Using U-Pb Carbonate Geochronology to constrain the timing of fault development and fluid source evolution in an inverted continental rift basin.

Abstract

The Bristol Channel Basin (BCB) is a small continental rift basin that developed along the eastern margin of the North-Atlantic during the Mesozoic. Relatively minor basin inversion occurred during the Cenozoic. It has been extensively studied due to the exceptional exposure of faults along both margins of the basin. Widespread calcite mineralisation of overprinting fracture networks documents fluid partitioning along these structures over time. However, the temporal evolution of deformation has been constrained solely from the relative timing of structures in the field and through comparisons with other basins in the region. This has made detailed modelling of how structures have evolved during basin development and later inversion problematic. In this study we succesfully use U-Pb carbonate geochronology on low U samples (<1ppm) to constrain the absolute timing of fault development, whilst also assessing fluid source evolution with stable isotope and fluid inclusion data. Absolute dating of calcite slickenfibres in the fault cores of extensional, thrust and strike-slip faults in the East-Quantoxhead - Kilve region reveals the precise timing of different phases of deformation within the BCB for the first time.New age data show that extensional faulting occurred from ca. 154-118 Ma. Strike-slip faults formed ca. 47-21 Ma, with thrust faults forming ca. 46-35 Ma. The results show Late Jurassic – Early Cretaceous E-W extension with a vertical σ1, followed by Eocene – Miocene N-S contraction with σ1 now being ~horizontal. New age determinations provide much greater insight into the longevity of these structural phases, as well as how fluid nature and composition has evolved over time. Stable isotope data captures fluid source evolution, with δ13C & δ18O values being more negative in the older extensional faults, and becoming more positive over time in the later contractional features. Reactivated structures show evidence for deeper fluid sources, shown by relatively hotter fluid inclusion temperatures within these faults.Constraining the development of different fault populations within the BCB increases our understanding of the evolution of regional stress within southern England and can be extrapolated to nearby basins. Understanding the historical partitioning of fluid flow through different fault populations has practical applications for understanding where produced and/or injected fluids will flow when a reservoir is exploited today, such as in an Enhanced Geothermal System (EGS).