Evidence for Cenozoic extensional basin development and tectonic inversion south of the flat-slab segment, southern Central Andes, Chile (33°–36°S.L.)

Abstract

The mainly volcanic Cenozoic deposits that make up much of the western part of the Principal Cordillera in Central Chile are generally subdivided into two major units: an older Abanico or Coya-Machalı́ Formation and a younger Farellones Formation. Difficulty in differentiating these units has led to considerable debate. On the basis of the wide distribution, great thickness, and presence of sedimentary intercalations, it has been postulated that these arc volcanics were deposited in an intermontane basin; more recently, it has been proposed that this basin developed under extensional conditions and underwent subsequent tectonic inversion. We present field, geochronologic, geochemical, and thermal maturity data that support the latter interpretation. Collectively, this new information clarifies the stratigraphic, tectonic, and paleogeographic evolution of these deposits. The vast geographic extent of the Abanico Formation and lateral equivalents, which reach from at least 32°30′ to 44°S along the Principal Cordillera, its great thickness, and the presence of repeated thick fluvial and lacustrine intercalations all indicate deposition in a large, strongly subsident, and probably north–south oriented basin, developed between middle to late Eocene and Oligocene. The unconformable contact with underlying Mesozoic units observed at several localities indicates that deposition followed a substantial erosional episode during late Cretaceous and/or early Cenozoic time. Basal deposits of the Abanico Formation near Termas del Flaco increase rapidly in thickness to the west. Still further to the west, a thick Abanico section contains, in its upper part, mammal fossils older than those found in the basal deposits near Termas. This evidence indicates a major space of deposition west of this locality, which had been filled before deposition took place at Termas. The east-vergent, high-angle El Fierro thrust fault on the east side of the westward-growing deposits is interpreted as an inverted normal fault associated with initial basin development and deposition. High-angle thrust faults observed elsewhere on the eastern outcrop margin of the Abanico Formation (i.e. the Chacayes-Yesillo Fault in the Maipo section and the Espinoza Fault in the Cachapoal–Las Leñas section) also have been interpreted as inverted normal faults. The irregular folding style of the Abanico Formation, with its highly variable amplitude, longitude, tightness, and vergency, suggests that deformation is attributable to the inversion of faults associated with basin development. Geochemical characteristics of the Abanico Formation indicate a relatively thin crust during early basin development. Thermal maturity data reflect a deep burial of the deposits during accumulation, and thermal modeling indicates high heat flow conditions during burial. These data support a major extensional episode of the crust and the development of a large depositional space (basin) in this region. On the basis of this evidence, we suggest that deposition of the Abanico Formation is related mostly to crustal extension and its deformation to tectonic inversion. In the western Las Leñas river valley, a growth structure indicates that deformation occurred between 20.8 and 16.1 Ma, while the Abanico Formation was still being deposited. Deformation apparently did not occur coevally throughout the region; however, sedimentation and volcanic deposition in the basin apparently occurred uninterrupted. This argues against a single, obvious unconformity separating the Abanico and Farellones Formations. Instead, it supports the existence of local angular unconformities where fault inversion affected the basin fill. Comparison of the timing of extensional basin development and subsequent contraction (inversion) with the convergence rates between the Nazca and South American plates during the Cenozoic period shows a correspondence with periods of decreasing and increasing convergence rates, respectively. Tectonic and volcanic events on the east versant of the Andes [Journal of South American Earth Sciences 15 (2002)], which are coeval with the basin inversion and crustal thickening episodes presented herein and, therefore, with the previously mentioned period of increasing convergence rate, are assumed to correspond with the same episode of major tectonic accommodation of the crust in this Andean region. It is not yet possible to determine if the collisional event of the Juan Fernández ridge at approximately 15 Ma in the flat-slab segment region had a local or a more regional effect on the late Cenozoic tectonic evolution of the continental margin in the Central Andes.