Continental thickening can be associated at depth with strain localization into anas- tomosing shear zones. These shear zones, formed during metamorphism, are often obliterated by later thermal and tectonic events possibly related with the exhumation of mountain roots under retrograde conditions. Anastomosing shear zones exposed in the lower crust of the Kohistan arc, in Pakistan, are rather exceptional because they have parageneses indicating increasing pressure during progressive deformation. We investigated their seismic properties on rock samples with constant bulk chem- istry. The compressional wave velocity was experimentally determined at high con- fining pressure and temperature. We observed that density, average Vp and acoustic impedance increased from the protoliths through a gradient zone to the mylonitic rock, whilst seismic anisotropy reached a maximum in the gradient zone. These findings will be presented and discussed in terms of reflectivity of the lower crust in collision zones.
