Velocity Structure beneath the Southern Puna Plateau: Evidence for Delamination.

CALIXTO F.J., SANDVOL E., KAY S., MULCAHY P., HEIT B., YUAN X., COIRA B., COMTE D., CUBED G., ALVARADO P.

GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS

AMER GEOPHYSICAL UNION

Lugar: NY; Año: 2013 vol. 14 p. 4292 - 4305

1525-2027

The southern Puna plateau (25S-28S) offers an excellent natural laboratory to study the formation and evolution of a continental plateau along an active continental margin.  The Puna -Altiplano plateau has a number of important features that set it apart from much of the rest of the Andean mountain belt. Such features include a distinctive spatial and geochemical pattern of mafic lavas and giant ignimbrites, a high topography with a large deficit of crustal shortening, and a slab with a gap in intermediate depth seismicity. The slab has a steeper segment to the north and a flat to the south. This region is also believed to have hosted a series of delamination events starting 6-7 Ma with the most recent occurring around 2 Ma. We believe that we have imaged the delaminated block as a region of abnormally high shear wave velocities on top of the subducting slab at a depth of around 150 km slightly to the north of Cerro Galan ignimbrite. An array consisting of 73 broad band and short period seismic stations was deployed in the region for a period of two years starting in 2007. We inverted the data using the two plane wave approach and obtained 1D and 2D Rayleigh wave phase velocities. Our dispersion curve shows that at short periods the phase velocities are slightly higher than those of the Tibetan plateau and lower than those of the Anatolian plateau. At periods of around 150 s we observe an ultra-low velocity zone that might be remnant hot asthenosphere from when the slab was flat (7-10 Ma). We estimate the average lithospheric thickness for the region to be between 100 and 130 km. Our three dimensional Rayleigh wave phase velocities show an abrupt transition from high phase velocities at depths 150-190 km to low phase velocities at shallower depths slightly to the north of Cerro Galan. This would be consistent with the hypothesis of delamination in which a piece of lithosphere has detached and causes upwelling of hot asthenosphere which in turn causes widespread alkaline-collision related volcanism. This interpretation is also corroborated by our shear wave velocity model where a high velocity anomaly beneath the northern edge of Cerro Galan at 150 km depth is interpreted as the delaminated block on top of the subducting Nazca slab.