Shear wave properties and Poisson's ratios of ultrahigh‐pressure metamorphic rocks from the Dabie‐Sulu orogenic belt, China: Implications for crustal composition

Abstract

Shear wave velocities (Vs), anisotropy, and shear wave splitting have been measured at pressures up to 600–800 MPa for ultrahigh‐pressure (UHP) metamorphic rocks from the Dabie‐Sulu orogenic belt, China, with a focus on three types of eclogites. Type 1 eclogites are coarse‐grained, unaltered samples showing high densities and high Vs values (4.85 ± 0.06 km/s at 600 MPa); type 2 eclogites are fine‐grained, sheared samples with intermediate Vs values (4.53 ± 0.04 km/s at 600 MPa); and type 3 eclogites are overprinted by amphibolite facies metamorphism and display low Vs values (4.33 ± 0.09 km/s at 600 MPa). The compositional layering and retrograde metamorphism can result in significant anisotropy and shear wave splitting in eclogites, suggesting their plausible contribution to seismic anisotropy in the lower crust, upper mantle, and particularly in subducted slabs. Integrating our P and S wave velocity results with reliable data from previous studies, we estimated the pressure and temperature derivatives of Vp, Vs, and Poisson's ratios for common rock types in the UHP metamorphic belt. The geometric means were used as a mixture rule to invert the lithological and chemical compositions of the layered crust from seismic refraction velocities. The inferred crustal composition suggests that the eclogite‐bearing UHP rocks are tectonic slices of crust that have been thrust along a series of shear zones during the continental collision between the north China and Yangtze cratons, over a normal UHP‐free middle lower crust with overall intermediate composition.