Offsets in Brecciation and Structural Uplift

Impact craters on sedimentary targets have the advantage, in comparison with similar structures on crystalline targets, that the regular, pre-impact stratification of these targets provides reference horizons against which the impact-induced structures can be identified and mapped by seismic reflection studies. Such studies have provided an effective means of mapping the large-scale geometrical structure at depth with a high-degree of horizontal and vertical resolution (e.g., Morgan and Warner 1999).

At Mjolnir, seismic mapping and analysis of the deeper structure levels in combination with gravity and seismic-velocity modelling, and with detailed numerical simulations, have provided greater insight into several cratering processes, such as brecciation and excavation, gravitational collapse of the transient crater, and structural uplift (Tsikalas et al. 1998a-c; Shuvalov et al. 2002). The integrated analysis supports the lateral differentiation of the Mjolnir seismic disturbance into a central uplift and a peripheral region (Fig. 2). The primary effect of the impact event is an impact-induced porosity increase due to extensive fracturing and brecciation (Pilkington and Grieve 1992; Tsikalas et al. 2002b). Subsequent modification of the density field takes place as a result of mass transport during gravitational collapse and structural uplift of the crater floor displacing deep, denser strata to shallower levels beneath the central structure (Fig. 2) (Tsikalas et al. 1998a, c; Shuvalov et al. 2002).

The models for Mjolnir are directly supported by the observed free-air gravity and seismic velocity anomalies. In particular, the residual free-air gravity field exhibits a circular anomaly over the structure (Fig. 6a). The anomaly is divided into an annular low, with an outer diameter of 45 km, attaining minimum values of -1.5 mGal over the periphery, and a central 14 km-wide gravity high, with a maximum value of +2.5 mGal (Tsikalas et al. 1998c). It appears that the 0-mGal gravity anomaly contour exhibits a very distinct elongated-shape in the SW-NE direction (Fig. 6a). In addition, the annular gravity low (<-0.5 mGal), which is directly connected with the region of most intense fracturing and brecciation, closely resembles a U-shaped central pit open to the northeast (Fig. 6a).

73* 33'

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Gravity profile ------- Main ahmIi: n bramriary ^ G eom elri c era ler center

Fig. 6. (a, left) Residual free-air gravity anomaly map with 0.25 mGal contour interval, (b, right) Residual traveltime anomaly map at the Top Permian reflector (cfr. Fig. 2) with 10 ms contour interval (modified extensively from Tsikalas et al. 1998c).

The deep seismic profiles provide evidence of upward bending reflector segments beneath the central high and the annular basin, indicating elevation of deep strata to shallower levels (Fig. 4). By measuring the difference in depth between the extrapolated top of selected upward bending reflector segments beneath the central high and their most likely equivalent subhorizontal interfaces the structural uplift was estimated to be 1.0 -. 1.5 km and when decompacted ~1.5-2.0 km, fitting the theoretical expectations for the Mjolnir dimensions (Fig. 4) (Tsikalas et al. 1998a). The north-south profile in Figure 4 (profile a) clearly shows a maximum structural uplift lateral offset of 2 - 2.5 km towards the south from the geometric crater center. Similarly, the gravity central peak which corresponds to the maximum structural uplift (Tsikalas et al. 1998c) is laterally offset by ~1.5 - 2 km to the southwest from the geometric crater center (Fig. 6a). Furthermore, fracturing and brecciation are expected to induce changes in the seismic velocity expressed as pull-up or pull-down effects on continuous reflectors below the impact structure. Indeed, the seismic profiles reveal a small pull-up of the high-amplitude, originally planar Top Permian reflector beneath the structure (Fig. 2). The mapped traveltime anomaly is 16 km in diameter and rises to +80 ms beneath the central crater (Fig. 6b), corresponding to a +175 m/s seismic velocity anomaly (Fig. 2) (Tsikalas et al. 1998c). It appears that the central traveltime anomaly has a slightly elongated shape in the SW-NE direction and its top is slightly offset by ~2 km to the WSW from the geometric crater center (Fig. 6b).

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