Gilda Collo

U–Pb detrital zircon ages and Sm–Nd isotopic features in low-grade metasedimentary rocks of the Famatina belt: implications for late Neoproterozoic–early Palaeozoic evolution of the proto-Andean margin of Gondwana

Abstract: The Famatina belt, Central Andes, is part of an ancient accretionary margin built along Western Gondwana in the early Palaeozoic. U–Pb ion microprobe analysis of detrital zircons and Sm–Nd whole-rock analysis of two early Palaeozoic low-grade metasedimentary units record the early evolution of this region. Detrital zircons in the Negro Peinado and Achavil formations have ages ranging from Palaeoproterozoic to Cambrian, consistent with derivation from Gondwanan sources. TDM ages suggest that the sedimentary rocks were derived from a composite source area, which separated from the mantle during the Palaeoproterozoic (c. 1.8–1.6 Ga). Constraints from the youngest detrital grains indicate accumulation in a Mid- to Late Cambrian foreland basin adjacent to the inboard Pampean orogenic tract. The dominance of Cambrian ages in the Negro Peinado Formation suggests derivation principally from the eastern Pampean belt whereas the dominance of late Neoproterozoic ages in the Achavil Formation suggests that input from the Pampean belt was overwhelmed by older sources. The paucity of Palaeoproterozoic ages argues against direct input from older areas such as the Río de la Plata craton. The predominance of Meso- and Neoproterozoic ages over older sources suggests that a Brasiliano-age magmatic arc developed on a Mesoproterozoic basement, probably a southern extension of the Arequipa–Antofalla massif.

Geothermobarometry of very low-grade metamorphic pelites of the Vendian–Early Cambrian Puncoviscana Formation (NW Argentina)

The Vendian–Early Cambrian Puncoviscana Formation is a pelite-greywacke turbidite sequence affected by polyphase deformation cropping out extensively in the Cordillera Oriental of northwestern Argentina. Previous X-ray diffraction and analytical high-resolution TEM studies on southern locations found anchizonal grade and proposed medium–high pressure metamorphism followed by higher thermal conditions. We have determined the chemical composition of mica and chlorite with EDX on small uncontaminated areas selected using SEM in backscattered electron mode. The application of mica-chlorite geothermobarometry to mineral grains defining two different foliations has produced consistent pressure/temperature results based on the intersection of six reactions: 9 kbar/250 °C peak conditions were followed by an isothermal decompression to 3 kbar and a successive increase of temperature to nearly 350 °C at 1.5 kbar. The shales from the overlying Meson and Santa Victoria Cambro–Ordovician groups do not contain chlorite, thus chlorite-mica geothermobarometry could not be applied. According to the range of KICIS obtained (0.41–0.68Δ°2θ) and their mineralogical assemblage (illite ± kaolinite ± corrensite + Qtz + Pl ± Kfs), these rocks have not surpassed diagenetic conditions, which implies a minimum temperature difference with the Puncoviscana Formation of 100 °C, consistent with the sharp angular unconformity between the two units.

Mantle Influence on Andean and Pre-Andean Topography

Mantle convection can drive long-wavelength and low-amplitude topography, which can occur synchronously and superimposed to tectonics. The discrimination between these two topographic components, however, is difficult to assert. This is because there are still several uncertainties and debates in the geodynamic community, for example, the scales and rates of dynamic topography. Geological, geomorphological, geophysical measurements, and/or landscape analyses might assist to validate models. In this contribution, we provide new geological evidences along the Central and Patagonian Andes, which demonstrate that dynamic topography has been an important component on the South American landscape formation as well as in the ancient western Gondwana. Our examples in the Argentine Pampas show that dynamic topography is required to explain not only the basin subsidence but also the whole observed topography. We also suggest that the dynamic components in this region are much lower than numerical models (average dynamic subsidence rates of ~0.04 mm/yr—this work— which contrast with the ~0.1 mm/yr estimated in the US). We also propose two strategies to analyze ancient cases. The first requires of comparing a total elevation proxy, like the equilibrium lines (or ELA) in glaciated areas, with model topography derived from geochemical studies of mantle rocks. A second strategy was the analysis of the Triassic rifting evolution of western Argentina (post-rift sag deposits). Sag deposit thicknesses exceed 2 km, which do not correlate with the 100 m thick thermal calculated by rift subsidence modeling.

Basin Thermal Structure in the Chilean-Pampean Flat Subduction Zone

Flat-slab segments are considered refrigerated areas given that the asthenospheric wedge is forced to shift hundreds of kilometres away from the trench, and the flat and coupled subducting plate acts as a thermal insulator. Although lithospheric-scale thermal analysis based on numerical modelling and geophysical observations abound, studies on the thermal history of sedimentary basins are scarce. In this contribution, we present a temperature data compilation from more than 60 oil wells within the Chilean-Pampean flat-slab segment and the transitional zones to normal subduction to the north and south in the south-central Andes. The geothermal gradient data are correlated with basin-basal heat flow estimated from 1D modelling, Curie point depths derived from aeromagnetic surveys, and previous crustal and lithospheric thicknesses estimations. Their distribution evidences a quite good consistency and correlation from region-to-region. Our modelling demonstrates that sedimentation changes are not sufficient to explain the variations illustrated in the geothermal gradient map, and that basal heat flux variations are required to reproduce the reported values. According to our results, the coldest basins develop over the flat slab or cratonward regions, whereas the highest temperatures on areas where the slab plunges. This suggests that the flat-slab geometry as well as the lithospheric structure affects the thermal state within the upper crust and particularly the sedimentary basins. Further studies will allow improving our database as well as the knowledge about the radiogenic contribution of the lithosphere and the asthenospheric heat input to the basins basal heat flow.