The Pipanaco Basin, northern Argentine broken foreland, and its connection with the Andean plateau rise

JULIETA NÓBILE; FEDERICO M. DÁVILA; MARIO GIMENEZ; RICARDO A. ASTINI

Mendoza

Congreso; 18th International Sedimentological Congress, Sedimentology at the foot of the Andes; 2010

IAS, RAS

The Pipanaco basin (PB) is a flat, high-elevated (~1 km a.s.l.) alluvial intermontane basins with an areal extension of >10000 km2. It is located in the northern end of the Argentine broken foreland province -or Sierras Pampeanas- where basement-thrusting controls the topography and local basement uplift rates, basin formation and accommodation, climate and landscape. Although there are no exposures across PB, a large geological and geophysical database exists from the surrounding regions and subsurface. The aim of this contribution is to integrate a detailed stratigraphic survey with new geophysical approaches and DEM analysis to give support to some of the proposed models that explain high-elevated basins and plateau formation in the Andes. By using a gravity anomaly filter (densities <2.4 g/cm3) we found that the sedimentary thickness signals in the PB vary from W to E from ~1 km to ~3.5 km. The stratigraphic of the surrounding ranges allows to explain this accommodation geometry by ~2000 m of Neogene + ~1000 m of Quaternary. Such a W-E arrangement can be reproduced considering an elastic flexural model with the tectonic load being equivalent to the Plio-Pleistocene uplift of the Ambato range (~5.5 km), the effective elastic thickness 40 km and a density of loads of 2700 kg/m3. The western side of the basin was in turn, affected by the activity of a buried basement thrust detected by a gravity Euler deconvolution and analytical signal analysis. This hidden thrust drove an exhumation of ~1000 m after Neogene sedimentation, preventing Quaternary accumulation in this part of the PB. When we contrast our gravity results in the PB with available seismic sections in other surrounding intermontane depocenters (Puna boundary, Campo de Arenal Basin to the north) and in the core of the broken foreland (La Rioja Basin to the south), a remarkable increase in sediment filling is observed to the south. Records vary from 2000 m to 3500 m indicating an important accommodation difference. In addition, the top of basement elevation varies from ~2000 m a.s.l. in the N (Campo del Arenal) to -2700 m a.s.l. in the S (La Rioja Basin). A swath topographic profile along ~66ºW, between 26ºS and 29ºS, depicts southward reduction of altitudes, ~3500 m to the north and 400 m to the south. Also the PB shows a particularly high-elevated depositional surface (~1000 m a.s.l.) and sediment thicknesses larger than in the northern intermontane basins. Altogether these suggest no obvious relationships between sedimentary aggradation (including cumulative thickness), basin rim relief and the formation of high-elevated surfaces. Thus, we conclude that deep-seated geodynamic processes (e.g. delamination) are the driving mechanisms to form high-elevated basins and plateaus, supported by the clear elevation differences of top of basement from N to S and a compilation of low-temperature cooling ages along this transect indicating differential exhumation to the north. In addition, our data are consistent with a rapid uplifting of the plateau as suggested by current hypothesis.