Sedimentary and crustal velocity structure of Trans-North China Orogen from joint inversion of Rayleigh wave phase velocity and ellipticity and some implication for Syn-rift volcanism

Abstract

We present a high-resolution 3-D shear wave velocity model of the crust and uppermost mantle beneath the Trans-North China Orogen and its adjacent regions derived from a joint inversion of Rayleigh wave phase velocity and ellipticity computed from ambient noise data. The 3-D velocity model reveals large lateral variations in sedimentary and crustal structure across the study area. In addition to the large velocity contrast between the North China Basin and the Ordos Basin located in the east and west sides of the orogen, we found a northward thinning of sediments within the Weihe-Shanxi Rift System that sits in the middle of the orogen, suggesting that the rifting has likely propagated from the Weihe Rift in the south to the Shanxi Rift in the north during the Oligocene-Pliocene. While the north-south trending rift is underlaid by a low Vs across the entire range in the upper and middle crust, only the north segment, which houses the Datong volcanic complex, exhibits a substantial low velocity structure in the lower crust and the uppermost mantle. The center of the low velocity anomaly extends into the uppermost mantle at a location approximately 150 km west to the volcanic complex, which is speculated for a nearly horizontal channel in the lower crust for the upwelling of the hot materials from the asthenosphere. These observations suggest that Shanxi Rift in the north is a more active segment within the rift system, which is manifested by the Quaternary syn-rift Datong volcanic complex.