INTEGRATED GEOPHYSICAL ANALYSIS SUPPORTING THE IMPACT ORIGIN OF THE MJOLNIR STRUCTURE, BARENTS SEA

Gravity, magnetic and seismic traveltime anomalies observed at the 40- km-diameter Mjolnir impact structure reveal a distinct spatial corresp ondence with the radially zoned seismic structure. The gravity anomaly is dominated by a +2.5 mGal, 14-km-wide, centrally located high super imposed on a 45-km-diameter low that attains minimum values of -1.5 mG al. The magnetic anomaly field exhibits several local, low-amplitude a nomalies within the +/-100 nT range located towards the periphery, whi le seismic mapping of a prominent, originally planar reflector beneath the structure brings out a central, pull-up traveltime anomaly on the order of 80 ms. In terms of impact origin, the integrated geophysical modelling based on the characteristic bowl-shaped seismic disturbance beneath the structure supports the differentiation into a central upl ift and a peripheral region. Interaction of several impact cratering p rocesses, such as impact-induced porosity increase due to brecciation, mass transport during collapse, and structural uplift, explains the m odelled physical properties associated with the disturbance. The model ling further substantiates the interpretation of the Mjolnir structure as an impact crater and demonstrates the incompatibility of alternati ve geological origins, such as salt or clay diapirism and igneous intr usions. (C) 1998 Elsevier Science B.V. All rights reserved.