CIGEOBIO   24054
CENTRO DE INVESTIGACIONES DE LA GEOSFERA Y BIOSFERA
Unidad Ejecutora - UE
artículos
Título:
Shear wave splitting and shear wave splitting tomography of the southern Puna plateau
Autor/es:
CALIXTO, F.; ROBINSON, D.; SANDVOL, E.; KAY, S.; ABT, D.; FISCHER, K.; HEIT, B.; YUAN, X.; COMTE, D.; ALVARADO, P.
Revista:
GEOPHYSICAL JOURNAL INTERNATIONAL
Editorial:
WILEY-BLACKWELL PUBLISHING, INC
Referencias:
Lugar: Londres; Año: 2014 vol. 199 p. 688 - 699
ISSN:
0956-540X
Resumen:
We have investigated the seismic anisotropy beneath the Central Andean southern Puna plateau by applying shear wave splitting analysis and shear wave splitting tomography to local S waves and teleseismic SKS, SKKS and PKS phases. Overall, a very complex pattern of fast directions throughout the southern Puna plateau region and a circular pattern of fast directions around the region of the giant Cerro Galan ignimbrite complex are observed. In general, teleseismic lag times are much greater than those for local events which are interpreted to reflect a significant amount of sub and inner slab anisotropy. The complex pattern observed from shear wave splitting analysis alone is the result of a complex 3-D anisotropic structure under the southern Puna plateau. Our application of shear wave splitting tomography provides a 3-D model of anisotropy in the southern Puna plateau that shows different patterns depending on the driving mechanism of upper-mantle flow and seismic anisotropy. The trench parallel a-axes in the continental lithosphere above the slab east of 68W may be related to deformation of the overriding continental lithosphere since it is under compressive stresses which are orthogonal to the trench. The more complex pattern below the Cerro Galan ignimbrite complex and above the slab is interpreted to reflect delamination of continental lithosphere and upwelling of hot asthenosphere. The a-axes beneath the Cerro Galan, Cerro Blanco and Carachi Pampa volcanic centres at 100 km depth show some weak evidence for vertically orientated fast directions, which could be due to vertical asthenospheric flow around a delaminated block. Additionally, our splitting tomographic model shows that there is a significant amount of seismic anisotropy beneath the slab. The subslab mantle west of 68W shows roughly trench parallel horizontal a-axes that are probably driven by slab roll back and the relatively small coupling between the Nazca slab and the underlying mantle. In contrast, the subslab region (i.e. depths greater than 200 km) east of 68W shows a circular pattern of a-axes centred on a region with small strength of anisotropy (Cerro Galan and its eastern edge) which suggest the dominant mechanism is a combination of slab roll back and flow driven by an overlying abnormally heated slab or possibly a slab gap. There seems to be some evidence for vertical flow below the slab at depths of 200?400 km driven by the abnormally heated slab or slab gap. This cannot be resolved by the tomographic inversion due to the lack of ray crossings in the subslab mantle. Key words Seismic anisotropy  Seismic tomography  Subduction zone processes Continental margins: convergent Previous SectionNext Section 1 INTRODUCTION The southern Puna plateau of the central Andes offers an excellent natural laboratory to study the distribution of seismic anisotropy in a region with a spatially changing subduction angle (e.g. Cahill & Isacks1992; Mulcahy et al. 2014) and where crustal and mantle lithospheric delamination has been argued to have occurred (e.g. Kay et al. 1994; Calixto et al. 2013; Liang et al. 2014) to explain geologic and geophysical observations including the existence of mafic lavas and giant ignimbrites, a gap of intermediate-depth seismicity, and high topography with large deficit in crustal shortening. Regionally, the southern Puna constitutes the southernmost part of the central Andean Altiplano-Puna plateau (Figs 1and 2; see review by Allmendinger et al. 1997). The region is generally characterized by a thin 60?80-km-thick lithosphere with a crustal thickness varying from 40 to 70 km (Heit et al. 2014). Crustal thickening related to crustal shortening has been argued to account for 70?80 per cent of the crustal thickness observed (Allmendinger et al. 1997) with the rest being attributed to magmatic addition, crustal flow and thermal uplift (Isacks 1988; Whitman et al. 1996). The thermal uplift has been argued to be due to mechanical thinning of the lithosphere by delamination (e.g. Kay & Kay 1993; Kay et al. 1994; Bianchi et al. 2013; Calixto et al. 2013; Lianget al. 2014) with decompression melting after delamination in an expanding mantle wedge over a steepening subduction zone being the source of the thermal uplift (Kay & Coira 2009; Risse et al. 2013). The thermal effects of crustal and lithospheric delamination have been associated with the distinctive southern Puna mafic volcanism and giant ignimbrite eruptions, a gap in intermediate depth seismicity (e.g. Mulcahyet al. 2014) and a late Miocene change (Risse et al. 2008) from an uniformly compressive to more chaotic stress regime (e.g. Marrett et al.1994). A high velocity body above the slab under the Cerro Galan ignimbrite region has been interpreted as the most recent delaminated block (Calixto et al. 2013).