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

BURD AURORA; BOOKER, JOHN; MACKIE RANDI; POMPOSIELLO CRISTINA; FAVETTO ALICIA

GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS

AMER GEOPHYSICAL UNION

Año: 2013 vol. 14 p. 3192 - 3209

1525-2027

We present a three-dimensional (3D) interpretation of 117 long period (20 to 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 3D inversions using various smoothing parameters and data misfits were done with a Non-Linear 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 down-dip it slices through the plume at 250km depth. Removal of portions of the plume or blocking vertical current flow at 250km depth significantly change 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.