Exhumation of the Dabie UHP Terrane, China

摘要

Exhumation of deeply buried ultrahigh-pressure (UHP) terranes remains a puzzle in understanding lithospheric dynamic process. New evidence that constrains the exhumation process of the Dabie UHP terrane includes the following. (1) Structural data record the architecture of the Dabie UHP terrane. Three slices are recognized -- i.e., from top to bottom, the Susong blueschisl slice, the Huangzhen-Huangweihe (HH) HP slice, and the Jinheqiao-Shuanghe-Bixiling (JSB) UHP slice. These slices have been stacked and domed. (2) Geophysical data depict two offsets in the Moho. Offset-I separates the Dabie UHP terrane from the North China craton, and has served as a subduc-tion channel, as well as an exhumation channel. Offset-II, separating the Dabie UHP terrane from the foreland belt, developed due to geodynamic regime reorganization. (3) Geochronological data reveal that the HP slice was exhumed earlier that the UHP slice. A northward-younging exhumation polarity is implied, with younger exhumation age closer to the exhumation channel. These pieces of evidence point to a multistage exhumation process. The Yangtze craton collided with and subducted beneath the North China craton at the very beginning of Triassic time. By 230 Ma, the Susong blueschist slice and the southern end of the HH HP slice had arrived at a middle-crustal level, while its northern part was still at mantle depth. Such processes continued until the JSB UHP slice was exhumed to mid-crustal levels at 210 Ma. After geodynamic regime reorganization during 210-180 Ma, continuing compression of the Yangtze craton and underthrusting along offset-II in the Moho caused the dome structure, and eventually brought the UHP rocks to the Earth's surface.