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

E. BORZOTTA; M.J. MAMANI; J.E. VENENCIA

ACTA GEODAETICA ET GEOPHYSICA HUNGARICA

AKADEMIAI KIADO RT

Año: 2009 vol. 44 p. 157 - 166

1217-8977

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 F´ertil (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 F´ertil Range (Sierra de Valle F´ertil, 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/m2◦ S Lat., near the Andean Range, two Precambrian large faults or lineaments are present: Ambato (AL) (NE-SW) and Valle F´ertil (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 F´ertil Range (Sierra de Valle F´ertil, 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/m2∼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 F´ertil Range (Sierra de Valle F´ertil, 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/m22 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.