Evolution of a Paradoxical Landscape: New Constraints for Tectonic and Climatic Processes in the Forearc of Southern Peru

The Atacama Desert of the Western Andean margin spans the southernmost part of Peru and Chile. Unique preservation due to hyperaridity since at least 3Ma reveals a dynamic region shaped by active tectonic and climatic processes. The asymmetric, positive, Incapuquio flower structure is the most prominent structure dissecting the forearc of Southern Peru since the Jurassic. This fault zone encompasses a set of neotectonic transpressional structures connected at depth that contribute to topography building. Through a micro, outcrop-scale, and remote structural and geomorphic analysis of four major fault strands or fault zones within the Incapuquio flower structure (among them the Purgatorio fault), we provide new data on the architecture and kinematics of these faults, and discuss its implications for the spatiotemporal evolution of tectonic deformation in the forearc and pre-existing tectonophysical models. These data reveal a complex system encompassing faults with different maturities and diverse kinematics.

In the forearc, active tectonic processes are coupled with climatic processes in shaping the landscape. Despite the persistent hyperaridity, the Andean Western flank that hosts the Atacama is subject to extreme events and climatic variability from the El Niño Southern Oscillation (ENSO). Periods of hyperaridity are punctuated by flash events of high precipitation that lead to flooding that can significantly rework the landscape. The timing of these floods is probably associated with ENSO events, yet in general remains poorly understood. Through the analysis of the morphology and stratigraphy of a set of braided, intermittent channels and abandoned terraces cut by a recent rupture of the Purgatorio fault, as well as dating of organic material and volcanic ash, we reconstruct the timing and characteristics of some of the precipitation events that have shaped the area. This chronology of geomorphic features provides a link between historical climate records and a longer record retained in the arid landscape enabling us to build an extended record of high discharge events for the region. Combining these new tectonic and climatic findings with pre-existing data, we propose a model for forearc evolution in which tectonics and climate couple in supporting and maintaining the western wedge of the Andean orogen.