Elastic anisotropies of rocks in a subduction and exhumation setting

Abstract

Subduction and exhumation are key processes in the formation of orogenic systems across the world, for example, in the European Alps. For geophysical investigations of these orogens, it is essential to understand the petrophysical properties of the rocks involved. These are the result of a complex interaction of mineral composition and rock fabric including mineral textures (i.e., crystallographic preferred orientations). In this study we present texture-derived elastic anisotropy data for a representative set of different lithologies involved in the Alpine orogeny. Rock samples were collected in the Lago di Cignana area in Valtournenche, in the Italian northwestern Alps. At this locality a wide range of units of continental and oceanic origin with varying paleogeographic affiliations and tectono-metamorphic histories are accessible. Their mineral textures were determined by time-of-flight neutron diffraction. From these data the elastic properties of the samples were calculated. The data set includes representative lithologies from a subduction-exhumation setting. In subducted lithologies originating from the oceanic crust, the P-wave anisotropies (AVPs [%]) range from 1.4g% to 3.7g% with average P-wave velocities of 7.20-8.24gkm/s and VPg/gVS ratios of 1.70-1.75. In the metasediments of the former accretionary prism the AVPs range from 3.7g% to 7.1g%, average P-wave velocities are 6.66-7.23gkm/s and VPg/gVS ratios are 1.61-1.76. Continental crust which is incorporated in the collisional orogen shows AVP ranging from 1.4g% to 2.1g% with average P-wave velocities of 6.52-6.62gkm/s and VPg/gVS ratios of 1.56-1.60. Our results suggest that mafic and felsic rocks in subduction zones at depth may be discriminated by a combination of seismic signatures: lower anisotropy and higher VPg/gVS ratio for mafic rocks, and higher anisotropy and lower VPg/gVS ratio for felsic rocks and metasediments.