Figure 14: Simplified version of Figure 12C showing our deflection and cone height data plotted against pit crater long axis, coupled with the power-law trend of all literature data used. We define an arbitrary boundary between areas where original pit crater geometries are likely preserved and those where infilling may modify apparent pit crater depths. Inset: sketches showing how infilling may modify pit crater depths.
Can we relate the surface expression of pit craters to subsurface structures and processes?
The surface expression of pit craters observed on Earth and other planetary bodies has been used to infer how they formed and establish characteristics of subsurface geology (e.g., regolith depth) (e.g., Kling et al., 2021; Korteniemi et al., 2010; Martin et al., 2017; Mège & Masson, 1997; Smart et al., 2011; Whitten & Martin, 2019). For example, we may expect pit craters long axes, like that of volcanic vents, to align parallel to underlying dykes as they form within the same stress field (Bonini & Mazzarini, 2010; Magee et al., 2016; Paulsen & Wilson, 2010). Because we have established that pit crater formation was related to either be dyking or faulting, and because we can image these features in 3D, we can examine pit crater morphologies to see if magmatic and tectonic origins can be distinguished. Aside from cone height (pit crater depth) and long axis lengths, our data demonstrates that most pit crater properties are only weakly positively correlated or that there is no correlation at all (Figs 11 and 12). Critically, there are no significant differences in sizes of pit craters developed above dykes, dyke-induced faults, or tectonic faults (Figs 11 and 12). The pit craters we analyse also show no preferred long axis orientation, even those seemingly related to contemporaneous dykes and dyke-induced faults that are clearly aligned ~N-S (Fig. 12B). The disparity in dyke and dyke-induced fault orientation relative to the elongation of the pit craters implies formation of the latter was not sensitive to the prevailing stress field, perhaps because they formed in unconsolidated wet sediments. Overall, our data suggest that pit craters related to dyking, dyke-induced faulting, or tectonic faulting cannot be easily distinguished based on their surficial size and orientation (Figs 9A, C, and 12).
In addition to relating surface expression to pit crater formation, pit crater depths have been used as a proxy for regolith thickness on other planetary bodies (e.g., Whitten & Martin, 2019; Wyrick et al., 2004). This use of pit crater depth follows the inference that drainage of loose, unconsolidated material into an underlying cavity instigates development of an inverted cone section controlled by the host materials angle of repose (e.g., Whitten & Martin, 2019; Wyrick et al., 2004). Our pit craters formed during deposition of the marine Dingo Claystone (Tindale et al., 1998), so we assume their contemporaneous shallow sub-seabed material was unconsolidated and wet. Although the presence of pore fluids may alter the behaviour of host sediment relative to dry regolith on other planetary bodies, it seems reasonable to expect both materials to respond similarly to localised subsidence; i.e. they should drain into underlying pipes. However, we show that cone height (i.e. pit crater depth) varies non-systematically across the study area, including along individual chains, with adjacent pit craters of the same age often displaying different cone heights (Figs 6-8 and 10). Furthermore, some pit craters seemingly have no seismically resolved inverted cone section and appear simply to have a pipe-like geometry (Figs 6-8). Assuming that the transition from unconsolidated wet sediment to lithified rock (i.e. perhaps equivalent to a regolith-rock transition) occurred at a relatively constant depth across the study area, the observed variation in pit crater cone heights and their local absence suggest the changing rheology of the host material did not primarily control pit crater geometry (cf. Whitten & Martin, 2019; Wyrick et al., 2004).
Conclusions
Here, we use seismic reflection data from offshore NW Australia to image the entire 3D geometry of pit craters and underlying magmatic and tectonic structures. Our work demonstrates that pit craters link at depth to dykes and steep fault segments, confirming pit crater formation can occur in response to magmatic processes and dilatational faulting. We also show that pit crater depths strongly correlate with their long axis lengths, consistent with observations of pit craters elsewhere on Earth and other planetary bodies; deviation of pit crater populations from the power-law trend that defines these may be an indicator that pit craters have been infilled and/or modified. Our results suggest that we should be cautious when interpreting the origin of pit craters on other planetary bodies because: (i) the distribution and size of pit craters may not be diagnostic of the potential dyking and/or faulting processes driving their formation; and (ii) pit crater size may not simply relate to the mechanical properties of the host material (e.g., regolith) or their driving mechanism. Overall, our work shows that reflection seismology is a powerful tool for subsurface exploration on other worlds, as it allows us to examine the 3D structure of features on Earth thought analogous to those recognised on the surfaces of other planetary bodies.