Kinematic history of the retroarc thrust belt in the central Andes of Argentina at 24-25°S: significant Andean shortening and sporadic foreland-ward deformation propagation

The southward along-strike transition from major thin-skinned shortening of Bolivia to the significantly lower magnitude of thick-skinned shortening in northwestern Argentina has often been attributed to the presence of a thick mid to late Paleozoic section in Bolivia relative to a thin group of correlative rocks in northwestern Argentina that were affected by significant Cretaceous rifting. Despite the Andes being regarded as an archetype of ocean-continent convergence, the northwestern Argentine Andes have remained enigmatic in a structural and tectonic context. This study integrates regional geological mapping, structural analysis, and geo- and thermochronology from the Salta province of northwestern Argentina. Geological mapping in the Cachi range at ~25° S latitude revealed the presence of an ~60° west-dipping package of rocks, passing from low grade phyllites in the eastern part of the range into cordierite-bearing, anatectic and arc-related rocks in the core of the range (one anatectic pluton yielded a U/Pb zircon age of 488 ± 10 Ma). Detrital zircons record U-Pb ages demonstrating that the highest-grade, structurally highest rocks are the oldest (maximum depositional age (MDA) ~548 Ma), rocks at structurally lower levels are younger (MDA ~538 Ma), whereas the structurally lowest rocks are the youngest (MDA ~523 Ma). Double dating some of these same zircons using the low temperature U-Th/He system indicates that at least 6-8 km of Miocene (15.7 ± 0.4 Ma) exhumation occurred in the core of the range at this time, yet exhumation at the eastern range margin was insufficient to reset zircons. U/Pb zircon ages from a tuff within growth strata in the footwall of a major thrust fault ~50 km east of Cachi demonstrate that shortening was ongoing there at 9.4 ± 0.4 Ma, yielding a propagation rate of the thrust belt of ~8 km/Ma. Since ~9 Ma, deformation has jumped ~150 km eastward to the Santa Barbara ranges, yielding an average rate of >30 km/Ma. Many thrust belts are thought to behave in a simple, foreland-propagating manner, with deformation accommodated on faults progressively farther into the retroarc through time, responding to conditions imposed by a critical or supercritical orogenic wedge. Deviations in behavior from this simple model may reflect fundamental processes influencing the orogenic system. These preliminary data from northwestern Argentina suggest that in this region, the Cenozoic thrust belt is not simply a gradual, eastward-propagating system, but rather jumps sporadically, possibly due to feedback among geologic processes elsewhere in the orogen that perturb the orogenic wedge into a supercritical state of taper, promoting rapid foreland-ward propagation of the thrust front.