Distributed deformation along the subduction plate interface: The role of tectonic mélanges

International audienceRecent geophysical monitoring of subduction zones has unraveled a complete spectrum of plate coupling behaviors, from coupled portions rupturing during earthquakes to decoupled portions slipping aseismically. However, the deformation mechanisms and the exhumed rock corresponding to these contrasted behaviors are not yet identified. Tectonic mélange zones are thought to play a major role in the deformation of the plate interface as they represent remnants of the subducted plate scraped off by the overriding plate. In this work we examine several tectonic mélange zones (Hyuga, Okitsu, Mugi) from the Shimanto Belt, an accretionary prism in southwest Japan connecting to the active Nankai subduction zone. These tectonic mélange zones have a block-in-matrix structure, with lenses of sandstones and basalts within a metapelitic matrix, and their deformation is distributed over zones of hundreds of meters in thickness. In addition, the examples of mélange considered here are bounded by sharp faults, some of them bearing pseudotachylyte layers, so that distributed deformation within the mélange and localized deformation on its boundary are juxtaposed. Distributed deformation involves the development of a foliation, as well as of a pervasive network of macroscopic and microscopic shear zones. Along with slip on this network, strain proceeds by fracturing and precipitation of quartz, in long veins parallel to the foliation or in smaller cracks perpendicular to stretching and forming in the neck of competent lenses of sandstones or former quartz veins. The analysis of shear band kinematics shows in all three examples a dominant, top-to-the-trench sense of shear, consistent with deformation along the plate boundary during subduction. Moreover, most shear zones, when foliation is restored back to syn-subduction position, are extensional structures. Finally, the geometry and kinematics of the mélange-bounding faults, as well as radiometric constraints, show that in most cases the faults (=localized structures) were formed during a later stage than mélange internal deformation. These findings bear several consequences on the structure and dynamics of the subduction plate boundary at seismogenic depths. First, there is no support for a model of plate boundary fault zone composed simultaneously of localized slip zones and domains of more distributed deformation. Second, rather than proposed models of underplating, where all deformation is localized into the thrusts bounding the tectonic sheets, we suggest that underplating was to a large extent accommodated by distributed deformation within the mélange sheets. This underplating model accounts for (i) the large amount of strain within the mélange, (ii) the absence of contractional structures during underplating, (iii) thinning of the mélange required by the network of extensional shear bands and stretched boudins. Third, mélanges appear as likely candidates for portions of the plate interface deforming by aseismic slip. The seismic vs. aseismic character of the plate interface might depend on the ability of sediments on top of the subducting plate to undergo distributed strain, which in turn depends on the efficiency of pressure solution to operate