Tectonic evolution of the North Patagonian Andes (41-44°S) through recognition of syntectonic strata.

ECHAURREN, A; ORTS, D; FOLGUERA, A.; GIANNI, G

TECTONOPHYSICS

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2016

0040-1951

The North Patagonian fold-thrust belt (41°?44° S) is characterized by a lowtopography, reduced crustal thicknessand a broad lateral development determined by a broken foreland system in the retroarc zone. This particularstructural system has not been fully addressed in terms of the age and mechanisms that built this orogenic segment.Here, new field and seismic evidence of syntectonic strata constrain the timing of the main deformationalstages, evaluating the prevailing crustal regime for the different mountain domains through time. Growth strataand progressive unconformities, controlled by extensional or compressive structures, were recognized in volcanicand sedimentary rocks fromthe cordilleran to the extra-Andean domain. These data were used to construct abalanced cross section, whose deep structure was investigated through a thermomechanical model that characterizesthe upper plate rheology. Our results indicate twomain compressive stages, interrupted by an extensionalrelaxation period. The first contractional stage in the mid-Cretaceous inverted Jurassic?Lower Cretaceous halfgraben systems, reactivating the western Cañadón Asfalto rift border ~500 km away from the trench, at a timeof arc foreland expansion. For this stage, available thermochronological data reveal forearc cooling episodes,and global tectonic reconstructions indicate mid-ocean ridge collisions against the western edge of an upperplate with rapid trenchward displacement. Widespread synextensional volcanism is recognized throughoutthe Paleogene during plate reorganization; retroarc Paleocene-?Eocene flare up activity is interpreted as productof a slab rollback, and fore-to-retroarc Oligocene slab/asthenospheric derived products as an expression ofenhanced extension. The second stage of mountain growth occurred in Miocene time associated with NazcaPlate subduction, reaching nearly the same amplitude than the first compressive stage. Extensional weakeningof the upper plate predating the described contractional stages appears as a necessary condition for abnormallateral propagation of deformation.