A crustal model of the ultrahigh-pressure Dabie Shan orogenic belt, China,derived from deep seismic refraction profiling

We present a new crustal cross section through the east-west trending ultra high-pressure (UHP) Dabie Shan orogenic belt, east central China, based on a 400-km-long seismic refraction profile. Data from our profile reveal that the cratonal blocks north and south of the orogen are composed of 35-km-th ick crust consisting of three layers (upper, middle, and lower crust) with average seismic velocities of 6.0 +/- 0.2 km/s, 6.5 +/- 0.1 km/s, and 6.8 /- 0.1 km/s. The crust reaches a maximum thickness of 41.5 km beneath the n orthern margin of the orogen, and thus the present-day root beneath the oro gen is only 6.5 km thick. The upper mantle velocity is 8.0 +/- 0.1 km/s. Mo deling of shear wave data indicate that Poisson's ratio increases from 0.24 +/- 0.02 in the upper crust to 0.27 +/- 0.03 in the lower crust. This resu lt is consistent with a dominantly felsic upper crustal composition and a m afic lower crustal composition within the amphibolite or granulite metamorp hic facies. Our seismic model indicates that eclogite, which is abundant in surface exposures within the orogen, is not a volumetrically significant c omponent in the middle or lower crust. Much of the Triassic structure assoc iated with the formation of the UHP rocks of the Dabie Shan has been obscur ed by post-Triassic igneous activity, extension and large-offset strike-sli p faulting. Nevertheless, we can identify a high-velocity (6.3 km/s) zone i n the upper (<5 km depth) crustal core of the orogen which we interpret as a zone of ultrahigh-pressure rocks, a north dipping suture, and an apparent Moho offset that marks a likely active strike-slip fault.