Fluid-mediated reactivation of brittle faults in the Bristol Channel Basin, UK

Faults act as sites for preferential failure in the continental crust when it is subjected to sequential tectonic events. To do so, they will typically have favourable orientation, geometry and/or be weaker than the adjacent crust and are therefore prone to slip. However, fluid flow that is focussed along faults has a role in modifying their mechanical strength, especially where different fluids, and resulting mineralisation, are partitioned along particular structures. The East-Quantoxhead Fault (EQHF) in the Bristol Channel Basin (BCB), SW England has been identified as a reactivated normal fault with multiple slip events, the cause and precise timing of which are unknown. Using U-Pb geochronology of calcite veins located in the fault core we show that the timing of mineralisation (as a proxy for fluid flow) along the EQHF spans from 151-35 Ma. Microstructural analysis of different vein generations within the fault core shows that the longevity of this structure is a result of progressive weakening of the fault core. Initially this is represented by crystal plastic deformation of early-stage calcite mineralisation in the fault core, compatible with protracted phases of normal-sense slip. This is most likely mediated by the flow of syn-kinematic fluids that are hotter than the ambient temperature of the wall-rocks. However, later weakening of the fault core results from the precipitation of relatively weak fibrous celestine (SrSO₄) along the margins of older calcite veins. Celestine shows evidence of reverse-sense S-C fabrics and was therefore a site of strain localisation and fault reactivation during regional contraction. This later fluid is sourced from deeper formations within the BCB which are only accessed by larger faults like the EQHF. Smaller normal faults in the BCB containing no celestine do not show a protracted fluid history, however crystal plastic textures (GBM, SGR) can also be seen within these faults. Understanding the role different fluids play in altering fault core composition, and strength via the analysis of vein textures plays a key role in understanding the partitioning and significance of fluid flow along fractures.