Electrical conductivity of the Pampean Shallow Subduction Region of Argentina near 33S and of the Payunia region of Argentina near 36.5S

Abstract

I present a three-dimensional (3D) interpretation of long period magnetotelluric sites: 117 from 31 - 35 S and 37 from 35 - 38 S in western Argentina. The first field area covers the most horizontal part of the Pampean shallow angle subduction of the Nazca slab and extends south into the more steeply dipping region. The second field area covers the < ~ 2 Ma Payunia Basaltic Province (PBP). Data from each area were used in 3D Non-Linear Conjugate Gradient inversions. Three electrically conductive plumes occur at different locations in the crust and upper mantle: 1. A plume east of the horizontal Nazca slab rises from near the top of the mantle transition zone at 410 km, through the extrapolated location of the Nazca slab, and extends to the probable base of the lithosphere at 100 km depth. 2. A westward dipping plume beneath the PBP rises from roughly 130 km depth to within 7 km of the surface, with two "tendrils"' approaching the surface beneath the Caldera Payun Matru and the Volcan Tromen. These regions have volcanism younger than 0.1 Ma with some volcanism possibly within the last 7000 years. This plume remains above the subducted Nazca slab. 3. An eastward dipping plume rises from at least 410 km depth to within 5 km of the surface while remaining above the subducted Nazca slab, with its shallowest portion beneath the southern region of the PBP, which has no volcanism younger than ~ 0.8 Ma. Model assessment via both forward modeling and additional inversion tested the veracity of these features. I interpret the plume near the horizontal Nazca slab as an indication of a slab "window"' in the Nazca slab &mdash stress within the slab and seismic tomography support the likelihood of a slab window in this location. I propose that the two plumes beneath the Payunia Basaltic Province were previously a single structure, but the resurgence of mantle shear flow following steepening of a middle to late Miocene shallow slab caused the original plume to be pulled north-westward and eventually "decapitated" to form the two present-day plumes. The west-ward dipping plume likely represents the source of much of the recent Payunia Basaltic Province volcanism.