Detailed mapping of the rocks to the north and south of Plymouth reveals a sedimentary sequence deformed by a series of folds and thrusts. Two structural zones exist each with a different style of deformation. In the Lower to Upper Devonian rocks of the southern zone, slip vectors, vergence and facing of D1 folds indicate that the transport direction is to the north-west. An associated first phase cleavage dips to the south-east. The northern zone, of Upper Devonian and Lower Carboniferous strata, extends into central Devon and east Cornwall and the geometry and facing of the early folds in these rocks indicate a transport direction to the south or south-east. D1 folds generally verge north but are downward facing towards the south. A flat-lying, penetrative slaty cleavage occurs in the mudrocks and also indicates southward facing. Generally the Carboniferous rocks in this area are inverted. The two structural zones confront each other at an E–W-trending line which passes through Cargreen, 8 km north of Plymouth. Here the deformation is more intense as the cleavage, common to the southern zone, overprints that of the northern zone, resulting in the formation of D2 folds. The confrontation is interpreted as a northerly dipping backthrust produced by large-scale underthrusting. It is proposed that the northward-advancing Variscan thrust belt progressively underthrust the Carboniferous foreland basin flysch deposits which became inverted and backthrust towards the south.

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