Post-emplacement extension within a thrust sheet from the central Pyrenees

Abstract

Secondary faults and related veins in a small thrust sheet (the Pic de Port Vieux thrust sheet) in the footwall of the Gavarnie thrust in the central Pyrenees are used to infer the deformation sequence that accompanied its emplacement. Initial deformation was characterized by distributed bedding-parallel simple shear. This was superseded by localized deformation involving arrays of extensional faults. The first faults to form were X-shears and were succeeded first by conjugate faults (X-X' shears) and then arrays of R-shears that locally cut the thrust, i.e. developed subsequent to its emplacement. Extension veins are used to infer the orientation of the ambient stress system during the deformation sequence. A forward rotation of the stress sytem (towards the vertical) was associated with the X and X-X' shears and is considered to reflect loading of the 4 km thick overlying Gavarnie thrust sheet. A later, backward rotation of the stress system then occurred, returning σ 1 to 45° obliquity to the Pic de Port Vieux thrust, and was associated with the R-shear development. This later event produced extensional shearing of the thrust sheet and is equated with a proposed episode of spreading/gliding within the overlying thrust pile. A model for this spreading/gliding relates it to the active tectonics of the developing south Pyrenean thrust belt.