Introduction

The 40-km-diameter Mjolnir crater is a well-established marine impact crater in the central Barents Sea (Fig. 1) (e.g., Tsikalas et al. 1999). Both geophysical and geological data unequivocally substantiate a meteorite bolide impact at ~142 Ma into an epicontinental basin with 300-500 m paleo-water depth (Gudlaugsson 1993; Dypvik et al. 1996; Smelror et al. 2001; Tsikalas et al. 2002a). In particular, a total of ~2100 km of seismic reflection profiles (Fig. 1) clearly image the impact-related and post-impact structure and stratigraphy (Fig. 2) (Tsikalas et al. 1998a, b). In addition, free-air gravity and seismic velocity anomalies exhibit a close correspondence to the impact-

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Fig. 2. Geophysical type section along profile AA' in Fig. 1. (a) observed free-air gravity and seismic traveltime anomalies, (b) interpreted high-resolution single-channel profile, (c) interpreted multi-channel profile, and (d) impact crater model with calculated physical property distribution. SF, sea floor; URU, late Cenozoic upper regional unconformity; TD (impact horizon), the first continuous reflector above the disturbed seismic reflections; TP, Top Permian; (d) low-angle décollement. The crater model geometry in (d) is corrected for regional tilt, and the modelled density contrasts are given in kg/m3, the seismic velocities in m/s, and the porosity anomalies in %.

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Fig. 2. Geophysical type section along profile AA' in Fig. 1. (a) observed free-air gravity and seismic traveltime anomalies, (b) interpreted high-resolution single-channel profile, (c) interpreted multi-channel profile, and (d) impact crater model with calculated physical property distribution. SF, sea floor; URU, late Cenozoic upper regional unconformity; TD (impact horizon), the first continuous reflector above the disturbed seismic reflections; TP, Top Permian; (d) low-angle décollement. The crater model geometry in (d) is corrected for regional tilt, and the modelled density contrasts are given in kg/m3, the seismic velocities in m/s, and the porosity anomalies in %.

induced structure and physical property distribution (Fig. 2) (Tsikalas et al. 1998c). Two shallow boreholes, one near the center and another ~30 km from the crater periphery (Fig. 1) offer a detailed seismic stratigraphic correlation and confirmed the impact origin of the structure. The boreholes revealed brecciated sediments, including shocked quartz containing abundant planar fractures/deformation features (Sandbakken 2002), and a prominent ejecta layer with an iridium abundance peak of about 1,000 ppt, about 15 times above the background level, and shocked quartz grains (Dypvik et al. 1996; Dypvik and Attrep 1999).

The physical impact resulted in an extensive disturbance both in the sedimentary target and the water column, as: (1) a 850-1400 km3 seismically disturbed volume of target rocks at the impact-site (Fig. 2) is directly-related to the impact-driven processes of brecciation, excavation, structural uplift, gravitational collapse, and infilling (Tsikalas et al. 1998a); (2) a 180-230 km3 excavated/ejected material volume was displaced from the impact-site and re-deposited in the near-field vicinity (Tsikalas et al. 1998a; Shuvalov et al. 2002); and (3) collapse of the impact-induced water-cavity is expected to have given rise to large-amplitude tsunami waves (Tsikalas et al. 1998a; Shuvalov et al. 2002). Dissipation of the energy released during the Mjolnir impact is directly related to the ejected material volume and the tsunami wave generation, which are, in turn, connected to possible short-term, near field perturbations affecting the Barents Sea region and, possibly, adjacent areas in the Arctic.

Although most qualitative and quantitative impact models assume, for simplicity, axial symmetry created by a near-vertical collision, no meteorite is expected to strike a planetary surface exactly vertically as the probability for near-vertical (85°-90°) and grazing impacts (0°-5°) is almost zero (Shoemaker 1962). The most probable angle of impact of a randomly incident projectile is 45° (Shoemaker 1962; Shoemaker et al. 1990), with an estimated 25% of impactors striking at angles below 30° from the horizontal (Schultz and Gault 1990). Following the above estimates, the Mjolnir impact did not, most probably, occur at vertical incidence. In this study, based on the established Mjolnir structure, morphology, and gravity and seismic velocity signatures a search for evidence revealing the impact direction and angle is carried out. Such well-constrained parameters are important for setting boundary conditions to models of ejecta and tsunami-wave distribution, and thus refining the geographic distribution of possible impact-induced regional perturbations and environmental stress.

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