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Rapid mass movements are a major threat in populated landscapes, as they can cause significant loss of life and damage civil infrastructure. Previous work has shown that using environmental seismology methods to monitor such mass movements and establish monitoring systems offers advantages over existing approaches. The first important step in developing an early warning system for rapid mass movements based on seismic signals is automatically detecting events of interest. Though the approach, such as short-term average to long-term average ratio (STA/LTA) and machine learning model, was introduced to detect events (e.g., debris flow and rockfall), it is still challenging to calibrate input parameters and migrate existing methods to other catchments. Detection of debris flows, for instance, is similar to anomaly detection if we consider the seismic stations recording background signals as an overwhelming majority condition. Benford's law describes the probability distribution of the first non-zero digits in numerical datasets, which provides a functional, computationally cheap approach to anomaly detection, such as fraud detection in financial data or earthquake detection in seismic signals. In this study, seismic signals generated by rapid mass movements were collected to check the agreement of the distribution of the first digit with Benford's law. Subsequently, we develop a computationally efficient and non-site-specific model to detect events based on Benford's law using debris flows from the Illgraben, a Swiss torrent, as an example. Our results show that seismic signals generated by high-energy mass movements, such as debris flows, landslides, and lahars, follow Benford's law, while those generated by rockfall and background signals do not. Furthermore, our detector performance in picking debris-flow events is comparable to a published random forest and seismic network-based approach. Our method can be applied at other sites to detect debris-flow events without additional calibration and offers the potential for real-time warnings.

Duricrusts are hard elemental layers forming in tropical and subtropical regions. Different types of duricrusts exist, including calcretes, silcretes and ferricretes (or iron duricrusts), that differ by their composition and, most likely, their formation mechanism. In places such as Africa, Australia or Brazil, we can observe them capping hills and protecting landscapes, thus being an important part of regional geomorphology. Their formation is highly dependent on climate and requires strong seasonal precipitation cycles, enabling transport and accumulation of elements during wet seasons and precipitation and hardening during dry seasons. However, it is also known that duricrusts form in tectonically quiet environments, and multiple tens of thousands of years are needed to form centimetres to metres thick elemental layers. Two hypotheses for duricrust formation exist, namely the water table (or horizontal) hypothesis and the laterite (or vertical) hypothesis. Yet, until recently, no quantitative (numerical) model exists that represents either of them. We presented last year (Fenske et al, 2022) a model based on the water table hypothesis. Here we present a second model based on the laterite hypothesis and compare the predictions of the two models. Although the models we propose are potentially applicable to a wide range of duricrusts, we will concentrate here on the formation of ferricretes.Laterites are a type of regolith covering around 33\% of land surfaces. The thickest lateritic profiles are found in the centre of continental cratons, where they evolved for millions of years. The main process responsible for laterisation is weathering, the process transforming bedrock into regolith. During weathering rocks progressively lose their structure, elements are dissolved, re-precipitated, leached, or transported, leading to progressive porification and compaction. Subsequently, from the weathering front to the surface, a typical lateritic profile is made of coarse grained and then fine grained saprolite, a mottled zone and at the top, an indurated cover, the ferricrete. We recently developed a numerical model for duricrust formation according to the laterisation hypothesis (LT model). In this model, iron duricrusts are genetically linked to the bed rock. Material from the bedrock are transformed and leached within a given vertical column of rocks but are also moved vertically due to tectonic uplift. We assume that as the regolith ages, it undergoes a process of transformation that is proportional to mean fluid flow and that leads to hardening and compaction. As material is constantly removed from the regolith layer, the model is dependant on a constant material input (through uplift and, potentially, surface erosion) to reach sufficient laterisation for ferricrete formation. Comparing predictions of the laterisation (LT) model to those of the water table (WT) model, we observe that they require slightly different tectonic regime, with the WT model most efficient during period of complete quiescence and the LT model needing a slow constant rate of base level fall to produce duricrusts. The two models also differ by the geometry of the duricrusts they predict and, in particular, the depth at which duricrust forms in the regolith (at or above the water table level).

Erosion rates are commonly used to study tectonic uplift and sediment export from mountain ranges. However, the scarcity of erosion rate data often hinders detailed tectonic interpretations. Here, we present 25 new erosion rates from the Northern Andes of Colombia, determined from cosmogenic 10Be measurements, to study spatial and temporal patterns of uplift along the Central and Eastern Cordillera. These rates, along with published data and precipitation-corrected normalized channel steepness measurements, were used to create high-resolution erosion rate maps. The results show that the southern Central Cordillera has relatively uniform erosion rates, averaging around 0.3 mm/a, while the northern Central Cordillera exhibits rapidly eroding canyons dissecting slowly eroding low-relief surfaces. We interpret that long-term, steep slab subduction has led to an erosional steady-state in the southern Cordillera Central, while Late Miocene slab flattening caused an acceleration in uplift in the northern Cordillera Central which the landscape has not yet adjusted to. The Eastern Cordillera also displays pronounced erosional disequilibrium, with a slowly eroding central plateau rimmed by faster eroding western and eastern flanks. Our maps suggest recent topographic growth of the Eastern Cordillera, with deformation focused along the eastern flank, which is also supported by balanced cross-sections and thermochronologic data. Spatial gradients in predicted erosion rates along the eastern flank of the Eastern Cordillera suggest transient basin-ward migration of thrusts. By using our erosion maps to estimate sediment fluxes, we find that the Eastern Cordillera exports nearly four times more sediment than the Central Cordillera. Our analysis shows that accounting for spatial variations in erosion parameters and climate gradients reveals important variations in tectonic forcing that would otherwise be obscured in traditional river profile analyses. Moreover, given relationships between tectonic, and topographic evolution, we propose that the dynamic landscape evolution of the Northern Andes, as revealed by our erosion maps, is primarily linked to spatial and temporal variations in slab dip, with potentially additional influences from inherited Mesozoic rift structures.

Stratigraphy represents a physical record of the behavior of source to sink sedimentary systems in the geological past. Grain-size fining in stratigraphy toward the downstream direction is driven by the sediment supply, subsidence rate and accommodation space generation. In principle these grain size trends can be inverted to quantify the temporal and spatial variation in these driving forces. This work addresses this challenge to understand how stratigraphic grain size fining reflects sediment flux and accommodation space generation in a normal fault bounded sedimentary system in the Gulf of Corinth, Greece. A SW to NE 2.6 km exposed natural cliff cut section of the middle Pleistocene Kerinitis Gilbert type delta has been selected for its accessibility, where the stratigraphic units are traceable, and where the timing of sediment deposition is well constrained (500 to 800 ka). The qualitative evolution of the fault is also understood as the three mapped depositional packages (lower part, middle part and the upper part) in the Kerinitis delta are thought to represent the initiation, development, and termination of the Pirgaki-Marmoussia fault. Down-system grain size data has been collected using the Wolman point count method at 25 measurement stations, predominantly from fluvial top-sets following for several timelines. Furthermore, high-resolution grain size photographs have been captured to study the inaccessible higher stratigraphic units. Paleoflow direction is reconstructed using a pebble imbrication dataset, which indicates the source to sink direction. For the grain size datasets, we have applied several self-similarity tests to evaluate the mutual relationship among the datasets. Our result shows that almost all the datasets exhibit significantly self-similarity with each other. We reconstruct grain size fining trends for each of the units, which we quantitatively relate to the spatial distribution of subsidence and sediment flux using a self-similarity based fining model. Our analysis gives new insights into the evolution of the delta bounding Pirgaki-Marmoussia fault and show how grain size data can be used to reconstruct landscape dynamics in the past.

On Earth, the characteristics of fluvial erosion depends on two main parameters: climate (rain fall) and tectonic history. Mars is a planet that experienced erosion driven by liquid water but its geodynamics are vastly different from Earth’s. Mars therefore represents a unique opportunity to understand how landscape evolution differs on a planet with a “stagnant lid” tectonic regime. The formation of Tharsis dome, a vast volcanic province, during the early history of Mars represented a major magmato-tectonic upheaval for the planet. Over several hundreds of million years, the Tharsis region experienced large scale magmatic intrusions, crustal deformation and effusive volcanism resulting in crustal growth, dynamic uplift and true polar wander (TPW) that accounts for the present location of the Tharsis dome at the equator. This event occurred during a time when Mars had an active water cycle, although the total mass and relative proportion of ice, liquid water and vapor is not well constrained. The uplift and subsequent true polar wander of Mars have affected drainage systems across the planet with many being abandoned or modified due to the variable uplift or subsidence as a lithospheric response to the regional upheaval in the Tharsis region (load on the elastic lithosphere) and TPW. Here we present results from numerical simulations performed using a stream power law algorithm on Mars during the Noachian/Hesperian growth of Tharsis to assess how the patterns of erosion rate are affected by the distribution of atmospheric moisture and flow routing in an attempt to reproduce the observed distribution of valley networks and their geometry. For this, we adapted and used the fully-implicit and O(n)-complexity FastScape algorithm to perform the simulation at the planetary scale. The aims of this work are to quantify the effect of Tharsis dome formation on fluvial systems during the Noachian and early Hesperian, and to establish a first-order erosion rate for this period. This study could help to constrain how much water was cycling on Mars at this time.

To first order mountain belts grow by crustal thickening and gain their elevated topography through isostatic compensation. High and rising topography in turn modifies the global wind circulation system and is the main locus of (orographic) precipitation. The ensuing flow of water (or ice) redistributes mass through erosion and deposition, counteracts orogenic growth, shapes the appearance of the landscape, and most importantly provides a feedback-loop between surface processes and tectonics. However, it remains debated whether surface processes or lithospheric strength control mountain belt height, width, and longevity, reconciling high erosion rates observed for instance in Taiwan and New Zealand, low erosion rates in the Tibetan and Andean plateaus, and long-term survival of mountain belts for several 100s of million years. Here we use a tight coupling between a landscape evolution model (FastScape) and a thermo-mechanically coupled mantle-scale tectonic model (Fantom) to investigate mountain belt growth. Based on several end-member models and the new non-dimensional Beaumont number, Bm, we provide a quantitative measure of the interaction between surface processes and tectonics, and define three end-member orogen types: Type 1, non-steady state, strength controlled (Bm > 0.5); Type 2, flux steady state, strength controlled (Bm ≈ 0.4−0.5); and Type 3, flux steady state, erosion controlled (Bm

Climate is a primary driver of sedimentary processes from source to sink. A key challenge for stratigraphic studies in deep time is to constrain how perturbations of hydroclimate in the past have affected surface processes and source-to-sink dynamics. In modern fluvial systems, precipitation and the global hydrological cycles are fundamental determinants of sediment production, transport and deposition, and have also long been shown to exert major influence on river channel dynamics. For instance, recent works suggest that increased seasonality in precipitation could enhance lateral river dynamics, through less-frequent but higher-energy flood events and decreased bank stability associated to droughts and vegetation decline. In such settings, channel mobility has more impact than water discharge on the export of fine sediments downsystem, hence predicting specific stratigraphic patterns potentially diagnostic of hydroclimate perturbations in deep time.This hypothesis has been tested by paleohydraulic reconstructions and estimates of avulsion occurrences based on facies analyses, but lacks direct assessment of the intensity of floodplain reworking. Therefore, to further test this hypothesis, we propose to quantify the degree of floodplain reworking by looking at the amount and spatial distribution of floodplain elements contained in fluvial to coastal sandstones within a source-to-sink system during an important climate perturbation.Our study focuses on the Paleocene-Eocene Thermal Maximum (PETM, 56Myr) in the Southern Pyrenees (Tremp-Graus Basin, Spain). Several authors described the PETM in many localities over the area, showing a widespread deposition of an anomalously thick conglomeratic interval suggesting a perturbation of the fluvial channel dynamics at that time. We point-counted the content of 45 sandstone samples that span the pre-, syn- and post-PETM intervals in continental channels to deltas and carbonated platform depositional environments. Reworked floodplain was identified by the presence of microcodiums and carbonate nodules. To assess whether these elements were reworked from floodplain significantly older (erosion of underlying layers) or contemporaneous (lateral erosion by channel migration), we attempted LA-MC-ICPMS U-Pb dating on carbonates. Preliminary results show an increase of the amount of reworked floodplain in distal environments during the PETM climatic event. This observation is in adequation with other studies predicting a link between enhanced seasonality during global warming and channel migration rate, and provides a new way of assessing and testing fluvial dynamics in deep time as a proxy for perturbations of ancient hydroclimates.

Interpretation of the sedimentary record requires a thorough understanding of both external forcing and autogenic dynamics within sedimentary basins. This research analyzes the influence of autogenic and basin dynamics on grain size fining through landscape evolution modeling (Myr timescales) of a flexural foreland basin and observations from real-world basins with well-known underlying subsidence for comparison. In previous work, we have incorporated the Fedele and Paola (2007) self-similar gravel grain size method in two dimensions allowing for the formation of multichannels (2D) into the FastScape (Braun and Willett, 2013) landscape evolution model. We have validated this modelling approach against the self-similar grain size results of Duller et al. (2010) without multiple channels (1D). Here, we analyze different upstream precipitation and flexural elastic thicknesses (impacting the pattern and rate of subsidence) on the evolution of grain size fining, stratigraphic thickness, topography, and channel dynamics. We use real world scenarios with known underlying subsidence, grain size fining, and surface ages (such as the Grapevine Mountain fans of Death Valley) to compare with model results. Underlying subsidence controls on grain size have been well described by Duller et al. (2010), but never with the consideration of channel and topographic dynamics that vary the deposition rate relative to the subsidence rate across the basin. Braun (2022) has described the upstream area and precipitation influence relative to downstream on fan extents, but has not yet applied this to grain size fining trends. Our results show that multiple channels influence the grain size fining solution (more fining) compared to solutions with a single set channel pathway in conditions ranging from full sediment capture in the basin to full bypass, and these results vary depending on fan extents and the pattern of underlying subsidence. The model can be also used to estimate a variance range in grain size fining in which autogenic dynamics may be playing a greater role to statistically rule out or acknowledge a potential autogenic control induced by channel dynamics. Braun & Willet (2013). Geomorphology: 180. Braun (2022). ESP: 10(2). Duller et al. (2010). JGR: ES 115(F3). Fedele & Paola (2007). JGR: ES 112(F2).

Deep incised glacial valleys surrounded by high peaks form the modern topography of the Southern Patagonian Andes. Two Miocene plutonic complexes in the Andean retroarc, the cores of the Fitz Roy (49°S) and Torres del Paine (51°S) massifs, were emplaced at 16.7±0.3 Ma and 12.5±0.1 Ma, respectively. Subduction of ocean ridge segments initiated at 54°S, generating northward opening of an asthenospheric window with associated mantle upwelling and orogenic shortening since 16 Ma. Subsequently, the onset of major glaciations at 7 Ma caused drastic changes in the regional topographic evolution. To constrain the respective contributions of tectonic convergence, mantle upwelling and fluvio-glacial erosion to rock exhumation, we present inverse thermal modeling of a new dataset of zircon and apatite (U-Th)/He from the two massifs, complemented by apatite 4He/3He data for Torres del Paine. Our results show rapid rock exhumation recorded in the Fitz Roy massif between 10.5 and 9 Ma, which we ascribe to mantle upwelling and/or crustal shortening due to ridge subduction at 49°S. Both massifs record a pulse of rock exhumation between 6.5 and 4.5 Ma, which we interpret as the result of the onset of Patagonian glaciations. After a period of erosional quiescence during the Miocene/Pliocene transition, increased rock exhumation since 3-2 Ma to present day is interpreted as the result of alpine glacial valley carving promoted by reinforced glacial-interglacial cycles. This study demonstrates that along-strike thermochronological studies provide us with the means to assess the spatio-temporal variations in tectonic, mantle, and surface processes forcing on rock exhumation.

Nile River, the longest river in the world (>6,500 km), has been studied since long time ago tracing back to the fifth century yet the timing of origin of its present-day drainage is still disputed. There are two end-members of notions of the birth of the Nile, the first one believes that the present-day Nile has been connected to the Ethiopian Plateau since ~30 Ma, while the other supports the idea of young Nile around ~6 Ma. The Nile crosses today two former endorheic systems (Sudanese and Albertine “Basins”) and one exorheic system (Tethys Margin) before finally depositing sediments in the Mediterranean Sea. Our objective here focuses on deciphering the source-to-sink scenario of the Nile through relief growth, tectonic, and climate since the uppermost Cretaceous.The timing of the uplifts and deformation wavelengths are constrained by characterizing and mapping several generations of stepped pediments on DEM and satellite images which then dated using their geometrical relationships with dated magmatic rocks. Additionally, stratigraphic records of the sedimentary basins were studied to complement the dating of the pediments. The authenticity of the approaches is to integrate those data enclosing all the source-to-sink (S2S) systems to construct a coherent scenario of the Nile paleorouting systems especially on the dynamics of its sediment sources.We proposed the following model for the S2S of the Nile where its catchment grew larger southwards through time. First, a significant deformation occurred around the Cretaceous-Paleogene boundary (66 Ma) with the uplift of western limit of the Nile catchment, the Darfur-Ennedi-Tibesti domains, followed by the formation of a main large pediment. Second, this pediment was then flooded during late Paleocene (58-57 Ma) until a subtle high bounding northward the endorheic Sudanese “Basin”. Consequently, carbonate platforms were widely deposited from Paleocene until middle Eocene in the Egypt extending southwards to the former Hudi Lakes in northeast Sudan and there was no siliciclastic supply for the Nile during that time. Third, the first evidence of a fluvial system (“Pre-Eonile” according to Said, 1981) was discovered during late Eocene (~37 Ma) by large channel incisions on top of the carbonate platforms. Contemporaneously, the Uweinat Dome was uplifted and likely acted as the main siliciclastic source during late Eocene to Oligocene. Fourth, the initiation of Red Sea rifts during the Oligocene followed by a major uplift at the scale of north-east Africa (~10 Ma) provoked another plausible siliciclastic source, the Red Sea Hill’s flanks, in addition to the Uweinat Dome. Fifth, during the capture of the Sudanese endorheic system in the Early Pliocene (~4 Ma), the Nile catchment grew significantly larger and the sources were actively provided by the Darfur and Ethiopian Plateau. Finally, during middle-late Pleistocene (< 1 Ma) the Nile completed its present-day catchment by capturing the Albertine endorheic system and the siliciclastic sediments were supplied by the Ethiopian Plateau and the East African Dome.

Arctic river deltas define the interface between the terrestrial Arctic and the Arctic Ocean. They are the site of sediment, nutrient, and soil organic carbon discharge to the Arctic Ocean. Arctic deltas are unique globally because they are underlain by permafrost and acted on by river and sea ice, and many are surrounded by a broad shallow ramp. Such ramps may buffer the delta from waves, but as the climate warms and permafrost thaws, the evolution of Arctic deltas will likely take a different course, with implications for both the local scale and the wider Arctic Ocean. One important way to understand and predict the evolution of Arctic deltas is through numerical models. Here we present ArcDelRCM.jl, an improved reduced-complexity model (RCM) of arctic delta evolution based on the DeltaRCM-Arctic model (Lauzon et al., 2019), which we have reconstructed in Julia language using published information. Unlike previous models, ArcDelRCM.jl is able to replicate the ramp around the delta. We have found that the delayed breakup of the so-called “bottom-fast ice” (i.e. ice that is in direct contact with the bed of the channel or the sea, also known as “bed-fast ice”) on and around the deltas is ultimately responsible for the appearance of the ramp feature in our models. However, changes made to the modelling of permafrost erosion and the protective effects of bottom-fast ice are also important contributors. Graph analyses of the delta network performed on ensemble runs show that deltas produced by ArcDelRCM.jl have more interconnected channels and contain less abandoned subnetworks. This may suggest a more even feeding of sediments to all sections of the delta shoreline, supporting ramp growth. Moreover, we showed that the morphodynamic processes during the summer months remain active enough to contribute significant sediment input to the growth and evolution of Arctic deltas and thus should not be neglected in simulations gauging the multi-year evolution of delta features. Finally, we tested a strong climate-warming scenario on the simulated deltas of ArcDelRCM.jl, with temperature, discharge, and ice conditions consistent with RCP7–8.5. We found that the ramp features degrade on the timescale of centuries and effectively disappear in under 1 millennium. Ocean processes, which are not included in these models, may further shorten the timescale. With the degradation of the ramps, any dissipative effects on wave energy they offered would also decrease. This could expose the sub-aerial parts of the deltas to increased coastal erosion, thus impacting permafrost degradation, nutrients, and carbon releases.

Here I present a comparison between two of the most widely used reduced-complexity models for the representation of sediment transport and deposition processes, namely the transport-limited (or TL) model and the under-capacity (or ξ–q) model more recently developed by Davy and Lague (2009). Using both models, I investigate the behavior of a sedimentary continental system of length L fed by a fixed sedimentary flux from a catchment of size A0 in a nearby active orogen through which sediments transit to a fixed base level representing a large river, a lake or an ocean. This comparison shows that the two models share the same steady-state solution, for which I derive a simple 1D analytical expression that reproduces the major features of such sedimentary systems: a steep fan that connects to a shallower alluvial plain. The resulting fan geometry obeys basic observational constraints on fan size and slope with respect to the upstream drainage area, A0. The solution is strongly dependent on the size of the system, L, in comparison to a distance L0, which is determined by the size of A0, and gives rise to two fundamentally different types of sedimentary systems: a constrained system where LL0. I derive simple expressions that show the dependence of the system response time on the system characteristics, such as its length, the size of the upstream catchment area, the amplitude of the incoming sedimentary flux and the respective rate parameters (diffusivity or erodibility) for each of the two models. I show that the ξ–q model predicts longer response times. I demonstrate that although the manner in which signals propagates through the sedimentary system differs greatly between the two models, they both predict that perturbations that last longer than the response time of the system can be recorded in the stratigraphy of the sedimentary system and in particular of the fan. Interestingly, the ξ–q model predicts that all perturbations in the incoming sedimentary flux will be transmitted through the system, whereas the TL model predicts that rapid perturbations cannot. I finally discuss why and under which conditions these differences are important and propose observational ways to determine which of the two models is most appropriate to represent natural systems.

International audience; We use a Landscape Evolution Model including flexural isostasy to investigate the influenceof inherited foreland relief on the stratigraphic evolution of the retro-foreland domain during mountainbuilding. We show models with four different types of initial relief in the foreland domain: at sea level,elevated (+300 m), a 1 km-deep and 100 km-wide foreland basin associated with either a forebulge at sealevel or elevated at +300 m. During the first 10 Myr of simulation, the landscape evolution of the foreland issignificantly altered by its inherited bathymetry/topography. The impact is then smoothed out once the foreland slope has stabilized and develops a transverse drainage network. Models record a long-term shallowing-up mega-sequence driven by the increase in sediment production rate in the uplifting range and the decrease in the rate of flexural accommodation space creation in the foreland basin. The initial relief of the foreland domain alters the timing of its transition from the under-filled to the over-filled phase. An initially deep foreland basin is twice as thick as an initially elevated foreland. It records deep marine deposits while a foreland initially at sea level records thin shallow marine and an elevated foreland records continental deposits. The forebulge is buried by continental deposits in an initially elevated foreland while it is buried by marine sediments in other models. Alluvial fans at the foot of the range are more elevated in initially elevated forelands. We discuss our results of modeled stratigraphic architecture in comparison with the Pyrenean, Alpine and Andean retro-foreland basins.

Landscape Evolution Models (LEMs) are prime tools to simulate the evolution of source-to-sink systems through ranges of spatial and temporal scales. Plethora of different empirical laws have been successfully applied to describe the different parts of these systems: fluvial erosion, sediment transport and deposition, hillslope diffusion, or hydrology. Numerical frameworks exist to facilitate the combination of different subsets of laws, mostly by superposing grids of fluxes calculated independently. However the exercise becomes increasingly challenging when the different laws are inter-connected: for example when a lake breaks the upstream-downstream continuum of the amount of sediment and water it receives and transmits; or when erosional efficiency depends of the composition of a sediment flux affected by multiple processes. In this contribution, we present a method mixing the advantages of cellular-automata and graph theory to address such cases. We demonstrate how the former guarantees finite knowledge of all fluxes independently from the process-law implemented in the model while the latter offer a wide range of tools to process numerical landscapes, including landscapes with closed basins. We provide three scenario largely benefiting from our method: i) one where lake systems are primary controls on Landscape evolution, ii) one where sediment provenance is closely monitored through the stratigraphy and iii) one where heterogeneous provenance influences fluvial incision dynamically. We finally outline the way forward to make this method more generic and flexible.

International audience; We present a study to estimate the large-scale landscape history of a continental margin, by establishing a source-to-sink volume balance between the eroding onshore areas and the offshore basins. Assuming erosion as the primary process for sediment production, we strive to constrain a numerical model of landscape evolution that balances the volumes of eroded materials from the continent and that deposited in the corresponding basins, with a ratio imposed for loss of erosion products. We use this approach to investigate the landscape history of Madagascar since the Late Cretaceous. The uplift history prescribed in the model is inferred from elevations of planation surfaces formed at various ages. By fitting the volumes of terrigenous sediments in the Morondava Basin along the west coast and the current elevation of the island, the landscape evolution model is optimized by constraining the erosion law parameters and ratios of sediment loss. The results include a best-fit landscape evolution model, which features two major periods of uplift and erosion during the Late Cretaceous and the middle to late Cenozoic. The model supports suggestions from previous studies that most of the high topography of the island was constructed since the middle to late Miocene, and on the central plateau the erosion has not reached an equilibrium with the high uplift rates in the late Cenozoic. Our models also indicate that over the geological time scale, a significant portion of materials eroded from Madagascar was not archived in the offshore basin, possibly consumed by chemical weathering, the intensity of which might have varied with climate.

Erosion rates are widely used to assess tectonic uplift and sediment export from mountain ranges. However, the scarcity of erosion rate measurements often hinders detailed tectonic interpretations. Here, we present 25 new cosmogenic nuclide-derived erosion rates from the Northern Andes of Colombia to study spatio-temporal patterns of uplift along the Central and Eastern Cordillera. Specifically, we combine our new and published erosion rate data with precipitation-corrected normalized channel steepness measurements for building high-resolution erosion rate maps. We find that erosion rates in the southern Central Cordillera are relatively uniform and average ~0.3 mm/a, whereas rapidly eroding canyons dissect slowly eroding, low-relief surfaces in the northern Central Cordillera. We interpret that long-term, steep slab subduction has led to an erosional steady-state in the southern Cordillera Central, whereas in the northern Cordillera Central, Late Miocene slab flattening caused an acceleration in uplift, to which the landscape has not yet equilibrated. The Eastern Cordillera also displays pronounced erosional disequilibrium, with a slowly eroding central plateau rimmed by faster eroding western and eastern flanks. Our maps suggest recent topographic growth of the Eastern Cordillera, with deformation focused along the eastern flank, which is also supported by balanced cross-sections and thermochronologic data. Spatial gradients in predicted erosion rates along the eastern flank of the Eastern Cordillera suggest transient basin-ward migration of thrusts. Finally, using our erosion maps to infer millennial-scale sediment fluxes, we find that the Eastern Cordillera exports nearly four times more sediment than the Central Cordillera. Our analysis shows that accounting for spatial variations in erosion parameters and climate gradients reveals important variations in tectonic forcing that would otherwise be obscured in traditional river profile analyses. Moreover, given relationships between tectonic, and topographic evolution, we hypothesize that the dynamic landscape evolution of the Northern Andes revealed by our erosion maps is mostly linked to spatio-temporal variations in slab dip with potentially superposed effects from inherited Mesozoic rift structures.

Alpine landscapes form in mountain belts that likely experienced tectonic uplift during plate’s convergence, and efficient erosion dominated by glacial carving and circle retreat. In southern Patagonia N-S oriented late Miocene plutonic complexes are exposed in deep incised valleys with summits topographically above the glacial equilibrium line altitude. Two of the most emblematic ones are the Fitz Roy (Chaltén, latitude 49°S) and the Torres del Paine (latitude 51°S) plutonic complexes, ~2 km higher than the mostly flat bottom valley that is partially covered by the Southern Patagonian Icefield. This continental region is located above an asthenospheric window that opens and migrates from the latitude 54 °S towards the latitude 46 °S since ~16 Ma, and experienced dynamic uplift during episodes of spreading ridge collision with the continental margin. Here we present a new dataset of combined low-temperature thermochronometers from the Chaltén and Torres del Paine plutonic complexes, and their thermal history inversion numerical modeling, to identify the geodynamic processes forcing on the exhumation of the mountain belt. These complexes are separated by 200 km along the strike of the belt, and share a pulse of rapid exhumation at ca. 6 Ma, likely showing that glaciation was regionally starting at this moment. After a period of quiescence, in Torres del Paine the exhumation rate is accelerated from ~2 Ma to the present, interpreted as a signal of the Pleistocene climatic transition creating incise valleys. Only in the Fitz Roy a pulse of rapid exhumation is present at ca. 10 Ma, approximately coincident with the time range in which the ridge was subducting beneath the continent at that latitude. This allows us to separate the climatic from the tectonic/mantle forcing to the exhumation in southern Patagonia, and represents the first in-situ observation of the passage of the asthenospheric window in the low-temperature thermochronometric record of the region.

This is a poster presented at the Community Surface Dynamics Modeling System annual meeting (May 17-21, 2022). Abstract: Passive margin stratigraphy contains time-integrated records of landscapes that have long since vanished. Quantitatively reading the stratigraphic record using coupled landscape evolution and stratigraphic forward models (SFMs) is a promising approach to extracting information about landscape history. However, the most commonly used SFM, which is based on an approximation of local, linear slope-dependent sediment transport, fails to produce diagnostic features of passive margin stratigraphy such as long-distance transport from the continental shelf and slope onto the abyssal plain. There is no consensus about the optimal form of simple SFMs because there has been a lack of direct tests against observed stratigraphy in well constrained test cases. Here we develop a nonlocal, nonlinear one-dimensional SFM that incorporates slope bypass and long-distance sediment transport, both of which have been previously identified as important model components but not thoroughly tested. Our model collapses to the local, linear model under certain parameterizations such that best-fit parameter values can be indicative of optimal model structure. Using seven detailed seismic sections from the South African Margin, we invert the stratigraphic data for best-fit model parameter values and demonstrate that best-fit parameterizations are not compatible with the local, linear diffusion model. Fitting the observed stratigraphy requires parameter values consistent with important contributions from slope bypass and long-distance transport processes. The nonlocal, nonlinear model yields improved fits to the data regardless of whether the model is compared against only the modern bathymetric surface or the full set of seismic reflectors identified in the data. Results suggest that processes of sediment bypass and long-distance transport are required to model realistic passive margin stratigraphy, and are therefore important to consider when inverting the stratigraphic record to infer past perturbations to source regions.

Arctic river deltas define the interface between the terrestrial Arctic and the Arctic Ocean. They discharge sediments, nutrients, and soil organic carbon to the Arctic Ocean and provide key stratigraphic records of permafrost landscape evolution. As the climate warms, the future evolution of Arctic deltas will likely take a different course, with implications both local in scale and on the wider Arctic Ocean. One important way to understand and predict the evolution of Arctic deltas is through numerical models. Here we present ArcDelRCM.jl, an improved reduced complexity model (RCM) of arctic delta evolution based on the DeltaRCM-Arctic model (Lauzon et al., 2019). We have rewritten the DeltaRCM-Arctic model entirely in the Julia language and the final ArcDelRCM.jl model retains the option to execute as the former. Unlike previous models, ArcDelRCM.jl is able to replicate an important and ubiquitous feature observed in Arctic deltas — the underwater ramps extending from the shoreline of deltas tens of kilometres towards the ocean at a depth of roughly 2 m. This feature may form a buffer between ocean processes and the land portions of the deltas. We have found that the delayed breakup of bed-fast ice on and around the deltas is ultimately responsible for the development of the ramp feature. However, changes made to the modelling of permafrost erosion and protective effects of bed-fast ice are also important contributors. Through a simple graph analysis performed on ensemble runs, including the non-Arctic DeltaRCM (Liang et al., 2015a), we found that the Arctic processes considered in all the models and modifications did not lead to significant differences in the channel structures. Moreover, we found that the summer months contribute significantly to the growth and evolution of Arctic deltas, thus should not be neglected in simulations. Finally, we tested a strong climate-warming scenario on the simulated deltas of ArcDelRCM.jl. We found that the ramp features degrade on the time scale of centuries and effectively disappear in under a millennium. Ocean processes, which are not included in these models, may further shorten the time scale. With the degradations of the ramps, any dissipative effects on wave energy they offered would also decrease. This could expose the sub-aerial parts of the deltas to increased coastal erosion, thus impacting permafrost degradation, nutrients and carbon releases.

Anorogenic reliefs (plateaus and plains) made up about 70% of the total emerged reliefs on Earth. They are characterized by nearly flat erosional surfaces upstream bounded by escarpments, called pediments/pediplains. Africa’s landforms are constituted by very long (several thousands of kilometers) wavelength relief of broad “basins” (depressions) and swells (Holmes, 1944) possibly controlled by mantle dynamics. This "basin" and swell pattern caused Africa to be consisted of numerous endorheic and exorheic systems. Consequently, the Nile River, the longest river on Earth and main object of this study, crosses today a set of two former endorheic systems (Ugandese and Sudanese “Basins”) and one exorheic system (Egyptian Margin) along its courses to the Mediterranean Sea. Our objective is here to unravel the paleorouting systems of the Nile River through relief growth, tectonic, and climate since the uppermost Cretaceous.Several generations of stepped pediments, proxies of relief growth, were characterized and mapped on DEM and satellite images and dated using their geometrical relationships with dated magmatic rocks. To better constrain periods of relief uplift and the deformation wavelength through time, the stratigraphic record of the sedimentary basins located in between two swells were studied using biostratigraphy and sequence stratigraphy. The originality of the approaches is to integrate data of all the source-to-sink (S2S) systems to produce a coherent scenario of the evolution of the Nile.We proposed the following model for the evolution of the Nile River. First, a main large pediplain is formed during the uppermost Cretaceous (?75-66 Ma), acting as a base of any kind of landforms that would be formed afterwards, bounded westward by the Darfur-Ennedi crest. Second, a major marine flooding during late Paleocene time (58-57 Ma) reached a subtle high bounding northward the endorheic Sudanese “Basin”. Third, Pre-Eonile started to form during the uppermost Eocene (~37 Ma) with a divide limited to the Egyptian Margin. Fourth, the Eonile was incised during late Miocene (~10 Ma) at time of a major uplift at the scale of north-east Africa. Finally, the Nile captured first the Sudanese endorheic system in the Early Pliocene (~4 Ma) and altered the Ugandese one in the Middle-Late Pleistocene (less than 1 Ma).

It is widely recognized that mountain belt topography is generated by crustal thickening and lowered by river incision, linking climate and tectonics. However, it remains enigmatic whether surface processes or lithospheric strength control mountain belt height, width, and longevity, reconciling high erosion rates observed for instance in Taiwan and New Zealand and low erosion rates in the Tibetan and Andean plateaus, as well as long-term survival of mountain belts for several 100s of million years as observed in the Urals and Appalachians. Here we use a tight coupling between a landscape evolution model (FastScape) and a thermo-mechanically coupled mantle-scale tectonic model (Fantom) to investigate mountain belt growth and decay. Using several end-member models and introducing the new non-dimensional Beaumont number, Bm, we quantify how surface processes and tectonics control mountain growth and define three end-member types of growing orogens: Type 1, non-steady state, strength controlled; Type 2, flux steady state, strength controlled; and Type 3, flux steady state, erosion controlled. Orogenic decay is determined by erosional efficiency and can be subdivided into two phases with variable isostatic rebound characteristics and associated timescales: First short-wavelength relief is removed within a few Myr, followed by removal of long-wavelength topography and effectively local isostatic rebound. Comparing model and scaling results to natural orogens explains why different orogens on Earth are rheology or erosion- (climate)-limited, and why long-term survival of topography seems to be the norm rather than the exception.

We aim to determine the intensity of chemical weathering of detrital clays, as well as the lag time between the onset of the Paleocene-Eocene Thermal Maximum (PETM) and the chemical weathering response in a source-to-sink system. The PETM was a hyperthermal event characterized by an abrupt increase in global temperature (5–8 °C) over a short period (20 ka). A negative carbon isotope excursion marks the onset of the PETM, which reflects the fast injection of light carbon into the ocean-atmosphere system, triggering global climatic changes. Thus, physical and chemical erosion acted as feedback mechanisms to recover the global climate to pre-onset conditions. We focus on the continental section of the source-to-sink system, near the locality of Esplugafreda in the Southern Pyrenean foreland basin. We analyzed the evolution of the clay mineral assemblages in two clay-sized fractions (18O excursion at the onset and body of the PETM found in both size fractions. Further, we combine hafnium and neodymium isotope analyses of both clay fractions to track the silicate weathering intensity. This method will help us constrain the weathering regime and its response time relative to the onset and the body of the PETM. The results obtained in this project will serve to test numerical models of landscape evolution incorporating the chemical weathering response to climatic changes. This project has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie Grant Agreement No. 860383.

During the formation of rifted continental margins, a rift evolves through a number of stages that produce major sedimentary basins and distinct rifted margin domains. While these domains have been classified based on the resulting structures and crustal thickness seen in geophysical data, the evolution of the fault network that produces these domains is not as well understood. Further, margin architecture may be influenced by erosion and sedimentation. Previous studies have qualitatively examined how faults respond to sedimentation during rifting, but there has not been a quantitative study on how variable surface processes efficiency affects fault network properties and the effect this has on rift evolution.In this study we use a two-way coupling between the geodynamic code ASPECT (Kronbichler et al., 2012) and the surface processes code FastScape (Braun and Willett, 2013) to run 12 high-resolution 2D rift models that represent asymmetric, symmetric, and wide rift types (Neuharth et al., in review). For each rift type, we vary the surface process efficiency by altering the bedrock erodibility (Kf) from no surface processes to low (Kf = 10-6 m0.2/yr), medium (10-5), and high efficiency (10-4). To analyze these models, we use a novel quantitative fault analysis toolbox that extracts discrete faults from our continuum models and correlates them through space and time (https://github.com/thilowrona/fatbox). This toolbox allows us to track faults and their properties such as the number of faults, their displacement, and cumulative length, to see how they evolve through time, as well as how these properties change given different rifting types and surface processes efficiency.Based on the evolution of fault network properties, we find that rift fault networks evolve through 5 major phases: 1) distributed deformation and coalescence, 2) fault system growth, 3) fault system decline and basinward localization, 4) rift migration, and 5) continental breakup. Each of these phases can be correlated to the rifted margin domains defined from geophysical data (e.g., proximal, necking, hyperextended, and oceanic). We find that surface processes do not have a large impact on the overall evolution of a rift, but they do affect fault network properties by enhancing strain localization, increasing fault longevity, and reducing the total length of a fault system. Through these changes, they can prolong rift phases and delay continental breakup with increasing surface process efficiency. To summarize, we find that surface processes do not change the overall evolution of rifts, but they do affect fault growth and as a result the timing of rifting. Braun, J., and Willett, S.D., 2013, A very efficient O(n), implicit and parallel method to solve the stream power equation governing fluvial incision and landscape evolution: Geomorphology, v. 180–181, p. 170–179, doi:10.1016/j.geomorph.2012.10.008.Kronbichler, M., Heister, T., and Bangerth, W., 2012, High Accuracy Mantle Convection Simulation through Modern Numerical Methods.: Geophysical Journal International, v. 191, doi:doi:10.1111/j.1365-246x.2012.05609.x.Neuharth, D., Brune, S., Wrona, T., Glerum, A., Braun, J., and Yuan, X.P., (in review at Tectonics), Evolution of rift systems and their fault networks in response to surface processes, [preprint], doi: https://doi.org/10.31223/X5Q333

Source to sink systems involve processes happening at very large timescales and the current state of the earth only represents a brief snapshot of it through space and time. These processes encompass any of those involved in the removal, transport and deposition of material from sources to sinks. The relatively stable incising valleys of the upland source landscapes allow for very efficient mathematical expressions to model their evolution over geological timescales. The transport part of the system is more challenging to study and resists radical simplification. Softer and lower-relief compared to their upstream counterparts, they are much more dynamics and undergo convoluted cycles of erosion/deposition/remobilisation only leaving sparse clues. The presence (or absence) of materials from identified provenance in the transport zone is commonly used to interpret landscapes connectivity through time. However, recent studies suggest that the dynamic nature of the transport section can make apparent provenance data ambiguous and misleading.In this contribution, we leverage the ability of a newly developed cellular automata landscape evolution modelling framework, CHONK, to investigate the transport zone. CHONK is a landscape evolution modelling framework combining advantages of cellular-automata methods with common eulerian ones. Equations are implemented in a cell referential and cells are processed in a lake-aware multiple flow topological order. Because everything happens within a cell before communicating with downstream landscape, this framework allows fine tracking of sediment provenance through space and time, unconditionally to which law is implemented, or how complex the landscape structure.We set up a basic mountain range with its foreland, exhume a discrete pluton of different rock type and track the pathways of the sediments through time, including in a stratigraphy allowing remobilisation. We show how, even in this simplistic setting, complex and stochastic patterns of sediment pathways arise. We explore at which degree these patterns are predictable and at extents they are stochastic – in other word we track the variability of the proportion of sediment coming from the pluton at given locations through time. Finally, adding more complexity to the settings with variable tectonic and climatic cycles, intermediate sink (lakes) and heterogeneous lithologies, we offer a perspective on the framework’s potential to study the role of sediment flux in shaping the landscape and its record in novel ways.

Allogenic factors such as climate and tectonics are regulating the mechanisms involved in sediment generation, transport and deposition. A key challenge for modern society is to predict and better understand how sedimentary systems adapt to an abrupt change in climate. The geological record allows an insight on past climate crises and their registration in sediments.This study focuses on the changes in physical sedimentary processes during the Paleocene-Eocene Thermal Maximum (PETM, 56 Myrs ago) in the Southern Pyrenees (Spain). A lengthening of the siliciclastic system has been shown to occur coevally to the PETM. Yet, connections throughout the sedimentary system from source to sink and reconstructions of the processes involved in this short-term lengthening lack constraints. Although higher seasonality in pluviometry is usually invoked to explain the increase in sedimentary export, this hypothesis is based on very few continental outcrops only and do not include any system-scale quantification. Here we propose to test this hydrology-based hypothesis thanks to (i) a better understanding of the source-to-sink system’s paleogeography, (ii) paleohydraulic quantifications applied on supplementary outcrops and (iii) grain-size used as a down-system common thread.First, the sources are better constrained thanks to provenance analyses combining petrography and double dating on detrital zircons (U/Pb and (U-Th)/He). Then, reconstructions of paleohydraulics in several continental outcrops allow to compare paleoslopes and water discharges between pre- and syn-PETM deposits. Finally, studying grain-size along the whole system allows a quantitative estimation of the climate-related evolution of the system’s diffusivity.This system-scale study provides quantitative insights on the changes in sediment transport processes during an abrupt climate change. The deduced landscape evolution is then a valuable tool to be applied on today’s systems. This research is carried out in the scope of the lead author’s PhD project and is part of the S2S-FUTURE European Marie Skłodowska-Curie ITN.

The large-scale and long-term transport and deposition of sediment is commonly represented by one of two reduced-complexity models, namely the transport limited (or T-L) model or the under-capacity (or ξ−q) model more recently developed by Davy and Lague (2009). Using both models, I investigated the behavior of a sedimentary continental system fed by a fixed sedimentary flux from a nearby active orogen though which sediments transit to a fixed base level representing a large river, a lake or an ocean.Firstly, I will show that the two models share the same steady-state solution, for which a simple 1D analytical solution exists that reproduces the major features of many orogenic sedimentary systems: a steep fan adjacent to the mountain front that connects to a shallower alluvial plain. The resulting fan geometry obeys basic observational constraints on fan size and slope with respect to the upstream drainage area, A0. I will also show that the solution is strongly dependent on the size of the system, L, in comparison to a distance L0 that is determined by the size of A0. This gives rise to two fundamentally different types of sedimentary systems: constrained systems where L < L0 and open systems where L > L0. Constrained systems contain only a steep, conical fan that connects directly to the base level, whereas open systems are made of a steep fan connecting to a broad, low slope alluvial plain.Secondly, I will present simple expressions that show the dependence of the system response time on the system characteristics, such as its length, the size of the upstream catchment area, the amplitude of the incoming sedimentary flux and the respective rate parameters (diffusivity or erodibility) for each of the two models. The ξ−q model predicts significantly longer response times, which I relate to its greater efficiency at propagating signals through its entire length. I will also demonstrate that, although the manner in which signals propagates through the sedimentary system differs greatly between the two models, they both predict that perturbations that last longer than the response time of the system can be recorded in the stratigraphy of the sedimentary system and in particular of the fan. Interestingly, the ξ−q model predicts that all perturbations (regardless of their period) in incoming sedimentary flux will be transmitted through the system whereas the T-L model predicts that rapid perturbations cannot. I will finally discuss why and under which conditions these differences are important and propose observational ways to determine which of the two models is most appropriate to represent natural systems.Reference: Davy P. and Lague D., JGR-Earth Surface, 2009.

Late Miocene calc-alkaline intrusions in the back-arc of Southern Patagonia mark an eastward migration of the arc due to accelerated subduction velocity of the Nazca plate or slab flattening preceding active ridge subduction. Amongst these intrusions are the emblematic Torres del Paine (51°S) and Fitz Roy (49°S) plutonic complexes, crystalised at ca. 12.5 and ca. 16.5 Ma, respectively (Leuthold et al., 2012; Ramírez de Arellano et al., 2012). Both intrusions are located at the eastern boundary of the Southern Patagonian Icefield and form prominent peaks with steep slopes that are ~3 km higher in elevation than the surrounding low-relief foreland. Their exhumation has been proposed as a response to glacial erosion and associated glacial rebound since ca. 7 Ma (Fosdick et al., 2013), and/or by regional dynamic uplift between 14 and 6 Ma due to the northward migration of subducting spreading ridges (Guillaume et al., 2009). Here we present a new data set of apatite and zircon (U-Th)/He from both plutonic complexes, numerically modelled to unravel their late-Neogene to Quaternary thermal histories. Our results show three rapid cooling periods for the Fitz Roy intrusion: at ca. 9.5 Ma, at ca. 7.5 Ma, and since ca. 1 Ma. For Torres del Paine, inverse thermal modelling reveals short and rapid cooling at ca. 6.5 Ma followed by late-Quaternary final cooling. The 10 Ma cooling signal only evidenced in the northern plutonic complex (Fitz Roy) may represent an exhumation response to the northward migrating subduction of spreading ridge segments, causing localized dynamic uplift. Thus, the absence of exhumation signal before 6.5 Ma in the southern part (Torres del Paine) suggest that the spreading ridge subduction must have occurred before its 12.5 Ma emplacement. On the other hand, rapid cooling by similar magnitude in both plutonic complexes between ca. 7.5–6.5 Ma, likely reflects the onset of late-Cenozoic glaciations in Southern Patagonia. Finally, the late-stage Quaternary cooling signals differ between Torres del Paine and Fitz Roy, likely highlighting different exhumation responses (i.e. relief development vs. uniform exhumation) to mid-Pleistocene climate cooling. We thus identify and distinguish the causes of rapid exhumation periods in the Southern Patagonian Andes, and propose a first Late Miocene exhumation pulse due to subduction of spreading ridge dynamics, and two Late Cenozoic exhumation episodes due to regional climate changes that have shaped alpine landscapes in this region.References:Leuthold J., et al. 2012. Time resolved construction of a bimodal laccolith (Torres del Paine, Patagonia). EPSL.Ramírez de Arellano C., et al. 2012. High precision U/Pb zircon dating of the Chaltén Plutonic Complex (Cerro Fitz Roy, Patagonia) and its relationship to arc migration in the southernmost Andes. Tectonics.Fosdick J. C., et al. 2013. Retroarc deformation and exhumation near the end of the Andes, southern Patagonia. EPSL.Guillaume B. 2009. Neogene uplift of central eastern Patagonia: Dynamic response to active spreading ridge subduction? Tectonics.

Ferruginous duricrust formation takes place by dissolution and accumulation of iron during the wet season and precipitation during the dry season. However, the formation of iron duricrusts has always been the centre of much debate. They commonly form as hard iron layers in tropical and subtropical environments with strong climatic seasonality. They were first described in Africa, Australia and India in the 1950’s. They often cap landscapes, which can be explained by their extremely high iron content making them more resistant to mechanical weathering. However, in recent years, iron duricrusts were also described at depth in the regolith, like in South America.This has led to 2 distinct formation hypotheses: the first one relies on the vertical beating of the water table most likely in a stable environment combined to lateral hydraulic transport of iron. In this scenario, iron hydroxides accumulate in one specific region after being transported from surrounding source areas through the water table, and precipitate as Fe3+ during the dry season. Through time, this leads to duricrusts forming within the water table beating range. This model is compatible with duricrusts forming at depth within the regolith. The second hypothesis relies on the vertical, in situ leaching of elements and resulting compaction and surface lowering. During weathering, iron nodules accumulate with the progressive leaching of soluble elements from the parent rock, leaving only the iron oxides, ultimately forming a duricrust. It implies a genetic link between the duricrust, the lateritic profile and the underlying parent rock. This model explains why duricrusts are mostly observed near the surface. Nevertheless, there is no consensus on the conditions under which either of these hypotheses prevail. To address this issue, we developed two separate first-order numerical models representing the two hypotheses for duricrust formation. We used the models to demonstrate that the hypotheses lead to different scenarios of duricrust formation by running a large number of simulations. Especially, we show that the stability of a landform is a discerning element between the 2 models. In the first case, quiescent periods of time in between uplifting periods are crucial, while in the second case, constant dynamic uplift is needed to enable the accumulation of enough elements by compaction.

This is a presentation given remotely at the Geological Society of America online annual meeting (October 26-30, 2020). Abstract: Numerical models that explicitly link source to sink (S2S) are critical for inferring past tectonic and climatic perturbations to Earth’s surface from the sedimentary record. While models seeking predictive power must simulate surface processes at both source and sink, substantially more model formulations exist for landscape evolution than for seascape evolution and the building of stratigraphy in marine basins. Most marine sedimentation models operating at basin spatial scales and geologic temporal scales use local linear diffusion approximations for sediment transport. Such approximations produce realistic stratigraphy in shallow marine environments, but do not replicate observed deep marine deposition patterns. Full closure of S2S sediment budgets requires models with predictive power over the development of both shallow and deep marine stratigraphy. We present a new marine sedimentation model that incorporates two simple modifications allowing non-local sediment transport: 1) the possibility of sediment bypass on steep slopes, and 2) long-distance transport, even over near-zero slopes. We constrain, using Bayesian inference techniques, the four model parameters by comparing modeled against observed stratigraphy from 130 Ma of passive margin evolution in the Orange Basin, southern Africa. Best-fit modeled stratigraphy captures the form of the observed record. Best-fit parameter values imply important roles for both model elements we introduced: sediment bypass on steep slopes and long-distance transport over very gentle slopes. We attribute remaining model-data misfit to additional transport processes—hemipelagic sedimentation, grain size variations, or ocean bottom currents—that are not captured by our simple model but might be required to explain sediment runout distances longer than those produced by our best-fit model. Results suggest that predictive modeling of marine sedimentation requires departing from local diffusion approximations, even over ocean-basin-filling timescales. Simple treatment of nonlocal transport dynamics can improve S2S model predictive power, and may enable better recovery of environmental signals from the long-term stratigraphic record.

High-elevation, low-relief surfaces are widespread in many mountain belts. However, the origin of these surfaces has long been debated. In particular, the southeast Tibetan Plateau has extensive low-relief surfaces perched above deep valleys and in the headwaters of three of the world's largest rivers (Salween, Mekong, and Yangtze Rivers). Various geologic data and geodynamic models show that many mountain belts grow first to a certain height and then laterally in an outward propagation sequence. By translating this information into a kinematic propagating uplift function in a landscape evolution model, we propose that the high-elevation, low-relief surfaces in the southeast Tibetan Plateau are simply a consequence of mountain growth and do not require a special process to form. The propagating uplift forms an elongated river network geometry with broad high-elevation, low-relief headwaters and interfluves that persist for tens of millions of years, consistent with the observed geochronology. We suggest that the low-relief interfluves can be long-lived because they lack the drainage networks necessary to keep pace with the rapid incision of the large main-stem rivers. The propagating uplift also produces spatial and temporal exhumation patterns and river profile morphologies that match observations. Our modeling therefore reconciles geomorphic observations with geodynamic models of uplift of the southeast Tibetan Plateau, and it provides a simple mechanism to explain the low-relief surfaces observed in several mountain belts on Earth.

It is widely recognized that collisional mountain belt topography is generated by crustal thickening and lowered by river bedrock erosion, linking climate and tectonics. However, whether surface processes or lithospheric strength control mountain belt height, shape and longevity remains uncertain. Additionally, how to reconcile high erosion rates in some active orogens with long-term survival of mountain belts for hundreds of millions of years remains enigmatic. Here we investigate mountain belt growth and decay using a new coupled surface process and mantle-scale tectonic model. End-member models and the new non-dimensional Beaumont number, Bm, quantify how surface processes and tectonics control the topographic evolution of mountain belts, and enable the definition of three end-member types of growing orogens: type 1, non-steady state, strength controlled (Bm > 0.5); type 2, flux steady state, strength controlled (Bm ≈ 0.4−0.5); and type 3, flux steady state, erosion controlled (Bm

The birth and expansion of continental plateaus exert a strong control on our planet's climate and the distribution and evolution of its biodiversity. It has been proposed that the Tibetan Plateau has been steadily growing by southward expansion. Here we demonstrate that the shape of the southeastern margin of the plateau has remained unchanged for the last 10 Myr despite vast amounts of exhumation. Our finding is based on a new, high-resolution thermochronological dataset from the deep gorges of the Salween and Mekong rivers, which we interpret using a physics-based model combined with an optimization method. We show that our scenario also agrees with a wide range of other, independent geological and geophysical data. This finding demonstrates that plateau margins can reach large-scale topographic steady-state between outward growth and surface erosion, which has important implications for our understanding of the evolution of Earth's climate and biodiversity in the recent geological past.

Variations in fluvial grain size have long been used to decipher past climatic and tectonic events within stratigraphy. Thus, a thorough understanding of grain size fining response to external forcing and autogenic dynamics over long timescales (Myr) has implications within the interpretation of the sedimentary record. This work presents a new method (GSFast) that generalizes the Fedele and Paola (2007) self-similar gravel grain size method into multidimensions (downstream, across the basin, and overtime/depth) using the FastScape (Braun and Willett, 2013) landscape evolution model. The self-similar model results in a fining rate that is dominantly controlled by the rate of deposition relative to flux. Previously, the Fedele and Paola (2007) grain size fining model has been applied along a single river long profile (1D) to infer a subsidence pattern from observed fining rates such as within the Eocene Montsor Fans in the Southern Pyrenees (Duller et al. 2010; Whittaker et al., 2011). Here, using GSFast, we demonstrate the role of across basin dynamics on grain size fining through a sensitivity analysis comparison with the Duller et al. (2010) single river profile approach. For this, we performed a series of simple numerical experiments to predict grain size fining rate in sedimentary systems of varying spatial extents that are fed by an orogenic source area and undergoing subsidence at a prescribed rate.When applied in 1D or within shorter confined basins, where all the upstream catchment flux is deposited as gravel within the fan (no alluvial plain), GSFAST can capture the Duller et al. (2010) results under comparable subsidence and flux conditions. This is because both 1D systems and short basins are characterized by no or limited lateral channel mobility resembling sheet flow or incised channel flow.Conversely, in wide or long (unconfined) basins, the rate of grain size fining predicted by GSFast deviates from the Duller et al. (2010) single profile solution. This deviation occurs due to multiple mobile drainage channels that form when the gravel flux leaving the upstream catchment is unconfined and deposited in both the alluvial fan and adjacent plain. Deviations in grain size fining trend from the Duller et al. (2010) approach correlate with channel mobility dynamics that preferentially form in wide basins or long systems that connect a fan to a large alluvial plain.Thus, under the same tectonic and climatic boundary conditions, our more dimensionally complex model that incorporates channel mobility leads to different predictions of subsidence patterns from grain size fining curves. This is because our multidimensional approach leads to a growing disconnect between subsidence and deposition rates as channel mobility increases. It also predicts markedly different fining patterns between short systems (i.e., limited to a steep fan-like structure), and long systems (i.e., systems that also incorporate a low gradient alluvial plain).Duller et al. (2010). JGR: ES 115(F3).Fedele & Paola. (2007). JGR: ES 112(F2).Whittaker et al. (2011). Bulletin, 123(7-8).

International audience; High-elevation, low-relief surfaces are widespread in many mountain belts. However, the origin of these surfaces has long been debated. In particular, the Southeast (SE) Tibetan Plateau has extensive low-relief surfaces perched above deep valleys and in the headwaters of three of the world’s largest rivers (Salween, Mekong and Yangtze). Synthesized low-temperature thermochronologic data indicate that uplift of the Tibetan Plateau may have propagated progressively from NW to SE, with thermochronologic ages at ~50 Ma observed in the headwaters and interfluves of the rivers but young ages

International audience; Diffusion-based stratigraphic models are widely used to simulate sedimentary systems and margin deltas. Diffusion-based models assume that the topographic evolution primarily depends from its slope. Limited attention has however been given to the calibration of the transport coefficients. Here, we evaluate transport coefficient values from natural examples, the Ogooué and Zambezi rifted margin deltas over the last 5 to 12 Ma respectively. We developed a method to estimate transport coefficients based on high resolution seismic stratigraphy analysis of the stratigraphic architecture of these deltas. For each stratigraphic sequence, we calibrated the sand/shale ratios of the deposits, we restored their depositional slopes, we estimated their uncompacted accumulated volumes and we calculated the transport coefficient (Kd) from the sediment flux / slope ratio.Estimated values of Kd fall within one order of magnitude (x 0.1 km2/ka), a much narrower range than previously published values (x0.0001 to x100 km2/ka). We show that the diffusion approximation is optimal at 10 - 100 km scale and 0.5 - 1 Ma time resolution, independently of the stratigraphic context. We show that the diffusion assumption is appropriate for the formation of the clinoforms (mainly gravity driven). It is however not optimal for the shelf and distal domains where additional processes (e.g. wave, flood, hemipelagic, turbidites, oceanic current), not accounted for it the diffusion assumption, significantly impact sediment transport. We documented a significant increase of Kd values after 0.9 Ma, coeval of an increase in the amplitude of eustatic variations at this time indicating that the calibration of Kd from present day sedimentary systems might not be optimal for simulations of sedimentary systems before the last million years.

The uplift of the southern African Plateau with its average elevations of ~1000 m is often attributed to mantle processes, but there are conflicting theories for the timing and drivers of topographic development. Evidence for most proposed plateau development histories is derived from continental erosion histories, marine stratigraphic architecture, or landscape morphology. Here we use a landscape evolution model to integrate a large dataset of low-temperature thermochronometry, sediment flux rates to surrounding marine basins, and current topography for southern Africa. We explore three main hypotheses for surface uplift: 1) southern Africa was already elevated by the Early Cretaceous before Gondwana breakup, 2) uplift and continental tilting occurred during the mid-Cretaceous, or 3) uplift occurred during the mid to late Cenozoic. We test which of these three intervals of plateau development are plausible by using an inversion method to constrain the range in erosional and uplift model parameters that can best reproduce the observed data. Results indicate four regions of parameter space that fall into two families of uplift histories are most compatible with the data. Both uplift families have limited initial topography with some topographic uplift and continental tilting starting at ~90-100 Ma. In one acceptable scenario, nearly all of the topography, >1300 m, is created at this time with little Cenozoic uplift. In the other acceptable scenario, ~400-800 m of uplift occurs in the mid- Cretaceous with another ~500-1000 m of uplift in the mid-Cenozoic. The two model scenarios have different geodynamic implications, which we compare to geodynamic models.

Sedimentary dynamics and fluxes are influenced by both autogenic and allogenic forcings. A better understanding of the evolution of sedimentary systems through time and space requires us to decipher, and therefore to characterise, the impact of each of these on the Earth’s landscape. Given the current increase in the concentration of atmospheric carbon, studying the impact of rapid and global climate changes is of particular importance at the present time. Such events have been clearly defined in the geologic record. Among them, the Paleocene-Eocene Thermal Maximum (PETM) has been extensively studied worldwide and represents a possible analogue of the rapid current climate warming.The present project focuses on the Southern Pyrenees (Spain) where excellent exposures of the Paleocene-Eocene interval span a large range of depositional environments from continental to deep-marine. These conditions allow us to collect data along the whole depositional system in order to document changes in sediment fluxes and paleohydraulic conditions. Because hydrological conditions have an impact on sediment transport through hydrodynamics, paleoflow reconstructions can shed light on changes in sediment dynamics. This information is reconstructed from the statistical distributions of channel morphologies, characteristic system dimensions including bankfull channel depth and width, and grain-sizes.With this approach, our aim is to provide both qualitative and quantitative assessments of the magnitude and extent of the perturbation of sedimentary fluxes along an entire source-to-sink system during an episode of extreme climate change. This will lead to a better understanding of the impact of abrupt climate change on earth surface systems in mid-latitudinal areas, with possible implications for current climate adaptation policy.This research is carried out in the scope of the lead author’s PhD project and is part of the S2S-FUTURE European Marie Skłodowska-Curie ITN (Grant Agreement No 860383).