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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.

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.

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.

Continental rifting is responsible for the generation of major sedimentary basins, both during rift inception and during the formation of rifted continental margins. Geophysical and field studies revealed that rifts feature complex networks of normal faults but the factors controlling fault network properties and their evolution are still matter of debate. Here, we employ high‐resolution 2D geodynamic models (ASPECT) including two‐way coupling to a surface processes (SP) code (FastScape) to conduct 12 models of major rift types that are exposed to various degrees of erosion and sedimentation. We further present a novel quantitative fault analysis toolbox (Fatbox), which allows us to isolate fault growth patterns, the number of faults, and their length and displacement throughout rift history. Our analysis reveals that rift fault networks may evolve through five major phases: (a) distributed deformation and coalescence, (b) fault system growth, (c) fault system decline and basinward localization, (d) rift migration, and (e) breakup. These phases can be correlated to distinct rifted margin domains. Models of asymmetric rifting suggest rift migration is facilitated through both ductile and brittle deformation within a weak exhumation channel that rotates subhorizontally and remains active at low angles. In sedimentation‐starved settings, this channel satisfies the conditions for serpentinization. We find that SP are not only able to enhance strain localization and to increase fault longevity but that they also reduce the total length of the fault system, prolong rift phases and delay continental breakup., Plain Language Summary: Continental rifting is responsible for breaking apart continents and forming new oceans. Rifts generally evolve according to three types: wide rift, symmetric rift, and asymmetric rifts, which also shape the final geometry of the continental rifted margin. Geophysical data shows that the evolution of rifts depends on a multitude of factors including the complex interactions between fault networks that accommodate extension and the processes of erosion and sediment deposition. Here we run 2D computer simulations to investigate fault network evolution during active rifting that include changes to the surface through erosion and sedimentation. By using a new python tool box, we extract the fault network from the simulation and determine individual fault properties like the number of faults, displacement, age, and length through time. We find that regardless of the rift type, rifts evolve according to five phases that can be assessed through the evolution of the fault network properties. Additionally, we find that greater erosion and sedimentation can prolong rift phases and delay the breakup of continents., Key Points: We apply a new fault analysis toolbox to coupled numerical models of tectonics and surface processes. Fault network evolution of the major symmetric, asymmetric, narrow, and wide rift types can be described in five distinct phases. Surface processes reduce fault network complexity and delay breakup by enhancing strain localization and increasing fault longevity., Helmholtz Young Investigators, National Science Foundation, Deutsche Forschungsgemeinschaft (DFG), https://doi.org/10.5281/zenodo.5753144

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.

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.

Natural thermoluminescence (TL) in rocks reflects a dynamic equilibrium between radiation-induced TL growth and decay via thermal and athermal pathways. When rocks exhume through Earth's crust and cool from high to low temperature, this equilibrium level increases as the temperature dependent thermal decay decreases. This phenomenon can be exploited to extract thermal histories of rocks. The main advantage of TL is that a single TL glow curve has a wide range of thermal stabilities (lifetime 100 °C/Ma, whereas deeper traps, i.e. with higher activation energies, provide constraints on thermal histories for higher cooling rates (>300 °C/Ma). Finally, we show how the path of rock exhumation (i.e., depth vs. time) can be constrained using an inverse approach. The newly developed methodology is applied to rapidly cooled samples from the Namche Barwa massif, eastern Himalaya to suggest a trend in exhumation rate with time that follows an inverse correlation with global temperature and glaciers equilibrium altitude line (ELA).

International audience; Due to the challenges in upscaling daily climatic forcing to geological time, physically realistic models describing how rainfall drives fluvial erosion are lacking. To bridge this gap between short-term hydrology and long-term geomorphology, we derive a theoretical framework for long-term fluvial erosion rates driven by realistic climate by integrating an established stochastic-mechanistic model of hydrology into a threshold-stochastic formulation of stream power. The hydrological theory provides equations for the daily streamflow probability distribution as a function of climatic boundary conditions. The new parameters introduced are rooted firmly in established climatic and hydrological theory. This allows us to account for how fluvial erosion rates respond to changes in rainfall intensity, frequency, evapotranspiration rates, and soil moisture dynamics in a way that is consistent with existing theories. We use this framework to demonstrate how hydroclimatic conditions and erosion threshold magnitude control the degree of nonlinearity between steepness index and erosion rate. We find that hydrological processes can have a significant influence on how erosive a particular climatic forcing will be. By accounting for the influence of hydrology on fluvial erosion, we conclude that climate is an important control on erosion rates and long-term landscape evolution.

Constraining the Cenozoic uplift of Tian Shan is important for assessing the impact of the India-Asia collision to Central Asia. Here we estimate the uplift history of the Bogda Shan, northeastern Tian Shan, using a thermo-kinematic model which is constrained by previously reported apatite fission-track thermochronological data. By assuming that the growth of the mountain range propagates towards the basin as a classic critical wedge model, we show that the observed variation in the cooling ages on the mountain flank can be used to provide constraints on the timing and rate of the deformation along a series of south dipping thrust faults, which all root on a low-angle decollement. Inverse modeling confirms previous findings from thermal history models that the Cenozoic uplift in the Bogda Shan initiated during the Paleogene, no later than ~40 Ma. Since the early Miocene (~23 Ma), locus of uplift has expanded to the current southern margin of the Junggar Basin. Our kinematic model of the deformation of the Bogda Shan suggests a temporal stability in the shortening rate of the northeastern Tian Shan over the period of the India-Asia collision during the late Cenozoic.

One major and under-appreciated aspect of coupled erosion-deposition numerical modeling is the ranges of input parameter values used to simulate natural source to sink systems without considering their meaning in term of erosion, transport and deposition processes. Most of the time, numerical models are used as a semi-inversion tool based on a “best-fit” approach, especially in its marine part where it aims to reproduce well-constrained sedimentary architectures which are great recorders of landscape evolution through time.In this study, we performed several simulations using a new numerical landscape evolution model that accounts for both erosion and deposition onshore, as well as sediment deposition in the marine domain (Yuan et al., 2019; COLORS project, funded by Total). In the marine domain, sediment dynamic is described by a diffusion equation and the diffusion or transport coefficient has been calibrated from natural delta geometries. This model is highly efficient and allows the separation of the different processes involved and exploration of various setups and parameters values in order to address a large variety of questions. Its efficiency also allows inverse simulations that are powerful to determine the best possible scenarios in terms of climatic or tectonic reconstructions, or to determine the evolution of several key parameters.In order to evaluate the model reliability to reproduce realistic sedimentary geometries, we explore the impact of perturbations in climatic, eustatic or tectonic parameters of the model on the stratigraphic architecture of passive margins shelf-edge deltas and discuss its feedbacks with the erosion dynamic of the onshore domain. This sensitivity analysis also allowed us to define the most relevant geometrical parameters of observed or theoretical stratigraphic architectures that have to be include in the misfit function of the inversions and optimization scheme.This study is part of the COLORS project, funded by Total.

The marine sedimentary record contains unique information about the history of erosion, uplift and climate of the adjacent continent. Inverting this record has been the purpose of many numerical studies. However, limited attention has been given to linking continental erosion to marine sediment transport and deposition in large-scale surface process evolution models. Here we present a new numerical method for marine sediment transport and deposition that is directly coupled to a landscape evolution algorithm solving for the continental fluvial and hillslope erosion equations using implicit and O(N) algorithms. The new method takes into account the sorting of grain sizes (e.g., silt and sand) in the marine domain using a non-linear multiple grain-size diffusion equation and assumes that the sediment flux exported from the continental domain is proportional to the bathymetric slope. Specific transport coefficients and compaction factors are assumed for the two different grain sizes to simulate the stratigraphic architecture. The resulting set of equations is solved using an efficient (O(N) and implicit) algorithm. It can thus be used to invert stratigraphic geometries using a Bayesian approach that requires a large number of simulations. This new method is used to invert the sedimentary geometry of a natural example, the Ogooue Delta (Gabon), over the last similar to 5 Myr. The objective is to unravel the set of erosional histories of the adjacent continental domain compatible with the observed geometry of the offshore delta. For this, we use a Bayesian inversion scheme in which the misfit function is constructed by comparing four geometrical parameters between the natural and the simulated delta: the volume of sediments stored in the delta, the surface slope, the initial and the final shelf lengths. We find that the best-fit values of the transport coefficients for silt in the marine domain are in the range of 300 - 500 m(2)/yr, in agreement with previous studies on offshore diffusion. We also show that, in order to fit the sedimentary geometry, erosion rate on the continental domain must have increased by a factor of 6 to 8 since 5.3 Ma. (C) 2019 Elsevier B.V. All rights reserved.

We present here a new model for the formation of regolith on geological time scales by chemical weathering based on the assumption that the rate of chemical weathering is primarily controlled by the ability of groundwater to transport solute away from the reacting solid-fluid interface and keep the system from reaching equilibrium (saturation). This allows us to specify the rate of propagation of the weathering front as linearly proportional to the pore fluid velocity which we obtain by computing the water table geometry in the regolith layer. The surface of the regolith layer is affected by mass transport and erosion. The main prediction of the model is that the geometry of the regolith, i.e. whether it is thickest beneath topographic highs or topographic lows, is controlled by the value of a dimensionless number, which depends on the square of the surface slope, the hydraulic conductivity and local precipitation rate, but is independent of the chemical weathering rate. In orogenic environments, where regolith formation by chemical weathering competes with surface erosion, the model predicts that the existence and thickness of the regolith layer are controlled by the value of another dimensionless number which is the ratio between the time scale for erosion and the time scale for weathering. The model also predicts that in anorogenic areas, regolith thickness increases as the square root of time, whereas in orogenic environments, a steady-state regolith thickness can be achieved, when the propagation of the weathering front equals erosion rate.

A new efficient method to solve the stream power law Key Points: We present an efficient (O(N) and implicit) method to solve a river erosion model taking into account sediment deposition. We show how the foreland stratigraphy is controlled by the efficiency of river erosion and the efficiency of sediment transport by rivers. We observe autogenic aggradation and incision cycles in the foreland once the system reaches a dynamic steady state. The stream power law model has been widely used to represent erosion by rivers, but does not take into account the role played by sediment in modulating erosion and deposition rates. Davy and Lague (2009) provide an approach to address this issue, but it is computationally demanding because the local balance between erosion and deposition depends on sediment flux resulting from net upstream erosion. Here, we propose an efficient (i.e., O(N) and implicit) method to solve their equation. This means that, unlike other methods used to study the complete dynamics of fluvial systems (including the transition from detachment-limited to transport-limited behavior, for example), our method is unconditionally stable even when large time steps are used. We demonstrate its applicability by performing a range of simulations based on a simple setup composed of an uplifting region adjacent to a stable foreland basin. As uplift and erosion progress, the mean elevations of the uplifting relief and the foreland increase, together with the average slope in the foreland. Sediments aggrade in the foreland and prograde to reach the base level where sediments are allowed to leave the system. We show how the topography of the uplifting relief and the stratigraphy of the foreland basin are controlled by the efficiency of river erosion and the efficiency of sediment transport by rivers. We observe the formation of a steady-state geometry in the uplifting region, and a dynamic steady state (i.e., autocyclic aggradation and incision) in the foreland, with aggradation and incision thicknesses up to tens of meters.

International audience; Most mountain ranges are formed by the compression and folding of colliding tectonic plates. Subduction of one plate causes large-scale asymmetry while their layered composition (or stratigraphy) explains the multi-scale folded strata observed on real terrains. We introduce a novel interactive modeling technique to generate visually plausible, large scale terrains that capture these phenomena. Our method draws on both geological knowledge for consistency and on sculpting systems for user interaction. The user is provided hands-on control on the shape and motion of tectonic plates, represented using a new geologically-inspired model for the Earth crust. The model captures their volume preserving and complex folding behaviors under collision, causing mountains to grow. It generates a volumetric uplift map representing the growth rate of subsurface layers. Erosion and uplift movement are jointly simulated to generate the terrain. The stratigraphy allows us to render folded strata on eroded cliffs. We validated the usability of our sculpting interface through a user study, and compare the visual consistency of the earth crust model with geological simulation results and real terrains.

International audience; The processes driving uplift and exhumation of the highest Peruvian peaks (the Cordillera Blanca) are not well understood. Uplift and exhumation seem closely linked to the formation and movement on the Cordillera Blanca normal fault (CBNF) that delimits and shapes the western flank of the Cordillera Blanca. Several models have been proposed to explain the presence of this major normal fault in a compressional setting, but the CBNF and the Cordillera Blanca recent rapid uplift remain enigmatic. Whereas the Cordillera Blanca morphology demonstrates important erosion and thus a significant mass of rocks removal, the impact of erosion and isostasy on the evolution of the Cordillera Blanca uplift rates has never been explored. We address the role of erosion and associated flexural rebound in the uplift and exhumation of the Cordillera Blanca with numerical modeling of landscape evolution. We perform inversions of the broad features of the present-day topography, total exhumation and thermochronological data using a landscape evolution model (FastScape) to provide constraints on the erosion efficiency factor, the uplift rate and the temperature gradient. Our results evidence the not negligible contribution of erosion and associated flexural rebound to the uplift of the Cordillera Blanca and allow us to question the models previously proposed for the formation of the CBNF.

Many regions central to our understanding of tectonics and landscape evolution are active or ancient magmatic terranes, and robust interpretation of low-temperature thermochronologic ages in these settings requires careful attention to the drivers of rock heating and cooling, including magmatism. However, we currently lack a quantitative framework for evaluating the potential role of magmatic coolingthat is, post-magmatic thermal relaxationin shaping cooling age patterns in regions with a history of intrusive magmatism. Here we use analytical approximations and numerical models to characterize how low-temperature thermochronometers document cooling inside and around plutons in steadily exhuming environments. Our models predict that the thermal field a pluton intrudes into, specifically the ambient temperatures relative to the closure temperature of a given thermochronometer, is as important as the pluton size and temperature in controlling the pattern and extent of thermochronometer resetting in the country rocks around a pluton. We identify one advective and several conductive timescales that govern the relationship between the crystallization and cooling ages inside a pluton. In synthetic vertical age-elevation relationships (AERs), resetting next to plutons results in changes in AER slope that could be misinterpreted as past changes in exhumation rate if the history of magmatism is not accounted for. Finally, we find that large midcrustal plutons, such as those emplaced at similar to 10-15-km depth, can reset the low-temperature thermochronometers far above them in the upper crusta result with considerable consequences for thermochronology in arcs and regions with a history of magmatic activity that may not have a surface expression.

Glaciers and rivers are the main agents of mountain erosion. While in the fluvial realm empirical relationships and their mathematical description, such as the stream power law, improved the understanding of fundamental controls on landscape evolution, simple constraints on glacial topography and governing scaling relations are widely lacking. We present a steady state solution for longitudinal profiles along eroding glaciers in a coupled system that includes tectonics and climate. We combined the shallow ice approximation and a glacial erosion rule to calculate ice surface and bed topography from prescribed glacier mass balance gradient and rock uplift rate. Our approach is inspired by the classic application of the stream power law for describing a fluvial steady state but with the striking difference that, in the glacial realm, glacier mass balance is added as an altitude‐dependent variable. From our analyses we find that ice surface slope and glacial relief scale with uplift rate with scaling exponents indicating that glacial relief is less sensitive to uplift rate than relief in most fluvial landscapes. Basic scaling relations controlled by either basal sliding or internal deformation follow a power law with the exponent depending on the exponents for the glacial erosion rule and Glen's flow law. In a mixed scenario of sliding and deformation, complicated scaling relations with variable exponents emerge. Furthermore, a cutoff in glacier mass balance or cold ice in high elevations can lead to substantially larger scaling exponents which may provide an explanation for high relief in high latitudes., Key Points Glacial steady state topography is predicted based on the shallow ice approximation and a glacial erosion ruleIce thickness and surface slope in glacial topographic steady state depend on rock uplift rate and mass balanceCompared to the fluvial realm, altitude‐dependent mass balance may reduce sensitivity of glacial landscapes to rock uplift rates

International audience

The Nanga Parbat-Haramosh massif (NPHM; western Himalayan syntaxis) requires an influx of mass exceeding that in the adjacent Himalayan arc to sustain high topography and rapid erosional exhumation rates. What supplies this mass flux and feeds this “tectonic aneurysm?” We show, using a simple 3-D model of oblique orogen convergence, that velocity/strain partitioning results in horizontal orogen-parallel (OP) crustal transport, and the same behavior is inferred for the Himalaya, with OP transport diverting converging crust toward the syntaxis. Model results also show that the OP flow rate decreases in the syntaxis, thereby thickening the crust and forming a structure like the NPHM. The additional crustal thickening, over and above that elsewhere in the Himalayan arc, sustains the rapid exhumation of this “aneurysm.” Normally, velocity/strain partitioning would be minimal for the Himalayan arc where the convergence obliquity is no greater than ~40°. However, we show analytically that the Himalayan system can act both as a critical wedge and exhibit strain partitioning if both the detachment beneath the wedge and the bounding rear shear zone, which accommodates OP transport, are very weak. Corresponding numerical results confirm this requirement and demonstrate that a Nanga Parbat-type shortening structure can develop spontaneously if the orogenic wedge and bounding rear shear zone can strain rate soften while active. These results lead us to question whether the position of NPHM aneurysm is localized by river incision, as previously suggested, or by a priori focused tectonic shortening of the crust in the syntaxis region as demonstrated by our models.

The processes that control long term landscape evolution in continental interiors and, in particular, along passive margins such as in southern Africa, are still the subject of much debate (e.g. Braun, 2018). Although today the Namibian margin is characterized by an arid climate, it has experienced climatic fluctuations during the Cenozoic and, yet, to date no study has documented the potential role of climate on its erosion history. In western Namibia, the Brandberg Massif, an erosional remnant or inselberg, provides a good opportunity to document the Cenozoic denudation history of the margin using the relationship between rock cooling or exhumation ages and their elevation. Here we provide new apatite (UThSm)/He dates on the Brandberg Inselberg that range from 151 +/- 12 to 30 +/- 2 Ma. Combined with existing apatite fission track data, they yield new constraints on the denudation history of the margin. These data document two main cooling phases since continental break-up 130 Myr ago, a rapid one (similar to 10 degrees C/Myr) following break-up and a slower one (similar to 12 degrees C/Myr) between 65 and 35 Ma. We interpret them respectively to be related to escarpment erosion following rifting and continental break-up and as a phase of enhanced denudation during the Early Eocene Climatic Optimum. We propose that during the Early Eocene Climatic Optimum chemical weathering was important and contributed significantly to the denudation of the Namibian margin and the formation of a pediplain around the Brandberg and enhanced valley incision within the massif. Additionally, aridification of the region since 35 Ma has resulted in negligible denudation rates since that time. (C) 2019 Elsevier B.V. All rights reserved.

International audience; We present a new numerical model to calculate the surface deflection of a two-dimensional, yet variable thickness, thin elastic plate. The model is based on a multi-grid, finite difference solution of the fourth-order differential equation that incorporates the terms arising from the non-uniform thickness assumption. The model has been developed to calculate the flexural response of the continental lithosphere subjected to an arbitrary, instantaneous stretching. The flexural model is coupled to (a) a finite element, three dimensional thermal model incorporating the conduction, advection and production terms that allows the computation of the thermal subsidence resulting from the stretching-induced perturbation of the isotherms, assuming that the effective elastic thickness is controlled by the depth to a given isotherm; and (b) a finite difference surface process model that assumes that transport is linearly proportional to slope leading to a second-order, diffusion-type partial differential equation. The model also incorporates the effect of sediment compaction. We present a series of simple benchmarks that demonstrate the accuracy of the model. We also present results of simple 2D and 3D stretching experiments highlighting the importance of 3D flexural effects and the assumed variable elastic thickness on the development of a passive margin and its thermal evolution. Finally, we perform a numerical experiment based on a stretching geometry derived from the present-day geometry of the Western AfricaTransform Margin to predict sediment accumulation patterns and a stratigraphic architecture which we can compare to observations.

International audience; The Namibian margin evolution started 130 Myr ago after the breakup between South America and southern Africa. Today, it shows specific features: (i) an elevation and a bathymetry about 1000 m higher than the surrounding areas, (ii) a significant sediment supply during the Late Cretaceous following the first sedimentary peak that occurred during rifting and just afterwards, and (iii) a long-term progradation since the Mid- Cretaceous. This present configuration results in couplings between several processes and the inherited structure. We constrained this evolution using a numerical simulation applied both onshore and offshore integrating different couplings: i) a thermal adjustment of a lithosphere driven by conductivity, ii) a flexural isostasy, and iii) the loading/unloading effects of surface matter transfer. Three processeswere investigated: rock-uplift, a change in the erosion efficiency and a sea-level fall. The resultswere compared to some pertinent observationsmade on the Namibian margin such as the vertical displacements, the sediment transfers, the denudation and the stratigraphic architecture in the marginal basin. In each simulation, rift-related reliefs were rapidly relaxed resulting in an exponentially decreasing denudation and sediment accumulation. Sixty Myr after the breakup,we simulated an additional uplift, an increase in the transfer efficiency of the sediment and a sea-level fall. The subsidence increased by loading, inducing a new progradational trend. Nonetheless, the three types of events differed in the duration of this progradation trend. The larger the additional uplift, the longer the progradation. The transfer efficiency increase and sea-level fall only resulted in a transient progradation trend. From these results,we suggest that the post-rift evolution of the Namibian margin was associated, during the Upper Cretaceous, to the rejuvenation of its continental relief triggered by a climate change during the Turonian and by a slow long-term rock-uplift during the Cenozoic

[1] Geological observations of mantle flow-driven dynamic topography are numerous, especially in the stratigraphy of sedimentary basins; on the contrary, when it leads to subaerial exposure of rocks, dynamic topography must be substantially eroded to leave a noticeable trace in the geological record. Here, we demonstrate that despite its low amplitude and long wavelength and thus very low slopes, dynamic topography is efficiently eroded by fluvial erosion, providing that drainage is strongly perturbed by the mantle flow driven surface uplift. Using simple scaling arguments, as well as a very efficient surface processes model, we show that dynamic topography erodes in direct proportion to its wavelength. We demonstrate that the recent deep erosion experienced in the Colorado Plateau and in central Patagonia is likely to be related to the passage of a wave of dynamic topography generated by mantle upwelling.

Whereas the large-scale morphology and dynamics of orogenic wedges are well explained by critical-taper theory, many questions remain unanswered regarding the details of how deformation is accommodated internally. Here, we investigate the dynamics of a collisional orogenic wedge bounded by an over-thickened continental plateau, using two-dimensional thermo-mechanical numerical models. These models, applied to the Himalayan orogen and compared with reference cross-sections, lead us to propose a new hypothesis to explain along-strike variations in tectonic style, topography and exhumation patterns observed along the Himalayan range by a combination of two mechanisms. First, numerical models produce a cycle of crustal ramp formation and advection toward the rear of the wedge. The asynchronous evolution of this cycle along different segments of the range may account for the well-documented lateral variations in the geometry of the Main Himalayan Thrust (MHT) and for the existence of a well-defined topographic transition in some segments of the range. Second, the models suggest that the formation of duplexes leading to the isolation of klippen along the range front may be controlled by rheological contrasts between the Tibetan plateau and/or the Greater Himalayan Sequence and the colliding Indian plate.

International audience; A wide range of methods are available to quantify Earth's surface vertical movements but most of these methods cannot track low amplitude (5 Ma, e.g. cosmogenic isotope studies) vertical movements characteristic of plate interiors. The difference between the present-day elevation of ancient sea-level markers (deduced from well dated marine deposits corrected from their bathymetry of deposition) and a global sea-level (GSL) curve are sometimes used to estimate these intraplate vertical movements. Here, we formalized this method by re-assessing the reliability of published GSL curves to build a composite curve that combines the most reliable ones at each stage, based on the potential bias and uncertainties inherent to each curve. We suggest i) that curves which reflect ocean basin volume changes are suitable for the ca. 100 to 35 Ma “greenhouse” period ii) whereas curves that reflects ocean water volume changes are better suited for the ca. 35 to 0 Ma “icehouse” interval and iii) that, for these respective periods, the fit is best when using curves that accounts for both volume changes. We used this composite GSL curve to investigate the poorly constrained Paleogene to Neogene vertical motions of the Armorican Massif (western France). It is characterized by a low elevation topography, a Variscan basement with numerous well dated Cenozoic marine deposits scattered upon it. Using our method, we identify low amplitude vertical movements ranging from 66 m of subsidence to 89 m of uplift over that time period. Their spatial distribution argues for a preferred scale of deformation at medium wavelengths (i.e., order 100 km), which we relate to the deformation history of northwestern European lithosphere in three distinct episodes. i) A phase of no deformation between 38 and 34 Ma, that has been previously recognized at the scale of northwestern Europe, ii) a phase of low subsidence between 30 and 3.6 Ma, possibly related to buckling of the lithosphere and iii) a phase of more pronounced uplift between 2.6 Ma and present, which we relate to the acceleration of the Africa–Apulia convergence or to enhanced erosion in the rapidly cooling climate of the Pleistocene.

To infer exhumation rates from thermochronological data requires knowledge of the temperature field in the Earth's crust, which is principally determined by a balance between heat conduction and heat advection during exhumation. The rate of heat conduction is set by the thermal conductivity which is known to strongly vary among rock types and to be a function of depth, pressure and temperature, among other factors. Despite this, most methods used to extract exhumation rate from thermochronological datasets neglect the effect of the observed variability in conductivity. Here we have computed the effect of assuming a linear variation in thermal conductivity with depth, rock type and temperature on estimates of exhumation rates based on the interpretation of thermochronological data. We show how the relationship between depth and temperature is affected by variations in rock conductivity for a range of exhumation rates. Assuming that the conductivity is a linear function of depth but is not affected by rock advection leads to very large variations in apparent exhumation rate, especially in low exhuming environments. This case is to be considered where effective conductivity may be a function of pressure, or porosity or affected by pore fluid circulation. If the conductivity is a function of initial depth (a proxy for rock type) and is advected with rock motion, the perturbation caused by a linear change in conductivity is much less important. In this situation, we show that it leads to a systematic yet small over-estimation or under-estimation in exhumation rate, when the conductivity is assumed to decrease or increase with depth, respectively. We also estimated the effect of the temperature dependence of rock conductivity on thermochronologically-derived measures of exhumation rate and demonstrate that neglecting the temperature dependence of conductivity leads to an approximate 20% underestimation of the true exhumation rate.

Using a simple solution to the heat conduction equation, I show how, at the end of an orogenic event, the relaxation of isotherms from a syn-orogenic advection-dominated geometry to a post-orogenic conduction-dominated geometry leads to the creation of a thick iso-age crustal layer. Subsequent erosion of this layer yields peculiar age-elevation profiles and detrital age distributions that cannot be easily interpreted using traditional techniques. I illustrate these points by using a simple analytical solution of the heat equation as well as a transient, three-dimensional numerical model. I also demonstrate that the age of the end of an orogenic event is so strongly imprinted in the thermochronological record that it erases most of the information pertaining to the orogenic phase itself and the subsequent isostatically-driven exhumation. The concept is used to explain two thermochronological datasets from the Himalayas and demonstrate that their most likely interpretation involves the sudden interruption of extremely fast exhumation accommodated by movement along the South Tibetan Detachment in the Higher Himalayas around 15 Ma.

Thermochronology is a unique tool to reconstruct the long-term exhumation history of outcropping rocks. Pronounced (palaeo-) topography can markedly perturb near-surface isotherms, which can result in erroneous exhumation histories derived from age–elevation relationships but also offers the possibility to reconstruct palaeo-topography. Here we use a synthetic dataset to illustrate the complex non-linear relationships between the degree of topographic perturbation of thermochronological ages on one hand, and exhumation rate, geothermal gradient, and topographic wavelength and relief on the other. The dataset reveals that, in theory, relief changes can be retrieved for wavelengths as low as 5 km, and wavelength changes are possible to detect for relief as low as 0.5 km. In addition, the data attest that even in regions characterised by very slow exhumation rates (e.g. 0.03 km/Ma), changes in palaeo-topography can be successfully retrieved. Coupling of this dataset with a Fast Fourier Transform (FFT) algorithm to decompose complex 2D topography into sinusoidal functions allows a rapid and accurate estimation of the topographic perturbation and resulting thermochronological ages assuming steady-state exhumation. This coupled method was successfully implemented to (i) predict most promising sample sites for the estimation of palaeo-topography and (ii) correct exhumation rates derived from non-vertical age–elevation profiles.

© 2015 Elsevier B.V.. Faults are known to be important pathways for fluid circulation within the crust. The transfer properties along faults can evolve over time and space. The Little Grand Wash and Salt Wash normal faults, located on the Colorado Plateau, are well known examples of natural CO2 leakage systems from depth to surface. Previous studies dated and established a chronology of CO2-enriched fluid source migration along the fault traces and linked the aragonite veins observed close to Crystal Geyser to CO2-pulses. However, multiple circulation events recorded along a given fault segment deserve to be studied in minute detail in order to unravel the chronology of these events, precipitation processes and associated mechanisms. A combination of structural geology, petrography, U/Th dating, oxygen and carbon isotope analysis were used to study the fault related CO2-enriched paleo-circulations in order to build a conceptual model of CO2-circulation along the faults. This study resulted in the precise descriptions of the features attesting CO2-enriched fluid circulation by a characterization of their relationship and architecture at the outcrop scale. These features are witnesses of a large range of circulation/sealing mechanisms, as well as changes in fluid chemistry and thermodynamic state of the system, providing evidence for (i) the evolution of the fluid through a pathway from depth to the surface and (ii) different cycles of fault opening and sealing. Large circulation events linked with fault opening/sealing are observed and calibrated in nature with millennial circulation and sealing time-lapses. Numerical modelling indicates that such sealing timescale can be explained by the introduction of a fault sealing factor that allows modifying permeability with time and that is calibrated by the natural observations. ispartof: Tectonophysics vol:651 pages:121-137 status: published

A 2-D numerical model is used to investigate the evolution of accretionary wedges and fold-and-thrust belts. The numerical method is based on the distinct-element method (DEM). Unlike many continuum numerical models, DEMallows localization to occur even after substantial amounts of deformation. The method is used to study the evolution of simple accretionary wedges and thrust belts with a rigid backstop and base. Experiments are done with a large range of coefficients of interelement friction (μ e ) and element-wall friction (μ b ). Two modes of deformation, which depend mainly on μ b , are observed. For the weak base case (low μ b ), the dominant mode is frontal accretion by 'pop-up' structures at or near the toe of the wedge. For the strong base case (high μ b ), uplift is concentrated near the back of the wedge, and is accompanied by underthrusting along a flat-ramp-flat (or 'staircase') thrust fault structure. At intermediate values of μ b , the wedge oscillates between the two modes of deformation. During periods of frontal accretion, normal faulting sometimes occurs in regions where the material has thickened considerably. The transition between the two modes of deformation is found to be a strong function of μ b but a weak function of μ e . A simple explanation of the experimental results is made using the principle of work minimization. Comparisons between the results and some accretionary wedges/fold-and-thrust belts are also made.

We present an approximate analytical solution to the stream power equation describing the erosion of bedrock in an actively uplifting mountain range subject to periodic variations in precipitation rate. It predicts a time lag between the climate forcing and the erosional response of the system that increases with the forcing period. The predicted variations in the sedimentary flux coming out of the mountain are also scaled with respect to the imposed rainfall variations in a direct proportion to the discharge exponent, m, in the stream power law expression. These findings are confirmed by 1-D and 2-D numerical solutions. We also show that the response of a river channel is independent of its length and thus the size of its catchment area, implying that all actively eroding streams in a mountain belt will constructively contribute to the integrated signal in the sedimentary record. We show that rainfall variability at Milankovitch periods should affect the erosional response of fast uplifting mountain belts such as the Himalayas, Taiwan or the South Island, New Zealand, and predict 1–10 thousand years offsets between forcing and response. We suggest that this theoretical prediction could be used to independently constrain the value of the poorly defined stream power law exponents, and provide an example of how this could be done, using geochemical proxy signals from an ODP borehole in the Bengal Fan.

The tectonic and geomorphic evolution of the southeastern Australian highlands has been a subject of controversy among geophysicists, geologists, and geomorphologists for several decades. We employ a numerical surface process model that includes long-range fluvial transport, hillslope diffusion, and landsliding, in order to quantitatively assess the tectonic, lithological, and structural controls on the landscape evolution and denudation history of SE Australia. We constrain fluvial erosion parameters for our model by fitting river profiles. Rockfall and landsliding are the most important processes controlling the concentration of erosion in river valleys. We therefore model hillslope evolution using a very low diffusion coefficient and a threshold slope for landsliding. The “initial” (prebreakup) topography of the highlands provides a fundamental control on their subsequent evolution, together with the style and rate of fluvial incision. Secondary controls are exerted by lateral variations in lithology and local tectonic uplift along faults. Our modeling results indicate that (1) the observed highlands morphology requires that the drainage divide was established at its present location prior to opening of the Tasman Sea; whether the divide is inherited from Paleozoic orogeny or resulted from Mesozoic syn-rift uplift cannot be established, however; (2) in contrast to traditional interpretations, escarpment retreat does not appear to be the fundamental process eroding the highlands; and (3) the observed barbed river drainage may be imprinted as a result of lithological variation; it does not necessarily evolve through capture. The driving mechanism that is most consistent with the inferred uplift pattern of the southeastern highlands appears to be magmatic underplating.

SUMMARY We present here the results of numerical models of the Southern Alps in which the thermal and dynamical developments of the orogen are fully coupled to one another. This interlinkage allows the use of a wide range of thermal and physical features of the orogen as constraints on the applicability of our models. In particular, the thermal aspect of this method enables the prediction of the distribution of thermochronological ages at the surface of the model. Perturbation of the geothermal structure due to rapid uplift and exhumation causes an intrinsic weakening and concentration of strain along the equivalent of the Alpine Fault zone in these models. This thermal weakening of the crust also produces a zone of high strain antithetic to the Alpine Fault in the upper crust that is comparable to the Main Divide Fault Zone within the Southern Alps. The introduction of orographic rainfall and erosional processes into the model leads to the development of high, asymmetric topography comparable to that of the Southern Alps. This topographic profile results from the capture of available precipitation by the windward side of the orogen and the resultant rain shadow on the leeward side. Due to the time required to accumulate suYcient topography for this rain-capture eVect to become significant, the establishment of this high, asymmetric topography lags the initiation of surface uplift by several million years. Comparison of the observed thermal history of the Southern Alps with that seen in models based on diVering hypotheses of the tectonic evolution of the South Island shows that the present tectonic regime of the orogen most probably developed in a single rapid reorganization of plate motions at approximately 5 Ma with relative stability of the tectonic regime since that time. The variation in isotopic ages along the Southern Alps is consistent with that expected from variation in accumulated uplift and exhumation along the orogen arising from the obliquity of convergence of the Australian and Pacific plates.

In the past few decades, many studies have been dedicated to the understanding of the interactions between tectonics and erosion, in many instances through the use of numerical models of landscape evolution. Among the numerous parameterizations that have been developed to predict river channel evolution, the stream power law, which links erosion rate to drainage area and slope, remains the most widely used. Despite its simple formulation, its power lies in its capacity to reproduce many of the characteristic features of natural systems (the concavity of river profile, the propagation of knickpoints, etc.). However, the three main coefficients that are needed to relate erosion rate to slope and drainage area in the stream power law remain poorly constrained. In this study, we present a novel approach to constrain the stream power law coefficients under the detachment-limited mode by combining a highly efficient landscape evolution model, FastScape, which solves the stream power law under arbitrary geometries and boundary conditions and an inversion algorithm, the neighborhood algorithm. A misfit function is built by comparing topographic data of a reference landscape supposedly at steady state and the same landscape subject to both uplift and erosion over one time step. By applying the method to a synthetic landscape, we show that different landscape characteristics can be retrieved, such as the concavity of river profiles and the steepness index. When applied on a real catchment (in the Whataroa region of the South Island in New Zealand), this approach provides well-resolved constraints on the concavity of river profiles and the distribution of uplift as a function of distance to the Alpine Fault, the main active structure in the area.

[1] We have used dedicated sampling and analysis of apatite fission track (AFT) and apatite (U-Th)/He (AHe) thermochronological data sets in an attempt to quantify relief evolution and exhumation rates in the Pelvoux and Dora-Maira massifs (western European Alps). A dual approach comparing spectral analysis and thermal-kinematic model inversion was applied. We sampled age-elevation relationships at a range of topographic wavelengths along two north-south transects crossing these massifs. For the 40-km-long Pelvoux transect we report 35 new AFT ages that range between 3.0 ± 0.4 Ma and 12.6 ± 1.0 Ma, and 8 new AHe ages between 3.5 ± 1.5 and 4.7 ± 0.7 Ma. The Dora-Maira transect spans a distance of 60 km and includes 28 (23 new and 5 previously published) significantly older AFT ages, which vary between 13.1 ± 0.2 and 27.3 ± 0.3 Ma. Inferred exhumation rates of 0.7 km m.y.−1 since ∼8 Ma in the Pelvoux and only 0.1 km m.y.−1since ∼20 Ma in Dora-Maira are consistent between methods as well as with previous estimates, although they are associated with large uncertainties due to imperfect sampling and analytical errors. Neither massif displays clear evidence for relief change during the intervals constrained by the data. Data from the Dora-Maria massif suggest moderate recent relief increase, whereas the results from the Pelvoux are inconclusive but could imply relief decrease. Our study highlights the difficulties of applying thermochronology techniques to constrain relief changes. We show that an unreasonable number of samples is needed to perform reliable spectral analysis of age-elevation transects and discuss the overall limitations of both techniques to address relief evolution. We suggest that a more efficient approach would be to apply thermal-kinematic model inversion to relatively small study areas using spatially distributed sampling and multiple thermochronometer analyses per sample.

[1] Using numerical models that couple surface processes, flexural isostasy, faulting and the thermal effects of rifting, we show that fault-bounded escarpments created at rift flanks by mechanical unloading and flexural rebound have little potential to “survive” as retreating escarpments if the lower crust under the rift flank is substantially stretched. In this configuration, a drainage divide that persists through time appears landward of the initial escarpment in a position close to a secondary bulge that is created during the rifting event at a distance that depends on the flexural rigidity of the upper crust. Moreover, the migration of the escarpment to the secondary bulge occurs when the pre-rift topography dips landward, otherwise the evolution of the escarpment is guided by the pre-existing inland drainage divide. To illustrate this new mechanism for the evolution of passive margins, we study the examples of Southeastern Australia and Southeastern Brazil. We propose that a pre-existing inland drainage divide with rift related flank uplift can produce the double drainage divide observed in Southeastern Australia. On the other hand, we conclude that it is possible that the Serra do Mar escarpments on the Southeastern Brazilian margin originated as a secondary flexural bulge during rifting that persisted through time. In both cases, the retreating escarpment scenario is unlikely and the present-day margin morphology can be explained as resulting from rift-related vertical motions alone, without requiring significant post-rift “rejuvenation”.

[1] The late Neogene–Quaternary exhumation history of the European Alps is the subject of controversial findings and interpretations, with several thermochronological studies arguing for long-term steady state exhumation rates, while others have pointed to late Miocene–Pliocene exhumation pulses associated with tectonic and/or climatic changes. Here, we perform inverse thermal-kinematic modeling on dense thermochronological data sets combining apatite fission track (AFT) data from the literature and recently published apatite (U-Th-Sm)/He (AHe) data along the upper Rhone valley (Aar and Aiguilles Rouges massifs, Swiss Alps) in order to derive precise estimates on the denudation and relief history of this region. We then apply forward numerical modeling to interpret cooling paths quantified from apatite 4He/3He thermochronometry, in terms of denudation and relief-development scenarios. Our modeling results highlight the respective benefits of using AFT/AHe thermochronology data and 4He/3He thermochronometry for extracting quantitative denudation and relief information. Modeling results suggest a late Miocene exhumation pulse lasting until ∼8–10 Ma, consistent with recently proposed exhumation histories for other parts of the European Alps, followed by moderate (∼0.3–0.5 km Myr−1) denudation rates during the late Miocene/Pliocene. Both inverse modeling and 4He/3He data reveal that the late stage exhumation of the studied massifs can be explained by a significant increase (∼85–100%) in local topographic relief through efficient glacial valley carving. Modeling results quantitatively constrain Rhone valley carving to 1–1.5 km since ∼1 Ma. We postulate that recent relief development within this part of the Swiss Alps is climatically driven by the onset of major Alpine glaciations at the mid-Pleistocene climate transition.

We present a new algorithm to solve the problem of rock deformation. The method is based on a ‘classical’ Lagrangian finite element solver in which the connectivity between the nodes of the numerical mesh is updated at the end of each time (or deformation) step. The method, which we term Dynamic Lagrangian Remeshing (DLR), relies on the theory of Delaunay triangulation and produces optimal finite element meshes in the sense that nodes are always connected to their set of nearest neighbour nodes. Because it is based on a Lagrangian method in which the numerical grid is attached to material particles and advected with the deformation, the DLR algorithm is ideally suited to track material boundaries and properly represent free surfaces. In addition, in comparison to ‘classical’ Lagrangian solvers the DLR method is not limited to small strain problems and may be used to model discontinuities such as faults and narrow shear zones. We have used the DLR method to study the evolution of a crustal layer undergoing finite shortening driven by a velocity discontinuity along its base. The numerical model predicts the formation of a large fold that is progressively cut by a fault originating at the velocity discontinuity and propagating through the crustal layer. The model predictions provide support for the fault-propagation folding model of Suppe [1].

International audience; We assess the importance of thermochronometric data sampling and modeling strategies for correctly estimating mountain belt exhumation. Thermochronological age-elevation profiles have been widely used to infer orogenic exhumation histories; however, recent studies have shown that this sampling strategy may not be the most pertinent for quantifying both denudation and relief history. Here, we investigate the ability of combining different thermochronology data sampling schemes with numerical modeling to better constrain denudation rates and relief changes. We produce synthetic thermochronology datasets for real Alpine topography under a specific exhumation and relief scenario using the thermal-kinematic model Pecube. We then adopt an inverse approach based on the Neighborhood Algorithm to quantitatively assess the resolution of different thermochronology datasets collected following elevation profiles, long transects and valley bottom sampling. We also test the effect of the modeling approach on denudation and relief predictions, in particular the influence of the topographic grid resolution and of potential constraints on the geothermal gradient. Our results show that sampling along a single elevation profile does not allow to quantitatively constrain both denudation and relief histories. Numerical outputs clearly evidence tradeoffs that limit the capacity of simultaneously resolving denudation rates and relief change. Quantitative predictions are only slightly different when combining elevation profiles along different valleys, but are highly improved when using long transects or valley-bottom samples combined with an elevation profile. The resolution with which relief evolution can be predicted may be increased by a factor of 2 by using spatially distributed datasets. Results of thermal parameter inversions suggest that the geothermal gradient may be better estimated using elevation profiles or long-transect sampling rather than using valley bottom samples. Simulations with different model topography resolutions show that degrading the resolution for computational efficiency may result in a loss of quantitative information on denudation rates and relief history. In summary, we highlight that both thermochronological sampling strategies and the choice of thermal parameters or model topography resolution have a significant influence on predicted denudation and relief histories. Ideally, the sampling strategy should be designed using preliminary modeling of expected denudation and relief histories, and a sensitivity study on assumed thermal parameters and model resolution should be performed when modeling the data. Although our modeling is based on a particular case study of relief evolution in the French western Alps, we believe that these inferences have general relevance for thermochronological studies within mountain belts.

22p.; International audience; The Himalayan range is commonly presented as largely laterally uniform from west to east. However, geological structures, topography, precipitation rate, convergence rates, and low-temperature thermochronological ages all vary significantly along strike. Here, we focus on the interpretation of thermochronological data sets in terms of along-strike variations in geometry and kinematics of the main crustal detachment underlying the Himalaya: the Main Himalayan Thrust (MHT). We report new apatite fission track (AFT) ages collected along north-south transects in western and eastern central Nepal (at the latitudes of the Annapurna and Langtang massifs, respectively). AFT ages are consistently young (

International audience; Heat transfer in the crust Low-temperature thermochronology PECUBE is a three-dimensional thermal-kinematic code capable of solving the heat production-diffusionadvection equation under a temporally varying surface boundary condition. It was initially developed to assess the effects of time-varying surface topography (relief) on low-temperature thermochronological datasets. Thermochronometric ages are predicted by tracking the time-temperature histories of rockparticles ending up at the surface and by combining these with various age-prediction models. In the decade since its inception, the PECUBE code has been under continuous development as its use became wider and addressed different tectonic-geomorphic problems. This paper describes several major recent improvements in the code, including its integration with an inverse-modeling package based on the Neighborhood Algorithm, the incorporation of fault-controlled kinematics, several different ways to address topographic and drainage change through time, the ability to predict subsurface (tunnel or borehole) data, prediction of detrital thermochronology data and a method to compare these with observations, and the coupling with landscape-evolution (or surface-process) models. Each new development is described together with one or several applications, so that the reader and potential user can clearly assess and make use of the capabilities of PECUBE. We end with describing some developments that are currently underway or should take place in the foreseeable future.

International audience; Plate tectonic theory suggests that present-day topography can be explained by the repeated interactions between the tectonic plates moving along Earth's surface. However, mounting evidence indicates that a significant proportion of Earth's topography results from the viscous stresses created by flow within the underlying mantle, rather than by the moving plates. This dynamic topography is transient, varying as mantle flow changes, and is characterized by small amplitudes and long wavelengths. It is therefore often hidden by or confused with the more obvious topographic anomalies resulting from horizontal tectonic movements. However, dynamic topography can influence surface processes and thus enter the geological record; it has, for example, played a role in the establishment of Amazon drainage patterns. In turn, surface processes such as the erosion of topographical anomalies could affect mantle flow. This emerging view of dynamic topography suggests that the concept of plate tectonics as the driver of surface deformation needs to be extended to include the vertical coupling between the mantle and the surface. Unravelling this coupling back in time with the help of models and the geological record can potentially provide unprecedented insights into past mantle dynamics.

International audience; Although critical to a variety of issues in Earth Sciences, paleotopography remains poorly constrained over the geological past. Indeed, sediments preserve a record of the history of the Earth surface, but deconvolving these archives remains a challenge in the absence of a proper quantification of surface processes at the large spatial and temporal resolution imposed by these data. To solve for this, we propose a set of simple bedrock erosion and sediment transport laws that apply over large spatial (∼100 km) and temporal (∼1-10 Ma) scales. These laws are tested in light of physical experiments of landscape evolution under different tectonic and climatic forcings and are calibrated using present-day large-scale Earth topography and sediment fluxes in rivers. We subsequently implement these processes into a numerical code, TopoSed, that is able to predict the evolution of macroscale topography, sediment fluxes, paleogeographies, and bedrock exhumation given a tectonic and climatic input scenario. The results of such simulations can be directly compared to sedimentary or thermochronological data to test the plausibility of the input tectonics and predicted topography. A series of tests on the sensitivity of such predictions to the uncertainties on input parameters shows that it should be possible from sedimentary data to invert for paleouplift rates and also for paleotopographies during periods covering major tectonic or climatic events. Although this code is meant to be refined in the future as we improve our understanding of surface processes at these macroscales, TopoSed provides a powerful tool to put constraints on past geodynamic processes, such as dynamic topography, by extracting quantitative information on the evolution of the Earth surface from sedimentary data.

The constantly improving resolution of geophysical data, seismic tomography and seismicity in particular, shows that the lithosphere does not subduct as a slab of uniform thickness but is rather thinned in the upper mantle and thickened around the transition zone between the upper and lower mantle. This observation has traditionally been interpreted as evidence for the buckling and piling of slabs at the boundary between the upper and lower mantle, where a strong contrast in viscosity may exist and cause resistance to the penetration of slabs into the lower mantle. The distribution and character of seismicity reveal, however, that slabs undergo vertical extension in the upper mantle and compression near the transition zone. In this paper, we demonstrate that during the subduction process, the shape of low viscosity slabs (1 to 100 times more viscous than the surrounding mantle) evolves toward an inverted plume shape that we coin jellyfish. Results of a 3D numerical model show that the leading tip of slabs deform toward a rounded head skirted by lateral tentacles that emerge from the sides of the jellyfish head. The head is linked to the body of the subducting slab by a thin tail. A complete parametric study reveals that subducting slabs may achieve a variety of shapes, in good agreement with the diversity of natural slab shapes evidenced by seismic tomography. Our work also suggests that the slab to mantle viscosity ratio in the Earth is most likely to be lower than 100. However, the sensitivity of slab shapes to upper and lower mantle viscosities and densities, which remain poorly constrained by independent evidence, precludes any systematic deciphering of the observations.

International audience; Glaciers and rivers control the shape of the high relief topography of mountain ranges. However, their relative contribution in response to climatic oscillations and tectonic forcing and whether landscapes can reach equilibrium conditions during the Quaternary are still unclear. Here we introduce a new thermochronometer of exceptionally low closure temperature (ca. 30-35 °C) based on Optically Stimulated Luminescence (OSL) dating and illustrate how it may be used to measure relief evolution and exhumation rates within the last glacial cycle in the Southern Alps of New Zealand, one of the most tectonically active orogens and an area that has experienced rapid, high magnitude climate changes. We find that exhumation rates have remained steady over the last glacial cycle and match rates observed at a million year timescale. This suggests that, despite an extreme exhumation rate of the order of 800 m in 100 ka, and the fact that in the last ca. 11-18 ka most hillslope sides have changed from U to V-shape valleys and have been dissected by debris-flows, landslides and rock avalanches, the mean exhumation rates have remained nearly constant. This may imply that tectonics, not climate, has a primary control on the rates of exhumation in tectonically active and wet mountain belts. On the contrary, tectonically active mountain ranges might not attain equilibrium on similar timescales in weathering and/or transport limited landscapes as, for example, in arid regions.

International audience; Thermochronologic data collected along age-elevation profiles are commonly interpreted as recording temporally variant but spatially constant exhumation rates. However, thermochronologic age-elevation relationships are known to be perturbed by topographic effects and potential changes in relief, which are neglected in the inherently 1-D interpretation commonly applied. Such data thus potentially record both the denudation and relief history of the sampled region but extracting this information is challenging. In a companion paper, we develop a methodology for rigorously interpreting thermochronologic age-elevation profiles in terms of exhumation rates and relief development through time, and to independently quantify the resolution of these constraints. Here we test this approach using a thermochronological dataset consisting of apatite and zircon fission-track and (U-Th)/He data, collected at La Meije Peak in the Pelvoux-Ecrins massif (French Western Alps). Our data and models suggest a three-phase exhumation history in the Pelvoux-Ecrins massif, including a pulse of rapid exhumation at ˜ 6-5.5 Ma, preceded and followed by more moderate rates of denudation in the order of 0.3-0.4 km Myr‑ 1. This rapid exhumation event appears to occur coevally in other external crystalline massifs in the Alps but is not detected by qualitative inspection of the age-elevation relationships. Both our synthetic results and inversion of the La Meije data strongly suggest that apatite fission-track age-elevation relationships alone cannot resolve both denudation and relief histories independently and that multiple thermochronometers are required. Combining apatite fission-track and (U-Th)/He ages and, particularly, including fission-track-length data greatly improves the resolution of the inferred exhumation histories. Although denudation rates through time and the timing of rate changes are generally well resolved, our data have insufficient resolution to satisfactorily constrain the relief history. Synthetic results reported in the companion paper suggest that the reason for this limitation is that relief increase through valley carving has been insufficient with respect to the regional denudation rates to be unambiguously extracted from the data.