Uplift rates versus exhumation rates in the northern argentine broken foreland

JULIETA C. NÓBILE; FEDERICO M. DÁVILA

Heidelberg

Congreso; 23rd Latin American Colloquium LAK 2014; 2014

The Argentine broken foreland topography shows evidence of differential surface uplift fromS to N with a remarkably topographic step located at 28°SL (Dávila et al., 2012). How fast is therelief growing? The use of longitudinal river profiles to determine uplift rates has demonstratedto be a good quantitative technique (e.g., Roberts and White, 2010). We calculated the upliftvelocities of Sierra de Ambato and Sierra de Aconquija (northernmost Argentine broken foreland)using the inverse algorithm derived from the equation ∂z/∂x=U(t)-νxm(∂z/∂x)n + κ(∂2y/∂x2)(Roberts and White, 2010), where ν is the advective coefficient of erosion, m and n are positiveconstants that control the concavity of a river profile, κ is erosional diffusivity. Independent geologicalstudies (low-temperature thermochronology, stratigraphy and structure, Dávila et al.,2012) proposed that the main uplift episode would have occurred at <5 Ma. We determined thebest uplift rate solutions from a set of results on the base of the geological constraints. For theSierra de Ambato the uplift rates are between 0.13-0.4 mm yr-1, whereas for the Sierra deAconquija the uplift rates vary between 0.6 mm yr-1 -0.2 mm yr-1. Sobel and Strecker (2003),nevertheless, estimated values of exhumation rate between 0.7-1.3 mm yr-1 based on low temperaturethermochronology. Exhumation rates are, consequently, almost the double respect tothe uplift rates. In a semiarid-arid region, like the Andean foreland system, it would be expectedan incomplete exhumation of the mountain range. Then, why erosion (exhumation) is fasterthan uplift? A plausible explanation might be a slower isostatic recovery of the foreland surfacetopography that could be related to a less elastic rheological behavior of the crust (viscoelasticor even plastic). We also considered the possibility of sublithospheric and non-isostatic forcesaffecting such equilibrium (e.g., dynamic topography, Davila and Lithgow-Bertelloni, 2013) pullingthe topography down and preventing the generation of relief (as indicated by the uplift fromthe river profiles).