The North Patagonian orogenic front and related foreland evolution during the Miocene, analyzed from synorogenic sedimentation and U/Pb dating (∼42°S)

Abstract

Miocene sedimentary successions of the Ñirihuau and Collón Cura formations east of the El Maitén Belt constitute a partial record of the Andean exhumation, defining a synorogenic infill of the Ñirihuau Basin in the foothills of the North Patagonian fold and thrust belt. Gravimetric and seismic data allow recognizing the internal arrangement and geometry of these depocenters that host both units, separating a synextensional section previous to the Andean development at these latitudes, from a series of syncontractional units above. A series of progressive unconformities in the upper terms shows the synorogenic character of these units corresponding to the different pulses of deformation that occurred during the middle Miocene. New U–Pb ages constrain these pulses to the ∼13.5–12.9 Ma interval and allow reconstructing the tectonic history of this region based on the detrital zircon source populations. The U–Pb maximum ages of sedimentation give to the Ñirihuau Formation in particular a younger age than previously assumed. Additionally, synsedimentary deformation in strata of the upper exposures of the Collón Cura Formation associated with contractional structures and U–Pb ages allow identifying a younger paleoseismogenic pulse in ∼11.3 Ma. Thus, based on these data and a compilation of previous datasets, a tectonic evolution is proposed characterized by a contractional episode that migrated eastwardly since ∼19 to 15 Ma producing the Gastre broken foreland and then retracted to the eastern North Patagonian Precordillera, where out-of-sequence thrusts cannibalized the wedge top zone in the El Maitén belt at ∼13.5–11.3 Ma.