Bidirectional growth of the Altyn Tagh Fault since the Early Oligocene

Abstract

The Cenozoic evolution of the Altyn Tagh Fault (ATF) is still under debate. Here, we present new paleomagnetic rotation evidence on curvatures south of the ATF in the northeastern Tibetan Plateau to shed new light on this issue. A total of 498 core samples representing 53 sites and 6 time intervals (a-f) were collected from the post-Oligocene sequence at the Heishiqiu locality that in the junction of the northwestern and middle of the reverse S-type northern Qaidam marginal thrust belt. Rock magnetic analysis indicated that the main magnetic carriers were hematite and magnetite. After detailed thermal demagnetization analyses, the characteristic remanent magnetizations of 48 sites (387 samples), which exhibited dual polarities and passed a regional fold test, are most likely the primary magnetizations. Two periods of remarkable rotations are identified at Heishiqiu: a clockwise rotation of ~16.0 ± 10.3° during ~27–17 Ma and a subsequent counterclockwise rotation of ~24.3 ± 14.5° during ~17–10 Ma. The rotations were attributed to two periods of strike-slip faulting along the ATF during the Early Oligocene to Late Miocene, yielding at least ~290–380 km of offset at Heishiqiu. The integrated lines of paleomagnetic and geological evidence suggest that the deformation along the ATF has propagated both southwestward and northeastward from Heishiqiu in the middle ATF since the Early Oligocene. We propose a bidirectional growth model for the ATF that might be applicable to the evolution of the Tibetan Plateau.