High-pressure-high-temperature seismic velocity structure of the midcrustal and lower crustal rocks of the Ivrea-Verbano zone and Serie dei Laghi, NW Italy

The Ivrea-Verbano zone and the adjacent Serie dei Laghi, in the inner are of the western Alps (NW Italy), display an upended cross section through much of the thickness of the continental crust as it existed in that region in Triassic/Jurassic time. We collected a suite of oriented rock samples that represent most of the volume of the rocks of the region. P and S wave velocity measurements were made in three orthogonal directions, related to the mesoscopic fabric of the rocks, at room temperature and up to 550 MPa confining pressure. Combined high-temperature and high-pressure measurements were made up to 700 degrees C on a subset of the samples. The lithologic units were divided into 23 different principal rock types, and we present velocity data averaged in terms of these groups. Vertical and horizontal velocity and anisotropy sections are computed from the measured velocity data based on a restored geologic section. These show that V-P and V-S both increase and anisotropy decreases systematically with depth of burial, reflecting variations in rock type and metamorphic grade with depth. V-P and V-S anisotropy arises mainly because of crystallographic preferred orientation or mineralogical banding, with the slowest direction tending to be normal to the foliation or banding. The V-P velocity sections were used to compute synthetic seismic reflection profiles for the region when it lay at the bottom of the continental crust. These correspond well with contemporary deep reflection profiles for regions of comparable tectonic evolution.