Late Variscan structures on the coast between Perranporth and St Ives, Cornwall

Abstract

Structural mapping of the coast between Perranporth and St. Ives has shown that two generations of northnorthwest verging Variscan convergence related structures (D1 and D2) are postdated by a diverse range of structures showing top to the southeast sense of shear. These D3 structures are represented by the coeval development of ductile zones of distributed shear, brittleductile detachments and brittle southeast dipping listric extensional faults. Lithology of the preexisting anisotropy controls both the extent and the geometry of these structures. Extensional exhumation is suggested by the progressively more brittle nature of postD2 deformation. D3 deformation probably represents the late orogenic extensional reactivation of thrust faults. Subsequent northnorthwest dipping faults appear synchronous with granite emplacement. A period of eastnortheast—westsouthwest shortening occurred prior to the development of steep, commonly mineralized, faults which indicate renewed northnorthwest—southsoutheast extension. The infill of the northnorthwest—southsoutheast striking crosscourse faults relates to later approximately eastwest extension. This study suggests that following Variscan convergence there is a period of lateorogenic extension both predating and synchronous with granite emplacement and mineralization.