Seismic evidence of continental subduction and upper mantle deformation beneath the western Alps

Abstract:

The finding of ultra-high pressure minerals in continental orogens like the Western Alps implies that continental crust can subduct to depths as great as 100 km and then be exhumed to the Earth’s surface. The onset mechanism of continental subduction, including how the continental plate overwhelms the buoyancy resistance and how it is exhumed, however remains elusive. Using data of a new temporary seismic array deployed in the French-Italian Alps, we present here new evidence that in the Western Alps the European plate subducted deeply beneath the Adria plate. This study uses teleseismic P receiver functions and shear-wave splitting measurements from SKS phases.

 In the depth-migrated receiver function cross-section, the positive P to S (Ps) conversions (corresponding to velocity increase with depth) on the Moho interface can be continuously traced beneath the European plate. This Moho conversion fluctuates in depth, amplitude and dipping angle. Beneath the external zone, the Moho shows up strongly at depths of 25-40 km, exhibiting an eastward dip angle < 5°. Starting from beneath the outcrop of the Frontal Penninic Thrust (FPT) eastward, the dip of the European Moho strongly increases and Moho conversions can be traced to 70-80 km depth beneath the Adria plate.

Shear wave splitting measurements demonstrate that fast polarization directions of seismic anisotropy are parallel to the strike of the orogen, which is consistent with previous studies. The most prominent new result is that the delay time increases rapidly from the external zone to the internal zone and then decreases rapidly from the FPT to the westernmost Po Plain. This rapid change of delay time suggests that the mantle lithosphere, partly serpentinized, has a major contribution to the observed SKS splitting. The largest delay times in the vicinity to the west of the Frontal Penninic Thrust may suggest localized strong shear in the lithospheric mantle beneath the boundary zone between the European and Adria plates.

In summary, our receiver function cross-section is the first direct evidence of subduction of the European lower crust in the Adria mantle beneath the Western Alps; SKS splitting data demonstrate that the left-lateral shear due to the post-Miocene counterclockwise rotation of the internal zone may also be detected in the lithospheric mantle.