Jurassic remagnetization in the northwestern margin of the Yangtze Block and its tectonic implications

The study of remagnetization in orogenic zones is conducive to determine the timing of their formation, since the orogenesis is usually accompanied by ubiquitous remagnetization. To constrain the age of the thrust framework of the northwestern margin of the Yangtze Block, an integrated paleomagnetic, rock magnetic and petrographic study has been carried out on the Cryogenian-Cambrian strata, including the Xixiang area to the northwest of the Dabashan thrust belt and the Ningqiang-Guangyuan area in the northern part of the Longmenshan thrust belt. Extensive Jurassic remagnetization is revealed. Demagnetization characteristics, rock magnetic results and scanning electron microscopy observations of these rock units suggested a chemical remagnetization carried by authigenic magnetite and hematite, whose growth was related to fluid migration resulting from the Jurassic orogenesis. Together with the published Jurassic remagnetization data of the studied area, the consistent Jurassic remagnetization direction was discovered from the Dabashan thrust belt and the Xixiang-Ningqiang area and this implied that the thrust framework formed no later than Jurassic. Whereas, the remagnetization component of the Guangyuan area of the northern part of the Longmenshan thrust belt anticlockwise rotated similar to 60 degrees relatively and this suggested that the thrust framework formed after Jurassic. But the area close to the Sichuan basin in the Guangyuan area east to the Guanxian-Anxian fault was not affected, with no obvious rotation being observed.