Late Cretaceous-Cenozoic exhumation of the Helanshan Mt Range, western Ordos fold-thrust belt, China: Insights from structural and apatite fission track analyses

The western Ordos fold-thrust belt is an intracontinental chain predominately formed in the Jurassic and subsequently reworked in the Late Cretaceous and Cenozoic. The structural deformation of the western Ordos fold thrust belt was driven by the force transmitted from the plate boundary of Asian collage. This study presents new structural analysis, apatite fission track and thermal modeling data in the Helanshan Mt Range of the western Ordos fold-thrust belt. The apatite fission-track analysis and thermal modeling suggest that the Helanshan Mt Range underwent two stages of cooling and exhumation since the Late Cretaceous. The first fast cooling stage in the Late Cretaceous (>90-70 Ma) led to ca. 1-1.2 km exhumation, and the second fast cooling was after ca. 30 Ma with an exhumation of ca. 1.8-2.2 km. Structural studies indicate that the Late Cretaceous exhumation was related to NW-SE compression and the exhumation of after ca. 30 Ma was associated with WNW-ESE transtension. The WNW-ESE transtension produced high angle normal faults by reworking the Mesozoic preexisting factures. Comprehensive analyses of the synchronous structures around the boundaries of the Asian collage suggest that the Late Cretaceous exhumation of the Helanshan Mt Range might be the far-field results of the convergence between the Pacific and Eurasian plates. The varying rates of the Pacific plate subduction may transmit NW-SW horizontal stress across the North China Plate. In contrast, the post-Oligocene (< 30 Ma) exhumation of the Helanshan Mt Range was induced by the Tibet plateau northeastward expansion. The produced force resulted in dextral strike-slip movements of the Helanshan Mt Range and transtension of the Yinchuan graben.