Burial history and estimation of ancient thermal gradients in deep synorogenic foreland sequences: the Neogene Vinchina basin, south-central Andes

GILDA COLLO; FEDERICO M. DÁVILA; JULIETA NÓBILE; RICARDO A. ASTINI

Jujuy

Congreso; XVII Congreso geológico Argentino, Jujuy; 2008

AGA

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). In the analyzed samples of the Vinchina Basin, the dominant neoformed clay mineral phases are illite and interstratified illite/smectite (I/S), with lesser amounts of chlorite. Detailed analysis of expandable I/S allowed establishing the coexistence of interstratified with R0, R1 and R3 orderings. In the shallowest sample (uppermost section of the Toro Negro Formation, Fig. 1) the clay mineral assemblage is dominated by R0 (~70%), R1 and illite phases, with absence of R3 ordering. The appearance of R3 takes place at ~5 Km depth within the Vinchina Formation and, likewise R1 and illite, show an increment towards the base of this unit. Although randomly, mixed-layered R0 clearly decreases toward the deeper levels, significant proportions (~30%) in the I/S phases are still present in the lowermost analyzed sample (depth of ~7 Km). Distribution of I/S interstratified phases through the succession allow establishing a progressive smectiteillitization process (R0 —> R1 —> R3 —> I) related to the sedimentary burial history of the Tertiary succession, as shown by the increment in the I/S ordering and illite content from top to base of the units and a strong correlation between the degree of illitization and the stratigraphic age of the rocks (Davila et al., this congress). The presence of R0 randomly mixed-layered I/S even in the deepest levels (~7 Km) evidences that the base of the basin fill did not exceed the diagenetic field (cf. Frey and Robinson 1999). Given that R0 becomes unstable over temperatures of 120ºC a maximum paleogeothermal average gradient of 17ºC/Km can be estimated. This value is coherent with thermochronological studies that suggested the sequence would not have exceeded temperatures of ~90ºC (Coughlin 2000). Our values are also consistent with the coldest geothermal records reported in the modern Andean foreland (see Hamza et al. 2005). An Early Miocene-Pliocene age (between 19 - 3.4 Ma) for the Vinchina stratigraphy is interpreted from new geochronological data (Re y Barredo 1993, Dávila et al. this congress) and is coincident with the onset of the flat subduction at these latitudes (Kay et al. 1988, Dávila et al. 2004). Within this context, crustal refrigeration could associate to modifications in the thermal structure by reduction of the astenospheric wedge. Supporting this hypothesis, similar geothermal gradients (18-20ºC/Km) were calculated for 5-km depth oil boreholes in the Bermejo Valley (Precordillera de San Juan), which also above the modern flat-slab segment. Exhumation ages for the Tertiary package are, however, not well constrained. The presence of subhorizontal Pleistocene(?) conglomerates (Santa Florentina Fomation) unconformably lying above the Vinchina and Toro Negro Formation on the same thrust sheet allows interpreting that exhumation would have occurred between the youngest age of Toro Negro Formation (3.4 Ma) and the deposition of this coarse conglomeratic succession. Given that the age of deposition of the Santa Florentina Formation is unknown, but considered Pleistocene sensu lato, the maximum residence interval for the Tertiary under extreme burial conditions would be £3.4 my. Although the estimated maximum paleogeothermal gradient (~17ºC/Km) is consistent with those from other coldest foreland basins, with the available thermocronologic data we cannot discard the influence of other factors in the persistence of R0 I/S over the 7 km depth. "Effective K+ concentration" (Cuadros 2006) and "residence time" of the sequences at maximum burial conditions may have interfered in the evolution of clay mineralogy, retarding the progression in the I/S ordering. Modeling of smectite-illitization process and comparisons between correlative exhumed and buried successions will allow further evaluation of the influence of these factors.