Implications of Electrical Conductivity Structure Associated With the Nazca Slab Beneath Argentina

From 1998 to 2005 we have collected more than 100 magnetotelluric (MT) sites in the Sierras Pampeanas of Argentina between 27°S and 33°S. The main targets of these investigations were initially active structures within this thick-skinned tectonics province. However, it became clear as we progressed that the subducted Nazca slab, which is anonymously flat under this area was generating observable electrical structures at mantle depth. The high resistivity of the crystalline crust and the generally thin, but conductive sedimentary cover of much of this area provide a favorable situation for the penetration of electromagnetic energy deep into the mantle. Our longest profile extends from near Chile to near Uruguay at 31.5°S, where the flat slab is widest. The flattest portion of the slab is both electrically conductive and seismically active. This seismic activity terminates eastward at almost the same place that the slab conductor ends and the overlying lithosphere becomes seismically active and electrically conductive. A unifying explanation is that as the slab descends to its flat portion at 100 km it dehydrates. The free water raises pore pressure and triggers earthquakes. As the slab progresses eastward, the fluids leak upward out of the slab into the lithosphere, the slab pore pressure drops, electrical pathways close and the slab becomes resistive and quiet. An intriguing possibility is that the wet lithosphere created in this process sets the stage for a massive volcanic flare-up if the slab dip increases and an asthenospheric wedge reforms. Further east, the slab plunges nearly vertically into the mantle when it meets the electrically resistive and presumably mechanically strong backstop of the root of the Rio de la Plata Craton. A near vertical conductor is found between the plunging slab and the resistive root. This conductor rises to the base of the lithosphere, but does not penetrate. It is almost certainly due to partial melt, but whether this melting is local due to fluids coming from the slab or rises from the mantle transition zone is not clear. The answer to this question, however, bears directly on recent ideas about what role the transition zone plays in global chemical differentiation. We have been collecting new data to the east of the plunging slab to try to constrain the depth at which this melt must originate. In 2004 we undertook MT profiles to examine the transition of the flat slab to its more normal dip to the south. The seismological literature generally supports slab flexure in this region, but this is not based on seismicity, which is very low. Preliminary interpretation of our data seem to indicate, on the contrary, that the slab is simply torn.