Burial history and estimation of ancient thermal gradients in deep synorogenic foreland sequences: the Neogene Vinchina basin, South-Central Andes. International Symposium on Andean Geo-Dynamics.

COLLO, GILDA; DÁVILA, FEDERICO M.

Niza, Francia

Simposio; 7th International Symposium on Andean Geo-Dynamics; 2008

The Miocene to Pliocene Vinchina basin (Vinchina and Toro Negro formations) constitutes one of the thickest foredeep sequences of the Central Andes, between the High Cordillera and the Sierras Pampeanas province (~ 28° SL). In spite of its great thickness, locally >10 km, strata lacks of diagenetic signatures, even in the basal section of the stratigraphic column, where it is expected to be identified (assuming middle geothermal gradients of 20-30ºC/km) rocks with evidences of low-grade metamorphism. In order to unravel the burial history of the Vinchina basin, and to estimate a Mio-Pliocene paleogeothermal gradient for the Central Andean region, we evaluated the progression of the illitization processes on fine-grained rocks that affected this sandy-silty dominated alluvial succession. Relations between interstratified clay mineral distribution and temperature (eg., Arostegui et al., 2006; Srodon, 2007) allowed to estimate the diagenentic history and the maximun burial conditions. Clay mineral associations were identified by X-ray analysis in <2µm fractions of 5 samples from bottom to top in the Quebrada de los Colorados section (28°41´S, 68°16´W; La Rioja Province). Relative proportions of interstratified illite/semectite (I/S) within the neoformed phases were established from decomposition of the XRD diagrams (cf. Lanson, 1997).