Collapse, infilling, and postimpact deformation at the Mjølnir impact structure, Barents Sea
A dense composite grid of high-resolution single-channel, shallow multichannel, and conventional multichannel seismic reflection profiles defines in detail the structures and stratigraphy associated with impact-induced tectonics, and the postimpact evolution of the Mjolnir impact structure, Barents Sea. Clear seismic images of the well-preserved 40-km-diameter structure reveal that the primary, impact-induced deformation is bounded on top by discernible structural relief. At this stratigraphic level the structure exhibits a distinct radial zonation pattern composed of a 12-km-wide complex outer zone, including a marginal fault zone and a modestly elevated ring, a 4-km-wide annular depression, and an uplifted central high, 8 km in diameter. Key features of the Mjolnir structure are similar to the majority of large terrestrial complex craters. Sharp boundary faults form an ∼150-m-high, near-circular rim wall and separate highly deformed strata within the crater from intact platform strata. A 45–180-m-thick layer of disturbed and incoherent seismic reflectivity caused by the impact is confined by prominent fault blocks and the postimpact strata. The seismic data also provide evidence of crater-influenced sedimentation, and extensive secondary, postimpact deformation expressed by structural reactivation and differential subsidence. Reconstruction of the original crater relief reveals a subtle structure that has average depth variations of only ∼30–40 m. When compared to the current morphology of the crater floor, this demonstrates substantial enhancement of structural features by postimpact burial and deformation. We infer that impact in a shallow-marine sedimentary basin led to an atypically shallow crater depth, resulting from gravitational collapse on an apparent low-angle decollement at the crater periphery, and considerable infilling of the crater. The extensive postimpact deformation is closely related to the instability and lateral changes in physical properties within the impact-affected rock volume, triggered by prograding postimpact sediments.