Evidence for a crustal root beneath the Paleoproterozoic collision zone in the northern Ordos block, North China

Recent studies have shown that Paleoproterozoic rocks, called the Khondalite Belt, crop out along a nearly EW-trending zone in the northern edge of the Ordos block, North China. This widely distributed Khondalite rock suggests that they originated in the margin of a passive continent during the Paleoproterozoic Era. Seismic stations deployed in the northern Ordos block and its adjacent regions provided abundant information on crustal thicknesses and Vp/Vs ratios, which were determined by use of receiver function H-κ stacking. The H-κ stacked images show an approximately EW-trending anomalous zone with thickened crust (∼46.5 km) and a higher Poisson's ratio (0.28) beneath the Khondalite Belt in the Paleoproterozoic collision zone. Based on the long-wavelength information of gravity anomalies, a two-dimensional crustal density model indicated that a crustal root with a density contrast of 130 kg/m3 between the root and surrounding mantle is required for the isostatic gravity compensation. The ratio of surface elevation to crustal root thickness was 0.02, suggesting that the exhumation of the crustal root in response to surface erosion continued to occur over time scales of hundreds of millions of years. This crustal thickening model is evidence for the crustal root beneath the Paleoproterozoic collision zone in the northern Ordos block. As the density contrast becomes smaller, the amount of isostatic rebound in response to erosion also decreases, and therefore the crustal root is maintained.