Abstract

We compared the averages of P-wave velocities (Vp) for four lithologies (felsic gneiss, eclogite, retrograde eclogite, and amphibolite) collected from core material sampled by the Chinese Continental Scientific Drilling Project (CCSD) at depths to 5158 m, and from surface outcrops in the Dabie-Sulu (China) ultrahigh-pressure metamorphic terrane. The velocities were measured at pressures to 800 MPa for 69 CCSD samples and 145 surface samples. The Vp-pressure relationship is described by a four-parameter exponential equation, allowing the determination of the crack-closing pressure (Pc), the intrinsic pressure derivative of velocity (D), the intrinsic velocity of the crack-free compacted rock at zero pressure (V0), and the maximum velocity increase due to crack closure (B0). The shape of the nonlinear segment of the velocity-pressure curve is controlled by a decay parameter (k). The CCSD core samples are characterized by significantly higher V0, B0, and k values and lower D and Pc compared to their surface analogues. This suggests that extrapolation of the surface rock data to depth may result in substantial underestimates in seismic velocities, and that the intrinsic pressure derivatives obtained from the Vp-pressure relations for the core samples are more suitable for the determination of in situ velocities at great depth than those derived from measurements on rocks exposed at the surface.

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