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.