Onset of compressive exhumation of the northern Santa Bárbara System (NW Argentina). Tectonic implications from low-T thermochronology

The Santa Bárbara System (SBS), located at the Central Andes of northwestern Argentina, is a thick-skinned fold-and-thrust belt (FTB) that represents the outermost portion of the orogenic wedge and the western boundary of the undeformed Chaco-Paraná foreland basin. The present-day structural architecture of the SBS is mainly governed by the reactivation of basement anisotropies and the inversion of Cretaceous normal faults imprinting an overall vergence towards the west. Some of the major faults show evidence of active tectonics in the landscape, which also correlates with instrumental seismicity and destructive earthquakes recorded.

Studies based on seismic interpretation of growth strata in synorogenic deposits have shown that the basement ranges of the southern SBS began to be uplifted during the late Miocene, although the magnitude of exhumation has not yet been quantitatively established. On the other hand, thermochronological analyses of basement samples from the neighboring Eastern Cordillera (EC) have highlighted the relevance of a late Miocene (ca. 10 Ma) exhumation event that propagated the orogenic front into the Mojotoro range, west of the northern SBS.

In this contribution, we present the first (U-Th-Sm)/He cooling ages from Paleozoic rocks of the northern SBS and from the Neoproterozoic basement of the Reyes range, in the EC. The integration of these data with previously published cooling ages allows to conclude that the late Miocene compressional event reached the northern SBS, driving the exhumation of the basement-cored ranges by 7-8 Ma, much earlier than previous estimations. In addition, the Reyes range sample yielded a younger cooling age (ca. 4 Ma) which agrees well with the model of hinterland reactivation of faults due to the recovery of a sub-critical orogenic wedge, as proposed by previous publications.

Based on the available structural reconstructions, an average exhumation rate of ca. 0.7 mm/a can be estimated for the western frontal thrusts of the northern SBS. This value agrees with the late Pleistocene-Holocene rates obtained for neotectonic morphostructures of the Lerma Valley, the easternmost intermontane basin of the EC, suggesting the continuation of a similar deformation pattern throughout the Quaternary for this portion of the Andean back-arc. Additionally, our results shed light on the tectonic evolution style of thick-skinned FTB´s and broken foreland basins.