The anatomy of a buried complex impact structure: The Mjolnir Structure, Barents Sea

Analysis of the prominent seismic disturbance at the 40-km-diameter Mjolnir impact structure, based on an extensive seismic reflection database, shows that the observed broad-brimmed bowl-shaped disturbance was formed as a re sult of the impact of an asteroid or comet during Volgian-Berriasian time ( 149-141 Ma). Seismic mapping exhibits and visualizes a 850-1400 km(3) distu rbed volume and analysis of several structural features within the disturba nce provides insight into major cratering processes, such as brecciation an d excavation, melting, gravitational collapse of the transient crater, and structural uplift. A transient crater of 16 km in diameter and 4.5 km in de pth is determined. From transient and final crater dimensions we obtain an estimate of the degree of gravitational collapse of the order of 2.5, consi derably larger than the average expected values for typical terrestrial cra ters. The extensive collapse rook place by low-angle decollement surfaces a t the periphery and by chaotic debris mass flows toward the center. Further more, we estimate the Mjolnir projectile to have been 0.9-3 km in diameter and that the physical impact released energy in the range of 2.4-53 x 10(20 ) J corresponding to an earthquake of magnitude 8.3. The Mjolnir impact is not associated with a significant mass extinction. However, dissipation of the energy released during the Mjolnir impact was sufficient to have caused several short-term, near-field perturbations, such as large-amplitude tsun ami waves, affecting the Barents Sea region and possibly adjacent areas in the Arctic.