Interstratified illite/smectite in the MIoCene Central Andean foreland: evolution of clay minerals under very low-paleogeothermal gradients

COLLO, GILDA; DAVILA, FEDERICO; NIETO, FERNANDO; NÓBILE, JULIETA; ASITNI, RICARDO A.

Castellaneta Marina, Italia

Congreso; XIV International Clay Conference; 2009

The Vinchina basin (Vinchina and Toro Negro formations) constitutes one of the thickest foredeep sequences (locally >10 km) of the Central Andes of Argentina (~ 28° SL). Its Early Miocene-Pliocene depositional age (19-3.4 Ma) is coincident with the onset of the flat subduction at this latitude. Distribution of I/S interstratified phases through this alluvial succession are consistent with a progressive burial smectite-illitization (R0 ® R1 ® R3 ® I; Collo and Dávila, 2008). The clay mineral assemblage of the shallowest sample (~1.25 Km depth) is dominated by R0 (~70%), R1 and illite, with absence of R3 ordering. The I/S structural formula, calculated from AEM analyses on the basis of 11 oxygens, show compositional variations with Si contents ranging from 3.41 to 3.94 a.p.f.u., layer charges between -0.25 and -0.74 and K, Na and Ca as interlayer cations. The appearance of R3 takes place at ~5 Km depth and, likewise R1 and illite, shows an increment towards the base of the unit. Although R0 clearly decreases to the deeper levels, significant proportions (~30%) are still present in the lowermost analyzed sample (~7 Km depth), as is evidenced by the AEM showing phases with Si contents of 3.91 a.p.f.u. and layer charges of -0.28. Given that R0 becomes unstable over temperatures of ~120ºC its presence even in the deepest levels allows estimating a maximum paleogeothermal gradient of 17ºC/Km. Although similar low-gradients (~20°C/Km) have already been found in the modern Andean “flat-slab” foreland (Hamza et al., 2005), a short residence time of the sequence at maximum burial conditions (~3.4 Ma) and a reduction of the thermal regime as consequence of high sedimentation rates (blanketing effect, cf. Husson and Moretti, 2002) should be considered as factors retarding the progression of the I/S.   Collo, G. and Dávila, F. 2008. Burial history and estimation of ancient thermal gradients in deep synorogenic foreland sequences: the Neogene Vinchina basin, South-Central Andes. 7th ISAG, Nice, 152-155. Hamza V.M., Silva Dias, F.J.S., Gomes, A.J.L., and Delgadilho Terceros, Z.G. 2005. Numerical and functional representations of regional heat flow in South America. Physics of the Earth and Planetary Interiors 152: 223–256. Husson, L. and Moretti, I. 2002. Thermal regime of fold and thrust belts—an application to the Bolivian sub Andean zone. Tectonophysics 345: 253–280. Keywords: Illitization progress, low-paleogeothermal gradient, Paleogene Central Andean foreland.