Electrical conductivity of the Pampean shallow subduction region of Argentina near 33 S: Evidence for a slab window

Abstract

[1] We present a three-dimensional (3-D) interpretation of 117 long period (20-4096 s) magnetotelluric (MT) sites between 31 S and 35 S in western Argentina. They cover the most horizontal part of the Pampean shallow angle subduction of the Nazca Plate and extend south into the more steeply dipping region. Sixty-two 3-D inversions using various smoothing parameters and data misfit goals were done with a nonlinear conjugate gradient (NLCG) algorithm. A dominant feature of the mantle structure east of the horizontal slab is a conductive plume rising from near the top of the mantle transition zone at 410 km to the probable base of the lithosphere at 100 km depth. The subducted slab is known to descend to 190 km just west of the plume, but the Wadati-Benioff zone cannot be traced deeper. If the slab is extrapolated downdip it slices through the plume at 250 km depth. Removal of portions of the plume or blocking vertical current flow at 250 km depth significantly changes the predicted responses. This argues that the plume is not an artifact and that it is continuous. The simplest explanation is that there is a ''wedge''-shaped slab window that has torn laterally and opens down to the east with its apex at the plume location. Stress within the slab and seismic tomography support this shape. Its northern edge likely explains why there is no deep seismicity south of 29 S.

The nature of subslab slow velocity anomalies beneath South America

“…Constraints on the source of the slow velocities disrupting the slab are provided by a recent 3-D magnetotelluric inversion across the study area [Burd et al, 2013], which identifies a slab-disrupting low resistivity (high conductivity) anomaly colocated with the observed slab hole anomaly. The low amplitude of resistivity observed suggests interconnected partial melt or a high concentration of dissolved water (0.1%) within the conductive body [Burd et al, 2013].…”

“…Constraints on the source of the slow velocities disrupting the slab are provided by a recent 3-D magnetotelluric inversion across the study area [Burd et al, 2013], which identifies a slab-disrupting low resistivity (high conductivity) anomaly colocated with the observed slab hole anomaly. The low amplitude of resistivity observed suggests interconnected partial melt or a high concentration of dissolved water (0.1%) within the conductive body [Burd et al, 2013]. Overlap of the subslab slow velocity anomaly, slab hole slow velocity anomaly, and the resistivity anomaly suggests that they are related, and magnetotelluric evidence indicates that at least the slab hole material contains some degree of partial melt or water content in addition to a potential thermal anomaly.…”

“…The resulting buoyancy contrast causes tearing of the slab, allowing for the passive ascension of the hot, buoyant, subslab asthenosphere. Decompression from upward flow through the overriding Nazca slab may lead to partial melting of the hot asthenosphere, as suggested by magnetotelluric imaging [ Burd et al ., ]. Furthermore, hot upwelling asthenosphere may contribute to anomalous Pleistocene volcanism in the Sierras de San Luis (Figure a), directly above the imaged hole in the slab [ Kay and Mpodozis , ; Ramos et al ., ].…”

“…The resulting buoyancy contrast causes tearing of the slab, allowing for the passive ascension of the hot, buoyant, subslab asthenosphere. Decompression from upward flow through the overriding Nazca slab may lead to partial melting of the hot asthenosphere, as suggested by magnetotelluric imaging [Burd et al, 2013]. Furthermore, hot…”

Structure of the mantle beneath the Alboran Basin from magnetotelluric soundings

“…The code is essentially the same as that used by Burd et al . [], although in our case the requirement to include land and marine data made the process more complex and computationally expensive.…”