Electrical conductivity at the Pampean Shallow Subduction Region of Argentina near 33S: evidence for slab window

AURORA BURD; BOOKER JOHN; RANDALL MACKIE; CRISTINA POMPOSIELLO; ALICIA FAVETTO

Darwin

Workshop; 21 Electromagnetic Induction Workshop; 2012

We present a three-dimensional (3D) interpretation of 117 long period magnetotelluric (MT) sites between 31Sand 35S in Argentina. They cover the most horizontal part of the Pampean shallow angle subduction of theNazca Plate and extend into the more steeply dipping region to the south. More than sixty 3D inversions usingvarious parameters were done with a Non-Linear Conjugate Gradient (NLCG) algorithm. A dominant featureof the mantle structure east of the horizontal slab is a sub-vertical conductive sausage rising from near the topof the mantle transition zone at 400 km to the probable base of the lithosphere at 100 km depth. Removal ofthe sausage or blocking vertical current flow at either 200 or 350 km depth significantly changes the predictedresponses. This argues that the sausage is not an artifact and that it is continuous. The subducted slab isknown to descend to 190 km just west of the sausage, but the Wadati-Benioff zone cannot be traced deeper. Ifthe slab is extrapolated down-dip it slices through the sausage. The simplest explanation is that the slab deeperthan 200 km is missing because it has torn parallel or perpendicular to the trench and opened a slab window.