Electrical Conductivity Beneath the Payún Matrú Volcanic Field in the Andean Back-arc of Argentina near 36.5S: Insights into the Magma Source and Results of 3D Modeling

Southern Mendoza and northern Neuquen Provinces, south of the Nazca flat slab in western Argentina, have widespread, geologically young basaltic volcanism, but no historic activity. The youngest basalts, erupted in the vicinity of the large Payún Matrú volcanic center have essentially no arc signature. Kay, et al. (2006) and Folguera, et al. (2006) argue that this back-arc igneous province is the result of extension due to trench roll-back following steepening of a flat slab that existed in the middle to late Miocene. Magnetotelluric data collected in 2005 at 18 sites along an east-west profile from 70W to 67W have been used to probe the source of the Payún Matrú basalts. These data imply significantly 3D structure. However, preliminary analysis of an arguably 2D region at the center of the profile allows tentative identification of a conductive mantle plume surfacing at Payún Matrú that rises from below 200 km depth. This suggests that Payún Matrú Volcanic Field is sampling mantle deeper than 200 km, perhaps just above the subducted Nazca slab or perhaps from the mantle transition near where the Nazca slab penetrates into it at 400 km. The 2008 fieldwork extends the earlier profile to a spatial array extending from Laguna Llancanelo north of Payún Matrú to beyond the Cortaderas Lineament that bounds the basaltic province to the south. These 19 new sites consist of 5 to 10 days of 4 Hz horizontal electric and 3-component magnetic field time- series. Data processing in the field suggests that the deep crust or upper mantle has northwest-southeast striking structure increases in conductivity to the southwest of Payún Matrú. This underlies shallow structure with north-south strike between Payún Matrú and the Colorado River. This complexity explains our initial difficulty with 2D interpretation. We will display a preliminary 3D interpretation of these new data.