Jean Chéry

Study of plate deformation and stress in subduction processes using two‐dimensional numerical models

Two-dimensional finite element modeling is used to model subduction of an oceanic lithospheric plate beneath continental lithosphere. The subduction process is initiated along a preexisting inclined fault and continues until reaching 400 km of total convergence. The lithosphere is assumed to be underlain by an in viscid asthenosphere. Different rheological laws have been considered for the lithosphere, including elasticity and elastoplasticity. The modeling shows that both the stress system in the plates and the surface topography are strongly dependent on two main parameters: the density contrast between lithosphere and asthenosphere (Δρ = ρL - ρA) and the coefficient of friction along the subduction plane. Varying these two parameters allows explanation of the main characteristics of real subduction zones and results in two major regimes manifested by extension or compression in the arc-back arc system. Extension and back arc rifting corresponds to a positive density contrast and a low coefficient of friction, while negative Δρ values and/or high friction leads to a compressional regime. The coexistence of trench arc compression and back arc tension is only possible for a coefficient of friction lower than 0.1. The results of the numerical experiments agree with those of experimental modeling conducted under similar physical assumptions.

A simple parameterization of strain localization in the ductile regime due to grain size reduction: A case study for olivine

We propose a simple parameterization of the transition between dislocation creep and grain-size-sensitive creep under conditions characteristic of the lithospheric mantle and derived from the results of laboratory experiments on olivine-rich rocks. Through numerical modeling and linear stability analysis, we determine the conditions under which this transition takes place and potentially leads to strain localization. We pay particular attention to the effect of cooling rate and strain rate which are likely to be dominant parameters in actively deforming tectonic areas. We conclude that at constant temperature, strain localization can only take place if the rheology of the material is nonlinearly related to grain size; that strain localization is facilitated by syndeformation cooling; that there is only a narrow region in the strain rate versus cooling rate parameter space where localization is likely to take place; and that grain growth inhibits strain localization at fast cooling rates but may lead to “grain growth localization” at low cooling rates. We draw attention to the potential consequences of our analysis of strain localization for the style of plate motions at the Earths surface.

Large uplift of rift flanks : a genetic link with lithospheric rigidity?

Abstract Large and permanent uplifts may develop along the flanks of passive rifts, and the most likely mechanism for explaining them is flexural bending of the lithosphere due to an isostatic load resulting from necking of the lithosphere. A numerical model that accounts for the elastic and viscoplastic behaviour of the lithosphere is used to determine the important parameters of this mechanism, in order to explain large uplifts such as those observed in the Red Sea and the Transantarctic Mountains (more than 2000 m). Large uplifts of the flanks and large subsidence in the rift are associated with a thick mechanical lithosphere.

An integrated mechanical model of the San Andreas Fault in central and northern California

Several lines of evidence support the general view of the San Andreas fault system (SAFS) as a major lithospheric weakness in a generally transpressive plate margin setting. However, the influence of the weakness of the SAFS on the observed stress and deformation fields is not straightforward because factors such as interactions between the brittle upper crust and ductile lower crust, lateral fault strength variation, and the amount of convergence may all be important. The goal of this study is to model steady state deformation, relative fault-parallel velocity, and crustal stress orientations in central and northern California using realistic rheologies and boundary conditions. Using a simplified three-dimensional (3-D) finite element analysis in large strain, we model the SAFS in 2-D cross sections with no stress or strain variations along strike. We investigate the influence of different parameters such as the frictional properties of the fault zone and the adjacent crust, the viscous properties of the lower crust as determined by its thermal structure, and the thermal structure of the lithosphere. Our model appears to provide a good conceptual framework for some first-order aspects of actively deforming plate margins such as the SAFS. The following findings emerge from a variety of numerical experiments: (1) The difference in the manner of transpressive strain partitioning in central and northern California can be explained by different fault strengths and the manner in which heat flow varies with distance from the fault. (2) Only the combination of a weak fault (with an effective friction coefficient of ≈0.1) and a strong lateral heat flow variation predicts approximately correct stress directions in the crust adjacent to the SAFS.

Late Pleistocene to Holocene slip rates for the Gurvan Bulag thrust fault (Gobi-Altay, Mongolia) estimated with 10Be dates

[1] We surveyed morphotectonic markers along the central part of the Gurvan Bulag thrust, a fault that ruptured with the Bogd fault during the Gobi-Altay earthquake (1957, M 8.3), to document climatic and tectonic processes along the fault for the late PleistoceneHolocene period. The markers were dated using 10 Be produced in situ. Two major periods of alluviation ended at 131 ± 20 and 16 ± 4.8 ka. These appear to be contemporaneous with global climatic changes at the terminations of marine isotope stages (MIS) 6 and 2. The vertical slip rates, determined from offset measurements and surfaces ages, are 0.14 ± 0.03 mm/yr over the late Pleistocene-Holocene and between 0.44 ± 0.11 and 1.05 ± 0.25 mm/yr since the end of the late Pleistocene. The higher of these slip rates for the last � 16 kyr is consistent with paleoseismic investigations along the fault [Prentice et al., 2002], and suggests that, at the end of late Pleistocene, the fault evolved from quiescence to having recurrence intervals of 4.0 ± 1.2 kyr for surface ruptures with � 4 m vertical offset (similar to that of 1957). The inferred recurrence interval is comparable to that of the Bogd fault (3.7 ± 1.3 kyr) suggesting that the two faults may have ruptured together also earlier during the last � 16 kyr. INDEX TERMS: 7221 Seismology: Paleoseismology; 1208 Geodesy and Gravity: Crustal movements—intraplate (8110); 1824 Hydrology: Geomorphology (1625); 7230 Seismology: Seismicity and seismotectonics; 8107 Tectonophysics: Continental neotectonics; KEYWORDS: Late Pleistocene, Holocene, thrust fault, slip rate, 10Be dating, Mongolia

Time‐lapse microgravity surveys reveal water storage heterogeneity of a karst aquifer

Time-lapse microgravity surveying combined with absolute gravity measurements is used to investigate water storage changes in a karst aquifer of similar to 100 km(2) area. The survey consists of 40 gravity stations measured with a relative gravimeter; absolute gravity is measured at three stations for each survey. In total, four gravity surveys are performed over a 2 year time period during consecutive wet and dry periods. Survey precisions range between 2.4 and 5 mu Gal, enabling statistically significant detection of 10 mu Gal change, i.e., similar to 0.25 m equivalent water level change. Observed gravity changes are coherent between consecutive survey periods, i.e., net water withdrawal and net water recharge is observed, reaching changes as high as 22 mu Gal. Observed gravity changes allow refining evapotranspiration estimates, which may serve to improve the water budget of the aquifer. High-and low-gravity amplitude zones characterize the karst system, demonstrating spatially variable storage behavior. Geomorphologic considerations are invoked to explain the location of preferential zones of water storage, and a conceptual model of water storage is discussed for the studied karst.

Postseismic stress transfer explains time clustering of large earthquakes in Mongolia

Abstract Three M>8 earthquakes have occurred in Mongolia during a 52-year period (1905–1957). Since these earthquakes were well separated in space (400 km), the coseismic stress change is far too low (0.001 bar) to explain mutual earthquake triggering. By contrast, postseismic relaxation gradually causes a significant stress change (0.1–0.9 bar) over large distances. Using a spring-slider model to simulate earthquake interaction, we find that viscoelastic stress transfer may be responsible for the earthquake time clustering observed in active tectonic areas. Therefore, revealing postseismic strain by satellite geodesy and modeling earthquake clusters could improve our understanding of earthquake occurrence, especially in zones where large earthquakes have already struck in past decades.

THE ROLE OF BED-PARALLEL SLIP IN THE FORMATION OF BLIND THRUST FAULTS

Abstract A finite-element code is used to model the mechanical behaviour of elastoplastic sedimentary layers that are deformed by the movement of a faulted, deformable basement. The propagation of the blind thrust fault is analysed in terms of the evolution of strain localization in the elastoplastic layers of strain-softening and strain-hardening materials. The role of layer thickness, bedding-parallel slip, and fault dip are also studied. We found that, if no interlayer slip is allowed, blind thrusts tend to propagate with constant dip, regardless of the rheological properties of the sediment layers; however, if bedding-parallel slip is present, a mechanical decoupling appears between layers, allowing stress transfers along large distances and favouring the formation of backthrusts. Fault dip also has significant influence on the qualitative behaviour; when interlayer slip is possible, a steep fault will tend to split.

3D mechanical modeling of the GPS velocity field along the North Anatolian fault

The North Anatolian fault (NAF) extends over 1500 km in a complex tectonic setting. In this region of the eastern Mediterranean, collision of the Arabian, African and Eurasian plates resulted in creation of mountain ranges (i.e. Zagros, Caucasus) and the westward extrusion of the Anatolian block. In this study we investigate the effects of crustal rheology on the long-term displacement rate along the NAF. Heat flow and geodetic data are used to constrain our mechanical model, built with the three-dimensional finite element code ADELI. The fault motion occurs on a material discontinuity of the model which is controlled by a Coulomb-type friction. The rheology of the lithosphere is composed of a frictional upper crust and a viscoelastic lower crust. The lithosphere is supported by a hydrostatic pressure at its base (representing the asthenospheric mantle). We model the long-term deformation of the surroundings of the NAF by adjusting the effective fault friction and also the geometry of the surface fault trace. To do so, we used a frictional range of 0.0^0.2 for the fault, and a viscosity varying between 10 19 and 10 21 Pa s. One of the most striking results of our rheological tests is that the upper part of the fault is locked if the friction exceeds 0.2. By comparing our results with geodetic measurements [McClusky et al., J. Geophys. Res. B 105 (2000) 5695^5719] and tectonic observations, we have defined a realistic model in which the displacement rate on the NAF reaches V17 mm/yr for a viscosity of 10 19 Pa s and a fault friction of 0.05. This strongly suggests that the NAF is a weak fault like the San Andreas fault in California. Adding topography with its corresponding crustal root does not induce gravity flow of Anatolia. Rather, it has the counter-intuitive effect of decreasing the westward Anatolian escape. We find a poor agreement between our calculated velocity field and what is observed with GPS in the Marmara and the Aegean regions. We suspect that the simple lithosphere model is responsible for this discrepancy. Taking into account the weaknesses of these deforming regions should allow us to build a more realistic model that would match ground observations more appropriately. On the other hand, our results fit well GPS measurements in central Anatolia, setting the basis of modeling crustal strain in Turkey. B 2003 Elsevier Science B.V. All rights reserved.

Core complex mechanics: From the Gulf of Corinth to the Snake Range

The activity of subhorizontal decollements mapped by geologists in extensional provinces is not explained by rock mechanics principles, which predict that only steep faults can slip. These exposed decollements are therefore suspected to be inactive; they may correspond to rotated, formerly active, high-angle faults. However, growing seismological evidence of earthquakes with low-angle fault-plane mechanisms has forced us to revisit this viewpoint. Using the example of the Gulf of Corinth in Greece, we propose a mechanical model that explains how a weak, high-angle fault may form a low-angle decollement at depth. This decollement is uplifted in a second stage of extension, which allows the exhumation of a metamorphic core complex such as the northern Snake Range, Nevada.