Fluid Migration and Widespread Remagnetization in the Dabashan Fold and Thrust Belt, China

Abstract To better understand the fluid migration in orogenic zones and associated chemical remagnetization, we have conducted a detailed magnetic, petrographic, and strontium isotope study in an important orogenic belt of China, the Jurassic Dabashan fold and thrust belt. This belt formed by the continued collision of the North and South China blocks after the Late Triassic closure of the Paleo‐Tethys Ocean. Samples were collected in a variety of rock units of Ediacaran to Permian age, in both the thrust and the fold belts. Paleomagnetic analysis indicates that all the samples were remagnetized and carry a Middle‐Late Jurassic paleo‐direction. Rock magnetic data and scanning electron microscopy observations found that the proposed remagnetization is carried by framboidal magnetite, which likely formed by the replacement of pyrite. The pervasive nature of the chemical remagnetization in these units and belts and its temporal and spatial association with the orogeny suggest that it resulted from the alteration of orogeny‐induced fluids. Sr‐isotopic analysis of the units that are thought to be remagnetized suggests that the sediments in the thrust belt were altered by externally derived evolved fluids, whereas the Permian samples in the fold belt were altered by internal pore fluid mixing during the orogeny. Together with the lithological and structural features, we conclude that the external orogenic fluids migrated preferentially along thrust faults and unconformities but were blocked by layers of low‐permeability gypsum. Our results help to constrain the origin of widespread remagnetization in South China.