SEISMIC AND MAGMATIC EVIDENCE FOR LITHOSPHERIC DELAMINATION UNDER THE PUNA PLATEAU OF THE CENTRAL ANDES

Crustal and mantle lithospheric delamination have become popular models for understanding processes in many orogenic belts. Such a model was proposed in the early 1990’s for the southern Puna plateau of the Central Andes to explain low Sn mantle attenuation, a gap in intermediate depth slab seismicity, a low effective elastic thickness, a change in fault kinematics and the distribution and chemistry of mafic lava flows, glassy andesites, and the giant Galan ignimbrite. This proposal has recently been supported by findings from the 74 station 2007-2009 southern PUNA passive seismic experiment at 25-28°S and 65-70°W operated by the Universities of Cornell, Missouri, Jujuy and the GfZ in Potsdam, Germany. In particular, a Vs tomographic image reveals a high velocity region about a 100 km across above a somewhat lower velocity region interpreted as the slab, which is at a depth of 150-200 km under the Galan region. A virtual seismic gap in the slab and a weak arc geochemical signature in the overlying volcanic rocks are attributed to a high temperature mantle wedge, which is supported by attenuated S waves from the sparse slab earthquakes. Other support for a delaminated block comes from a circular pattern of mantle shear wave splitting vectors under the Galan region. A thin lithospheric mantle under this region of the elevated plateau is indicated by receiver function images showing a lithosphere-asthenosphere boundary at 70-90 km, which is in accord with calculated mantle equilibration depths of 60-80 km at temperatures over 1350°C for young lavas. Evidence for a hot crust comes from a high crustal Vp/Vs ratio and a seismic swarm under the Galan caldera, and for a shallow brittle-ductile transition from low magnitude shallow earthquakes at <5-10 km. across the region. Overall, the crust has a thickness of 45-55 km thick crust under much of the southern Puna and a Vp/Vs ratio < 1.70 implying a felsic composition crust under at least part of the plateau.