Late Carboniferous to early Permian deformation
|From: Stone, P, Millward, D, Young, B, Merritt, J W, Clarke, S M, McCormac, M and Lawrence, D J D. 2010. British regional geology: Northern England.
Fifth edition. Keyworth, Nottingham: British Geological Survey.
By Stephanian times, the thermal subsidence that had controlled sedimentation patterns during the Namurian and Westphalian Regional Stages had all but ceased. Far to the east, the Ural Ocean had closed whilst to the south, Gondwana had collided with Laurussia; the supercontinent of Pangaea was the result. The Laurussia–Gondwana collision generated the Variscan Orogeny with a mountain fold belt produced as its culmination. The approximate northern limit of the fold belt extends from the mainland of Europe across southern England and Wales and into Ireland, and is referred to as the Variscan Front. In Stephanian times, northern England lay in the foreland region to the north of the deformation front in an equational latitude (P916033).
In northern England, deformation and magmatism took place during an interval of approximately 15 Ma that is now represented by the major unconformity between upper Carboniferous and lower Permian strata. Although sedimentation in the region had probably continued into the deformation phase, with the developing structures influencing depositional thicknesses, little evidence of the resulting rocks has been preserved. However, studies of conodont colouration (as an indicator of burial depth and consequent heating) suggest that up to about 2000 m of upper Carboniferous strata were removed at this time from north Northumberland. Although tectonic activity in the foreland region was much weaker than in the fold belt to the south some folds were formed, whilst reactivation of preexisting, long-lived faults caused the inversion of the Carboniferous basins. Field evidence suggests that the style of the deformation was profoundly influenced by the distribution of established fault-bound blocks with their granite cores, and the region’s position above the Iapetus Suture Zone.
Traditionally, the late Carboniferous tectonic event across northern England was attributed to east–west shortening, a model based on fold orientations and attributed generally to lateral ‘extrusion’ of fault-bound basement blocks under north–south compression. Associated strike-slip movements arose from reactivation of pre-existing faults that were orientated obliquely to the imposed stress regime. North–south extension towards the end of the tectonic episode then allowed large volumes of tholeiitic basaltic magma to ascend east–west faults and intrude the sedimentary succession to form the Whin Sill-swarm and its associated dykes. Though the intrusions cut late Carboniferous structures in places, elsewhere magma appears to have been emplaced during deformation. This contrast has led to alternative explanations involving wholly separate compression and extension events. Most recently, the contrasting styles of late Carboniferous deformation seen in the Northumberland Basin and adjacent Cheviot Block, have been explained by a single event involving oblique dextral extension. In this model, existing structures within the Acadian basement partitioned strain, allowing compressive structures to develop in some places whilst the region as a whole experienced extension. The extensional regime permitted huge volumes of tholeiitic magma, generated through mantle melting, to invade the sedimentary sequences not only in northern England and the Midland Valley of Scotland, but also across much of north-west Europe.
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