Quaternary deformation and stress field in the Río de la Plata Craton (Southeastern South America)

Abstract

The aim of this research was to analyse the deformation in an intraplate region of southeastern South America. We analysed the motion velocities of GNSS permanent stations to characterise the stress field in the Río de la Plata Craton (RPC). To this end, a multi-year solution of the SIRGAS-CON continuous monitoring geodetic network was analysed. Seven stations in the cratonic region in the Pampean flat-slab subduction latitudes were considered. The solution encompasses the weekly solutions from January 2000 up to one day before the Maule earthquake on February 26th, 2010. The GNSS stations that produce significant results are located in Argentine territory and have a time span of at least 2 years. The stresses inferred from the strain rate tensors obtained from triangle arrays and relative displacement velocities between stations (baselines) suggest a compressive/transpressive regimen in the central-western RPC. On the basis of the GNSS velocity field, we can estimate maximum compression axes ranging from 3.5 to 5.3 nano-strain with W-E trends (90.3°-93.8°E), which differs slightly from the convergence vector between the Nazca and South American plates (ENE/78°E). This result strongly suggests the influence on the South America mid-plate from far-field stresses generated in the plate margins and the rotation of the maximum compression axes due to the growing influence eastward from the Mid-Atlantic ridge push. Satellite gravimetric data allow us to interpret the presence of upper crustal structures, which match surface morphostructural features. It is reasonable to think that the structures concentrate the recent deformation. The geometry of the subsurface structures inferred appears to favour reverse slips, compatible with the stress field interpreted from geodetic data.Regional wetland layers interpreted to be planar geomorphic markers, currently situated at elevations higher than the maximum sea levels reached over the Quaternary, can be interpreted as evidence of deformation and regional uplift. The compressive stresses appear to have substantial control over the uplift in the wide lowland areas of the Chaco-Parana intracratonic basin. The geomorphologic analysis suggests lineal uplift rates from 0.025 to 0.050 mm yr-1 for the central RPC in the Quaternary time scale and 0.1-0.35 mm yr-1 for the western RPC in the Late Pleistocene time scale.Although geodetic data are not conclusive evidence of seismic hazards, the confluence of geodetic and geologic data allows us to presume that deformation energy is being accumulated in the main fault systems.