Eocene onset and late Miocene acceleration of Cenozoic intracontinental extension in the North Qinling range-Weihe graben: Insights from apatite fission track thermochronology

Intracontinental extension in central China is accommodated by Cenozoic grabens flanking the Ordos Block, such as the Weihe graben, which accommodate 5-6 mm/yr of extension. To define the onset and evolution of major Cenozoic extension, we examine the cooling history of exhumed footwall rocks in the North Qjnling range adjacent to the Weihe graben. We present results from 47 new apatite fission track (AFT) samples primarily from two vertical transects of 1.5-2.5 km relief located on the northern flank of the North Qjnling footwall. AFT ages that get progressively older with increasing distance from the active, range bounding, normal fault suggest that Cenozoic uplift and southward tilting was a response to range-parallel extension. Correlations between AFT ages and both elevation and track lengths, combined with thermal modeling of representative samples, reveal that the North Qinling experienced two major stages of Cenozoic exhumation: (1) relatively slow exhumation in response to a small magnitude of extension that initiated at similar to 50 Ma; (2) relatively rapid exhumation in response to a large magnitude of accelerated extension that initiated at similar to 10 Ma. In addition, small changes in cooling rate at similar to 35 Ma and similar to 25 Ma may reflect minor changes in faulting rate. We interpret initial extensional at similar to 50 Ma to be a far-field effect of initial India-Asia continental collision. In contrast, accelerated extension after similar to 10 Ma along the Qinling is likely linked to the upward and outward growth of the Tibetan Plateau. Alternatively, lower crustal flow beneath the Qjnling may have progressively accelerated exhumation in late Miocene time. (C) 2012 Elsevier B.V. All rights reserved.