From Crustal Thickening to Orogen‐Parallel Escape: The 120‐Myr‐Long HT‐LP Evolution Recorded by Titanite in the Paleozoic Famatinian Backarc, NW Argentina

Abstract

Exposed sections of accretionary orogens allow reconstruction of their tectonic evolution. Most commonly, orogens are characterized by two‐dimensional shortening perpendicular to the orogenic front. We describe the midcrustal section of the backarc of the early Paleozoic Famatinian accretionary orogen, exposed in the Sierra de Quilmes. Here crustal deformation evolved from a typical two‐dimensional shortening with tectonic transport toward the west, to a noncoaxial constrictional strain with a southward tectonic transport parallel to the orogen. During the early phase of deformation, high‐temperature and low‐pressure (HT‐LP) metamorphic complexes were juxtaposed by west directed thrusting on remarkably thick shear zones forming a thrust duplex. Deformation of the buried footwall complex continued after the exhumed hanging wall ceased to deform. We suggest that the thermally weakened footwall complex responded by initiating a phase of south verging thrusting, parallel to the orogen, associated with strong constriction, associated with L‐tectonites, and sheath folds. This late phase of deformation defines a noncoaxial constrictional regime characterized by simultaneous east‐west and vertical shortening and strong north‐south, orogen‐parallel stretching. Titanite ages and Zr‐in‐titanite thermometry demonstrate that this backarc remained above 700°C for 120 Myr between 500 and 380 Ma. Combined with regional geology, the new data suggest that west verging thrusting interrupted an early, back‐arc extensional phase, and lasted from ~470 to 440 Ma and that footwall constriction and south verging thrusting continued for another 40 to 60 Ma. The Famatinian backarc exposed in Sierra de Quilmes thus is an example of how shortening and orogenic growth in a hot orogen was counterbalanced by lateral flow.