Tectonic styles and crustal shortening of the Central Andes “Pampean” flat-slab segment in northern Chile (27–29°S)

Abstract

The Andean orogenic belt, located in the Central Andes “Pampean flat-slab” segment in northern Chile (27–29°S), is composed of two major tectonic regions: the Coastal Cordillera and the Frontal Cordillera. To understand their internal tectonic styles, history of growth and the shortening absorbed by the upper crustal structure of this segment, we combined regional geological mapping data, new ages obtained from radiometric U–Pb dating, and a semibalanced and restored cross-section 225.18km in length. The results as shown in the previous Mesozoic extensional fault systems, established in northern Chile by the Gondwana breakup, have played a fundamental role in the orogenic buildup. The central structure is characterized by an asymmetric basin (Upper Cretaceous–Paleocene) confined by a doubly vergent fault system composed of inverted faults related to the edges of the Mesozoic Chañarcillo and Lautaro Basins. The U–Pb geochronological data obtained from synorogenic volcano-sedimentary deposits and the angular unconformities recorded between the Cenozoic geological units have revealed that the compressive deformation in this segment started at around ~80Ma by tectonic inversion in the eastern Coastal Cordillera and western Frontal Cordillera, however, the presence of Paleocene and Miocene synorogenic successions at the footwall of the basement reverse faults of the Frontal Cordillera suggests a migration of Andean deformation from the west to the east during the Paleocene–Miocene by propagation of ramps involving inherited basement highs. The pre-compression restoration makes it possible to estimate 40.94km of minimum shortening, concentrated by inversion anticlines and fault-controlled basement highs across the Frontal Cordillera.