DECOUPLING ALONG A HIGH STRAIN ZONE DURING THE TECTONIC INVERSION OF A BACK-ARC BASIN AND FORMATION OF THE PATAGONIAN ANDES, CHILE

During the Late Cretaceous inversion of the Late Jurassic Rocas Verdes back-arc basin (RVB) in southern Chile, a high strain zone (HSZ) formed in continental crust, accommodating NE-directed thrusting of RVB rocks onto the continental margin and transferring displacement into a nascent Patagonian-retroarc fold-thrust belt. New mapping, EBSD fabric analysis and microstructural data document the kinematic evolution and deformation conditions of the HSZ in two locations within Cordillera Darwin (CD).

Toward the hinterland, at Seno Martínez, mafic schists of the RVB are thrust above garnet-bearing chlorite schist and psammite of the CD. The CD schist shows a quartz/chlorite schistose foliation (S₁) folded by noncylindrical, recumbent, isoclinal folds (F₂) that are progressively refolded by inclined (F₃) folds. About 1 km beneath the thrust contact, deformation intensifies in a ~5 km thick HSZ defined by the tightening of F₂ and F₃ folds, pronounced SW-plunging quartz lineations (L₁), and F₂ sheath folds that are subparallel to and fold L₁. F₂ sheath folds are refolded by F₃. C-S fabrics, C-C’ shear bands and rotated porphyroblasts show top-NE, foreland directed transport. Kink-folds (F₄) with N-dipping axial planes (S₄) and top-SW asymmetry overprint F₂ and F₃ folds and are related to Paleogene backthrusting. Quartz textures are typical of subgrain rotation and grain boundary migration recrystallization equivalent to Regime 3. Quartz CPO patterns indicate basal <a> and mixed<a> slip systems.

At Bahía Gallant, ~75 km toward the foreland, volcanoclastic rocks coeval with the RVB are thrust above CD schist along a > 1 km thick HSZ defined by the tightening of F₂ recumbent isoclinal folds and a SW plunging quartz lineation. C-S fabrics, C-C’ shear bands and asymmetric quartz boudins indicate top-NE transport. Quartz textures indicate subgrain rotation recrystallization typical of Regime 2. Quartz CPO patterns suggest basal <a> slip. Quartz deformation mechanisms suggest the HSZ formed in the middle crust and dipped shallowly (<10°) toward the hinterland. The HSZ decouples the upper-crust from the underthrust continental margin forming a décollement below a nascent retroarc fold-thrust belt.