The Varillar Basin: An example of basement-involved tectonism in the Central Andes of northern Chile

Abstract

The Frontal Cordillera in the Central Andes of northern Chile is a basement-involved fold-and-thrust belt characterized by large ranges composed of Paleozoic crystalline rocks, and intermontane contractional basins that have recorded much of the Mesozoic to Cenozoic deformation history of the orogen. At the core of the Frontal Cordillera, along the transition zone between the normal and flat-slab subduction segments, the Varillar Basin serves as a natural laboratory for studying the styles and mechanisms of basement-involved deformation resulting from Andean orogenesis. In this study, we present a novel structural analysis of the basement-involved structures present in this basin by combining field and seismic data with balanced and restored cross-sections. On the surface, the dominant structural features include NNE-striking basement-cored anticlines associated with steeply east and west-dipping reverse faults. These faults uplifted and exhumed large Paleozoic blocks, which are interpreted to be the pre-rift basement for Late Permian to Jurassic rift-related basins. The available seismic line revealed the existence of partially inverted half-graben structures beneath the basin, suggesting that the tectonic inversion of preexisting normal faults is responsible for the basement-involved deformation. This is further evidenced by the occurrence of folded Triassic and Jurassic syn-rift strata, which have been partially expelled from their original depocenters and are now elevated nearly 3 km above their regional datum. Restoration of three cross-sections to their pre-shortening state indicates that steeply dipping reverse faults (e.g., Varillar and Cerro Guerrita faults) accommodated approximately 3 km of crustal shortening on average. Some of these faults have also truncated and overthrust inverted normal faults (e.g., Border Fault), resulting in complex structural arrays. While the exact timing of basement-involved deformation remains uncertain, previous K-Ar age determinations of synorogenic strata suggest that the contractional structures were active during the Paleocene. However, other previously reported low-temperature thermochronological data (apatite fission track) from surrounding areas indicate an Eocene age for the basement-involved tectonism of the Frontal Cordillera.