Implications of 3D electrical conductivity beneath the Payun Matru basalt province in western Argentina (36.5S, 69.5W)

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

San Francisco, CA, USA

Congreso; 2009 AGU Fall Meeting; 2009

AGU

To understand geologic processes that shaped western North America in the early Cenozoic, it is useful to look at southern South America, where similar processes such as flat-slab subduction and intraplate basaltic volcanism are active today. Payún Matrú is a very large shield volcano east of the Andes at 36.5 S 69.5 W. It is the largest feature of the “Payunia” basalt province that covers about 15,000 sq km. The most recent activity may have been about 1,000 years ago. Lava chemistry has evolved with time. Despite being only 150 km east of the axis of the Andean Southern Volcanic Zone, the younger lavas are essentially OIB. We have collected 38 longperiod magnetolluric (MT) sites in an array that extends beyond the Payunia basalts to investigate the source of magma. MT impedance tensor data (including vertical to horizontal magnetic field transfer functions) have been inverted for smoothest log resistivity using a 3D non-linear conjugate gradient (NLCG) algorithm. The model includes the Atlantic and Pacific oceans, which both significantly affect the data. The image shows the isosurface at 30 Ohm-m together with the locations of sites (inverted triangles), earthquakes deeper than 50 km (filled circles) and volcanoes with geological recent activity (normal triangles). The result is that Payún Matrú (the large triangle) lies at the northern end of a conductive finger at the top of the mantle. This finger has a pimple that rises into the upper crust just west of the caldera. The finger appears to originate in anomalously conductive mantle deeper than 150 km that extends south and to the east (away from the Andes). If this conductor is due to fully interconnected basalt partial melt, the region inside this isosurface has more than 3% partial melt. This structure appears to rule out such processes as crustal delamination or a vertical plume for this volcanic province.It also suggests that the lithosphere east of the asthenospheric wedge under the Andes is quite thin. Perhaps not much more than the continental crust. The anomalous mantle to the south and east could be a consequence of the early Miocene Somun Cura “hot spot” volcanism about 500 km south-southeast of Payún Matrú and the conductive finger could be related to the subsequent motion of South America relative to the hot spot frame of reference.