Variability of active deformation of the Cordillera de la Sal fold-and-thrust belt, Salar de Atacama, Central Andes, Chile. Preliminary data on deformed fluvial features.

The Salar de Atacama (SdA) endorheic basin is a low topographic anomaly located in the Central Andes forearc, and it has been suggested as an independent block that subsides with respect to their neighbouring morpho-structures: Cordillera de Domeyko at the west, and the Altiplano and Puna volcanic plateaus to the east. Within the SdA depression, we focus on the Cordillera de la Sal (CdS), a ridge that emerges at its western margin and extends to the northeast for more than 100 km towards the volcanic arc, where the SdA basin closes. The core of the CdS ridge is formed by a fold-and-thrust belt affecting the Oligocene-Miocene continental sedimentary sequences of the San Pedro Formation. Unconformably overlaying this sequence, Upper Miocene-Pliocene tuffs and clastics with varying intensities of deformation are recognised along the northern segment of CdS, where it ends covered by the volcanic arc.

The deformation of CdS and the western border of the SdA have been suggested as a consequence of the inversion of a normal fault that delimits the basin or as an eastward propagation of the thrusting of Cordillera de Domeyko. Moreover, the presence of salt intervals and domes within the San Pedro Formation made some authors propose the existence of halokinesis. In the present work, we aim to investigate the actual tectonic regime of the CdS fold-and-thrust belt. Our objective is to determine spatial and temporal strain variability of CdS to contribute to the understanding of how this mountain belt evolved and how deformation is partitioned at its northern prolongation under the volcanic edifices.

Detailed geological mapping and the construction of seriated cross-sections will allow us to determine variable spatial patterns of deformation affecting the tuff-rich succession, spanning from 9 to 1 Ma. In addition, we will obtain temporal patterns of deformation at the scale of 10³ to 10⁵ yr using tectonic geomorphology indicators, such as deformed strath terraces and Holocene salt cave conduits.

Our preliminary results suggest that a compressional tectonic regime is progressively deforming the Upper Miocene-Pleistocene succession of CdS. Moreover, the evolution of drainages from the south-facing slope of the volcanic arc towards the SdA competed with the folds and thrusts, and the major channels developed along thrusts and synclines. This competition is going on also in the Middle to Late Pleistocene as documented by deformed fluvial strath terraces, which we are currently dating with Infra-Red Stimulated Luminescence. The age assessment of deformed terraces and cave conduits will allow us to model the slip rates of the thrust structures at different time scales.