Holocene shortening rates of an Andean-front thrust, Southern Precordillera, Argentina

Abstract

A significant part of the Quaternary shortening between the Chilean trench and the relative stable interior of the South American plate at the Pampean flat slab (27–33°S), has been accommodated at the eastern foothills of the Andes and mainly within a narrow neotectonic belt along the eastern side of the Argentine Precordillera. Tectonic geodesy results point out that this area is being shortened at a ~2–4 mm/a rate, whereas shortening rates estimated over longer time periods (1–20 Ma) suggest values ranging from 1 mm/a to 16 mm/a. Geomorphic and geologic evidence indicate that the east-directed Las Higueras Thrust System is one of the main structures that has accommodated Quaternary deformation at this section of the Andean orogenic front (32° 05′–32° 35′S). An outcrop exhibiting the thrust propagation into fluvial sediments allows the Holocene shortening rates at the northern end of this structure to be estimated, based on retrodeformation of Holocene strata and radiocarbon dating of two charcoal-bearing beds. Estimated shortening rates yielded mean values of 1.90 ± 0.28 mm/a for the last 4495 ± 143 cal yr BP and 1.53 ± 0.26 for the last 8245 ± 48 cal yr BP. These results pose some uncertainties due to the incompleteness of the exposed deformation at the hanging wall. However, they correspond to a key timescale which helps to bridge the gap between rates derived from the short-term GPS data and the longterm permanent deformation rates obtained through geologic studies. Although the estimated rates suggest that slip on the thrust could have accelerated during the last ~4 ka, more data are necessary to reliably address this key issue.