Figure 8: (A-C) Uninterpreted and interpreted, depth-converted seismic sections showing the structure of pit craters (X1-X5) associated with tectonic normal faults. See Figure 4 for interpreted section key and Figure 7 for line locations.
Plan-view geometry
Within dyke-induced graben, funnel-like pit crater tops have long axes of 150–740 m, areas of 0.05–1.24 km2, and are quasi-circular to elliptical with aspect ratios of 1.04–3.05 (Fig. 9A; Supporting Table 1). The orientation of these pit crater long axes is variable, with a mean axis trend of 080° and a mode of 005–015° (Fig. 9B). At the Top Athol Formation, typically located below inverted cone sections, the pipes of these funnel-like pit craters are smaller in plan-view: long axis lengths are 108–585 m, aspect ratios are 1.01–3.14, and areas are 0.02–0.10 km2 (Figs 9A and C; Supporting Table 1). For pipe-like pit craters within dyke-induced graben, their plan-view outlines at their tops are characterised by long axes of 170–390 m with no clear alignment, aspect ratios of 1.23–1.63, and areas of 0.06–0.39 km2 (Figs 9A and C; Supporting Table 1). At the Top Athol Formation, pipe-like pit crater long axes within dyke-induced graben are 117–364 m, have aspect ratios of 1.22–1.47, and have areas of 0.04–0.32 km2 (Figs 9A and C; Supporting Table 1).
The area and aspect ratio of the 29 pit craters situated directly above dykes broadly decrease (R2 = 0.55) and seemingly increase (R2 = 0.29) with depth, respectively, and tend to be larger than those above dyke-induced faults (Figs 9A, C, and D). Although the 29 pit craters above dykes cumulatively have a mean long axis orientation of 077°, six are oriented between 005–015°, sub-parallel to the mean axis (011°) of dyke segments in the study area (Fig. 9B). The size and shape of the 25 pit craters above dyke-induced faults varies non-systematically with depth; i.e. the correlation between area and aspect ratios measured at the pit crater tops and at the structurally deeper Top Athol Formation are R2 = 0.26 and R2 = 0.08, respectively (Figs 9A, C, and D).
Pit craters with a funnel-like morphology, but associated with tectonic normal faults (X1 , X2 , X4 , and X5 ), are typically larger than those within dyke-induced graben and have long axes of 400–740 m, aspect ratios of 1.17–1.55, and areas of 0.33–1.47 km2 at their tops (Fig. 9A; Supporting Table 1). At the Top Athol Formation, these funnel-like pit craters have long axes of 358–790 m, aspect ratios of 1.10–2.24, and areas of 0.18–1.66 km2 (Fig. 9; Supporting Table 1). The only pit crater associated with a tectonic normal fault that does not have a funnel-like morphology (X3 ) is 140 m long, has an aspect ratio of 1.3, and an area of 0.05 km2 at its top (Fig. 9A; Supporting Table 1). At the Top Athol Formation, this pipe-like pit crater is 98 m long, has an aspect ratio of 1.6, and an area of 0.02 km2(Fig. 9A; Supporting Table 1). Of the pit craters above tectonic faults, some increase in area or aspect ratio with depth, whereas others decrease (Figs 9C and D). There is no preferred orientation of pit crater long axes above tectonic faults (Fig. 9B).