A major mid-crustal decollement of the Paleozoic convergent margin of western Gondwana: The Guacha Corral shear zone, Argentina

The Guacha Corral shear zone (GCSZ) is one of the most significant and widest thrusts in the western Gondwana margin of Argentina. It is 10–16 km wide and thrusts ∼525-515 Ma forearc migmatites of the Pampean Orogeny eastwards onto greenschist facies rocks metamorphosed at 460-450 Ma during the Famatinian Orogeny. It is divided into two sections. At the highest structural levels, the shear zone is >7.5 km wide and records high-T, amphibolite facies deformation. At lower structural levels, this is overprinted by a >2.5 km-wide, low-T, greenschist facies shear zone. The two are separated by a transitional zone where overprinting relationships are preserved. Motion in the two sections had the same kinematics. Syn-anatectic structures in migmatites indicate that thrusting started during Pampean forearc melting and continued into subsolidus, during hanging wall exhumation and cooling. Greenschist facies reactivation, at the end of the Famatinian Orogeny, thrust the Pampean migmatites over Famatinian-age metamorphic rocks. At this stage the shear zone became part of a system of contractional shear zones, defining the eastern boundary of a 300 km-wide belt of Famatinian foreland shortening. Thus, the uniquely wide GCSZ is a major tectonic boundary developed in two steps: first as an amphibolite facies mid-crustal detachment controlling the widening of the compressional Pampean forearc, then as the easternmost foreland thrust system of the Famatinian Orogeny.