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

Abstract

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

An appraisal of the Serra da Cangalha impact structure using the Euler deconvolution method

“…These depth values are in good agreement with depths from boreholes located not too far from the crater site, and are coherent with previous studies by Adepelumi et al (2005). Figure 4c represents the structural interpretation of the Euler solutions for η = 3 of the gridded data.…”

“…Local stratigraphy comprises upper Silurian to Cretaceous sedimentary rocks roughly 1200 m thick (Góes and Feijó 1994;Adepelumi et al 2005). The youngest sedimentary formation, Pedra de Fogo, lies over the Piaui formation, followed by the Poti and the Longá formations.…”

“…Furthermore, in the inner ring, there are severely deformed, steeply dipping (>60°) sedimentary structural patterns. More recently, Adepelumi et al (2005) evaluated the magnetotelluric data acquired across the crater and suggested that impact-induced structural deformation is limited to a 2 km depth. Further evidence from the Geological Survey of Brazil (CPRM 1972) shows that the annular inner depression of the crater is filled with impact-derived materials made up of very minute allochthonous monomictic breccias and minor impact melt bodies.…”

“…4, since line and point dipole sources are appropriate for estimating the depth of a structure with circular geometry. Depth limits, obtained through a magnetotelluric method and downward continuation of the aeromagnetic data by Adepelumi et al (2005), and well logs by Góes and Feijó (1994) have been used here to constrain the Euler results obtained. Subsequently, solutions that fall outside these ranges have been rejected.…”

“…1). The structure is characterized by a distinct uplifted annular trough and a prominent circular central ring with a diameter of 5 km and a height of about 300 m above the surrounding terrains (Adepelumi et al 2005).…”