Evidence of an imbricate crustal thrust belt in the southern British Variscides: Contributions of southwestern approaches traverse (SWAT) deep seismic reflection profiling recorded through the English Channel and the Celtic Sea

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Abstract
The Southwestern Approaches Traverse (SWAT) seismic reflection profiles recorded through the Celtic Sea and the English Channel provide unexpected data concerning, notably, the deep structure of the Irish and south British Variscan crust. One of the most significant results is the recognition of prominent deep southerly dipping reflectors, regarded as large‐scale foreland directed Variscan thrusts which constitute, in the southern innermost zones, a crustal stacking wedge. The resulting overthickened crust may have induced the petrogenesis of the Cornubian granitic batholith by an anatectic melting process. The roots of this crustal duplex are not imaged on the SWAT lines because of their subsequent assimilation by the more recent layering of the present‐day lower crust. Upward, the deep crustal ramps are assumed to flatten out and to join a roof thrust which acts as a major midcrustal decoupling zone, not revealed by the SWAT profiles, and which underlies the thick Devonian allochthonous high‐strained units of Cornwall. Northward, this shallow northerly verging shear zone is overstepped by the Tintagel antithetic back thrusts which are closely related to deeper northward dipping basement faults considered as Caledonian features. Southward, the imbricate crustal thrust unit is overridden by the Lizard ophiolitic suture, which appears as a 8‐km‐thick gently southerly dipping sheared zone, crosscuttting the entire crust and rooting deep beneath the present‐day Moho. Its hanging wall is constituted by the Channel Cadomian block, characterized by an almost seismically featureless upper crust which is not involved in the main Variscan thrust stacking. Northward, the imbricate thrust unit is delineated by a moderately southerly dipping ramp which penetrates straight down into the upper crust without any evidence of an intervening flat‐lying decollement level. This frontal ramp emerges along a broadly 110°N trending discontinuous thrust front, located above the northern downfaulted margins of the Munster (Ireland) and Pembroke (Wales) Devonian basins. The SWAT deep seismic data relate the overall Variscan crustal shortening undergone by the south British domain to an oblique converging process involving a continental shelf pinched between the rigid allochthonous Channel block and the northern autochthonous foreland. This crustal shortening occurred during a Carboniferous intracontinental collisional context subsequent to the closure and the obduction of a small oceanic crust‐floored basin (Lizard).