Triassic to Neogene Evolution of the Andean Retroarc: Neuquén Basin, Argentina

Abstract

The Andes Mountains provide an ideal natural laboratory to analyze the relationship between the tectonic evolution of a subduction margin, retroarc shortening, basin morphology, and volcanic activity. Timing of initial shortening and foreland basin development in Argentina is diachronous along strike, with ages varying by 20-30 million years. The Neuquén Basin (32°S-40°S) of southern-central Argentina sits in a retroarc position and provides a geological record of sedimentation in variable tectonic settings from the Late Triassic to the early Cenozoic including: 1.) active extension and deposition in isolated rift basins in the Late Triassic-Early Jurassic; 2.) post-rift back-arc basin from Late Jurassic-Late Cretaceous; 3.) foreland basin from Late Cretaceous to Oligocene; and 4.) variable extension and contraction along-strike from Oligocene to present. The goal of this study is to determine the timing of the transition from post-rift thermal subsidence to foreland basin deposition in the northern Neuquén Basin and then assess volcanic activity and composition during various tectonic regimes. The Aconcagua and Malargüe areas (32°S and 35°S) are located in the northern segment of the Neuquén Basin and preserve Upper Jurassic to Miocene sedimentary rocks, which record the earliest phase of shortening at this latitude. This study presents new sedimentological and detrital zircon U-Pb data from the Jurassic to latest Cretaceous sedimentary strata to determine depositional environments, stratigraphic relations, provenance, and maximum depositional ages of these units and ultimately evaluate the role of tectonics on sedimentation in this segment of the Andes. The combination of provenance, basin, and subsidence analysis shows that the initiation of foreland basin deposition occurred at ~100 Ma with the deposition of the Huitrín Formation, which recorded an episode of erosion marking the passage of the flexural forebulge. This was followed by an increase in tectonic subsidence, along with the appearance of recycled sedimentary detritus, recorded in petrographic and detrital zircons analyses, as well development of an axial drainage pattern, consistent with deposition in the flexural forebulge between 95 and 80 Ma. By ca. 70 Ma the volcanic arc migrated eastward and was a primary local source for detritus. Growth structures recorded in latest Cretaceous units very near both the Aconcagua and Malargüe study areas imply 35-40 km and 80-125 km of foreland migration between 95 and 60 Ma in the Aconcagua and Malargüe areas, respectively. Strata ranging in age from Middle Jurassic to Neogene were analyzed to determine their detrital zircon U-Pb age spectra and Hf isotopic composition to determine the relationship between magmatic output rate, tectonic regime, and crustal evolution. When all detrital zircon data are combined, significant pulses in magmatic activity occur from 190-145 Ma, and at 128 Ma, 110 Ma, 69 Ma, 16 Ma, and 7 Ma. The duration of magmatic lulls increased markedly from 10-30 million years during back-arc deposition (190-100 Ma) to ~40-50 million years during foreland basin deposition (100-~30 Ma). The long duration of magmatic lulls during foreland basin deposition could be caused by flat-slab subduction events during the Late Cretaceous and Cenozoic or by long magmatic recharge events. There are three major shifts towards positive Hf isotopic values and all are associated with regional extension events whereas compression seems to lead to more evolved isotopic values.