The Abanico extensional basin: Regional extension, chronology of tectonic inversion and relation to shallow seismic activity and Andean uplift

The main volcanic Cenozoic deposits in the cordillera of Central Chile (33°-36°S) correspond to the middle?-late Eocene to early Miocene Abanico Fm., and the early-middle Miocene Farellones Fm. The wide distribution of the Abanico Fm., its great thickness, the frequent and thick lacustrine intercalations, the unconformable stratigraphic relation with underlying units, and its tectonic features suggest deposition in an extensional basin that underwent subsequent tectonic inversion. This interpretation is supported by recent field observations, and geochronologic, geochemical, and thermal maturity data. Extensional basin development began before 36 Ma, while the crust was relatively thin, and went on during Oligocene. Contraction occurred during and following late depositional stages of the Abanico Fm. and was controlled by inversion of extensional faults associated with basin development. This event began before 21 Ma, ended at ∼ 16 Ma, was not simultaneous throughout the region, and coincided with a thickened crust. Sedimentation and volcanism continued throughout the contractional episode along the axis of the preexistent basin and formed the Farellones Fm., while the inverted and exhumed west and east parts of the Abanico Fm. were eroded. This situation explains the absence of a clear structural boundary between the two formations at some localities, the existence of one or more angular unconformities at various localities, and that radioisotopic dates for the Abanico Fm. at some regions are younger than those obtained in low levels of the Farellones Fm. in other regions. The extensional and contractional episodes coincide with periods of slow and rapid convergence rates between the Nazca Plate and South-America respectively. The oldest ages for the base of the Farellones Fm. (25 and 21 Ma) were obtained to the north of the study region, and the youngest ages for the Abanico Fm. (16.1 Ma) to the south of it suggesting a N to S progression of deformation. This can be attributed to the southward progression of the intersection of the Juan Fernández Ridge with the continental margin.