Lithospheric and upper mantle structure of central Chile and Argentina

Abstract

SUMMARY The tectonics of central Chile and Argentina have been greatly affected by the shallow dips of the subducting Nazca plate, which controlled patterns of magmatism and deformation nearly 1000 km away from the plate boundary. We calculate receiver functions from data recorded by the CHARGE array, which transected the Andes and Sierras Pampeanas in central Chile and Argentina, to better constrain the crustal structure of this region. Beneath the northern transect of the CHARGE array, where the Nazca slab flattens near 100 km, we find the crust is over 60 km thick beneath the Andes and thin to the east. The thick crust, however, extends ∼200 km to the east of the high elevations. Estimates of VP/VS obtained from receiver functions vary along ancient terrane boundaries exhibiting higher values to the west. Interestingly, we observe that the amplitude of the phase corresponding to the Moho on receiver functions diminishes to the west, complicating our images of crustal structure. We proposed that the observations presented here of thickened crust within a region of low elevations, diminished receiver function arrivals, and reports of high shear-wave speeds atop of the mantle wedge overlying the shallowly subducted Nazca slab, can be explained by partial eclogitization of the lower crust. The Moho appears simpler across the southern transect where it can be identified near 50 km depth at its deepest point beneath the Andes and shallows eastwards. Volcanism remains active near the latitudes of our southern transect and we observe multiple crustal lowvelocity zones indicative of regions of partial melt near the centres of volcanism. Signals related to the Nazca slab remain more elusive, suggestive of a small impedance contrast between the slab and overlying mantle.