The Cenozoic evolution of the Lundy Pull-Apart Basin into the Lundy Rhomb Horst

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Abstract
The NW–SE Cornubian fault zones (FZs) continue to southwest Wales, where they caused dextral offsets during the late Stephanian. A fault bounded area of Devonian–Carboniferous strata, intruded by the Tertiary Lundy Igneous Complex, forms the Lundy Rhomboid. It originated as the Lundy Rhomb Graben, created by 28–40 km of Tertiary sinistral strike–slip faulting along the left-stepping NW–SE Sticklepath–Lustleigh FZ (SLFZ). Tertiary sedimentation in the graben was accompanied by intrusion of the 2.5–4.0 km thick igneous complex into its basement. Sedimentation later spread beyond the graben to form a ‘steer's horns’ basin, of which the Stanley Bank and West Lundy basins are remnants. The strata of the rhomb graben and the ‘steer's horns’ basin constituted the perhaps 4 km deep Lundy Pull-Apart Basin. Subsequent relatively minor dextral faulting on the left-stepping SLFZ inverted the graben into the Lundy Rhomb Horst; the basement of the graben was uplifted ≮ 1–2 km (net) and exposed by erosion of the Tertiary strata. The pull-apart basin developed at the intersection of reactivated Hercynian NW–SE transform FZs, and E–W subduction zone thrusts. Cenozoic N–S tensional and compressional reactivations of the thrusts were respectively associated with transtensional development of the pull-apart basin, transpressional of the rhomb horst.