Extensional tectonics during Late Cretaceous evolution of the Southern Central Andes: Evidence from the Chilean main range at ~35°S

The study of Late Cretaceous-earliest Paleogene series in the Chilean Andes at ~35°S provides new constraints on the coeval evolution of the area and the Southern Central Andes. These correspond to the continental series of BRCU and the overlying Plan de los Yeuques Formation (PYF). Zircon age determinations reveal that these represent an almost continuous record of deposition, respectively from the Cenomanian-early Campanian to the Campanian-Danian. According to paleogeographic reconstructions, the BRCU series corresponds to the coeval foreland basin deposits, in a near arc position, which was westerly bounded by the earliest Late Cretaceous orogen. Detrital zircon age spectra for this reveal only a Cretaceous component, thus indicating a depocenter disconnected to that represented by coeval synorogenic deposits in Argentina. The volcanic rocks of the PYF record an eastern migration of the magmatic loci into the area at least since ~80 Ma. Its outcrops display a series of syn-sedimentary normal faults indicating extensional conditions throughout its deposition. Its volcanic rocks conform an apparent bimodal suite characterized by arc-like affinities, low pressure conditions for the corresponding magmas, and juvenile isotopic signatures for all the compositional spectra. Comparison and modeling of such compositional characteristics indicate a derivation from parental magmas similar to those currently sourcing the southern segment of the Andean Southern Volcanic Zone (~37–42°S). Altogether these characteristics suggest an extensional intra-arc basin setting for the deposition of this unit probably framed by a normal to thin crust of ~30–35 km. The extensional conditions observed for this unit are also recorded in numerous Late Cretaceous arc deposits along the Andean margin, thus indicating that this corresponds to a regional scale feature. This evidence records a change from Late Cretaceous from compressional to extensional conditions in the arc area which agrees well with major plate tectonic configuration proposed for this time span in the Andean margin.