Moho-mapping in the Dabie ultrahigh-pressure collisional orogen, central China

Abstract

Eight wide angle reflection/refraction seismic profiles (with a total length of ∼1700 km) and near-vertical reflection seismic profiles (with a total length of ∼340 km) across the Dabie Orogen, eastern China have been completed in recent years, and allow the Moho depth and uppermost-mantle velocity to be mapped across ∼40,000 km². These data reveal the detailed structure of the crust and upper mantle of this orogen that resulted from collision of the North China craton (NCC) with the Yangtze Craton (YC). The Dabie Orogen is an asymmetric orogen, with a thin crustal root (∼6 km) preserved in its northern part, highlighted by a north-dipping Moho. Compared to the nearly seismically-transparent lower crust, the Moho crust-mantle transition zone under the Dabie Orogen is a prominent north-dipping, strongly layered reflection, which is inferred to reflect the remnants of subduction of YC under the edge of the NCC or post-collisional exhumation of ultrahigh-pressure (UHP) rocks, and indicates the northward polarity of subduction of the YC. The Moho of the YC and NCC meet beneath the northern part of the Dabie Mountains, and the NCC Moho (north) is uplifted by ∼4 to 5 km (Offset 1) over the YC Moho (south), marking the NCC-YC collision zone. Along the southern margin of the Dabie Mountains, the Moho below the Yangtze foreland is underthrust beneath the Moho of the Dabie Mountains, forming another Moho overlap (Offset 2) with a depth difference of 5 to 6 km. The two Moho overlaps (offsets) at both flanks of the Dabie Mountains formed two crustal-scale boundaries at depth between the NCC and the Dabie Orogen, and the Dabie Orogen and the YC respectively. Offset 1 is considered to be the Triassic suture between the YC and NCC as supported by the metamorphic ages of UHP rocks. This displays a wedge-shaped offset zone opening towards the east that acted as a channel for exhumation of UHP rocks. Offset 2 under the southern margin of the Dabie Orogen is connected to the Xiangfan-Guangji fault that gave rise to a large-scale thrust detachment that propagated towards the foreland of the Dabie Orogen during the Jurassic. Thus our Moho mapping confirms that there are structural remnants of the Triassic deep continental subduction preserved, despite the superimposed Jurassic deformation.