Abstract

In the Andean forearc of northern Chile, the interplay between deformation and magmatism is not fully understood. Outcrops in the Precordillera of northern Chile record a long and complex history of magmatism and overprinting deformational events. The growth of the Precordillera is predominantly associated with the Late Eocene-Early Oligocene transpressive Domeyko fault system (Incaic Phase), however, a series of compressive late Cretaceous folds and faults have also been recognized. In this study, we combined structural mapping, stereo graphic fold analysis, and geochronologic data of both deformed rocks and post-tectonic hypabyssal bodies to reveal the distribution and kinematics of the late Cretaceous thrust system exposed in the Precordillera between 24 degrees 30'S and 25 degrees 30'S. We identified an approximately NS-striking belt of reverse faults and folds along the Precordillera, which was cut and displaced by the Late Eocene-Early Oligocene Domeyko fault system. This deformation corresponds to thick-skinned structures affecting Paleozoic basement and thin-skinned structures which include folding with a general sub-horizontal NNW-SSE shortening direction with westward and eastward vergence. The ages of post-tectonic hypabyssal bodies cutting the deformed rocks indicate that the timing of the contractional structures occurred before 86 Ma taking place in the westernmost margin of Precordillera and continued between 72 Ma and 65 Ma. Our results agree with the timing proposed for regional Peruvian and 'K-T' deformations. We also used the distribution of the syn-kinematic late Cretaceous volcanic rocks to identify late Cretaceous contraction that evolved into a back-arc fold-and-thrust belt. Its structures are currently distributed along the northern Precordillera and were obliterated by younger strike-slip Incaic deformation. Moreover, this deformation must have forced the topography upward, creating an early late Cretaceous topographic relief related to the tectonic activity along these structures. These results indicate that late Cretaceous deformation had a role in the early history of the proto-Precordillera of northern Chile, prior to development of the strike-slip Domeyko fault system in the Paleogene.