Force-Balance Analysis of Stress Changes During the Subduction-Collision Transition and Implications for the Rise of Mountain Belts

Mountain height at convergent plate margins is limited by the megathrust shear force, but it remains unclear how this constraint affects the topographic evolution and mountain building at the transition from subduction to collision. Generally, mountain height increases during the subduction-collision transition in response to crustal thickening or processes like mantle delamination and slab breakoff, but the main parameters controlling how much mountain height increases remain poorly understood. Here we show, based on analytical and finite-element force-balance models, that the increase in mountain height depends on the magnitude of the megathrust shear force and the reduction of submarine margin relief. During the subduction stage, the shear force is balanced by the gravitational effect of the margin relief and the deviatoric stresses in the upper plate are low. When the submarine margin relief is reduced during the closure of the ocean basin, the effect of the gravitational force decreases and the upper plate experiences enhanced deviatoric compression, which allows the mountain height to increase until a near-neutral stress state beneath the high mountains is restored. If the increase in mountain height cannot keep pace with the submarine relief reduction, the compression of the upper plate increases by a few tens of MPa, which promotes tectonic shortening and mountain building. Our analysis implies that mountain height can increase by hundreds of meters to a few kilometers during collision, depending primarily on the trench depth during the subduction stage and possible syncollisional changes of the megathrust shear force

Megathrust Stress Drop as Trigger of Aftershock Seismicity: Insights From the 2011 Tohoku Earthquake, Japan

Numerous normal-faulting aftershocks in subduction forearcs commonly follow large megathrust earthquakes. Postseismic normal faulting has been explained by stress changes induced by the stress drop along the megathrust. However, details of forearc stress changes and aftershock triggering mechanisms remain poorly understood. Here, we use numerical force-balance models combined with Coulomb failure analysis to show that the megathrust stress drop supports normal faulting, but that forearc-wide aftershock triggering is feasible within a narrow range of megathrust stress drop values and preseismic stress states only. We determine this range for the 2011 Tohoku earthquake (Japan) and show that the associated stress changes explain the aftershock seismicity in unprecedented detail and are consistent with the stress released by forearc seismicity before and after the earthquake

Cretaceous to Tertiary paleogeographic reconstructions of the Alps-Pyrenees linking zone

The northwestern Mediterranean subduction systems underwent an important phase of reorganization between Late Cretaceous and Eocene. The mode and timing of this reorganization are still under debate. Great uncertainties mainly derive from the poorly preserved record of the early phases of orogenic evolution in both the Alps and Pyrenees and the distruction of the orogenic system between the Pyrenees and Alps by the Oligo-Miocene opening of the Gulf of Lion due to backarc rifting. Vestiges are nevertheless preserved in the Pyreneo-Provençal fold-and-thrust belt and associated basins in southern France and Corsica-Sardinia. In this work we first review published plate kinematic models for Iberia, Apulia and Europe from 83 Ma, focusing in particular on the restoration of the Corso-Sardinia block using the free software GPlates. Second, we characterize the Upper Cretaceous to Eocene depositional systems at the junction between the Alps, Pyrenees and Apennines, reviewing previous paleogeographic restorations for the Western Alpine and Eastern Pyrenean foreland basins. Last, we compare the kinematic models with reconstructed basin dynamics. We critically assess the implications of newly proposed paleogeographic reconstructions (at 83, 65, 50, 37 and 30 Ma) for the validity of various plate kinematic models. The information derived from the sedimentary basins help to define the mode and timing of the subduction reorganization that occurred between 83 and 30 Ma in the northwestern Mediterranean.This study is part of the Orogen research program funded by Total, the BRGM (Bureau de Recherches Géologiques et Minières), the CNRS (Centre National de la Recherche Scientifique).Peer Reviewe

Global Constraints on Intermediate‐Depth Intraslab Stresses From Slab Geometries and Mechanisms of Double Seismic Zone Earthquakes

Abstract Double seismic zones (DSZs), parallel planes of intermediate‐depth earthquakes inside oceanic slabs, have been observed in a number of subduction zones and may be a ubiquitous feature of downgoing oceanic plates. Focal mechanism observations from DSZ earthquakes sample the intraslab stress field at two distinct depth levels within the downgoing lithosphere. A pattern of downdip compressive over downdip extensive events was early on interpreted to indicate an unbending‐dominated intraslab stress field. In the present study, we show that the intraslab stress field in the depth range of DSZs is much more variable than previously thought. Compiling DSZ locations and mechanisms from literature, we observe that the “classical” pattern of compressive over extensive events is only observed at about half of the DSZ locations around the globe. The occurrence of extensional mechanisms across both planes accounts for most other regions. To obtain an independent estimate of the bending state of slabs at intermediate depths, we compute (un)bending estimates from slab geometries taken from the slab2 compilation of slab surface depths. We find no clear global prevalence of slab unbending at intermediate depths, and the occurrence of DSZ seismicity does not appear to be limited to regions of slab (un)bending. Focal mechanism observations are frequently inconsistent with (un)bending estimates from slab geometries, which may imply that bending stresses are not always prevalent, and that other stress types such as in‐plane tension due to slab pull or shallow compression due to friction along the plate interface may also play an important role

Iberia-Europe convergence and Adria subduction initiation unraveled by basins evolution

European Geosciences Union General Assembly 2018 Vienna | Austria | 8–13 April 2018In the northwestern Mediterranean, microplates configuration at early stages of convergence is matter of debate. Two are the main reasons. First, magnetic anomalies in the Atlantic cannot give any control on plates motion. Reversals in Earth’s magnetic field are indeed missing for large part of the Cretaceous. Second, structures due the early interaction between the microplates have been largely destroyed by the more recent evolution in Alps, Pyrenees and Apennines. Sediments were nevertheless deposited along long-living structures in the whole northwestern Mediterranean. The sedimentary basins are thus the only large-scale feature available for the comprehension of the system at the end of the Cretaceous. We review the Late Cretaceous to Eocene depositional systems at the junction between the Alps, Pyrenees and Apennines. We build paleogeographic maps integrated in a tectonic and plate kinematics framework on GPlates. The paleogeographic maps synthetize the regional pattern of uplift and subsidence and show also the distribution of compression and extension in the northwestern Mediterranean. We integrate metamorphic and volcanic data and we use the whole database to identify the spatial and temporal evolution of the plate boundaries during the Late Cretaceous to Eocene. Finally, we suggest timing and mode of Adria subduction initiation.This study is part of the "Orogen" research program funded by Total, BRGM (Bureau de Recherches Géologiques et Minières) and CNRS (Centre National de la Recherche Scientifique)

Megathrust Stress Drop as Trigger of Aftershock Seismicity: Insights From the 2011 Tohoku Earthquake, Japan

Abstract Numerous normal‐faulting aftershocks in subduction forearcs commonly follow large megathrust earthquakes. Postseismic normal faulting has been explained by stress changes induced by the stress drop along the megathrust. However, details of forearc stress changes and aftershock triggering mechanisms remain poorly understood. Here, we use numerical force‐balance models combined with Coulomb failure analysis to show that the megathrust stress drop supports normal faulting, but that forearc‐wide aftershock triggering is feasible within a narrow range of megathrust stress drop values and preseismic stress states only. We determine this range for the 2011 Tohoku earthquake (Japan) and show that the associated stress changes explain the aftershock seismicity in unprecedented detail and are consistent with the stress released by forearc seismicity before and after the earthquake