Crustal structure in Dabieshan UHP metamorphic belt and its tectonic implication

Abstract

The model of Dabieshan crustal structure has been obtained on the basis of the deep seismic sounding data in this area. The 2-D crustal structure shows the feature of the collision orogens and provides some deep geophysical evidences of the ultra-high pressure (UHP) metamorphic belt. The 3-D upper-crustal velocity structure reveals that the velocity distribution at 2 km deep obviously relates to the surface geological setting and the UHP metamorphic belt has the higher velocity at 5–10 km deep. The observed data of Bouguer gravity anomalies reveal a larger range of negative anomalies in Dabieshan area while the positive anomalies in the UHP metamorphic belt is calculated from the 3-D upper-crustal velocity structure. The 2-D crustal model along the seismic profile shows that the “root” beneath the orogen is only 4–5 km thick and the velocity in the uppermost mantle changes a little in the lateral direction. The inconsistency between the observed and calculated Bouguer gravity anomalies mainly results from the crust, and at least the middle-upper crust should yield the negative anomalies. The material density of the crust in the UHP metamorphic belt should be lower than that in the surrounding areas. This material with lower density relates to the collision processes in which Yangtze crust subducted northward to 100 km deep and then returned to the crust.