Deep crust and mantle electrical conductivity structure of the Northern Sierras pampeanas, Argentina

POMPOSIELLO,M.C; FAVETTO,A; BOOKER,J; SAINATO; LI.S; SMITH.J Y GIORDANENGO,G.

Calafate-Santa Cruz-Argentina

Congreso; XV Congreso Geologico Argentino; 2002

Asociación Geologica Argentina

DEEP CRUST AND MANTLE ELECTRICAL CONDUCTIVITY STRUCTURE OF THE NORTHERN SIERRAS PAMPEANAS, ARGENTINA M. Cristina Pomposiello1, Alicia Favetto2, John Booker3, Claudia Sainato4, Shenghui Li3, Jeremy Smith3 and Gabriel Giordanengo1 1 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. 1 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. 1 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. Shenghui Li3, Jeremy Smith3 and Gabriel Giordanengo1 1 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. 1 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. 1 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. Shenghui Li3, Jeremy Smith3 and Gabriel Giordanengo1 1 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. 1 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. 1 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. 1, Alicia Favetto2, John Booker3, Claudia Sainato4, Shenghui Li3, Jeremy Smith3 and Gabriel Giordanengo1 1 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. 1 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. 1 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. 3, Jeremy Smith3 and Gabriel Giordanengo1 1 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. 2 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 3 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 4 Cátedra de Física. Univ.de Buenos Aires. Av. San Martín 4453 1417. Buenos Aires. Argentina. Argentina. Argentina. Cátedra de Física. Univ.de Buenos Aires. Av. San Martín 4453 1417. Buenos Aires. Argentina. Keywords: Sierras Pampeanas, magnetotelluric, electrical conductivity , lower crust.: Sierras Pampeanas, magnetotelluric, electrical conductivity , lower crust. INTRODUCTION Interpretation of new magnetotelluric (MT) data reveals a highly conductive zone in the Pre-Cambrian crystalline basement under the Tucumán Plain in the Andean foreland. This structure dips eastward into the upper mantle. 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 33º S and are east of a quiescent volcanic arc and the thin-skinned thrust belt of the Eastern Cordillera. They are underlain by a 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 Tucumán Plain (TP) is at the northern end of the SP, where the plate dip is steepening northward. In this region both the subducted plate and overlying mantle appear anomalous. The subducted plate is seismically quiet except at great depth (Cahill & Isacks, 1992) and the upper mantle has inefficient seismic wave propagation (Whitman, 1994). In the southeast, the TP is bordered by the Guasayán Range, which reaches an altitude of approximately 600 m. On the northwest, the TP is bordered by the Aconquija Range, which has been elevated more than 5000 m along an east-dipping high angle thrust on its western side. The TP sedimentary units are inferred from outcrops at the basin edges (Battaglia, 1982) and seismic reflection data (Cristallini et al., 1997) to be primarily Pliocene and Miocene unconformably overlying a thin Cretaceous package, which in turn unconformably overlies the crystalline basement. The depth to basement is constrained by both seismic and MT data (Cristallini, et al., 1997; Pomposiello, et al., 2001).