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

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