Seismic properties and densities of middle and lower crustal rocks exposed along the North China Geoscience Transect

Abstract

In the central segment of the Xiangshui-Mandal Geoscience Transect Archean basement rocks of the North China craton are well exposed. The metamorphic grade of the tilted rock units, which are suggested to represent an oblique cross-section through the middle and lower crust, shows a northwestward progressive increase, from the subgreenschist facies to low amphibolite facies Wutai terrain, through the high amphibolite facies Henshan/Fuping terrains, to the granulite facies Jining terrain. We measured compressional wave (V P) and shear wave (V S) velocities and densities at confining pressures up to 600 MPa at room temperature and at temperatures up to 600°C at 600 MPa on 12 representative rock samples from these terrains and determined the pressure and temperature derivatives of the P- and S-wave velocities and densities. The petrophysical data were correlated with chemical and mineralogical characteristics of the rocks. Based on a regional geotherm for a surface heat flow density of 40 mW/m 2, we calculated velocity-depth profiles for the various lithologies in order to provide clues for a lithological interpretation of the seismic refraction data reported by Ma et al. [1]. The two-dimensional interpretation shows four mega-layers (upper crust, middle crust, upper lower crust and lowermost crust) exhibiting different seismic characteristics. The petrophysical and geochemical studies indicate that the middle crust and upper lower crust of the North China craton are dominated by felsic and mafic rocks that are similar to the Archean amphibolite-granulite facies tonalitic-trondhjemitic-granodioritic gneisses, amphibolites and mafic granulites exposed in this area. The mafic granulites have lower velocities than usual, due to high contents of opaque minerals. The experimental and geochemical data give hints that the rocks representing the high-velocity part of the lowermost crust (V P > 7 km/s) are not exposed at the surface. The bulk lower crust is suggested to have a relatively evolved, more felsic composition compared to recent estimates for shield/platform areas.