The question of lateral and/or vertical continuity of subducted slabs in active orogens is a hot topic partly due to poorly resolved tomographic data. The complex slab structure beneath the Alpine region is only partly resolved by available geophysical data, leaving many geological and geodynamical issues widely open. Based upon a finite‐frequency kernel method, we present a new high‐resolution tomography model using P wave data from 527 broadband seismic stations, both from permanent networks and temporary experiments. This model provides an improved image of the slab structure in the Alpine region and fundamental pinpoints for the analysis of Cenozoic magmatism, (U)HP metamorphism, and Alpine topography. Our results document the lateral continuity of the European slab from the Western Alps to the central Alps, and the downdip slab continuity beneath the central Alps, ruling out the hypothesis of slab break off to explain Cenozoic Alpine magmatism. A low‐velocity anomaly is observed in the upper mantle beneath the core of the Western Alps, pointing to dynamic topography effects. A NE dipping Adriatic slab, consistent with Dinaric subduction, is possibly observed beneath the Eastern Alps, whereas the laterally continuous Adriatic slab of the Northern Apennines shows major gaps at the boundary with the Southern Apennines and becomes near vertical in the Alps‐Apennines transition zone. Tear faults accommodating opposite‐dipping subductions during Alpine convergence may represent reactivated lithospheric faults inherited from Tethyan extension. Our results suggest that the interpretations of previous tomography results that include successive slab break offs along the Alpine‐Zagros‐Himalaya orogenic belt might be proficiently reconsidered.

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The first discovery of ultrahigh-pressure coesite in the European Alps 30 years ago led to the inference that a positively buoyant continental crust can be subducted to mantle depth; this had been considered impossible since the advent of the plate tectonics concepts. Although continental subduction is now widely accepted, there remains debate because there is little direct (geophysical) evidence of a link between exhumed coesite at the surface and subducted continental crust at depth. Here we provide the first seismic evidence for continental crust at 75 km depth that is clearly connected with the European crust exactly along the transect where coesite was found at the surface. Our data also provide evidence for a thick suture zone with downward-decreasing seismic velocities, demonstrating that the European lower crust underthrusts the Adriatic mantle. These findings, from one of the best-preserved and long-studied ultrahigh-pressure orogens worldwide, shed decisive new light on geodynamic processes along convergent continental margins.*

Lithos publishes original research papers on the petrology, geochemistry and petrogenesis of igneous and metamorphic rocks. Papers on mineralogy/mineral physics related to petrology and petrogenetic problems are also welcomed. Types of papers published include: regular articles, letters, reviews (normally invited by the editors), comments/replies, book reviews and (very occasionally) synthesis papers. Please click here for more information on Letters and Synthesis AUDIENCE. Mineralogists, petrologists and geochemists.

Direct absolute dating of the Penninic Frontal Thrust tectonic motion is achieved using the 40Ar/39Ar technique in the Pelvoux Crystalline Massif (Western Alps). The dated phengites were formed syn‐kinematically in shear zones. They underline the brittle‐ductile stretching lineation, pressure‐shadow fibres and slickensides consistent with underthrusting of the European continental slab below the propagating Penninic Thrust. Chlorite–phengite thermobarometry yields 10–15 km and T∼280 °C, while 40Ar/39Ar phengite ages mainly range between 34 and 30 Ma, with one younger age at 27 Ma. This Early Oligocene age range matches a major tectonic rearrangement of the Alpine chain. Preservation of prograde 40Ar/39Ar ages is ascribed to passive exhumation of the Pelvoux shear zone network, sandwiched between more external thrusts and the Penninic Front reactivated as an E‐dipping detachment fault. Partial resetting in the Low Temperature part of argon spectra below 24 Ma is ascribed to brittle deformation and alteration of phengites.

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Abstract. We tackle the numerical simulation of reaction-diffusion equations modeling multiscale reaction waves. This type of problems induces peculiar difficulties and potentially large stiffness which stem from the broad spectrum of temporal scales in the nonlinear chemical source term as well as from the presence of steep spatial gradients in the reaction fronts, spatially very localized. In this paper, we introduce a new resolution strategy based on time operator splitting and space adaptive multiresolution in the context of very localized and stiff reaction fronts. It considers a high order implicit time integration of the reaction and an explicit one for the diffusion term in order to build a time operator splitting scheme that exploits efficiently the special features of each problem. Based on recent theoretical studies of numerical analysis such a strategy leads to a splitting time step which is not restricted neither by the fastest scales in the source term nor by stability constraints of the diffusive steps, but only by the physics of the phenomenon. We aim thus at solving complete models including all time and space scales within a prescribed accuracy, considering large simulation domains with conventional computing resources. The efficiency is evaluated through the numerical simulation of configurations which were so far, out of reach of standard methods in the field of nonlinear chemical dynamics for 2D spiral waves and 3D scroll waves as an illustration. Future extensions of the proposed strategy to more complex configurations involving other physical phenomena as well as optimization capability on new computer architectures are finally discussed. Key words.Reaction-diffusion equations, multi-scale reaction waves, operator splitting, adaptive multiresolution AMS subject classifications. 33K57, 35A18, 65M50, 65M081. Introduction. Numerical simulations of multi-scale phenomena are commonly used for modeling purposes in many applications such as combustion, chemical vapor deposition, or air pollution modeling. In general, all these models raise several difficulties created by the high number of unknowns, the wide range of temporal scales due to large and detailed chemical kinetic mechanisms, as well as steep spatial gradients associated with very localized fronts of high chemical activity. Furthermore, a natural stumbling block to perform 3D simulations with all scales resolution is either the unreasonably small time step due to stability requirements or the unreasonable memory requirements for implicit methods. In this context, one can consider various numerical strategies in order to treat the induced stiffness for time dependent * This research was supported by a fundamental project grant from ANR (French National Research Agency -ANR Blancs) Séchelles (project leader S. Descombes -2009Descombes - -2013, by a CNRS PEPS Maths-ST2I project MIPAC (project leader V. Louvet -2009Louvet - -2010, and by a DIGITEO RTRA project MUSE (project leader M. Massot -2010Massot - -2014 problems. The most natural id...

Active and fossil mantle flows in the western Alpine region unravelled by seismic anisotropy analysis and highresolution P wave tomography.