Effects of pre‐orogenic tectonic structures on the Cenozoic evolution of Andean deformed belts: Evidence from the Salar de Punta Negra Basin in the Central Andes of Northern Chile

Abstract

AbstractWe integrated new field observations, two‐dimensional (2‐D) seismic profiles and new and previously reported chronological data to understand the effects of pre‐orogenic structures on the tectonic evolution of the Salar de Punta Negra in the Central Andes. For first time a series of restored geological cross‐sections are presented, thus showing the pre‐orogenic tectonic architecture of the region and new ideas about the tectonic evolution of the inner forearc of the Central Andes. Our results show a series of east‐dipping normal faults as the main pre‐orogenic structures in the region, which resulted from lithospheric stretching of the western continental margin during the Paleozoic to Mesozoic (Triassic–Jurassic). These were later incorporated into the Andean orogen by tectonic inversion, forming west‐verging inversion anticlines. The beginning of the tectonic inversion is constrained by the first on‐lap of the Upper Cretaceous‐Palaeocene syn‐kinematic deposits on the top of the Mesozoic syn‐rift successions, highlighting that inversion occurred during this period. These syn‐kinematic deposits display zircons with older age peaks between ca. 200 and 300 Ma, thus indicating that some Carboniferous to Triassic sources of sediments were eroded during the uplift of the orogen. Other basement reverse faults affect the footwalls of normal inverted faults and the shoulders of ancient half‐graben structures. These truncate and decapitate previous inverted faults and completely cut the infill of the basin, leading to exhumation of the pre‐rift basement rocks. We propose that the propagation of these structures was favoured by the modified thermal‐tectonic state of the lithosphere from the eastward migration of the volcanic arc, and not by the previous pre‐orogenic structures. The structural and stratigraphic relationships recognized both in the field and 2‐D seismic profiles indicate that many reverse faults originated after the initial tectonic inversion and continued to be active from the Eocene until the Pleistocene period.