Mjolnir Impact Obliquity Constrains Models for Near Field Perturbations

Numerical simulations and experimental analogues have shown that obliquity is accompanied by less energy transfer from the projectile to the target (e.g., Gault and Wedekind 1978; Hayhurst et al. 1995; Schultz 1996; Burchell and Mackay 1998; Ivanov and Artemieva 2002). The Mjolnir energy release estimates of Tsikalas et al. (1998a) were made considering an elevation impact angle of 45° based, at that time, on well-known probability arguments (Shoemaker 1962; Shoemaker et al. 1990). The energy release was estimated to be in the order of 16 x 1020 J (range of 2.4-53 x 1020 J), translating into 3.8 x 105 megatons TNT equivalent (range of 5.7 x 104 - 1.2 x 106) (Tsikalas et al. 1998a). An oblique impact at a ~45° (possibly 30°-45°) angle, as estimated in this study, is expected to have resulted in very similar energy release.

Energy release dissipation at the proposed trajectory and angle for the Mjolnir impact may have a direct consequence on the distribution of proximal ejecta and tsunami-waves following the cessation of the impact-related processes at the impact-site. This is because the oblique impact most probably have created a down-range sector/corridor of thicker ejecta deposits and greater water column disturbance (Fig. 7). Such a sector/corridor may have been responsible for a geographic variation of short-term perturbations/environmental stress magnitude on the Barents Sea and adjacent regions, as it may have intensified the stress at a specific location and left the others almost unaffected.

Nature and Distribution of Proximal Ejecta

Theoretically, the volume of material displaced from the crater equals the volume of excavated cavity (Croft 1985; Melosh 1989). Geophysical observations constrain the volume of a parabolic excavated cavity to 180 km3 (Tsikalas et al. 1998a), whereas numerical simulations indicate a volume of ~230 km3 (Shuvalov et al. 2002). The ejecta layer is expected to be thickest close to the crater rim, decreasing rapidly with distance from the crater center (Tsikalas et al. 1998a; Shuvalov et al. 2002). Accepting an oblique Mjolnir impact from the south/southwest, ejecta iso-thickness contours will probably not be circular around the crater site, but rather elongated towards the north/northeastern direction (Fig. 7).

Borehole evidence for the Manson crater case (e.g., Anderson et al. 1996) substantiate that ejecta deposits are thinnest in the up-range direction and that only the top target layers are ejected due to shallower excavation as a result of oblique impact (Schultz and Anderson 1996). At Mjolnir, possible shallower penetration and excavation may be the reason for the small discrepancies in the estimated transient cavity depth of 4-5 km versus 6 km and the excavated/ejected volume of180 km3 versus 230 km3, based on geophysical observations and numerical simulations, respectively (Tsikalas et al. 1998a; Shuvalov et al. 2002). Note that numerical simulations have for simplicity considered a vertical incidence for the Mjolnir impact (Shuvalov et al. 2002). Furthermore, the fact that the shocked quartz grains and the iridium anomaly peak are located at the base and top, respectively, of the 80-cm-thick ejecta deposit at borehole 7430/10-U-01 (Fig. 1) (Dypvik et al. 1996) may have

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Fig. 7. Mjelnir impact location with possible range of impact direction azimuth and down-range area of possible maximum ejecta deposits and water column disturbance, shown at a ~142 Ma plate reconstruction based on Lawver et al. (1999) and overlaid on a simplified paleogeographic synthesis based on Brekke et al. (2001) approximately at the time of impact. FJL, Franz Josef Land.

Fig. 7. Mjelnir impact location with possible range of impact direction azimuth and down-range area of possible maximum ejecta deposits and water column disturbance, shown at a ~142 Ma plate reconstruction based on Lawver et al. (1999) and overlaid on a simplified paleogeographic synthesis based on Brekke et al. (2001) approximately at the time of impact. FJL, Franz Josef Land.

possibly resulted from a multistage ejecta emplacement, similar to those attributed to oblique impacts as revealed by laboratory experiments and planetary impact crater studies (Schultz and Gault 1990; Schultz 1992; Schultz and D'Hondt 1996).

Magnetic modelling has indicated that only low quantities of dispersed-character melts localized in the crater periphery may have been produced during the water-covered, sedimentary target Mjolnir impact (Tsikalas et al. 1998c). A similar absence of considerable impact glass and melts at Manson crater has been also attributed to the sedimentary target and, more importantly, to the obliquity of the impact as this results in shallower target penetration and less direct energy transfer from the projectile to the target (Izett et al. 1993; Schultz and Anderson 1996). Recent geochemical analyses of samples from the Mjolnir central crater core (borehole 7329/03-U-01, Fig. 1) have shown absence of (Cr, Co, Ni) or weak (Ir) siderophile-element anomalies (Sandbakken 2002). This translates into a low abundance or total absence of projectile material in the crater itself, being consistent with oblique impact models, where a large fraction of the projectile material retains a net down-range motion and fragments of it may survive the impact, due to higher ejection velocity and lower shock compression, and may be deposited outside the crater proper (Schultz 1996; Pierazzo and Melosh 2000; Artemieva and Shuvalov 2001).

Tsunami-wave Distribution

The growing crater rim and ejecta curtain following the Mjolnir impact form a water surge that eventually breaks up and causes the formation of several waves that, in turn, together with reflected waves, will generate tsunamis. Tsunami wave heights resulting from a vertical incidence Mjolnir impact at various radii from the impact site have been calculated based on different approaches (Tsikalas et al. 1998a; Shuvalov et al. 2002). Although the impact tsunami theory for vertical incidence impacts is well understood (e.g., Ward and Asphaug 2002), there is an almost total absence of computational experiments of tsunami-wave distribution resulting from oblique impacts. Due to the relatively shallow water-target depth, I visualize an oblique Mjolnir impact (south/southwest azimuth at -45° angle, possibly 30°-45°) to have generated a greater down-range water column disturbance, probably giving rise to faster travelling tsunami-waves at the down-range rather than the up-range region (Fig. 8). Such a scenario is better approximated with the non-axis-symmetrical propagation of tsunamis resulting from submarine slides (Ward 2001).

The ~80-cm-thick ejecta layer at borehole 7430/10-U-01 (Fig. 1) is the thickest Mjolnir ejecta detected so far. The minor thickness ejecta detected on Svalbard (<1 cm; H. Dypvik, pers. comm.) and the absence (undetected so far despite the intense efforts) of tsunami-deposit signatures on NE Greenland (H. Dypvik, per. comm.), are additional evidence for the obliquity of the Mjolnir impact and possible geographic selectivity in ejecta and tsunami-waves distribution patterns. The proposed conceptual model (Fig. 7, 8) envisages thickest ejecta distribution and faster travelling (thus most devastating) tsunami waves concentrated in the area between Svalbard and

Fig. 8. Schematic cross-section diagrams showing the proposed formation sequence of the Mjelnir crater, with focus on the ejecta distribution and water-column disturbance, resulting from an oblique impact from south/southwest. Detailed hydrocode simulations of a vertical incidence Mjelnir impact are provided by Shuvalov et al. (2002).

Fig. 8. Schematic cross-section diagrams showing the proposed formation sequence of the Mjelnir crater, with focus on the ejecta distribution and water-column disturbance, resulting from an oblique impact from south/southwest. Detailed hydrocode simulations of a vertical incidence Mjelnir impact are provided by Shuvalov et al. (2002).

Novaya Zemlya. The analysis clearly shows the importance of impact direction and angle in the distribution pattern of ejecta and tsunamis, and further research must, therefore, focus on the proposed down-range region (Fig. 7).

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