The start-perranporth zone- transpressional reactivation across a major basement fault in the variscan orogen of S.W. England

Abstract

The Start-Perranporth Zone is one of a number of E-W trending zones in S.W. England, which are characterised by an anomalous and structurally complex deformation history, and which are thought to reflect the influence of pre-existing basin architecture. The SPZ straddles the Start Complex in S. Devon, and is approximately coincident with the northern margin of the Gramscatho Basin in S. Cornwall. It appears to coincide with significant sedimentological, geochemical and metamorphic transitions, and may mark the site of a pre- to Early Devonian terrane boundary. This terrane boundary may have formed the northern margin to a series of small possibly transtensional basins, including the Start and Gramscatho Basins, in which thick successions accumulated prior to inversion during the Variscan orogeny. The pelitic sequences in these basins (Gramscatho Group sandstones, Start greyschists) are geochemically similar to one another, and to other Rhenohercynian basinal sequences in mainland Europe. Both the Gramscatho and Start basins are characterised by the presence of incipient ocean crust (Lizard ophiolite, Start greenschists), with a strongly depleted N-type MORB signature and evidence of ridge-related sub-oceanic early deformation. The interlayered green and grey schists of the Start Complex are separated from the shallow marine Meadfoot shales to the north by a steep north dipping normal fault, the Start Boundary Fault, which bears evidence of a long-lived movement history. This fault is intimately associated with large volumes of highly altered and replaced basic intrusives, and appears to be the surface manifestation of the basin bounding fault at depth. Approaching the SBF, the strain intensifies, primary folds tighten, the primary cleavage steepens to sub-vertical and mineral stretching lineations switch from SSE plunging (sub-parallel to the Variscan transport direction) to sub-horizontal approximately E-W trending. Immediately adjacent to the SBF, sheath folds occur, suggesting very high along strike shear strains. Small scale structures, e.g. shear bands, refold relationships, etc. consistently indicate that dextral simple shear is important during Variscan shortening. Similar, though somewhat more cryptic, evidence for dextral shear is also seen in the L. Devonian shales north of the SBF. In S. Cornwall there is a similar focusing of high strain along the northern Gramscatho margin, with a tightening of folds, a backsteepening of the primary cleavage, and the development of overprinting late crenulations. Primary stretching lineations lie E-W. There is no evidence for sheath folding on either coast, although broad phyllonite zones bearing dextrally asymmetric quartz augen provide evidence of long-lived dextral shear. Many of the high strain fabrics on the east coast are absent, probably faulted out along a major NW-SE dextral strike-slip fault (the Pentewan Fault).The small scale structural evidence along this zone consistently indicates that dextral transpression was the dominant deformation mechanism during Variscan orogenesis. The structural transitions are also suggestive of fault butttressing, e.g. secondary backfolding, backthrusting, etc. and it appears that the ~E-W trending basin bounding fault acted as an oblique buttress to the NNW directed Variscan nappes, the high angle obliquity of this collision inducing dextral transpression in the shortening cover sequence. This fault buttressing mechanism readily accounts for all of the observed anomalous small scale structures, and the marked along strike persistence of the anomalous zone.