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

PEREZ LUJAN SOFIA; AMMIRATI, JEAN BAPTISTE; ALVARADO PATRICIA; VUJOVICH GRACIELA

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Oxford; Año: 2015 vol. 64 p. 325 - 338

0895-9811

Elastic properties in twelve representative rock samples from central and western Precordillera in the Andean backarc region of Argentina between 30ºS and 31ºS were estimated from detailed petrological analysis. The samples were first collected and examined in order to quantify their modal mineralogical composition. Ourpetrological results overall show mafic and ultramafic outcropping lithologies represented by gabbros, leuco-gabbros and wehrlites, in greenschist and amphibolite metamorphic facies. P and S seismic velocities (Vp, Vs) as well as Poisson´s ratio (σ) among other parameters were derived for these lithologies using a framework of a wide variety of empirical observations from active continental margins. In addition, Vs lithospheric models along two west-east cross sections were obtained using a joint inversion of teleseismic Rayleigh waves and receiver functions; these models clearly delineate the flat-slab subduction of the Nazca plate at about 100-km depth in this region. Our petrological and seismological results collectively support a crustal model of a mafic-ultramafic composition extending to middle and lower crustal levels beneath central and western Precordillera; interestingly, this zone correlates with the intermediate region between the eastern Cuyania terrane and thewestern Chilenia terrane. The suggested seismic velocity structure shows a relative low (< 3.3 km/s) Vs layer located in the first 15-18 km depth, then an increase of it from 3.3 km/s to 4 km/s between 20 km and 55 km depth with a mayor change at 40 km depth beneath Precordillera (the shear wave velocity increases from 3.3 km/s to 3.8 km/s). The region nucleates seismicity up to 40-km depth. Using this seismological model, Vs estimations derived from the petrological analyses for the 12 collected samples can be projected at depths greather than 30 km. The Moho discontinuity was identified at around 65 km depth beneath the Precordillera (Vs = 4.3 km/s) and shows a low shear-wave velocity contrast with the upper continental mantle´s parameters. These geophysical and petrological results agree with the hypothesis of a mafic thickened and partially eclogitized lower crust beneath the Precordillera, which has been predicted previously on a base of seismological studies. A comparison with a similar petrological-seismic modeling in the Western Sierras Pampeanas, located 100 km to the east of the Precordillera region, provides evidence for a mountain developing process which is built upon a common mafic basement lower crust mapping the composite Cuyania terrane. Notably, although similar elevations the western Precordilleran crust is at least 10 to 15 km deeper than the crust beneath the Western Sierras Pampeanas. This can be a result of a previous subduction zone that seem to have existed by Ordovician time between Chilenia and Cuyania terranes, which may have favoured large duplication of crustal blocks beneath the Precordillera.