Constraining a mafic thick crust model in the Andean Precordillera of the Pampean flat slab subduction region

PÉREZ LUJÁN, S.B.; AMMIRATI, J.B.; ALVARADO, P.; VUJOVICH, G.I.

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2015 vol. 63 p. 325 - 338

0895-9811

Elastic properties in twelve representative rock samples from Central and Western Precordillera in theAndean backarc region of Argentina between 30 and 31S were estimated from detailed petrologicalanalysis. Thus, P and S seismic-wave velocities (Vp, Vs) as well as Poisson´s ratio (s) among other parameterswere derived for gabbros, leuco-gabbros and wehrlites, in greenschist and amphibolitemetamorphic conditions using a framework of a wide variety of empirical observations from activecontinental margins. In addition, Vs lithosphere models along two westeeast cross sections were obtainedusing a joint inversion of teleseismic Rayleigh waves and receiver functions. These models clearlydelineate the upper-plate crustal structure and the flat-slab subduction of the Nazca plate at about 100km depth in this region. The suggested seismic velocity structure shows a relatively low (<3.3 km/s) Vslayer located in the first 15e18 km depth, then an increase of it from 3.3 to 4 km/s between 20 and 55 kmdepth with a mayor change at 40 km depth beneath the Precordillera showing an increase in Vs from 3.3to 3.8 km/s. The Moho discontinuity was identified at around 65 km depth beneath the Precordillera(Vs ¼ 4.3 km/s) and shows a low shear-wave velocity contrast in comparison with the upper continentalmantle´s parameters. Using this seismological model, Vs estimations derived from the petrological analysesfor the 12 collected samples can be projected at depths greater than 30 km. These geophysical andpetrological results agree with the hypothesis of a mafic thickened and partially eclogitized lower crustbeneath the Precordillera, which has been predicted previously on a base of seismological studies only.Our petrological and seismological results collectively support a thick crustal model of a maficeultramafic composition extending to middle-to-lower crustal levels beneath Central and WesternPrecordillera; this region correlates with a suture zone between the eastern Cuyania terrane and thewestern Chilenia terrane.