APATITE (U-TH)/HE CONSTRAINTS ON THE CENOZOIC THERMAL HISTORY OF THE DABIE SHAN, EASTERN CHINA

The Dabie Shan lies at the eastern end of the ~2000km WNW-ENE trending Qinling-Dabie orogen that marks the collisional suture between the Yangtze and North China cratons (Hacker et al., 1996). The eastern margin of the Qinling-Dabie orogen is the NNE-SSW trending Tan-Lu fault, a prominent structure that has been assigned significant roles in Asian tectonic development. Few constraints exist on its Cenozoic motion, particularly to the south. Hsiao et al., (2004) documented Cenozoic strike-slip motion along the northern Tan-Lu with little or no dip-slip component. In contrast, evidence for Cenozoic dip-slip motion on the southern Tan-Lu comes from 1) the formation of the late Cretaceous-Eocene Qianshan basin east of the Dabie, and 2) apatite fission track temperature-time path modeling indicating relatively rapid cooling in the southeastern Dabie at 45 ± 10 Ma (Grimmer et al., 2002). New apatite (U-Th)/He ages from the eastern Dabie Shan record the late Cretaceous to Cenozoic thermal history of the range and offer insight into the timing and degree of normal displacement of the southern Tan-Lu fault. An age-elevation thermochronologic transect in the northeastern Dabie Shan suggests very slow exhumation since the Cretaceous: apatite (U-Th)/He ages decrease linearly from 124Ma to 60Ma over 400m of elevation change, suggesting a very slow uplift rate of ~0.007 km/my. The distribution of ages is consistent with rapid cooling in the Cretaceous following emplacement of the Northern Orthogneiss Unit and subsequent quiescence. Apatite fission track and (U-Th)/He cooling ages decrease towards the south (Grimmer et al., 2002) and west (Reiners et al., 2003) in the Dabie Shan, suggesting differential uplift across the range that may not necessarily be related to the Tan-Lu fault. This observation is consistent with Cenozoic Tan-Lu strain measurements farther to the north, but it does not help explain the formation of the Qianshan basin.