finke’s upper reaches preserve an enigmatic set of intertwined active and relict gorges that suggest a complex history of incision, aggradation and re-incision. We measured cosmogenic 10Be and 26Al in fluvial gravels stored in the gorges, and we applied a Markov chain Monte Carlo-based inversion model to test two limiting-case hypotheses about the timing of the gravel deposition and exhumation. Our results suggest that the nuclide memory contained within the gravels was essentially erased during protracted sediment storage. Previous studies attribute landscape evolution to the intensified post-Miocene aridity in tune with the perception that central Australia experienced limited deformation during the Cenozoic. However, the close correlation between drainage network patterns and the gravity field leads us to propose, instead, that incision/aggradation phases in the upper Finke are driven by a flexural response (at ~102 km length scales) to extreme uncompensated loads embedded in the crust. Further, we suggest that dynamic mantle processes have deformed the central Australian topography over longer (~103 km) wavelengths via the in-situ stress field, with horizontal stress variations of order 1–10 MPa. Acting together, these crustal and sub-lithospheric structures have imposed to-and-fro tilting on the Finke, triggering the phases of incision/aggradation on a million-year timescale that created the unusual intertwined bedrock gorges. The amplitude of topographic responses in the upper Finke to inferred variations in end-loading on the plate helps resolve an ongoing debate about the effective elastic thickness of the central Australian lithosphere to no more than 35 km.

How to cite: Jansen, J., Sandiford, M., Fujioka, T., Cohen, T., Struck, M., Anderson, S., Anderson, R., and Egholm, D.: Geomorphic imprints of dynamic topography and intraplate tectonism in central Australia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9165, https://doi.org/10.5194/egusphere-egu21-9165, 2021.

15:56–15:58
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EGU21-16512
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ECS
Conor O'Malley, Nicky White, Gareth Roberts, and Simon Stephenson

A range of geological evidence documents the growth of African topography as a result of sub-plate support throughout Cenozoic times. Recent studies used inverse modeling of drainage networks governed by the linear stream power law to quantify the spatio-temporal history of uplift and erosion across the continent. Here, we test predictions of this uplift rate history by applying it as tectonic forcing to naturalistic landscape evolution simulations. These simulations parameterise dynamic drainage reorganisation, track sedimentary flux, and permit variable erodibility, none of which are feasible in inverse models. Modelled topography, river profiles, drainage planforms and sedimentary flux patterns broadly match observations. We test the sensitivity of forward model prediction to variations in erodilibity by employing spatio-temporally variable precipitation rate. Forward model predictions are relatively robust to even large excursions, suggesting landscapes contain internal feedbacks which modulate these effects and permit close recovery of the geomorphic record of uplift. Wavelet power spectral analysis reveals observed African river profiles are self-similar at wavelengths >~ 100 km, meaning longest-wavelength features have the highest amplitudes. At shorter wavelengths, spectral slopes increase, implying sharper features are generated only at wavelengths <~ 100km. Synthetic fluvial profiles recovered from simple landscape evolution models driven by uplift forcing obtained from inverse modeling of observed river profiles are self-similar across all wavelengths. This self-similarity solely reflects the tectonic forcing applied. When noise in erodibility or uplift rate forcing is added to forward simulations, power spectra of resulting fluvial profiles more closely approximate spectra of observed profiles. Thus sharp signals in observed river profiles arise from factors which do not correlate between neighbouring tributaries, e.g. lithological constrasts, self-forming hydraulic shocks, or human alteration. The recoverability of regional uplift from observed fluvial profiles is made possible by the fact that most topographic power is generated by regional uplift and resides at long-wavelengths.

How to cite: O'Malley, C., White, N., Roberts, G., and Stephenson, S.: African Epeirogeny in the Geomorphic Record, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16512, https://doi.org/10.5194/egusphere-egu21-16512, 2021.

15:58–16:00
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EGU21-12331
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ECS
Gregory Ruetenik and Robert Moucha

Rift escarpments have long been the subject of coupled geodynamic/landscape evolution studies.  Many of these studies have shown that the flexural unloading response of the lithosphere plays a significant role in the rate of divide migration and the longevity of the escarpment topography, with lower elastic thickness values allowing for more localized isostatic rebound of the lithosphere in response to erosional unloading. However, rift escarpments are thought to last hundreds of millions of years, and therefore the lithosphere may exhibit viscoelastic behavior on this timescale.  Here we present a simplified model of a viscoelastic response to erosional unloading during escarpment evolution, and show that this drastically alters the behavior of the escarpment system.  Specifically, the escarpment retreat rate is significantly reduced, and topography maintained, when compared to a purely flexural model. Additionally, the area in front of the retreating the scarp (i.e., seaward of the scarp) experience delayed uplift response and topographic rejuvenation many millions of years after the divide passes.      

How to cite: Ruetenik, G. and Moucha, R.: Escarpment retreat on a viscoelastic lithosphere , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12331, https://doi.org/10.5194/egusphere-egu21-12331, 2021.

16:00–16:02
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EGU21-1562
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ECS
Suzanne Atkins and Nicolas Coltice

Net rotation is the process whereby the entire lithosphere can rotate with respect to the Earth’s mantle. The plates and continents retain their location with respect to each other, but they change their position with respect to global reference frames such as the Earth’s magnetic dipole, and structures in the Earth’s mantle such as plumes and hotspots. Constraining lithospheric net rotation is therefore one factor in building an absolute plate motion model. However, the amount of net rotation occurring at present day is poorly contained, and the drivers of net rotation are very poorly understood. Many absolute plate motion models therefore attempt to minimise net rotation, because there is no way to constrain rotation in the geological past. 

 

In previous geodynamical studies, the presence of thick continents and large viscosity contrasts were found to be controlling factors in the development of net rotation. We investigate the effects of different convection parameters and tectonic states on the magnitude and evolution of net rotation in 2D simulations. The use of 2D simulations allows us to run enough simulations to study a wide range of model parameters. We intend to compare our 2D conclusions with 3D simulations, to investigate how much of a difference the third dimension makes.

 

We find that net rotation varies on much shorter timescales than any other geodynamic feature. Net rotation is not cleanly correlated with any tectonic behaviours or settings, and that the magnitude and duration is unpredictable. We do however find that the distribution of net rotation within the lifetime of a particular simulation is Gaussian, with standard deviation dependent on the viscosity structure and contrasts of the simulation, in agreement with previous studies. However, in contrast to previous studies, the presence and thickness of continents makes very little difference to the speed of lithospheric rotation, although this may be because we are working in 2D. If the 2D results are also relevant in 3D, net rotation is a continuously varying and unpredictable value, but with a predictable statistical range. This may provide a way to better constrain net rotation for plate motion models.

How to cite: Atkins, S. and Coltice, N.: A search for constraints on lithospheric net rotation in 2D convection simulations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1562, https://doi.org/10.5194/egusphere-egu21-1562, 2021.

16:02–16:04
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EGU21-13574
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ECS
Kevin Gaastra and Richard Gordon

To improve modeling of deep-earth dynamics it is important to understand changes in the arrangements of plate boundaries, especially trenches accommodating subduction, and major changes in tectonic plate motion. Here we focus on the sequence of key surface events in Eocene time that likely coincide with changes in deep-earth dynamics. In particular, we develop methods of analysis of seamount locations and age dates using a small number of adjustable parameters (10 per chain) on the Pacific plate with a focus on the timing of the Hawaiian-Emperor bend relative to the timing of other major Eocene tectonic changes.

We find that motion between hotspots differs insignificantly from zero with rates of 2±4 mm/a (±2σ) for 0-48 Ma and 26±34 mm/a (±2σ) for 48-80 Ma. Relative to a mean Pacific hotspot reference frame, nominal rates of motion of the Hawaii, Louisville, and Rurutu hotspots are ~5 mm/a and differ insignificantly from zero. We conclude that plumes underlying these Pacific hotspots are more stable in a convecting mantle than previously inferred.

We estimate the locations and ages (with uncertainties) of bends in Pacific hotspot chains using a novel inversion method. The location of the ~60° change in trend at the Hawaiian-Emperor bend is well constrained within ~50-80 km (=2σ), but the location of the bends in the Louisville and Rurutu hotspots are more uncertain. If the uncertainty in the location of the bend in the Louisville chain is included, we find no significant difference in age between the bends of different Pacific hotspot chains. The best-fitting assumed-coeval age for the bends is 47.4±1.0 Ma (±2σ), which is indistinguishable from the age of the C21o geomagnetic reversal. The age of the bend is younger than the initiation of subduction in the Western Pacific, but approximately coeval with changes in Pacific and circum-Pacific relative plate motion. Changes to the tectonic system near the age of the bend are not limited to the Pacific basin. The smooth-rough transition flanking the Carlsberg Ridge records a threshold in the decreasing spreading rate between India and Africa, thought to record the onset of the collision of India with Eurasia, and is constrained to be between C21y and C20o (46 Ma and 43 Ma) in age. Nearly simultaneously, South America and Australia began to diverge more rapidly from Antarctica. The Eocene bend in Pacific hotspot chains may be the most evident feature recording a global re-organization of plate motions and mantle circulation possibly caused by the earlier collision of India and Eurasia or initiation of western Pacific subduction.

How to cite: Gaastra, K. and Gordon, R.: Relative Timing of the Eocene Global Reorganization of Plate Motions: New Results for Pacific Plate Hotspot Tracks, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13574, https://doi.org/10.5194/egusphere-egu21-13574, 2021.

16:04–16:06
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EGU21-13842
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ECS
Andres Felipe Rodriguez Corcho, Sara Morón, Rebecca Farrington, Romain Beucher, Louis Moresi, and Camilo Montes

Arc-continent collision is the process by which intra-oceanic arc crust is accreted to continental margins and the most important mechanism that enables the growth of the continental crust since Phanerozoic times. We use numerical visco-plastic mechanical models to explore: (i) the role of lithospheric-mantle dynamics in controlling the spatio-temporal evolution of stress in arc-continent collision settings, and (ii) the role of density contrasts in the evolution of the stress regime. We performed a series of simulations only varying the thickness of the arc based on natural examples as the arc thickness controls the buoyancy of intra-oceanic arcs. Therefore, we investigated a range of density contrasts between the arc and the continental plate. Modelling results show that arc-continent collision can evolve into two contrasting scenarios: (i) slab-anchoring and arc transference in dense arcs where the density contrast between the arc and the adjacent continental lithosphere is above -3% (15-31 km in thickness); and (ii) slab break-off in buoyant arcs where the density contrast between the arc and the adjacent continental lithosphere is below -3% (32-35 km in thickness). We conclude that the large-scale mantle return flow emerged from slab-anchoring facilitates the simultaneous occurrence of compression and extension in the margin by enhancing: (i) compression and lithospheric thickening of the buoyant intra-oceanic arc crust; and (ii) the density contrast between the accreted arc and the continental margin that triggers the release of a gravitational flow. In the particular case of buoyant arcs, the compressional body force applied by the deformed arc to the subducting plate drives its passive retreat. The results of our numerical modelling highlight the importance of the role of lithospheric-mantle dynamics on controlling the spatio-temporal evolution of stress.

How to cite: Rodriguez Corcho, A. F., Morón, S., Farrington, R., Beucher, R., Moresi, L., and Montes, C.: Dynamics of arc-continent collision: the role of crustal-mantle dynamics on controlling the spatio-temporal evolution of stress , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13842, https://doi.org/10.5194/egusphere-egu21-13842, 2021.

16:06–16:08
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EGU21-14684
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ECS
Han Shi, Jinyao Gao, Mingju Xu, and Qingsheng Guan

The South China Sea (SCS) is situated at the junction of Eurasian, Indo-Australian, and Philippine sea plates. Its stress state provides significant information about the regional tectonic structure associated with interaction among the three plates. The stress field of the SCS is composed of horizontal and vertical stress fields. We calculate the vertically averaged deviatoric stress field using horizontal gradients of gravitational potential energy obtained by high-resolution sea-surface height data (SSH) from satellite Haiyang-2A. The vertical tectonic stress field is computed based on the Bouguer gravity anomaly derived from SSH and topographic data.

The vertically averaged deviatoric stress field is consistent with the GPS velocity field, the focal mechanism, and the mantle flow stress field of the South China Sea. Moreover, it also indicates the Red River-Ailaoshan Fault zone on the west of the SCS and the Manila subduction on the east. The vertical tectonic stress field removing the influence of sediment indicates upward stress of the lithosphere in the SCS ocean basin. The stress field model therefore provides a powerful tool for understanding regional tectonic activities around the SCS.

How to cite: Shi, H., Gao, J., Xu, M., and Guan, Q.: The stress field model of the South China Sea calculated by sea level data from satellites, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14684, https://doi.org/10.5194/egusphere-egu21-14684, 2021.

16:08–16:10
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EGU21-5494
A. M. Celal Şengör, Nalan Lom, Cengiz Zabcı, Gürsel Sunal, and Tayfun Öner

We describe a major new Neoproterozoic orogenic system belonging to the larger Pan-African deformational realm, the Saharides, in North Africa, by using various tools such as magnetic maps and our own remote-sensing based structural interpretation to aid us in following the orogenic trend-lines in addition to a large compilation of geochronological data. The Saharides, a Turkic-type orogenic complex similar to the Altaids of central and northwestern Asia, involved major subduction accretion complexes occupying almost the entire Arabian Shield and much of Egypt and Sudan and the small inliers of such complexes farther west to and including the Ahaggar mountains. These complexes are formed at least by half from juvenile material representing at least 5 million km2 new continental crust formed during the Neoproterozoic from about 900 to 500 Ma ago. Contrary to conventional wisdom in the areas they occupy, the Saharides involved no continental collisions until the very end of their history, but evolved by subduction and strike-slip stacking of arc material mainly by pre-collisional coast-wise transport of arc fragments shaved off the Congo/Tanzania cratonic nucleus in a manner very similar to the development of the Nipponides in east Asia, parts of the North American Cordillera and the Altaids. The entire Sahara is shown to be underlain by a double orocline much like the Hercynian double orocline in western Europe and northwestern Africa and not by an hypothetical ‘Saharan Metacraton’. The method here followed may be a fruitful procedure to untangle the structure of some of the Precambrian orogenic belts before life evolved sufficiently to make biostratigraphy feasible.

How to cite: Şengör, A. M. C., Lom, N., Zabcı, C., Sunal, G., and Öner, T.: The Saharides: Turkic-Type Orogeny in Afro-Arabia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5494, https://doi.org/10.5194/egusphere-egu21-5494, 2021.

16:10–17:00