ELECTRICAL CONDUCTIVITY STRUCTURE BENEATH CÓRDOBA PROVINCE,

JOHN BOOKER1, M. CRISTINA POMPOSIELLO2, ALICIA FAVETTO3,BARRY NAROD AND GABRIEL GIORDANENGO

Calafate-santa Cruz-Argentina

Congreso; XV Congreso Geologico Argentino; 2002

AGA

ELECTRICAL CONDUCTIVITY STRUCTURE BENEATH CÓRDOBA PROVINCE, ARGENTINA John Booker1, M. Cristina Pomposiello2, Alicia Favetto3, Barry Narod4 and Gabriel Giordanengo2 1 Department of Earth and Space Science, Box 351310, University of Washington, Seattle WA, USA. booker@ess.washington.edu WA, USA. booker@ess.washington.edu WA, USA. booker@ess.washington.edu 1 Department of Earth and Space Science, Box 351310, University of Washington, Seattle WA, USA. booker@ess.washington.edu WA, USA. booker@ess.washington.edu WA, USA. booker@ess.washington.edu 1 Department of Earth and Space Science, Box 351310, University of Washington, Seattle WA, USA. booker@ess.washington.edu WA, USA. booker@ess.washington.edu WA, USA. booker@ess.washington.edu Barry Narod4 and Gabriel Giordanengo2 1 Department of Earth and Space Science, Box 351310, University of Washington, Seattle WA, USA. booker@ess.washington.edu WA, USA. booker@ess.washington.edu WA, USA. booker@ess.washington.edu 1 Department of Earth and Space Science, Box 351310, University of Washington, Seattle WA, USA. booker@ess.washington.edu WA, USA. booker@ess.washington.edu WA, USA. booker@ess.washington.edu 1 Department of Earth and Space Science, Box 351310, University of Washington, Seattle WA, USA. booker@ess.washington.edu WA, USA. booker@ess.washington.edu WA, USA. booker@ess.washington.edu Barry Narod4 and Gabriel Giordanengo2 1 Department of Earth and Space Science, Box 351310, University of Washington, Seattle WA, USA. booker@ess.washington.edu WA, USA. booker@ess.washington.edu WA, USA. booker@ess.washington.edu 1 Department of Earth and Space Science, Box 351310, University of Washington, Seattle WA, USA. booker@ess.washington.edu WA, USA. booker@ess.washington.edu WA, USA. booker@ess.washington.edu 1 Department of Earth and Space Science, Box 351310, University of Washington, Seattle WA, USA. booker@ess.washington.edu WA, USA. booker@ess.washington.edu WA, USA. booker@ess.washington.edu 1, M. Cristina Pomposiello2, Alicia Favetto3, Barry Narod4 and Gabriel Giordanengo2 1 Department of Earth and Space Science, Box 351310, University of Washington, Seattle WA, USA. booker@ess.washington.edu WA, USA. booker@ess.washington.edu WA, USA. booker@ess.washington.edu 1 Department of Earth and Space Science, Box 351310, University of Washington, Seattle WA, USA. booker@ess.washington.edu WA, USA. booker@ess.washington.edu WA, USA. booker@ess.washington.edu 1 Department of Earth and Space Science, Box 351310, University of Washington, Seattle WA, USA. booker@ess.washington.edu WA, USA. booker@ess.washington.edu WA, USA. booker@ess.washington.edu 4 and Gabriel Giordanengo2 1 Department of Earth and Space Science, Box 351310, University of Washington, Seattle WA, USA. booker@ess.washington.edu WA, USA. booker@ess.washington.edu WA, USA. booker@ess.washington.edu Department of Earth and Space Science, Box 351310, University of Washington, Seattle WA, USA. booker@ess.washington.edu 2 Instituto de Geocronología y Geología Isotópica. Ciudad Universitaria, Pabellón INGEIS,1428- BUENOS AIRES, Argentina. cpomposi@ingeis.uba.ar INGEIS,1428- BUENOS AIRES, Argentina. cpomposi@ingeis.uba.ar INGEIS,1428- BUENOS AIRES, Argentina. cpomposi@ingeis.uba.ar Instituto de Geocronología y Geología Isotópica. Ciudad Universitaria, Pabellón INGEIS,1428- BUENOS AIRES, Argentina. cpomposi@ingeis.uba.ar 3 Dpto. de Física, Univ. Buenos Aires, Ciudad Universitaria, Pabellón I,1428 Buenos Aires Argentina. favetto@df.uba.ar Argentina. favetto@df.uba.ar Argentina. favetto@df.uba.ar Dpto. de Física, Univ. Buenos Aires, Ciudad Universitaria, Pabellón I,1428 Buenos Aires Argentina. favetto@df.uba.ar 4 Narod Geophysics, 4413 W. 7th Ave., Vancouver B.C. Canada V6R 1X1. narod@geop.ubc.ca narod@geop.ubc.ca narod@geop.ubc.ca Narod Geophysics, 4413 W. 7th Ave., Vancouver B.C. Canada V6R 1X1. narod@geop.ubc.ca Keywords: flat slab, magnetotelluric, electrical conductivity, Sierras Pampeanas Interpretation of magnetotelluric (MT) data reveals a conductive zone in the mantle that is probably the asthenospheric wedge east of the flat subducted Nazca Plate. Interpretation of magnetotelluric (MT) data reveals a conductive zone in the mantle that is probably the asthenospheric wedge east of the flat subducted Nazca Plate. Interpretation of magnetotelluric (MT) data reveals a conductive zone in the mantle that is probably the asthenospheric wedge east of the flat subducted Nazca Plate. : flat slab, magnetotelluric, electrical conductivity, Sierras Pampeanas Interpretation of magnetotelluric (MT) data reveals a conductive zone in the mantle that is probably the asthenospheric wedge east of the flat subducted Nazca Plate. GEOLOGICAL AND TECTONIC SETTING The Andean region is an active convergent margin between the oceanic Nazca Plate (NP) and the stable South American (SA) continent. Along the Andean Cordillera, the variations in tectonic style are associated with changes in the geometry of the subducted Nazca Plate (Jordan, et al., 1983). Characteristic of this subduction process is the northsouth variation in the dip angle of the oceanic plate, and changes in the rate of convergence and direction with respect to the continental margin (Barazangi & Isacks, 1976; Jordan et al., 1983; Cahill & Isacks, 1992, Dewey & Lamb, 1992). The Sierras Pampeanas (SP) lie between 27º and 34º S and are east of a quiescent volcanic arc and the thin-skinned thrust belt of the Eastern Cordillera. They are underlain by a nearly flat segment of the subducting Nazca Plate. The SP consist of Proterozoic- Paleozoic granitic and metamorphic basement blocks that have been tilted and uplifted along broad, high-angle, thick-skinned thrust systems and represent a late Cenozoic eastward progression of Andean deformation into cratonic SA (Jordan & Allmendinger, 1986). The Sierras de Córdoba (SC) are the most easterly ridges of the SP. There is little Wadati-Benioff Zone seismicity in this area, but projection of seismicity to the north and south suggests that the SC are located approximately above where the flat slab curves over to a more normal suduction angle (Cahill & Isacks, 1992). With more normal subduction geometry, one would expect corner flow between the dipping portion of the slab and the overlying lithosphere. This is usually associated with an asthenospheric wedge and arc volcanism. The closest volcanism is the Pocho center just west of the Sierra Grande de Córdoba. The youngest eruption products at Pocho have a mantle signature as might be expected for an ultimate source in an asthenospheric wedge, but they are about 4.5 MA old (Kay & Gordillo, 1994). DATA COLLECTION AND ANALYSIS The Magnetotelluric (MT) method employs low frequency electromagnetic energy diffusing into the Earth to image electrical resistivity structure. In August and September 2001, we collected 18 sites along a 215 km long east-west profile through the city of Alta Gracia at about 31.5º S. This profile extends from the top of the Sierra Grande de Córdoba east across the plains and its western end is above the most easterly extension of the flat slab. These data are the first collected with a new generation of low power, long period GPS-

 
  


 [Artículo] controlled systems. The data were processed using robust statistical time series methods (Egbert and Booker, 1987) and remote reference (Gamble, et al. 1979). The regional strike was determined to be very close to north-south at all sites using impedance tensor decomposition for electric and magnetic distortion (Chave & Smith, 1994). We inverted the MT data in the bandwidth from 20 to 4100 s. We used electric (E) field polarized both parallel and perpendicular to strike, plus the complex transfer function between the vertical and horizontal magnetic field. The vertical magnetic field data enhance our ability to constrain structure off the end of the profile. We used the two-dimensional NLCG algorithm of Rodi & Mackie (2001). This inversion looks for a model that has less structure than other models fitting the data at the same level of misfit. We expect such a model to have only features that are required by the data. Because the data magnitude with E polarized parallel to strike (perpendicular to our profile) can be distorted by off-profile structure, we used only the impedance phase for this polarization.