Using U-Pb carbonate dating to constrain the timing of extension and fault reactivation within the Bristol Channel Basin, SW England

Abstract

The Bristol Channel Basin is a Mesozoic continental rift basin. The basin is an important analogue for offshore reservoirs. Relative cross-cutting relationships and correlation with adjacent sedimentary basins have previously been used to constrain the timing of basin development. In-situ U-Pb carbonate geochronology has been used to date calcite slickenfibre development in the cores of normal, thrust and strike-slip faults in the East Quantoxhead and Kilve region of Somerset for the first time. Protracted N-S extension from c a. 150–120 Ma formed normal faults. Subsequent N-S shortening from ca. 50–20 Ma was accommodated by (i) mutually cross-cutting strike-slip faults (ii) minor E-W-striking thrust faults and (ii) the reactivation of pre-existing normal faults. Throughout Cenozoic contraction, σ 2 and σ 3 remained similar in magnitude and periodically flipped to become vertical, this was likely controlled by local stress permutations and changes in fluid pressure. The timing of inversion is contemporaneous with dominant Pyrenean and later Alpine orogenic events, as well as the opening of the Mid Atlantic Rift. Early inversion of the Bristol Channel Basin was likely driven by far-field Pyrenean deformation, with later contraction caused by Alpine forces. Ridge push from the Mid Atlantic Rift exacerbated the reactivation of the basin.