Paleomagnetic evidence of the brittle deformation of the Central Patagonian Batholith at Gastre area (Chubut Province, Argentina)

Abstract

The Central Patagonian Batholith (CPB) in the Gastre area, central Patagonia, constitutes a set of two I-type Late Triassic plutonic suites, the Gastre superunit with a U-Pb zircon age of 221 ± 2 Ma, and the Lipetrén superunit with a U-Pb SHRIMP age of 215 ± 1 Ma. The source of this calc-alkaline batholith is characterized by crustal and mantle contributions, and it registers NW-SE subvertical magmatic and solid-state fabrics. These features, together with its intraplate position away from the inferred proto-Gondwana margin, have made it the focus of different tectonic interpretations. Early studies have interpreted it as the record of major dextral motions along the transcontinental NW-SE-striking Gastre fault system in Jurassic times. Later interpretations have proposed that the magmatic and tectonic foliations of the CPB were formed during a sinistral transpressive regime which was aided by Late Paleozoic widespread NW-SE subvertical structures in the North Patagonian Massif. Paleomagnetism, a unique tool to detect the presence of tectonic block rotations on vertical axes, was applied in the CPB in order to constrain the timing and type of deformation in the area. A paleomagnetic pole was obtained, which statistical parameters are: N = 45, Lat. = 81.4°S, Long. = 207°E, K = 11.5, A95 = 6.6°. Although the position of this pole does not coincide with equivalent Late Triassic poles, this position can be reconciled with the presence of a NE tilting of about 11° of the sampled block of the batholith. The tilting would have been aided by the NW-SE subvertical structures that affect the area. These paleomagnetic results rule out the possibility of vertical axis rotations from Late Triassic to present times and suggest that the ductile syn- to post-emplacement deformation of the CPB in Gastre area occurred during this period (Late Triassic), being the later brittle deformation triggered by the Andean Orogeny a possible explanation for this tilting.