Variscan strain pattern in the Paleozoic series at the Lizard Front, southwest England

Abstract

A structural analysis of the Devonian sedimentary Gramscatho formations, South Cornwall, allows us to specify the Variscan strain pattern at the Lizard front. This deformation is consistent with a NNW verging tangential shear which first induces a synmetamorphic folding phase (F1S1) and whose noncoaxial character is emphasized, on different scales, by the existence of a positive deformation gradient toward narrow shear zones, the development of syncleavage noncylindrical folds whose axes tend to be reoriented, in the S1 slaty cleavage plane, toward a pervasive stretching lineation (L1) which trends N160°E and by the existence of NNW verging sheath folds. This folding phase evolves finally to tangential structures which induce, over the parautochthonous Gramscatho formations, subhorizontal shear planes bearing strong slickenside lineations trending parallel to L1, lineations which thus represent, on a regional scale, the “tectonic transport” trend of allachthonous units migrating northwards. The existence of such allochthonous units in the South Cornubian area is now well established by the discovery, at the Lizard front, of a Paleozoic (Carboniferous ?) limestone lens wedged under the Lizard hornblendeschist. The structural evolution of the South Cornubian domain is consistent with the “scheme” of a continental platform progressively implicated in a tangential deformation propagating northward. This compression leads, in the internal southern zones, to an earlier (Mid‐Upper Devonian) crustal tangential tectonic which later (Carboniferous) reaches the external domain of England, and it expresses in the Devono‐Carboniferous cover by a tegumentary tectonic. This northward verging tangential pattern is now testified by SWAT (South West Approaches Traverses) profile interpretation.