Philippe Steer

Erosion-induced isostatic rebound triggers extension in low convergent mountain ranges

Mechanisms that control seismic activity in low strain rate areas such as western Europe remain poorly understood. For example, in spite of low shortening rates of <0.5 mm/ yr, the Western Alps and the Pyrenees are underlain by moderate but frequent seismicity detectable by instruments. Beneath the elevated part of these mountain ranges, analysis of earthquake focal mechanisms indicates extension, which is commonly interpreted as the result of gravitational collapse. Here we show that erosional processes are the predominant control on present-day deformation and seismicity. We demonstrate, using fi nite element modeling, that erosion induces extension and rock uplift of the elevated region of mountain ranges accommodating relatively low overall convergence. Our results suggest that an erosion rate of ~1 mm/yr can lead to extension in mountain ranges accommodating signifi cant shortening of <3 mm/yr. Based on this study, the seismotectonic framework and seismic hazard assessment for low strain rate areas need to be revisited, because erosion-related earthquakes could increase seismic hazard.

Erosion influences the seismicity of active thrust faults

Assessing seismic hazards remains one of the most challenging scientific issues in Earth sciences. Deep tectonic processes are classically considered as the only persistent mechanism driving the stress loading of active faults over a seismic cycle. Here we show via a mechanical model that erosion also significantly influences the stress loading of thrust faults at the timescale of a seismic cycle. Indeed, erosion rates of about ~0.1-20 mm yr(-1), as documented in Taiwan and in other active compressional orogens, can raise the Coulomb stress by ~0.1-10 bar on the nearby thrust faults over the inter-seismic phase. Mass transfers induced by surface processes in general, during continuous or short-lived and intense events, represent a prominent mechanism for inter-seismic stress loading of faults near the surface. Such stresses are probably sufficient to trigger shallow seismicity or promote the rupture of deep continental earthquakes up to the surface.

Surface Lagrangian Remeshing: A new tool for studying long term evolution of continental lithosphere from 2D numerical modelling

In this paper we present a new local remeshing algorithm that is dedicated to the problem of erosion in finite element models whose grid follows the movement of the free surface. The method, which we name Surface Lagrangian Remeshing (SLR), is adapted to 2D Lagrangian models which couple surface erosion with deformation of Earth materials. The remeshing procedure preserves nodes defining the surface submitted to erosion and removes nodes belonging to surface elements whose internal angles or area is critically low. This algorithm is ideally suited to track long term surface evolution. To validate the method we perform a set of numerical tests, using triangular finite elements, which compare the results obtained with the SLR algorithm with global remeshing and with analytical results. The results show good agreements with analytical solutions. Interpolation errors associated with remeshing are generated locally and numerical diffusion is restricted to the remeshed domain itself. In addition this method is computationally costless compared to classical global remeshing algorithm. We propose to couple the SLR method with the Dynamical Lagrangian Remeshing (DLR) algorithm to enable local remeshing only of Lagrangian models coupling large deformation of Earth materials with large erosion.

Three-dimensional numerical simulations of crustal systems undergoing orogeny and subjected to surface processes

As several modeling studies indicate, the structural expression and dynamic behavior of orogenic mountain belts are dictated not only by their rheological properties or by far-field tectonic motion, but also by the efficiency of erosion and sedimentation acting on its surface. Until recently, numerical investigations have been mainly limited to 2-D studies because of the high computational cost required by 3-D models. Here, we have efficiently coupled the landscape evolution model Cascade with the 3-D thermomechanically coupled tectonics code FANTOM. Details of the coupling algorithms between both codes are given. We present results of numerical experiments designed to study the response of viscous-plastic crustal materials subjected to convergence and to surface processes including both erosion and sedimentation. In particular, we focus on the equilibration of both the tectonic structures and on the surface morphology of the orogen. We show that increasing the efficiency of fluvial erosion increases the frontal thrust angle, which in turn decreases the width of the orogen. In addition, the maximum summit elevation of the orogen during transient evolution is significantly higher in those models showcasing surface processes than those that do not. This illustrates the strong coupling between tectonics and surface processes. We also demonstrate that an along-strike gradient of erosion efficiency can have a major impact upon the landscape morphology and the tectonic structure and deformation of the orogen, in both the across-strike and along-strike directions. Overall, our results suggest that surface processes, by enhancing localization of deformation, can act as a positive forcing to topographic building.

Evidence for Eocene–Oligocene glaciation in the landscape of the East Greenland margin

Assessing the onset and extent of Northern Hemisphere glaciation is required to understand Cenozoic climate change and its impact on topography. While the onset of accelerated Cenozoic erosion is generally associated with the Quaternary at mid-latitudes, some high-latitude passive margins may have undergone earlier glaciation starting at 38–30 Ma or even 45 Ma. Here we document a rapid phase of exhumation in the East Greenland margin between 68°N and 76°N starting at 30 ± 5 Ma. The timing is coincident with the dramatic worldwide fall of surface temperature at the Eocene-Oligocene transition. Our inference is based on apatite fission track and apatite helium data. We suggest that a transition from an Eocene fluvial to an Oligocene glacial-dominated landscape triggered a period of enhanced erosion. This study provides the first onshore potential evidence of the onset of continental ice in East Greenland margin at the Eocene-Oligocene transition (ca. 34 Ma), contemporaneously with the onset of Antarctica glaciation and erosion. Our interpretation is consistent with that based on the oldest ice-rafted debris found in the sedimentary records offshore East Greenland and implies that East Greenland exhibits the oldest onshore record of Cenozoic glacial erosion on Earth.

A precipiton-based approach to model hydro-sedimentary hazards induced by large sediment supplies in alluvial fans

Mountain ranges are frequently subjected to mass wasting events triggered by storms or earthquakes and supply large volumes of sediment into river networks. Besides altering river dynamics, large sediment deliveries to alluvial fans are known to cause hydro-sedimentary hazards such as flooding and river avulsion. Here we explore how the sediment supply history affects hydro-sedimentary river and fan hazards, and how well can it be predicted given the uncertainties on boundary conditions. We use the 2D morphodynamic model Eros with a new 2D hydrodynamic model driven by a sequence of flood, a sediment entrainment/transport/deposition model and a bank erosion law. We first evaluate the model against a natural case: the 1999 Mount Adams rock avalanche and subsequent avulsion on the Poerua river fan (West Coast, New Zealand). By adjusting for the unknown sediment supply history, Eros predicts the evolution of the alluvial riverbed during the first post-landslide stages within 30 cm. The model is subsequently used to infer how the sediment supply volume and rate control the fan aggradation patterns and associated hazards. Our results show that the total injected volume controls the overall levels of aggradation, but supply rates have a major control on the location of preferential deposition, avulsion and increased flooding risk. Fan re-incision following exhaustion of the landslide-derived sediment supply leads to sediment transfer and deposition downstream and poses similar, but delayed, hydro-sedimentary hazards. Our results demonstrate that 2D morphodynamics models are able to capture the full range of hazards occurring in alluvial fans including river avulsion aggradation and floods. However, only ensemble simulations accounting for uncertainties in boundary conditions (e.g., discharge history, initial topography, grain size) as well as model realization (e.g., non-linearities in hydro-sedimentary processes) can be used to produce probabilistic hazards maps relevant for decision making.