ELECTRICAL-CONDUCTIVITY AND CRUSTAL DEFORMATION FROM MAGNETOTELLURIC RESULTS IN THE REGION OF THE ARAGUAINHA IMPACT, BRAZIL

In order to obtain information about the deep structure of large meteo rite craters, a series of MT soundings were carried out in the region of the Araguainha impact crater, located in the northern section of th e Parana Basin (Mato Grosso, Brazil). The MT responses in the short pe riod range from 0.001 to 1s show one-dimensional behaviour, in contras t to the longer periods where data are multi-dimensional. At periods b etween 1 and 10s the splitting of the apparent resistivity and phase c urves of different polarisations is accompanied by a strong increase o f the phase-sensitive regional skew parameter to values up to 0.5. The results of two- and three-dimensional modelling reveal a disk-shaped body embedded within a layer of 5000 Ohm m. The resistivity of the bod y, ranging between 20 Ohm m and 500 Ohm m, lies significantly below th e bulk value of the upper crust. The anomaly is believed to be caused by impact-induced faulting and brecciation of the crust, reaching to d epths of 3 to 7km. The horizontal extent of this zone is 16km, corresp onding to about 40% of the total crater diameter at the surface. The l ong-period data (T > 10 s) reveal a heterogeneous lower crust of enhan ced conductivity at depths between 15 and 30km, which is not related t o the impact event. It is likely to be a feature typical of the baseme nt in the area of the Araguainha structure. (C) 1997 Elsevier Science B.V.