The timing and direction of Permo-Triassic extension in southern Britain

Abstract

Abstract The Permo-Triassic sedimentary basins of southern Britain form a roughly N-S-trending linear rift system which extends more than 400 km from the northern English Channel to the Irish Sea. The component basins of the rift system typically contain 1000–4000m of dominantly continental red-beds. The basin margins are clearly imaged by seismic reflection data, and typically comprise a major syndepositional normal fault or faults lying in the hanging-wall block of a reactivated basement thrust. In southern England, basin-controlling faults have a dominant E-W (Variscoid) trend, in the Midlands the faults have a dominant N-S (Malvernoid) trend and in northern England NE-SW (Caledonoid) basin fault trends are prominent. Thus the rift system can be structurally divided into three segments, dependent on the structural gain of the underlying basement. Because the orientations of the basin-margin faults were controlled by basement structures, they did not develop perpendicular to the extension direction and consequently suffered varying degrees of oblique-normal displacements. An analytical model is developed which relates the displacement components of a set of variably oriented faults to a single regional extension direction. Application of the model to the rift basin-margin faults indicates that faults trending N-S had displacements which most closely approximated to dip-slip. Conversely, faults trending E-W suffered strongly oblique-slip displacements. This is consistent with a cumulative Permo-Triassic extension vector oriented 060°–240°, roughly ENE-WSW. Analysis of thickness changes across the basin-margin faults, and backstripped burial history plots from the basin depocentres, indicate at least two phases of crustal extension. The first occurred in late Permian times, with a second major episode in early Triassic times. They were characterized by rapid fault-controlled basin subsidence and coarse clastic sedimentation. Each was followed by a period of more gradual, regional subsidence, depositional overlap of the basin margins and a tendency towards finer-grained sedimentation.