Abstract. In subduction zones the geotherm is thought to vary as a function of the subduction rate and the age of the subducting lithosphere. Along a single subduction zone the rate of subduction can strongly vary due to changes in the angle between the trench and the plate convergence vector, namely the subduction obliquity. We currently observe such a configuration all around the Pacific (e.g. Marianna, Chile, Aleutians). Subduction obliquity is also supposed in the geological record of Western and Central Turkey. In order to investigate this effect, we designed and computed simple thermo-kinematic finite element 3D 5 numerical models. We prescribe the trench geometry by means of a simple mathematical function and compute the mantle flow in the mantle wedge only by solving the equation of mass and momentum conservation. We then solve the energy conservation equation until steady-state is reached. We analyse the results (i) in terms of mantle wedge flow with emphasis on the trenchparallel component and (ii) in terms of temperature along the plate interface by means of maps and depths-temperature path at the interface. We show that the effect of the trench curvature on the geotherm is substantial. A small obliquity yields a small but 10 not negligible trench parallel mantle flow leading to differences of 50• C along strike of the model. With increasing obliquity, the trench parallel component of the velocity consequently increases and the temperature variation can be as important as 200• C along strike. This can even be larger with varying plate velocity. Finally, we discuss the implication of our simulations for the ubiquitous oblique systems that are observed on Earth, the limitation of our modeling approach and the significance for the geological record with an emphasis on the case study of Western and Central Turkey.
<div> <div> <div> <div> <p>The objective of the present study is to use potential lithologic analogues sampled in the European crust units exhumed in the Alps to predict the seismic properties of the buried continental crust panel. To this end, from the chemical compositions of representative rock samples, we calculate seismic velocities (Vp, Vs or Vp/Vs) at any P and T, under the assumption that the rocks have completely re-equilibrated during burial.</p> <p>The sample catalog comprehend (1) the mafic intercalations, present in the Variscan basement series of the External Crystalline Massif; (2) the rocks involved in the Grand Paradis - Schistes Lustr&#233;s contact (metabasites and garnet bearing micaschists of the upper unit, mylonite and gneiss of the lower unit); (3) those along the Lanzo-Canavese contact (serpentinites, blue schist facies mylonites and biotite bearing gneiss); (4) lithologies of the Ivrea domain (peridotites, garnet bearing gabbros, textured mafic rocks, amphibolitic and mylonitic paragneiss), (5) those from the Gruf massif (biotite bearing orthogneiss, deformed leucogranites and charnockites from the Gruf complex and amphibolites and serpentinites from the Chiavenna unit); (6) lithologies from Alpine Corsica (pelitic gneisses of the granulite facies and more or less foliated metagabbros, from the San Petrone and Farinole unit).</p> <p>In these diagrams, the main seismic contrasts appear to correspond to the early stages of jadeite crystallization (mainly in the Vp/Vs diagram), as well as to the boundaries of the garnet and clinopyroxene stability fields. Considering the selected rocks as relevant analogues, we then compare the evolution of seismic properties along the top of the Alpine dipping panel with profiles inferred from recent Vp and Vs tomography models (CIFALPS 1 and AlpARRAY), varying the effective thermal profile of the Alpine panel, its reaction degree and overall chemistry. Preliminary results suggest that the lower crust of the plunging panel has a seismic velocity too low to be eclogitized. Its velocity rates are closer to those of an underreacted quartzo-felspathic gneiss. At first sight, observed velocities are too low compared to values predicted for any lithology fully reacted during subduction. The best-fitting scenario turns out to be that of a lower crust thermally relaxed in the variscan without significant mineralogical footprint of subduction. If detected, the velocity rise due to eclogitization might offset of several tenth along the slab, implying a sensible impact of reaction kinetics.</p> </div> </div> </div> </div>
<p>Fragments of subducted slow-spreading oceanic lithosphere are exposed continuously in the Liguro-Piemont domain of the Western Alps. By combining new and literature petrological data, interpolated maps of maximum temperatures, maximum Si contents of phengite as a proxy for peak pressure and thermodynamic modelling, we provide a detailed framework of the peak metamorphic conditions experienced by the distinct subduction slices. High-resolution mapping confirms the marked eastward increase in metamorphic grade throughout the domain, as well as within some slices. The compilation of lithostratigraphic, structural and radiochronological data and the estimation of sediment/mafic-ultramafic ratio for each slice allow refining the origin of these tectonometamorphic units within the former oceanic domain. The refined structural sketchmap allows to restore the geometries of the Alpine subduction at peak burial conditions. Results point to a trimodal distribution of the units with an increase in metamorphic conditions from the Upper (LPU; 320-400&#176;C- 1.2-1.9 GPa) to the Middle (LPM; 415-475&#176;C- 1.7-2.2 GPa) and to the Lower units (LPL; 500-580&#176;C- 2.2-2.8 GPa). The blueschist-facies LPU and LPM units are dominated by sediments (>90%), whereas the eclogitic LPL units are far richer in mafic-ultramafic rocks (>40%). These characteristics, along with lithostratigraphic differences, reflect major differences in their initial paleogeography and/or in the mechanisms responsible for material offscraping from the downgoing slab. The peak burial depths of the LPU, LPM and LPL units are similar to those inferred for slicing and underplating in both modern and fossil subduction zones. Petrological and lithostratigraphic data suggest that the offscraping of the LPU and LPM units was mostly controlled by lithological contrasts, within pelagic shales or along contacts with the serpentinized mantle. In contrast, major dehydration reactions (such as lawsonite breakdown in sediments) likely controlled the offscraping of the LPL units at eclogite-facies conditions, possibly through high fluid pressure conditions and rocks embrittlement.</p>
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