Conduit or Barrier? Insights From Rock Magnetic Properties of the Red River Fault, China

Fault zones, with their dense networks of fractures and microfractures, often serve as primary pathways for fluid migration. However, their role as conduits or barriers depends on their development stages. Previous studies on the fluid transport properties of fault zones have primarily relied on permeability measurements, microstructural analyses and numerical simulations. In this study, magnetic measurements were conducted on the fault rocks from the Middle Valley Fault (MVF) of the Red River Fault (RRF) system, supplemented by scanning electron microscope observations. Results show that the protoliths are dominated by paramagnetic pyrite, while weakly paramagnetic siderite and ferrimagnetic magnetite are unevenly distributed across the fault zone. Along with the widespread presence of barite, this suggests that hydrothermal fluids have circulated within the fault zone. The healing effects of these fluids, coupled with the enrichment of clay minerals, significantly reduce the permeability of the fault gouge. Consequently, the ∼90-cm-thick fault gouge zone acts as a barrier, inhibiting fluid migration across the fault zone. Fluid-rock interactions varied across fault compartments, producing diverse magnetic assemblages in the fault rocks. Moreover, integrating outcrop-scale and microstructural observations with relevant geological data, this reveals that the MVF has undergone at least three tectonic events since the Paleogene, with hydrothermal fluids transitioning from sulfate-dominated during the Paleogene and early Neogene to CO₂-rich since the Neogene. These findings provide unique insights into the evolutionary history and contemporary deformation of the RRF system and offer a novel magnetic perspective for studying fluid migration in fault zones.