Tectonic Evolution of a Doubly-Vergent, Thick-Skinned Fold and Thrust Belt in the Patagonian Andes, Southernmost South America

The Magallanes fold and thrust belt in southernmost Patagonia records the Cretaceous-Neogene compressional closure of the early Mesozoic Rocas Verdes extensional basin. The internal part of this belt, represented by the Darwin Complex, contains the largest exposure of Mesozoic, moderate to high-P (8-10 kbar) metamorphic rocks in the Andes south of Ecuador. We show that these high-P rocks constitute a wedge of mid-crustal metamorphic basement between two foreland-vergent thrusts. The thrusts formed at 25-30 km depth as the floor of the Rocas Verdes basin was underthrust southward beneath the Patagonian batholith. At the top of the wedge, several hinterland-vergent back thrusts place greenschist facies metaturbidites on top of the high-grade rocks. These back thrusts, together with the foreland-vergent faults, define a thick-skinned doubly-vergent crustal structure. U-Pb analyses of zircon (LA-ICPMS) indicate that prior to ~90 Ma the oceanic floor and infill of the Rocas Verdes basin were shortened and thickened in a narrow, <100 km-wide belt within the hinterland. The doubly-vergent structure and the high-grade thrust wedge formed later during the interval 90-72 Ma. Zircon fission track analyses indicate that these latter events coincided with a period of Late Cretaceous cooling. From ~61 to ~30 Ma, new mostly thin-skinned thrust sheets formed and propagated into the foreland. Ductile transpression also affected the hinterland mostly after ~61 Ma. These results indicate that basin closure involved two major cycles of deformation exhibiting contrasting structural styles. Prior to ~61 Ma, thick-skinned deformation in the hinterland was decoupled from the foreland, possibly as a result of strength contrasts created by the high residual temperatures and thin crust of the Rocas Verdes basin. By ~61 Ma cooling appears to have accentuated the strength contrast between hinterland and foreland and allowed the orogen to grow laterally by thin-skinned thrust faulting.