Geological investigations at the southern tip of the Americas :the development of the Patagonian Orocline and uplift of the Cordillera Darwin metamorphic complex, southernmost Chile




Cunningham, W. D.

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The Patagonian Orocline is the 90° bend in the southernmost Andes between 50°S and 56°S. New paleomagnetic data indicate that the orocline is, at least in part, the product of tectonic rotation. Field work in the Beagle Channel region of southernmost Chile provides evidence for widespread left-lateral strikeslip faulting in the internal zones of the mountain belt. Both arms of the Beagle Channel are interpreted to be left-lateral strike-slip faults based on detailed study of mesoscale strike-slip faults (Riedel shears) in coastal outcrops. Although much of the evidence indicates Cenozoic brittle strike-slip faulting, Late Cretaceous brittle-ductile strike-slip and oblique-slip shear zones and S1, L1 fabric trends in southern Cordillera Darwin indicate that a component of strikeslip deformation accompanied Late Cretaceous deformation. Detailed mapping of D1 and D2 structural trends in three separate areas in southern Cordillera Darwin and the identification of an uplifted upper ophiolitic sequence on Isla Gordon immediately south of Cordillera Darwin suggest that the mid-Cretaceous Andean orogeny involved the transpressional inversion of the Rocas Verdes marinal basin. Progressive transpression appears to have been the dominant deformational regime in the region for the last 120 Ma years. This conclusion is supported by quantitative analysis of southern South America-Antarctic Peninsula relative motion for 150-0 Ma that indicates a significant strike-slip component of relative motion has existed between southern South America and the Antarctic Peninsula since the Early Cretaceous opening of the South Atlantic. A new model is proposed that integrates the development of strike-slip faulting and the structural evolution and uplift of the southernmost Andes with the rotational development of the orocline. The Patagonian Orocline appears to be the product of broad interplate shearing accommodated by strike-slip faulting, block rotation and contraction and is probably continuing to evolve today. The uplift of the Cordillera Darwin metamorphic complex may have been due to hinged unroofing in a transpressional restraining bend setting.