Preliminary magnetotelluric study of Ambato and Valle Fértil lineaments in Bermejo Basin and Sierra de Valle Fértil, San Juan, Argentina

Borzotta, E. and Mamaní, M. and Venencia, J. (2009) Preliminary magnetotelluric study of Ambato and Valle Fértil lineaments in Bermejo Basin and Sierra de Valle Fértil, San Juan, Argentina. Acta Geodaetica et Geophysica Hungarica, 44 (2). pp. 157-166. ISSN 1217-8977

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Abstract

In the western region of Argentina, 30° S Lat., near the Andean Range, two Precambrian large faults or lineaments are present: Ambato (AL) (NE-SW) and Valle Fértil (VFF)(∼NS). They are deep-seated faults in the crust, with extend develop in time, crossing each other in the region where Bermejo Basin and Valle Fértil Range (Sierra de Valle Fértil, SVF) are present. The study of these lineaments is important to better know the tectonic evolution of this region. Three deep magnetotelluric soundings were carried out in this zone using induction coils and a flux-gate magnetic variometer. Principal results show a thickness up to 7000 m and saline character for the Bermejo Basin, with resistivities range frm 3 Ωm to 13 Ωm, and an integrated conductivity of 1600 Siemens. High conductivity is also suggested for the AL from the top of basement to higher depths. Westward of VFF, beneath the Bermejo Basin, a conductive lower crust is suggested with a longitudinal conductance at least of 1600 Siemens and seated at 43 km depth; but this layer does not seem to be present eastward of the fault. A possible phase transition zone at 300 km depth is also suggested. From these conductive layers, a heat flow estimate of 36–40 mW/m 2 is made for the region, using empirical formulas. Graphite is suspected to be the cause of lower crust conductivity. If correct, it would be suggesting a strong ancient tectonic activity, with probable dipping and lifting of significant amplitude in the region.

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