Progressive Development of a Distributed Ductile Shear Zone beneath the Patagonian Retroarc Fold-Thrust Belt, Chile

Abstract The southern Patagonian Andes record Late Cretaceous–Paleogene compressional inversion of the Rocas Verdes backarc basin (RVB) and development fold-thrust belt (FTB). A ductile décollement formed in middle crust accommodated underthrusting, thickening, tectonic burial continental margin (Cordillera Darwin Metamorphic Complex (CDMC)) beneath RVB. We present new geologic mapping, quartz microstructure, crystallographic preferred orientation (CPO) fabric analyses to document kinematic evolution deformation conditions décollement. Within CDMC, is defined by a quartz/chlorite composite schistose foliation (S1-2) that progressively refolded two generations noncylindrical, tight, isoclinal folds (F3–F4). Strain intensifies near top forming >5 km thick shear zone penetrative L-S tectonite (S2/L2) progressive noncylindrical folding (F3). Younger kink steeply inclined tight (F4) with both north- south-­dipping axial planes (S4) overprint D2 D3 structures. Quartz textures from fabrics show subgrain rotation grain boundary migration recrystallization equivalent regime 3, CPO patterns indicate mixed prism <a> [c] slip systems c-axis opening angles indicative temperatures between ~500° >650°C. Approximately 40 toward foreland, thins (~1 thick) tightening recumbent typical 2 dominantly basal slip, are consistent ~375° 575°C. Deformation occurred under greenschist amphibolite facies foreland hinterland, respectively, indicating dipped shallowly hinterland. Magallanes an example regional distributed lower crustal thickening below retroarc FTB.