Faculté des sciences

An integrated magnetotelluric and aeromagnetic investigation of the Serra da Cangalha impact crater, Brazil

Schnegg, Pierre-André ; Adepelumi, A. A. ; Flexor, J. M. ; Fontes, S. L.

In: Physics of the Earth and Planetary Interiors, 2005, vol. 150, p. 159-181

The research is aimed at delineating the post-impact structural characteristics across the Serra da Cangalha impact crater in Brazil using a combination of magnetotelluric (MT) and aeromagnetic data. The MT survey was carried out along three radial MT profiles trending NW–SE, ENE–WSW and NNE–SSW across the crater. For MT sites located further away from the centre of the crater, isotropic MT... Plus

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    Summary
    The research is aimed at delineating the post-impact structural characteristics across the Serra da Cangalha impact crater in Brazil using a combination of magnetotelluric (MT) and aeromagnetic data. The MT survey was carried out along three radial MT profiles trending NW–SE, ENE–WSW and NNE–SSW across the crater. For MT sites located further away from the centre of the crater, isotropic MT responses were observed, suggesting a 1D conductivity distribution in the subsurface in the frequency range of 100–10 Hz. For sites located in the vicinity of the inner ring of the crater, anisotropic responses were observed for the same frequency range. We believe that this zone probably represents the areas of structural disturbance. A 2D resistivity inversion of these data reveals a four-layer model, representing a thin resistive layer underlain by a conductive layer, a weathered basement and a resistive crystalline basement. The depth to the top of the basement is estimated to be about 1.2 km. This is in good agreement with the estimation of depth to the basement of about 1.1 km, calculated using the aeromagnetic data. However, in view of the circular geometry of the crater, we have carried out a 3D forward modeling computation to supplement the derived 2D model. The 3D resistivity forward model, fitting the MT responses by trial-by-error revealed a five-layer model, showing a significant reduction in the basement resistivity. This, perhaps, could be due to the structural disturbances that have been caused by the impact on the crater, resulting in brecciation, fracturing, alteration and shocked zone filled with weak-magnetic materials and fluids. We have calculated the effect of the impact on the overall structural deformation beneath the Serra da Cangalha crater, following the classical crater scaling relation of Holsapple and Schmidt [Holsapple, K.A., Schmidt, R.M., 1982. On the scaling of crater dimensions. II. Impact processes. J. Geophys. Res. 87, 1849–1870] and found to be about 2 km.