Composition of the crust beneath southeastern China derived from an integrated geophysical data set

摘要

The nonuniqueness in determining the composition of the crust can be reduced by comparing the data for P-wave velocity (Vp), S-wave velocity (Vs), Vp/Vs ratio, mass density (ρ) and Lamé impedances (ρλ, ρμ) to data determined from laboratory measurements of these physical variables on a variety of crustal rock samples. The composition of the crust along the Tunxi-Wenzhou transect, southeastern China, is presented as a model based on a complete set of geophysical data involving seismic P-wave and S-wave velocity, density, gravity, heat flow, and temperature surveys, which allow us to place tighter constraints on possible crustal models. The Yangtze and the Cathaysia blocks have a crustal structure characterized by remarkable differences between the upper, middle, and lower crust. Integrated geophysics data are here interpreted to indicate: (1) an average composition of granite gneiss for the upper crust, with presence of mica quartz schist, felsic granulite, paragranulite, and granite-granodiorite beneath the Yangtze block, and basalt down to the upper part of the middle crust just over the Jiangshan-Shaoxing fault, followed laterally by granite gneiss beneath the Songyang, Qintiang, and Wenping depressions; (2) granite-granodiorite and biotite gneiss for the upper half of the middle crust, with presence of mica quartz schist beneath the Cathaysia block; (3) gabbro and in less proportion basalt for the lower part of the middle crust; and (4) amphibolite for the lower crust, with presence of mafic garnet granulite just over the Jiangshan-Shaoxing fault and beneath the Wenping depression.