Crustal structure in Dabieshan UHP metamorphic belt and its tectonic implication

The model of Dabieshan crustal structure has been obtained on the basis of the deep seismic sounding data in thisarea. The 2-D crustal structure shows the feature of the collision orogens and provides some deep geophysicalevidences of the ultra-high pressure (UHP) metamorphic belt. The 3-D upper-crustal velocity struCture reveals thatthe velocity distribution at 2 km deep obviously relates to the surface geological setting and the UHP metarnorphicbelt has the higher velocity at 5~10 km deep. The observed data of Bouguer gravity anomalies reveal a largerrange 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 thatthe "root" beneath the orogen is only 4-5 km thick and the velocity in the uppermost mantle changes a little in thelateral direction. The inconsistency between the observed and calculated Bouguer gravity anomalies mainly resultsfrom the crust, and at least the middle-upper crust should yield the negative anomalies. The material density of thecrust in the UHP metamorphic belt should be lower than that in the surrounding areas. This material with lowerdensity relates to the collision processes in which Yangtze crust subducted nor'thward to 100 km deep and thenreturned to the crust.