The tectonic evolution of the plate boundary between Iberia and Europe since the Variscan and more clearly since the Mesozoic rifting is at the origin of heterogeneities of densities and structure, in the crust and the mantle, which have an impact on the distribution of the current stresses and post-orogenic uplift in the Pyrenees. Here, we investigate the lithosphere structure across the Pyrenees and Western Europe using LitMod2D that integrates geophysical and petrological data sets to produce the thermal, density, and seismic velocity structure of the lithosphere and upper mantle. Of particular interest is the chemical composition of the mantle, including the degree of serpentinization near the North Pyrenean Fault (>10 km), and the shape of the lithosphere-asthenosphere boundary at a larger scale (>100 km). The topography and geophysical constraints, including LAB geometry, Vs, Vp data are well reproduced for a weak fertile Phanerozoic lithosphere. Our results suggest that accounting for serpentinization allows fitting second-order gravity and seismological features in the lithosphere, but not topography which is controlled to first-order by high lateral variability in crustal thickness and lithosphere strength.
Abstract The relationships between the serpentinized continental mantle in orogens, its geophysical signature at depth and hydrogen seepages are poorly understood. A petro‐physical modeling approach accounting for serpentinization shows that a large domain of serpentinized mantle (1,800 km 2 ) is present in the northern Pyrenees. The serpentinization reached a maximum of 40% during the mid‐Cretaceous rifting, according to the predicted temperature and pressure. Although high‐temperature serpentinization could have generated large quantify of hydrogen during the Mesozoic, the shallow and inactive faulting in Northern Pyrenees make this process unlikely to explain the entire serpentinization inferred by petro‐physical modeling. A combination of low‐temperature alteration of mafic and ultramafic rocks in the North Pyrenean Zone, active normal faulting in the North Pyrenean Fault, accumulation in local traps and transport of H 2 ‐rich fluids along inactive but permeable fault may explain the hydrogen seepages observed today.
<p>Long-term tectonics numerical modelling at accretionary margin scale is a powerful tool to retrieve the influence of many parameters such as the spatial variations of the frictional properties along a simplified interface and its feedback on the deformation. Despite significant analogue and numerical studies on the evolution of accretionary prism, none of them account for heat conservation or temperature-dependent rheological transitions. Since Makran is one of the thickest accretion prisms in the world, the contribution of heat to the rheology of the prism cannot be ignored. Here, we solve for advection-diffusion of heat with imposed constant heat flow at the base of the model domain to allow the temperature to increase with burial. We start with a simple setup of one d&#233;collement layer to capture how the brittle-ductile transition affects the structures and geometry of the accretionary prism.</p><p>Our results show that a mature brittle-ductile wedge forms four different structural segments that can be distinguished based on topographic slope and deformation. An initial purely frictional segment is characterized by an imbricated zone and active in-sequence thrusts faults at the toe of the wedge. Its topographic slope is controlled by the basal friction of the d&#233;collement and is consistent with the critical taper theory prediction. The presence of the smectite-illite transition (dehydration reaction) leads to a flat topographic slope by the drop of friction. This flat segment produces little internal deformation and appears during the early stage of the accretionary prism formation. The third segment is marked by an increase of the topographic slope that begins with the onset of internal distributed viscous deformation in between brittle structures. Viscous deformation appears once the base of the model reaches 180&#176;C while the d&#233;collement remains brittle. We refer to that segment as the brittle-ductile transition where both brittle and ductile deformation co-exists within the wedge together with high internal deformation. The last segment of deformation corresponds to the onset of the ductile deformation along the d&#233;collement by reaching a temperature of 450&#176;C with an approximate flat zone without effective internal deformation. The topographic slope is again consistent with the critical taper theory, considering that a viscous d&#233;collement is equivalent to a brittle d&#233;collement of extremely low friction.</p><p>Knowing the impact of temperature transitions, we include more complexity in our simulations to increase the relevance of the models with the Makran accretionary prism. We calibrate the basal heat flow from BSR visible along seismic profiles. An intermediate d&#233;collement, essential for underthrusting to occur at the rear of the wedge, is added to the simulations. We show that the onset of underthrusting is controlled by the brittle-ductile transition. As tomographic models on land indicate packages with a higher velocity at depth, seamount subduction is another hypothesis tested. We conclude that the subduction of large seamount is accompanied by deep-rooted listric normal faults, whose location migrates through time. Seamount subduction also permits the formation of a large thrust slice zone and lateral variation of basal-erosion which can be followed in seismic profiles of Nankai and Makran subduction zones.</p>
<p>Western Makran is one of the few subduction zones left with a largely unconstrained seismogenic potential. According to the sparse GPS stations, the subduction is accumulating some strain to be released during future earthquakes. Mechanical modelling is first used to retrieve the spatial variations of the frictional properties of the megathrust, and discuss its seismogenic potential. To do so, we first build a structural map along the Iranian part of the Oman Sea and investigate three N-S seismic profiles. The profiles are characterized by a long imbricated thrust zone that takes place at the front of the wedge. A diapiric zone of shallow origin lies in between the imbricated zone and the shore. Along the eastern and western shores, active listric normal faults root down to the megathrust. Eastern and western domains have developed similar deformation, with three zones of active faulting: the normal faults on shore, thrusts ahead of the mud diapirs, and the frontal thrusts. On the contrary, no normal faults are identified along the central domain, where a seamount is entering into subduction. From mechanical modelling, we show that along the eastern and western profiles, a transition from very low to extremely low friction is required to activate the large coastal normal fault. To propagate the deformation to the front, an increase of friction along the imbricated zone is necessary. These along-dip transitions could either be related to a transition from an aseismic to seismic behavior or the brittle-viscous transition.</p><p>To decipher, we run 2-D thermo-mechanical modelling incorporating temperature evolution, with a heat flow boundary condition. Our simulations are first calibrated to reproduce the heat flow estimates based on the BSR depth. Then the effects of the illite-smectite and brittle-viscous transitions on the deformation are investigated. The decrease in heat flow landward is due to the landward deepening of the oceanic plate and thickening of sediments of the accretionary wedge. Deformation starts at the rear of the model and migrates forming in-sequence, forward verging thrust sheets. The two brittle-viscous and illite-smectite transitions affect the topographic slope and friction. A reduction of friction due to the illite-smectite transition reduces the slope by normal faulting that does not appear in the brittle-viscous transition simulations. Therefore, the presence of normal faults could permit to distinguish viscously creeping segments from segments that deform seismically. As a consequence, the normal fault is most probably related to the presence of a seismic asperity, and the difference in deformation along strike would thus reveal the existence of two different patches, one along the eastern domain and a second along the western domain. Since no large earthquake has been historically reported and given the high convergence rate, a major earthquake will strike the Makran region. We suggest that the magnitude of this event will depend on the behavior of the Central region, and the ability of the earthquake to propagate from the eastern to the western asperity or the Pakistani Makran.</p>
Abstract. The local topographic slope of the accretionary prism is often used together with the critical taper theory to determine the effective friction on subduction megathrust. In this context, extremely small topographic slopes associated with extremely low effective basal friction (μ ≤ 0.05) can be interpreted either as seismically locked portions of megathrust, which deforms episodically at dynamic slip rates or as a viscously creeping décollement. Existing mechanical models of the long-term evolution of accretionary prism, sandbox models, and numerical simulations alike, generally do not account for heat conservation nor for temperature dependant rheological transitions. Here, we solve for advection-diffusion of heat with imposed constant heat flow at the base of the model domain. This allows the temperature to increase with burial, and therefore to capture how the brittle-ductile transition and dehydration reactions within the décollement affect the dynamic of the accretionary prism and its topography. We investigate the effect of basal heat flow, shear heating, thermal blanketing by sediments, the thickness of the incoming sediments. We find that while reduction of the friction during dewatering reactions result as expected in a flat segment often in the fore-arc, the brittle-ductile transition result unexpectedly in a local increase of topographic slope. We show that this counter-intuitive backproduct of the numerical simulation can be explained and by the onset of internal ductile deformation in between the active thrusts. Our models, therefore, implies significant viscous deformation of sediments above a brittle décollement, at geological rates, and we discuss its consequences in term of interpretation of coupling ratios at subduction megathrust. We also find that, with increasing burial and ductile deformation, the internal brittle deformation tends to be accommodated by backthrusts until the basal temperature becomes sufficient to form a viscous channel, parallel to the décollement, which serves as root to a major splay fault and its back-thrust and delimits a region with small topographic slope. Morphologic resemblances of the brittle-ductile and ductile segments with fore-arc high and fore-arc basins of accretionary active margins respectively allow us to propose an alternative metamorphic origin of the fore-arc crust in this context.
