P wave velocities, anisotropy and hysteresis in ultrahigh‐pressure metamorphic rocks as a function of confining pressure

The Chinese Continental Drilling Project (CCSD) has drilled to a depth of 5100 m at Maobei (N34.40, E118.67), Donghai County, Jiangsu Province in the eastern segment of the Dabie‐Sulu ultrahigh pressure (UHP) metamorphic terrane. The borehole, which penetrated through all of the high velocity layers and seismic reflectors observed within the uppermost crust on seismic refraction and reflection profiles, reveals the main lithologies to be coesite‐bearing felsic gneisses, metabasic rocks (i.e., amphibolite, retrogressed, and non‐retrogressed eclogites) and ultramafic rocks (i.e., garnet peridotite and serpentinite). P wave velocities, anisotropy, and hysteresis of 31 typical CCSD core samples and 35 representative surface samples collected from the Sulu UHP belt were measured at hydrostatic confining pressures up to 800 MPa. The velocity‐pressure curves can be well described by a four‐parameter exponential equation derived from theory: V ( P ) = V 0 + DP − B 0 exp(− kP ), where V 0 is the projected velocity at zero pressure if pores/cracks were absent; D is the intrinsic pressure derivative of velocity in the linear elastic regime; B 0 is the initial velocity drop caused by the presence of pores/cracks at zero pressure; and k is the decay constant of the velocity drop in the nonlinear poro‐elastic regime. The seismic hysteresis is caused by irreversible changes in grain contact, increases in microcrack aspect ratios and reduction of void space during the pressurization‐depressurization cycle. The statistical properties of P wave velocities in the UHP rocks provide an important set of basic information for the interpretation of field seismic data from the root zones of continental convergent orogenic belts and modern and ancient subduction zones.