Isotopic and thermochronologic evidence of extremely cold lithosphere associated with a slab flattening in the Central Andes of Argentina

Abstract

We present mineralogic, isotopic and thermochronologic analyses on psammopelitic andtuffaceous levels from the Bermejo and Vinchina basins ? both foreland depocentres of theCentral Andes of Argentina ? that define a low-temperature regime for the crust akin to a slabshallowing and flattening process. The contents of illite in illite/smectite interstratified (I/S)show a progressive illitization into the deeper parts of both basins. The distribution of I/S iscompatible with theoretical simulations and predicted heat flow values of ca. 26 mW m2 in the8?3.4 Ma interval for the Vinchina Basin and ca. 42 mW m2 since 9 Ma for the BermejoBasin. The latter shows heat flow values that are comparable to those reported by magnetotel-luric analysis (36?40 mW m2) in agreement with previously published heat flow calculationsalong the modern Andean foreland. The Rb?Sr isochrones in psammopelites (<2 lm fractions)show ages between 125 and 165 Ma, whereas the K?Ar ages decrease as the grain size is smaller(136?224 Ma for 1?2 lm, 112?159 Ma for 0.2?1 lm, 76?116 Ma for <0.2 l and 39.3?42 Mafor <0.1 lm). These ages are significantly older than the sedimentation in the basins (ca. 16 Mafor the Vinchina Basin; U?Pb age), and can be explained by the presence of a significantamount of detrital components, mainly illite, even in the finer fractions. The preservation ofdetrital ages is consistent with the shallow diagenesis related to a low-temperature regime,proposed here for the basins. Younger K?Ar ages (21.3?12 Ma) were obtained for a basaltuffaceous level. Clay mineralogy and R0 ordering in the deepest part of the Vinchina Basin,together with the evolution model of I/S with depth, suggest that the burial temperatures wouldhave not exceeded ca. 100°C in agreement with (U?Th)/He analyses performed on apatiteextracted from two tuffaceous units. Thermal indicators from both studied basins confirm theexistence of a low-temperature regime during flat subduction.