The Closure Of The Rocas Verdes Basin And Early Tectono-Metamorphic Evolution Of The Magallanes Fold-And-Thrust Belt, Southern Patagonian Andes (52-54 Degrees S)

The Western Domain of the Magallanes Fold-and-Thrust Belt (MFTB) between 52 degrees-54 degrees S is part of a poorly studied hinterland region of the southernmost Andean Cordillera. This domain consists of NNW-SSE trending tectonic slices of pre-Jurassic basement units and Upper Jurassic-Lower Cretaceous ophiolitic complexes and volcanosedimentary successions of the Rocas Verdes Basin (RVB). New detrital zircon U-Pb ages of metatuffs and metapsammopelites constrain episodes of Late Jurassic rift-related volcanism (ca. 160 Ma) followed by Early Cretaceous sedimentation (ca. 125 Ma) during the opening of the RVB. Shear bands developed in the RVB units further record the initial phases of the Andean Orogeny. The 30-km wide thrust stack located on top of the Eastern Tobifera Thrust consists of mylonitic metatuffs, metapelites and metabasalts with a NE-verging brittleductile S-1* foliation. Phengite-bearing metatuffs commonly record pressure-temperature (P-T) conditions between similar to 3-6 kbar and similar to 210-460 degrees C, consistent with underthrusting of the RVB beneath the parautochthonous magmatic arc in the west. Peak metamorphic conditions of similar to 6 kbar and 460 degrees C are derived from a metapsammopelite with textures of contact metamorphism overprinting early mylonitic structures (at least S-1*). A back-arc quartz-diorite, intruded at ca. 83 Na, is in contact with the metapsammopelite and constrain the minimum age of deformation at deep crustal depths. Campanian-Maastrichtian (ca. 70-73 Ma) Ar-40/Ar-39 phengite dates from a mylonitic metapelite indicate the timing of thrusting and backthrusting during the initial uplift of the underthrusted crustal stack. These findings reveal a similar to 400 km along-strike connection of mylonite belts in a continent-verging thrust structure that became active at the onset of the Andean omgeny during the closure of the Rocas Verdes back-arc marginal basin.