Modelo petrofísico del borde oriental de las sierras de Valle Fértil-La Huerta, Argentina, a partir de datos sísmicos y petrológicos

  • Ma. Florencia Ahumada Instituto de Investigaciones en Energías No Convencionales - Consejo Nacional de Investigaciones Científicas y Técnicas, Unidad de Recursos Geológicos y Geotérmicos. Universidad Nacional de Salta, Av. Bolivia No. 5150, C.P. 4400, Salta, Argentina.
  • Brígida Castro de Machuca Departamento de Geofísica y Astronomía, Centro de Investigaciones de la Geósfera y la Biósfera - Consejo Nacional de Investigaciones Científicas y Técnicas, Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de San Juan, Meglioli No. 1160 S Rivadavia, C.P. 5400, San Juan, Argentina. Instituto de Geología, Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de San Juan, Meglioli No. 1160, C.P. 5400, San Juan, Argentina.
  • Patricia Alvarado Departamento de Geofísica y Astronomía, Centro de Investigaciones de la Geósfera y la Biósfera - Consejo Nacional de Investigaciones Científicas y Técnicas, Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de San Juan, Meglioli No. 1160 S Rivadavia, C.P. 5400, San Juan, Argentina.
  • Jean-Baptiste Ammirati Departamento de Geofísica y Astronomía, Centro de Investigaciones de la Geósfera y la Biósfera - Consejo Nacional de Investigaciones Científicas y Técnicas, Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de San Juan, Meglioli No. 1160 S Rivadavia, C.P. 5400, San Juan, Argentina.
  • María Gimena López Departamento de Geofísica y Astronomía, Centro de Investigaciones de la Geósfera y la Biósfera - Consejo Nacional de Investigaciones Científicas y Técnicas, Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de San Juan, Meglioli No. 1160 S Rivadavia, C.P. 5400, San Juan, Argentina.
Keywords: petrophysical model, crystalline basement, flat slab subduction, Western Sierras Pampeanas, Argentina

Abstract

This paper is a contribution to the knowledge of the crustal structure of the eastern flank of the Valle Fértil - La Huerta ranges (Western Sierras Pampeanas, San Juan, Argentina) at 31°S, in the Andean foreland region, where the Nazca plate is subducting horizontally at about 100 km depth. A 1D velocity model was constrained, combining petrographic and seismological observations from analysis of 19 igneous and metaigneous plutonic rocks belonging to the Famatinian (Ordovician) magmatic arc, which make up most of the crystalline basement of these ranges. Granitoid lithologies predominate in the northern region whereas mafic lithologies are more common to the south. The seismological analysis consisted of modeling teleseismic receiver functions near three seismological stations: LUNA, MAJA and CHUC, in places where those rocks are dominant. Thus, P and S seismic-wave velocities (Vp and Vs) and Poisson´s coefficient (ν), among other elastic parameters, were obtained. The seismic velocity model indicates an overthickened crust with an average thickness between 55 and 60 km, which matches with global average values (~41km); this agrees well with the hypothesis of partial eclogitization in the lower crust. The presence of two seismic velocity discontinuities at mid-crustal levels (12 and 28 km depths), likely associated to décollements, might be related to the accretion of the Cuyania terrane to the Pampia terrane. We obtained low P seismic-wave velocities (Vp ~5.8 km/s), Vp/Vs ratio (~1.70) in upper crust levels consistent with granitoid lithologies, as well as high P seismic-wave velocities (Vp ~6.76 km/s), Vp/Vs ratio (~1.78) in lower crust levels; these figures match with mafic lithologies of a more dense (~3.00 g/cm3),  lower crust with respect to other back-arc Andean regions. Also, these values are consistent with the existence of mafic rocks composed of olivine, ortho- and clinopyroxene, which constitute the root of the Famatinian magmatic arc. These results indicate high-grade metamorphic conditions and depths corresponding to geophysical properties of middle to lower crust and correlate with the hypothesis of a dehydrated, cool and magnesium-enriched mantle located in the region between the subducted Nazca slab and the bottom of the Cuyania terrain crust.
Published
2017-04-01
Section
Articles