Inversion tectonic in the Carboniferous basins of northern England : with special reference to Northumberland

The Northumberland basin is a member of the Carboniferous syn-orogenic complex that developed in the nonhern continental foreland of the Variscan orogeny. The basin occupies pan of the Iapetus province, forming a generalised half-graben between the Southern Uplands and the Alston block. It straddles the Caledonian Iapetus suture, over lower Palaeozoic crust in the north and lower Palaeozoic and older crust in the south. The basin was initiated in the Early Tournaisian by major extension along E/W normal syn-sedimentary growth faults. These dominated the southern margin of the basin where up to 6km of sediment accumulated. Fault activity was accompanied by localised dewatering and gravitational folding and slumping. Basin asymmetry was maintained in the Late Carboniferous when the basin suffered inversion in the widespread Asturian compression event. Over 3km of pre-Permian erosion was achieved over the northern half of the basin compared with <lkm in the south. The inversion event is characterised by four structural elements: Reactivated basement controlled regional scale reverse fault bounded NE/SW to N/S anticlines (confined exclusively to the north): Minimal strike-slip reactivation and butressing around the southern margin and E/W faults: Positive flower structures associated with surface monoclines and original basin hinge-lines: Extensive transpressional deformation along the North Pennine Fault line. A basin synthesis aided by palaeomagnetism suggests local dextral strike-slip modified NW/SE compression best explains the deformation. Basin modelling suggests mechanisms for the process e.g. thick skinned shortening for the north and foot-wall butressing against the southern margin. Modelling further indicated that subsidence was resumed after the onset of inversion prior to the intrusion of the Whin Sill.