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\begin{document}

\title{Seismic reflection data reveal the 3D subsurface structure of pit
craters}



\author[1]{Craig Magee}%
\author[2]{Corbin L. Kling}%
\author[3]{Paul K. Byrne}%
\author[4]{Christopher Jackson}%
\affil[1]{University of Leeds}%
\affil[2]{Smithsonian Institution}%
\affil[3]{Washington University in St. Louis}%
\affil[4]{University of Manchester}%


\vspace{-1em}



  
  \date{May 27, 2024}


\begingroup
\let\center\flushleft
\let\endcenter\endflushleft
\maketitle
\endgroup





\selectlanguage{english}
\begin{abstract}
Pit craters are enigmatic, quasi-circular depressions observed on rocky
(e.g., Earth and Mars) and icy (e.g., Enceladus) planetary bodies, and
numerous asteroids. These depressions are not related to meteorite
impacts. Instead pit craters are thought to be generated by overburden
collapse into a subsurface cavity (e.g., created by dilatational
faulting), or volumetrically depleted zone (e.g., an evacuated magma
reservoir or conduit). The surface size and distribution of pit craters
should thus provide an important record of otherwise inaccessible
subsurface processes. Because we cannot access the subsurface structure
of pit craters observed on planetary surfaces, we rely on physical and
numerical models to infer the subsurface processes linked to their
formation. Here, we use 3D seismic reflection data to quantify the
palaeosurface and subsurface geometry of pit craters. These data enable
us to identify whether pit craters connect to and their formation thus
related to underlying structures. We map 59, Late Jurassic pit craters
deeply buried within a sedimentary basin, offshore NW Australia. We
demonstrate that some pit craters extend down and appear physically
connected to igneous dykes, whereas others connect to dyke-induced and
tectonic faults, seemingly where faults are steeply dipping and
dilatational. There are no apparent relationships between pit crater
geometries (e.g., palaeosurface expression or vertical extent) and the
different magmatic and fault-related mechanisms we infer drove their
formation. Reflection seismology is a powerful tool for understanding
how pit craters form and whether their surface expression can be used to
reconstruct subsurface structures and processes on other planetary
bodies.%
\end{abstract}%



\sloppy


\textbf{Seismic reflection data reveal the 3D subsurface structure of
pit craters}

Craig Magee\textsuperscript{1}, Corbin Kling\textsuperscript{2}, Paul
Byrne\textsuperscript{3}, Christopher A-L Jackson\textsuperscript{4}

\textsuperscript{1}School of Earth Science and Environment, University
of Leeds, Leeds, LS2 9JT, UK

\textsuperscript{2}Center for Earth and Planetary Studies, Smithsonian
Institution, Washington, D.C., 20560, USA

\textsuperscript{3}Washington University in St. Louis, St. Louis, MO
63132, USA

\textsuperscript{4}Department of Earth and Environmental Sciences,
University of Manchester, Williamson Building, Oxford Road, Manchester,
M13 9PL, UK

\textbf{Key points}

\begin{itemize}
\tightlist
\item
  We image and quantify the palaeosurface and subsurface 3D geometry of
  pit craters formed in the Late Jurassic
\item
  Pit craters physically connected to upper tips of igneous dykes or
  steeply dipping, dilatational portions of normal faults
\item
  No apparent relationship between pit crater geometry and formation
  mechanism
\end{itemize}

\textbf{Abstract}

Pit craters are enigmatic, quasi-circular depressions observed on rocky
(e.g., Earth and Mars) and icy (e.g., Enceladus) planetary bodies, and
numerous asteroids. These depressions are not related to meteorite
impacts. Instead pit craters are thought to be generated by overburden
collapse into a subsurface cavity (e.g., created by dilatational
faulting), or volumetrically depleted zone (e.g., an evacuated magma
reservoir or conduit). The surface size and distribution of pit craters
should thus provide an important record of otherwise inaccessible
subsurface processes. Because we cannot access the subsurface structure
of pit craters observed on planetary surfaces, we rely on physical and
numerical models to infer the subsurface processes linked to their
formation. Here, we use 3D seismic reflection data to quantify the
palaeosurface and subsurface geometry of pit craters. These data enable
us to identify whether pit craters connect to and their formation thus
related to underlying structures. We map 59, Late Jurassic pit craters
deeply buried within a sedimentary basin, offshore NW Australia. We
demonstrate that some pit craters extend down and appear physically
connected to igneous dykes, whereas others connect to dyke-induced and
tectonic faults, seemingly where faults are steeply dipping and
dilatational. There are no apparent relationships between pit crater
geometries (e.g., palaeosurface expression or vertical extent) and the
different magmatic and fault-related mechanisms we infer drove their
formation. Reflection seismology is a powerful tool for understanding
how pit craters form and whether their surface expression can be used to
reconstruct subsurface structures and processes on other planetary
bodies.

\textbf{Plain language summary}

Linear chains of small craters are commonly observed on planetary bodies
like Mars, Enceladus, or various asteroids. These craters, called pit
craters, are not related to meteorite impacts. Based on comparison to
similar craters observed on Earth and created in laboratory models, we
think pit craters form because rock collapses into holes present beneath
the surface. There are many suggestions for how these holes form, but
all suggest their presence destabilises overlying rock, which drains
into the hole and leaves a crater at the surface. We have not been able
to test suggestions about how these holes form, because we cannot easily
look beneath the surface of other planetary bodies. We use a technique
called seismic reflection, which uses sound waves to create an
ultrasound-like image of Earth's subsurface. With this technique, we
image the 3D shape of pit craters found offshore Australia. These data
allow us for the first time to measure what pit craters look like in the
subsurface and see whether they link to any structures that could have
caused their formation. We show that some pit craters are connected to
igneous dykes (solidified vertical sheets of molten rock) and others to
faults (cracks in the rock).

\textbf{Introduction}

Quasi-circular topographic depressions are observed on the surface of
Earth and many planetary bodies and asteroids (e.g., Figs 1A and B)
(e.g., Abelson et al., 2003; Ferrill et al., 2004; Ferrill et al., 2011;
Frumkin \& Naor, 2019; Horstman \& Melosh, 1989; Kling et al., 2021;
Martin et al., 2017; Okubo \& Martel, 1998; Sauro et al., 2020; Scott \&
Wilson, 2002; Whitten \& Martin, 2019; Wyrick et al., 2004). These
depressions, termed `pit craters', have diameters of meters to thousands
of meters and commonly arranged in linear chains (e.g., Kling et al.,
2021; Whitten \& Martin, 2019). The lack of raised rims and ejecta
deposits around pit craters suggest they are not formed by meteorite
impacts (e.g., Figs 1A and B) (e.g., Halliday, 1998; Poppe et al., 2015;
Wyrick et al., 2004). Instead, pit craters are thought to reflect
collapse of overlying rock and/or regolith into subsurface cavities or
volumetrically depleted zones generated by (Fig. 1C) (see also Wyrick et
al., 2004): (i) the dissolution of carbonate or salt (e.g., sinkholes;
Abelson et al., 2003; Poppe et al., 2015; Spencer \& Fanale, 1990); (ii)
local porosity reduction of the host material following hydrothermal
fluid flow or fault-related overpressure release (e.g., pockmarks;
Velayatham et al., 2019; Velayatham et al., 2018); (iii) evacuation of
lava tubes (see Sauro et al., 2020 and references therein); (iv) opening
of tensile fractures (e.g., Ferrill et al., 2011; Smart et al., 2011;
Tanaka \& Golombek, 1989); (v) local dilation where faults are steeply
dipping (e.g., Ferrill \& Morris, 2003; Ferrill et al., 2011; Ketterman
et al., 2015; Kettermann et al., 2019; Smart et al., 2011; Von Hagke et
al., 2019); (vi) dyke intrusion (e.g., M\selectlanguage{ngerman}ège \& Masson, 1996; Okubo \&
Martel, 1998; Scott \& Wilson, 2002; Wall et al., 2010); (vii) magma
migration out of a reservoir (e.g., Mège et al., 2003; Poppe et al.,
2015); and/or (viii) explosive volcanism (e.g., Hughes et al., 2018).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

\emph{Figure 1: (A) A pit crater chain in the Natron Basin, Tanzania
(Magee et al., 2019; Muirhead et al., 2015). (B) Pit crater chains and
graben-bounding fault traces in the Noctis Labyrinthus area of Mars
(Kling et al., 2021). Basemap is THEMIS Daytime-IR. (C) Conceptual
models of pit crater formation (modified from Kettermann et al., 2019;
Sauro et al., 2020; Velayatham et al., 2019; Velayatham et al., 2018;
Wyrick et al., 2004).}

If the surface expression of pit craters reflects their formation
mechanism(s), we could use their morphology to interrogate inaccessible
subsurface processes and structures on Earth, as well as other planetary
bodies and asteroids (e.g., Kling et al., 2021; Korteniemi et al., 2010;
Martin et al., 2017; M\selectlanguage{ngerman}ège \& Masson, 1997; Smart et al., 2011; Whitten
\& Martin, 2019). However, imaging pit craters on other worlds is
difficult (Wyrick et al., 2004), and even our knowledge of the
subsurface geometry of pit craters on Earth remains limited (cf. Abelson
et al., 2003; Frumkin \& Naor, 2019; Halliday, 1998; Wall et al., 2010).
We thus rely on physical experiments and numerical modelling to predict
how pit crater formation may translate into surface deformation (e.g.,
Poppe et al., 2015; Smart et al., 2011). Identifying pit craters on
Earth, the subsurface structure of which we \emph{can} study, is crucial
to validating these models and thus determining whether pit crater
surface expressions can be used to distinguish formation mechanisms.

Reflection seismology provides unprecedented insight into Earth's
subsurface and can uniquely image the three-dimensional structure of pit
craters (Abelson et al., 2003; Magee et al., 2019; Magee \& Jackson,
2020a; Wall et al., 2010). Here, we use seismic reflection data imaging
a sedimentary basin on the Exmouth Plateau, offshore NW Australia to
conduct the first-ever quantitative, 3D analysis of pit craters and
their underlying structure. Seismic-stratigraphic analyses reveal the
now-buried pit craters developed during a phase of Late Jurassic igneous
dyking (Magee \& Jackson, 2020a). Our data allow us to: (i) quantify the
palaeosurface expression and subsurface structure of pit craters in 3D;
and (ii) identify underlying structures that may be related to pit
crater formation. Overall, we can thus test whether the (palaeo)surface
expression of pit craters are diagnostic of subsurface structure and
processes.

We map 59 pit craters, which typically have a funnel-like appearance
comprising an upper inverted cone that is underlain by a pipe, and are
commonly arranged in linear chains. Most (54) pit craters occur in
chains located along the floor of \selectlanguage{english}[?]2 km wide, buried graben that are
bound by dyke-induced faults and underlain by what were
northwards-propagating dykes (Magee \& Jackson, 2020a); five other pit
craters are connect to steeply dipping portions of tectonic normal
faults. The link between pit craters and dykes and faults confirms that
magmatic processes and overburden collapse into dilatational fault jogs.
By recognising that some pit craters above dykes occur at different
stratigraphic levels and broadly get younger southwards, we specifically
suggest these pit craters may have formed when a waning of magma
pressure and potential backflow led to a local volume reduction of the
dyke. In addition to obtaining insight into pit crater formation
mechanisms, we also show that pit crater depths are variable across the
study area. Pit crater depth are one of the few morphological properties
of pit craters that can be measured from surficial data, and have been
suggested to equal and thus be a proxy for regolith thickness on other
planetary bodies (e.g., Whitten \& Martin, 2019; Wyrick et al., 2004).
The variability in pit crater depths we record suggest this
characteristic may \emph{not} always control regolith thickness.
Overall, our work demonstrates that seismic reflection data provides a
unique insight into the structure and growth of pit craters on Earth.
Further seismic-based studies will help us understand extraterrestrial
pit craters and, thus, probe the subsurface structure and composition of
other planetary bodies for which direct, in situ data are not yet
available.

\textbf{Geological setting}

The North Carnarvon Basin, offshore NW Australia (Fig. 2A) formed during
periodic rifting between Australia and Greater India in the Late
Carboniferous-to-Early Cretaceous (e.g., Direen et al., 2008; Longley et
al., 2002; Stagg et al., 2004). The Exmouth Plateau, where the studied
pit craters are situated, lies within the North Carnarvon Basin and
itself began in the Rhaetian (Late Triassic) to Callovian (Middle
Jurassic) rift phase, which produced an array of
\textasciitilde{}N-striking, large-throw (up to \textasciitilde{}1 km)
normal faults (Figs 2B and C) (e.g., Bilal et al., 2018; Black et al.,
2017; Gartrell et al., 2016; Marshall \& Lang, 2013; Stagg \& Colwell,
1994; Tindale et al., 1998). These tectonic faults displace a thick
pre-rift succession, including the fluvio-deltaic Mungaroo Formation,
and accommodated deposition of a relatively condensed (\selectlanguage{english}[?]100 m thick),
clastic-dominated, syn-rift succession (i.e. the Brigadier and North
Rankin formations, the Murat Siltstone, and the Athol Formation; Figs 2B
and C) (Hocking, 1992; Hocking et al., 1987; Stagg et al., 2004; Tindale
et al., 1998).

The Callovian unconformity caps the Athol Formation and underlies the
Oxfordian-to-Tithonian, marine Dingo Claystone, marking the end of major
Late Triassic-to-Middle Jurassic rifting (Figs 2B and C) (e.g., Tindale
et al., 1998; Yang \& Elders, 2016). Renewed rifting in the Tithonian
(Late Jurassic) to Valanginian (Early Cretaceous) involved (Figs 2B and
C): (i) sub-aerial development of the regionally developed, Base
Cretaceous unconformity at \textasciitilde{}148 Ma (latest Tithonian);
(ii) rapid and significant subsidence to accommodate deposition of the
\selectlanguage{english}[?]3 km thick, fully marine Barrow Group; and (iii) relatively limited
upper crustal faulting, which was restricted to minor reactivation of
older faults and generation of an array of N-S to NE-SW striking,
low-throw (\textless{}0.1 km) normal faults (e.g., Driscoll \& Karner,
1998; Magee et al., 2016; Paumard et al., 2018; Reeve et al., 2016).
Continental break-up occurred along the western and southern margin of
the Exmouth Plateau in the Valanginian-to-Hauterivian
(\textasciitilde{}135--130 Ma; Fig. 2B) (e.g., Direen et al., 2008;
Reeve et al., 2021; Robb et al., 2005). Following break-up, thermal
subsidence accommodated deposition of a thick post-rift succession that
hosts several tiers of polygonal faults (e.g., Paganoni et al., 2019;
Velayatham et al., 2019).

During the Late Jurassic, at \textasciitilde{}148 Ma, a radial dyke
swarm, up to \textasciitilde{}170--500 km long and \textasciitilde{}300
km wide, was emplaced across much of the Exmouth Plateau (Figs 2A and B)
(Magee \& Jackson, 2020a). Associated with this dyke swarm is an array
of dyke-induced faults that extend up from and dip towards the upper
tips of dykes, offsetting Late Triassic-to-Late Jurassic strata and
bounding dyke-parallel graben (e.g., Figs 2D and 3A) (Magee \& Jackson,
2020a; Magee \& Jackson, 2020b). Within these graben, linear chains of
sub-circular depressions are recognised (e.g., Figs 2D and 3A) (Magee \&
Jackson, 2020a; Velayatham et al., 2019; Velayatham et al., 2018). These
depressions, which are interpreted to have formed at the contemporaneous
free surface, are underlain by pipe-like features that extend down
towards dykes, dyke-induced normal faults, or tectonic normal faults
(e.g., Figs 2D and 3A) (Magee \& Jackson, 2020a; Velayatham et al.,
2019; Velayatham et al., 2018). The depressions have previously been
interpreted as pockmarks formed by fluid escape from an overpressured
horizon when faulting locally reduced the overburden pressure (e.g.,
Fig. 1B {[}ii{]}) (Velayatham et al., 2018). However, the spatial and
temporal association between the depressions and underlying dykes
suggests they may be analogous to pit craters observed elsewhere on
Earth and other planetary bodies (Magee \& Jackson, 2020a); henceforth
we refer to these depressions as `pit craters'.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

\emph{Figure 2: (A) Map of the southern portion of the North Carnarvon
Basin highlighting principal tectonic elements, including the: ESB =
Exmouth Sub-basin, CT = Carnarvon Terrace, BSB = Barrow Sub-basin, DSB =
Dampier Sub-basin, and PS = Peedamullah Shelf. The location of the
Exmouth Dyke Swarm is also shown (Magee \& Jackson, 2020a). Elevation
data are based on the 2009 Australian Bathymetry and Topography grid
(Geoscience Australia). Inset: Location map of the North Carnarvon Basin
(NCB) relative to Australia. (B) Stratigraphic column for the Exmouth
Plateau summarising the age, dominant lithology, and generalised
depositional environment for key units (after Hocking et al., 1987;
Partington et al., 2003; Tindale et al., 1998). Key tectonics and
magmatic events are also shown for (see Reeve et al., 2021 and
references therein). (C) Interpreted seismic section, showing
generalised stratigraphic architecture of the Exmouth Plateau and
Exmouth Sub-basin (modified from Reeve et al., 2016). See Figure 2A for
location. (D) Interpreted time-structure map and seismic section showing
pit crater-like features arranged in chains and associated with normal
faults in the western portion of the Exmouth Plateau (modified from
Velayatham et al., 2018). See Figure 2A for map location.}

\textbf{Dataset and Methods}

The zero-phase, time-migrated Chandon 3D seismic reflection survey
covers an area of \textasciitilde{}951 km\textsuperscript{2}, has a bin
spacing of 25 m, and a record length of 6 seconds two-way time (s TWT).
We present seismic images where a trough (black) reflection has a
positive polarity and corresponds to a downward increase in acoustic
impedance, and a peak (white) reflection has a negative polarity and
represents a downward decrease in acoustic impedance (e.g., Fig. 3A). To
help identify and map the pit craters, their underlying dykes, and
spatially associated faults, we extract a variance volume attribute;
this accentuates spatial variations in waveform similarity (e.g., Chopra
\& Marfurt, 2005; Marfurt \& Alves, 2015).

We use data from five boreholes (Chandon-1, Chandon-2, Chandon-3,
Mercury-1, and Yellowglen-1) within the survey area to establish
estimates for: (i) the age and intervening lithology of three mapped
stratigraphic horizons, which correspond to the Top Mungaroo Formation
(TM; \textasciitilde{}209 Ma), Top Athol Formation (TA;
\textasciitilde{}164 Ma), and Base Cretaceous unconformity (BC;
\textasciitilde{}148 Ma); (ii) an average time-depth relationship for
the subsurface (see Supporting Figure 1), allowing us to depth-convert
(from ms TWT to metres) measurements taken from the top 4 s TWT of the
survey; and (iii) the seismic velocity (\emph{v} ) range of the interval
of interest within which the pit craters are imaged. Seismic velocity
increases with depth, from 2.64(\selectlanguage{ngerman}±10\%) km s\textsuperscript{-1} to
3.66(±10\%) km s\textsuperscript{-1}, as the dominant frequency
(\emph{f} ) of the data broadly decreases from \textasciitilde{}33 Hz to
\textasciitilde{}23 Hz; with these data we calculate the wavelength (\selectlanguage{greek}λ
\selectlanguage{english}=\emph{v} /\emph{f} ) of the data and estimate that its vertical seismic
resolution can be characterised by limits of separability (\selectlanguage{greek}λ/\selectlanguage{english}4) of
37(\selectlanguage{ngerman}±7)--17(±3) m and limits of visibility (\selectlanguage{greek}λ/\selectlanguage{english}30) of 5(\selectlanguage{ngerman}±1)--3(±1) m
(e.g., Brown, 2011). The limits of separability and visibility
correspond to the minimum vertical distance between two features
required for them to be imaged, respectively, as: (i) two discrete
reflections; and (ii) a tuned reflection package, created by convolution
of the two reflections on their return to the surface (Brown, 2011). The
horizontal resolution of the time-migrated seismic reflection data is
estimated to be at least 25 m, which is equivalent to the bin spacing,
and likely increases to 37(±7) m (i.e. \selectlanguage{greek}λ/\selectlanguage{english}4) with depth.

Plan-view measurements

We identify 59 pit craters that we label A\emph{n} --I\emph{n} if they
overlie a dyke, with A--I corresponding to the dyke name (Magee \&
Jackson, 2020a; Magee \& Jackson, 2020b), or \emph{Xn} if they appear
spatially related to tectonic normal faults; \emph{n} denotes the pit
crater number, which increases along each chain from south to north. We
map the plan-view outline of each pit crater at: (i) their uppermost
expression in the seismic reflection data; and (ii) where they intersect
the Top Athol Formation; this allows us to characterise how pit crater
plan-view morphology changes with depth. For each mapped pit crater
outline, we use image analysis software (ImageJ) to define a best-fit
ellipse and centroid position; where pit craters appear to have merged,
we define an encompassing best-fit ellipse but manually determine
centroid positions (Fig. 3B). We use the outline of each pit crater, or
merged pit craters, to measure their area and the best-fit ellipses to
determine their long axis lengths and azimuths, and short axis lengths
(Fig. 3B). From these values, we calculate the aspect ratio of the pit
crater long and short axes.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

\emph{Figure 3: (A) 3D view of a funnel-like pit crater (F4), the top of
which occurs at a horizon between the Base Cretaceous unconformity (BC)
and Top Athol Formation (TA) (Magee \& Jackson, 2020a). The pit crater
occurs with a graben and extends down to the top of Dyke F, below the
Top Mungaroo Formation (TM). (B and C) Schematics showing the
measurements of pit crater plan-view (B) and cross-sectional (C)
morphologies.}

Cross-section measurements

The only cross-section attributes commonly available to measure for most
pit craters on Earth and other planetary bodies are the depth and slope;
together these can be used to estimate volume, of their surface
expression (e.g., Gwinner et al., 2012; Sauro et al., 2020; Scott \&
Wilson, 2002; Wyrick et al., 2004). Most (54; i.e. 92\%) pit craters we
study display a funnel-like morphology, comprising an upper, inwardly
inclined conical surface, which we refer to as an `inverted cone',
underlain by a sub-vertical cylindrical `pipe' (e.g., Fig. 3A). Some pit
craters lack an inverted cone and we refer to their morphology as
pipe-like. Stratigraphic reflections within many of the inverted cone
sections appear continuous with in the flanking host rock, although they
are often locally deflected downwards within the pit crater (e.g., Fig.
3C). Note that for some pit craters it is difficult to correlate
internal and external stratigraphic reflections. Where possible, we
measure both the maximum `cone height' and `deflection height' of pit
craters (Fig. 3C). By integrating these measurements with those of pit
crater long axis lengths, we estimate slopes and volumes of the inverted
cone sections and deflected reflections, assuming they are axisymmetric
(Fig. 3C) (Wyrick et al., 2004). We also measure `pipe height', which
for funnel-like pit craters we define as the vertical distance from the
base of inverted cones down to where the pit crater structure is no
longer seismically resolved (Fig. 3C). We calculate pipe volume from the
pipe height and the pit crater long axis length as measured at the Top
Athol Formation. To aid comparison between pit craters with and without
a funnel-like morphology, we describe each as having a `total height';
for pipe-like pit craters their total height is equal to pipe height,
but for funnel-like pit craters their total height includes the cone
height and pipe height (e.g., Fig. 3C). Our measured pipe and total
heights are minimum estimates as a reduction in imaging with depth and
the presence of other structures (e.g., faults), limits our ability to
determine the true base of pit craters (Velayatham et al., 2019;
Velayatham et al., 2018; Wall et al., 2010). We measure the depth of
dykes or normal faults directly beneath the top of each pit crater (Fig.
3C).

Sources of error

The primary error source in our study relates to uncertainty in the
seismic velocities used to depth-convert the seismic data from seconds
two-way time to depth in metres. For example, our depth conversion
assumes that seismic velocities do not vary laterally across the study
area. The similarity in the time-depth relationships for all four
boreholes wells within the Chandon 3D survey supports our assumption
that there is little lateral variation in seismic velocity (Supporting
Fig. 1), but we conservatively consider that calculated velocities and
measured dominant frequencies may vary by up to \selectlanguage{ngerman}±10\% (Magee \& Jackson,
2020a). Measurements of limits of separability and visibility,
deflection height, cone height, pipe height, and the depth to underlying
dykes thus have assumed depth conversion-related errors of ±10\%. Manual
mapping and measurement can introduce human errors (e.g., in defining
pit crater pipe bases) that cannot be quantified; however, we consider
that these errors may be up to ±5\% (Magee \& Jackson, 2020a). Overall,
plan-view measurements are presented with ±5\% errors as they do not
require depth-conversion, but may be susceptible to human error. For
measurements of pit height, cone height, pipe height, and the depth to
underlying dykes, we estimate cumulative errors of ±15\%; this based on
the uncertainty in calculated seismic velocities and potential human
errors.

\textbf{Results}

Stratigraphic and structural framework

Stratigraphic reflections within the interval of interest typically have
a moderate amplitude and laterally continuous character (Fig. 4A). In
places these reflections are: (i) offset by \textasciitilde{}N-striking,
linear-to-curvilinear, moderate-to-high throw (up to
\textasciitilde{}500 m) tectonic normal faults (e.g., Figs 4A-D); (ii)
cross-cut by sub-vertical, \textasciitilde{}N-trending dykes (named A to
I) that are expressed as \textgreater{}100 m wide zones of disrupted,
low-amplitude reflections (Figs 4A and E); (iii) displaced by
graben-bounding, low-throw (\selectlanguage{english}[?]0.1 km), dyke-induced normal faults that
dip towards and converge upon the upper tips of the underlying dykes at
depths of \textasciitilde{}3--3.5 km (Fig. 4A); or (iv) intersected by
pit craters (see below) (Figs 3A and 4A). Below the Base Cretaceous
unconformity, pit craters and dykes are marked by high variance, as are
areas where strata are folded and/or offset by tectonic faults (e.g.,
Fig. 4A).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

\emph{Figure 4: (A) Interpreted composite section capturing the
tectono-stratigraphy of the area, intersecting all dykes (A-I) and
revealing the structure some dyke-induced normal faults and pit craters.
The line also intersects the Yellowglen-1 and Chandon-3 boreholes. The
time-migrated data is shown in the top panel, and in the lower panels we
show selected depth-converted, reflectivity data overlain by a variance
attribute. See Figure 4B for line location. (B-C) Time-structure maps of
the three major horizons (Base Cretaceous unconformity, Top Athol
Formation, and Top Mungaroo Formation) interpreted in Figure 4A.
Uninterpreted and interpreted versions of the maps are shown,
highlighting underlying dyke traces, faults, and boreholes. The
boreholes shown are: 1 = Chandon-1; 2 = Chandon-2; 3 = Chandon-3; 4 =
Yellowglen-1; and 5 = Mercury-1. (E) Uninterpreted and interpreted RMS
(root-mean squared) amplitude time-slice at 4.5 seconds two-way time
(TWT) showing the seismic expression of dykes A-I. For part of their
length, dykes B and C merge and we cannot determine if both dykes are
continuous, or if one cross-cuts and displaces the other.}

Pit crater morphology

Pit craters occur across the dataset, primarily arranged in chains
aligned parallel to and occurring within dyke-induced graben (Fig. 5).
Five pit craters (\emph{X1} --\emph{X5} ) are located outside these
graben and are instead situated in the hanging wall of tectonic normal
faults (Fig. 5). Of the 54 pit craters within dyke-induced graben, 50
have a funnel-like morphology in cross-section, as do four of the pit
craters associated with tectonic normal faults (Figs 5-8). The remaining
four pit craters within dyke-induced graben (A4, F5, G7, and H1), as
well as\emph{X3} , have a pipe-like geometry and no inverted cone
section (Figs 5-8).

At the Top Athol Formation, we mapped 61 pit craters (Fig. 5); three pit
craters observed at this structural level merge (e.g., E1a and b, F10a
and b, G10a and b; Figs 6E, 7A, and B) and one splits (G2/3; Fig. 8B) at
shallower depths. Of the 57 pit craters within dyke-induced graben at
the Top Athol Formation, 30 occur directly above dyke traces and 27 have
centroids situated \textasciitilde{}75--653 m orthogonally from the
associated dyke trace (Fig. 5C). The only pipe-like pit crater located
directly above a dyke trace is A4 (Fig. 5).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

\emph{Figure 5: (A) Composite map showing the outlines of each pit
crater at the shallowest stratigraphic level they occur at, relative to
dyke and fault traces. (B) Map showing the expression of each pit crater
at the Top Athol Formation.}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

\emph{Figure 6: (A-F) Uninterpreted and interpreted, depth-converted
seismic sections showing the structure of pit craters associated with
dykes A-E. See Figure 4 for interpreted section key. VE = vertical
exaggeration Inset: map showing line locations.}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image7/image7}

\end{center}
\end{figure}

\emph{Figure 7: (A-G) Uninterpreted and interpreted, depth-converted
seismic sections showing the structure of pit craters associated with
dykes F-I. See Figure 4 for interpreted section key and Figure 7 for
line locations.}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image8/image8}

\end{center}
\end{figure}

\emph{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.}

\emph{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 km\textsuperscript{2}, 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\selectlanguage{ngerman}° 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 km\textsuperscript{2} (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 km\textsuperscript{2} (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 km\textsuperscript{2} (Figs 9A
and C; Supporting Table 1).

The area and aspect ratio of the 29 pit craters situated directly above
dykes broadly decrease (R\textsuperscript{2} = 0.55) and seemingly
increase (R\textsuperscript{2} = 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 R\textsuperscript{2} =
0.26 and R\textsuperscript{2} = 0.08, respectively (Figs 9A, C, and D).

Pit craters with a funnel-like morphology, but associated with tectonic
normal faults (\emph{X1} , \emph{X2} , \emph{X4} , and \emph{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
km\textsuperscript{2} 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
km\textsuperscript{2} (Fig. 9; Supporting Table 1). The only pit crater
associated with a tectonic normal fault that does not have a funnel-like
morphology (\emph{X3} ) is 140 m long, has an aspect ratio of 1.3, and
an area of 0.05 km\textsuperscript{2} 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
km\textsuperscript{2}(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).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image9/image9}

\end{center}
\end{figure}

\emph{Figure 9: (A) Box and whisker plots comparing the long axes,
aspect ratios, and areas of pit craters measured at two stratigraphic
levels, distinguishing those that have different geometries (i.e.
funnel-like and pipe-like) and/or associated with different structures
(i.e. dykes, dyke-induced faults, and tectonic faults) (Supporting Table
1). n = number. The plots also highlight data median (thin black line),
mean (dashed black line), and outliers. See Figure 3B for explanation of
measured parameters. (B) Rose plots showing the orientation of pit
crater long axes, grouped based on their morphology and/or association
with different structures (Supporting Table 1). Arrows show the mean
vector of select populations. Orientation of dyke segments is also shown
(Magee \& Jackson, 2020a). (C and D) Graphs comparing the area (C) and
aspect ratio (B) of pit crater expressions at their tops and at the Top
Athol Formation. Two plots are provided for area and aspect ratio
comparison, with one grouping the data based on the associated dyke or
tectonic faults, and the other highlighting different between pit
craters located directly above dykes, dyke-induced faults, and tectonic
faults.}

\emph{Cross-section geometry}

In cross-section, pit crater walls typically bound zones within which
stratigraphic reflections locally have a lower amplitude and are
deflected (e.g., E4b) or displaced (e.g., F13) downwards relative to
those in the flanking host rock (Figs 4A, 6-8). Pit crater centroids
typically occur within the Dingo Claystone, \textasciitilde{}16--188 m
(i.e. 0.5--5.5 seismic cycles where one cycle is a peak and trough pair)
beneath the Base Cretaceous unconformity (Figs 4A, 6-8, 10A, and B).
Only B1-B3 and D1-D2 are expressed at the Base Cretaceous unconformity
(Fig. 10A). Stratigraphic reflections overlying some pit craters appear
locally thickened (e.g., F13; Fig. 7B) whereas others seem to onlap onto
crater walls (e.g., G10a; Fig. 7D).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image10/image10}

\end{center}
\end{figure}

\emph{Figure 10: (A and B) Plots showing how deep (A) or how many
seismic cycles (B) pit crater centroids are beneath the Base Cretaceous
unconformity along dyke-strike (Supporting Table 1). Inset into (B):
schematic depicting what constitutes a seismic cycle. (C) Plot of cone
height along dyke strike (Supporting Table 1). See Figure 3B for
explanation of measured parameters.}

For each pit crater chain situated within dyke-induced graben, the depth
of their pit crater centroids beneath the Base Cretaceous unconformity
varies by [?]60 m along their length (Fig. 10A). Several pit crater chains
(A, D, F, G, and I) show a broad northwards increase in centroid depth
(Fig. 10A). Above some dykes, pit crater centroids all occur along the
same stratigraphic reflection (i.e. D and G), even though the number of
cycles between their level and the Base Cretaceous unconformity may vary
(i.e., the post-overburden varies in thickness over a length-scale
longer than that of an individual crater or chain of craters Figs 6D,
7B, and 10B). For other chains, pit crater centroids coincide with
different (deeper or shallower) reflections along their length (i.e. A,
B, C, E, F, H, and I; Figs 6, 7, and 10B). The pit craters associated
with tectonic normal faults have centroids that occur at the same
stratigraphic reflection above the Top Athol Formation, except
for\emph{X3} which is the only pit crater to terminate below the Top
Athol Formation (Fig. 8).

The total height of the pit craters varies across the data, with most
extending down into the Mungaroo Formation (Figs 4A and 6-8); pit
craters G5, G11, and G12 appear to terminate at or above the Top
Mungaroo Formation (Fig. 7C). Our measurements show pit crater total
height ranges from 114--868 m (Supporting Table 1). In places, mapped
bases of pit crater pipes coincide with the upper tips of underlying
dykes (G2, G4, G6, G10a, G12, and H2) or dyke-induced fault planes
(B/C2, C2, C3, F9, F13, I2, and I3) (Figs 6-7). The \emph{X1}
-\emph{X5}pit crater pipes all extend down to tectonic faults (e.g.,
Fig. 8). Where the base of pit crater pipes intersect dyke-induced or
tectonic faults, they typically do so where the fault planes are
relatively steep or sub-vertical (e.g., Figs 6C, 7B, F, 8B, and C).
Other pit craters appear to terminate within strata overlying dyke upper
tips or fault planes (Figs 4A and 6-8).

Where pit craters display a funnel-like morphology, we measure the
height of their inverted cone section and occasionally the deflection of
the uppermost stratigraphic reflection within them (i.e. the deflection
height) (Fig. 3B). We show inverted cone heights are
\textasciitilde{}18--245 m, with slopes of 7--51\selectlanguage{ngerman}°, and that these values
vary across the study area and along individual pit crater chains (Figs
10C, 11A, and B; Supporting Table 1). The corresponding pipe height of
these funnel-like pit craters are 27--789 m (Fig. 11A; Supporting Table
1); the heights of pipe-like pit craters are 107--254 m (Table 2). There
is no correlation between cone height and total height or pipe height
for pit craters within dyke-induced graben (either those above dykes or
dyke-induced faults), or for those associated with tectonic normal
faults (Figs 11A and B; Supporting Table 1). Deflection heights are
\textasciitilde{}4--21 m, with slopes \textless{}12°, and do not
correlate with either total height or cone height (Figs 11C and D;
Supporting Table 1).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image11/image11}

\end{center}
\end{figure}

\emph{Figure 11: (A-D) Graphs comparing how the total height, pipe
height, cone height, and deflection height relate (Supporting Table 1).
See Figure 3B for explanation of measured parameters.}

\emph{Comparative analyses}

With our 3D seismic reflection data we are able to quantitatively assess
relationships between the plan-view and cross-sectional geometry of the
pit craters. Between pit crater total heights and long axis lengths
there is a very weak positive correlation between for those above dykes
(R\textsuperscript{2} = 0.17), no meaningful correlation for pit craters
above dyke-induced faults (R\textsuperscript{2} = 0.04), but a strong
positive correlation for those above tectonic faults
(R\textsuperscript{2} = 0.71) (Fig. 12A). Visually similar pit craters
recognised elsewhere across the Exmouth Plateau within dyke-induced
graben (e.g., Fig. 2D) display a moderate (R\textsuperscript{2} =
0.37--0.50) positive correlation between total height and long axes
(Fig. 12A) (see Velayatham et al., 2019). For all measured pit craters
across the Exmouth Plateau there is no correlation
(R\textsuperscript{2}[?]0.06) between total height and the aspect ratio of
their long and short axes (Fig. 12B).

For pit craters above dykes, dyke-induced faults, and tectonic faults,
there are positive, power-law correlations between cone height and pit
crater long axes with R\textsuperscript{2} values of 0.61, 0.24, and
0.77 respectively (Fig. 12C); these trends are consistent with power-law
correlations that define pit crater depth and long axes of craters
observed elsewhere on Earth and on other planetary bodies (Fig. 12C)
(e.g., Ferrill et al., 2011; Gwinner et al., 2012; Okubo \& Martel,
1998; Scott \& Wilson, 2002; Whitten \& Martin, 2019; Wyrick et al.,
2004). There is no correlation (\textless{}R\textsuperscript{2} = 0.08)
between deflection height and pit crater long axes (Fig. 12C). Inverted
cone volumes (1.34\selectlanguage{ngerman}×10\textsuperscript{-4} km\textsuperscript{3} to
2.71×10\textsuperscript{-2} km\textsuperscript{3}), and those of the
deflected reflections (7.89×10\textsuperscript{-5}
km\textsuperscript{3}to 1.62×10\textsuperscript{-3}
km\textsuperscript{3}), are typically smaller than corresponding pipe
volumes (1.41×10\textsuperscript{-3}km\textsuperscript{3} to
2.79×10\textsuperscript{-1}km\textsuperscript{3}) (Supporting Table 1).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image12/image12}

\end{center}
\end{figure}

\emph{Figure 12: (A and B) Plots comparing total height to pit crater
long axes (A) and aspect ratio (B) for those located above dykes,
dyke-induced faults, and tectonic faults (Supporting Table 1). Data is
also shown from two other 3D seismic surveys (Io-Jansz and Thebe) from
the Exmouth Plateau, although the exact relation of each pit crater to
underlying structure is unknown (Velayatham et al., 2019). (C) Log-log
plot of deflection and cone height, one of which is expected equivalent
to pit crater depth (e.g., Wyrick et al., 2004), compared to pit crater
long axis. Insets show a zoomed-in view of our data and the power-law
best-fit trendlines for each plotted dataset. The power-law best-fit
trendline shown (black line) with standard errors was calculated from
all literature data plotted, with the exception of data from Frumkin and
Naor (2019) and Kling et al., (2021) as these pit craters show evidence
of post-formation modification.}

\textbf{Discussion}

Pit crater structure

Seismic reflection data provides unique opportunities to image and
quantify the entire 3D geometry of pit craters. Like pit craters
observed elsewhere on Earth and other planetary bodies, those on the
Exmouth Plateau have manifest as the as quasi-circular depressions,
which in plan-view are commonly arranged in chains (e.g., Figs 3A and
5). Offset of stratigraphic reflections within most mapped pit craters
indicate that these structures form because of spatially restricted host
rock subsidence (Figs 3A, 4A, and 6-8). This subsidence is confined to
cylindrical pipe-like structures with sub-vertical walls, which towards
their tops commonly widen and become inwardly inclined, broadly
describing an inverted conical shape (i.e. they are funnel-like) (Figs
3A, 4A, 6-8, and 9C). These observations are consistent with the
inferred subsurface geometry of pit craters recognised elsewhere and
those modelled using physical or numerical approaches (e.g., Ferrill et
al., 2011; Kettermann et al., 2019; Wyrick, 2004; Wyrick et al., 2015).
We note that the subsided strata within the studied pit craters
typically have a lower amplitude and/or higher variance expression
compared to reflections in the flanking host rock (Figs 2D, 3A, 4A, and
6-8). These seismic attribute changes indicate less seismic energy was
reflected from within the pit crater, perhaps due to increased
scattering of seismic energy from and/or local decreases in acoustic
impedance across disrupted beeding (Brown, 2011). Both controls on the
amount of seismic energy reflected could be linked to disaggregation of
and fluid infiltration through rock during or after subsidence, which
are likely common processes during pit crater formation (e.g., Frumkin
\& Naor, 2019; Halliday, 1998; Velayatham et al., 2018).

Pit crater age

Where data resolution allows, some reflections we observe above the pit
craters appear thickened (e.g., F13; Fig. 7B) and/or onlap onto the
conical walls of pits (e.g., G10a; Fig. 7D). These seismic-stratigraphic
relationships imply that the strata represented by these reflections
were deposited \emph{within} the pit craters; i.e. the pit craters were
surficial features, similar to those recently formed on Earth (Abelson
et al., 2003; Frumkin \& Naor, 2019; Okubo \& Martel, 1998; Whitten \&
Martin, 2019). Having established that the shallowest expression of the
pit craters likely marks the contemporaneous surface to their formation,
we can use biostratigraphic data from local boreholes to estimate their
age.

The youngest pit craters (B1-3, D1-2) formed coincident with the Base
Cretaceous unconformity at \textasciitilde{}148 Ma (Figs 6B and D).
Critically, the uppermost expression of pit craters across the study
area and along individual chains (e.g., A--C, E, F, H, and I) often
occur at different stratigraphic horizons above the \textasciitilde{}165
Ma Top Athol Formation (Figs 6-7, 10A, and B). These observations
indicate the pit craters developed periodically during deposition of the
marine Dingo Claystone in the Late Jurassic (Figs 2B, 6-7, 10A, and B).
For some pit crater chains, specifically D1-D2 and G1-G13, the tops of
individual pits occur along the same stratigraphic horizon suggesting
they formed near-synchronously (Figs 6D, 7B, 10A, and B).

Our inferred Late Jurassic timing of pit crater formation is consistent
with seismic-stratigraphic constraints on the age of the Exmouth Dyke
Swarm and associated dyke-induced faults (Fig. 2) (Magee \& Jackson,
2020a). As most pit craters are found below the \textasciitilde{}148 Ma
Base Cretaceous unconformity, but all dyke-induced faults offset this
horizon, it seems likely that pit crater formation generally ceased
before dyking and associated faulting ended (Figs 4B and C) (Magee \&
Jackson, 2020a). Overall, we suggest that the oldest pit craters
occurred during the early development of the Exmouth Dyke Swarm and
associated dyke-induced faults, with dyking and fault growth occurring
(periodically) up until \textasciitilde{}148 Ma.

Pit crater origin

Numerous processes involving underlying cavity collapse or the
volumetric reduction of a subsurface body have previously been proposed
to generate space for overburden subsidence and pit crater formation
(Fig. 1B) (see Wyrick et al., 2004 and references therein). We identify
no clear cavity-like structures at the pit crater bases (Figs 3A, 4A,
and 6-8), and disregard the following possible mechanisms of pit crater
formation: (i) carbonate or salt dissolution (Fig. 1C {[}i{]}) (Abelson
et al., 2003; Spencer \& Fanale, 1990), because the Triassic-to-Late
Jurassic strata hosting the pit craters contains no (or only very
little) carbonate rocks and no salt (Fig. 2B) (e.g., Exon et al., 1992;
Stagg et al., 2004; Tindale et al., 1998); (ii) evacuation of lava tubes
(see Sauro et al., 2020 and references therein), as there is no evidence
for high-amplitude, sinuous, strata-concordant reflections that could be
attributed to lava flows or tubes (Figs 3A, 4A, and 7-9) (e.g., Sun et
al., 2019); and (iii) magma migration from a reservoir (e.g., M\selectlanguage{ngerman}ège et
al., 2003; Poppe et al., 2015), as there is no evidence for underlying
tabular igneous intrusions (e.g., sills or laccoliths) (Figs 3A, 4A, and
6-8), which are typically expressed as high-amplitude, positive,
sub-horizontal-to-inclined reflections (e.g., Planke et al., 2005).
Instead, we find that some pit craters directly emanate from either the
upper tips of dykes (\emph{n} = 6), dyke-induced fault planes (\emph{n}=
6), or tectonic faults (\emph{n} = 5) (Figs 6-8). Where we observe pit
crater bases situated some distance above dyke upper tips or
dyke-induced faults (Figs 6-8), we consider it plausible that
seismically unresolved or obscured portions of their pipes extend down
to these underlying structures. Our observations allow for the
generation of these pit craters in response to the opening of vertical
tensile fractures (Fig. 1C {[}iv{]}) (e.g., Ferrill et al., 2011; Smart
et al., 2011; Tanaka \& Golombek, 1989), which cannot be imaged in
seismic reflection data.

The generation of pit craters linked to dyke tips may be driven by (Fig.
1C {[}vi{]}): (i) a volume reduction of the dyke itself, perhaps as
magma pressure wanes or volatiles escape (e.g., Patterson et al., 2016;
Scott \& Wilson, 2002); (ii) escape of heated pore fluids from the
tip-adjacent host rock and subsequent porosity collapse (e.g., Schofield
et al., 2010); (iii) phreatic eruption (Hughes et al., 2018); and/or
(iv) subsidence of material into a tensile fracture that opens above an
inflating and widening dyke (similar to Fig. 1C {[}iv{]}) (e.g., Ferrill
et al., 2011; Smart et al., 2011; Tanaka \& Golombek, 1989). We show
that within some pit crater chains, the tops of individual pits located
directly above dykes occur at deeper stratigraphic levels and are thus
older towards the north of our study area (e.g., above dykes A and I;
Figs 10A and B). These occurrences of pit crater tops at multiple
stratigraphic levels above single dykes indicate that periods of
sediment deposition separated pit crater formation (Fig. 13); this
interpretation is consistent with fault displacement data, which reveals
the dyke-induced faults likely grew incrementally via segment linkage
(Magee \& Jackson, 2020b). As the dykes are part of a radial dyke swarm
that intruded laterally northwards (Fig. 2A) (Magee \& Jackson, 2020a),
the apparent southwards decrease in age of the pit craters suggests that
they did not develop above propagating dyke tips (Fig. 13A). Dyke
thickness also decreases gradually northwards (Magee \& Jackson, 2020a),
so it seems unlikely that pit crater formation occurred in response to
dyke widening and tensile fracturing of the overburden; i.e. we should
expect areas where dyke width is greatest to generate pit craters first
(Fig. 13B). We suggest that the possible southwards decrease in age of
pit craters directly above dykes may have occurred due to localised
volume reductions of the intrusion as driving pressure periodically
waned (Fig. 13C). Such volume reductions could have been driven by magma
backflow and retreat (e.g., Philpotts \& Asher, 1994; Philpotts \&
Philpotts, 2007), or solidification and contraction (e.g., Caricchi et
al., 2014). Cyclical periods of intrusion and driving pressure waning
could create complex trends of pit craters along individual dykes.

Pit craters linked to faults observed elsewhere on the Exmouth Plateau
have been attributed to local reduction of confining pressure and fluid
escape from an overpressured horizon in the Mungaroo Formation during
faulting (e.g., Fig. 1C {[}ii{]}) (Velayatham et al., 2019; Velayatham
et al., 2018). We show that some pit craters link to
steep-to-sub-vertical fault portions, suggesting their formation may
also be associated with the collapse of dilatational jogs (Figs 1C
{[}v{]}, 7B, 8B, and C) (e.g., Ferrill \& Morris, 2003; Ferrill et al.,
2011; Ketterman et al., 2015; Kettermann et al., 2019; Smart et al.,
2011; Von Hagke et al., 2019).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image13/image13}

\end{center}
\end{figure}

\emph{Figure 13: Schematic models describing how pit crater may be
expected to relate to different dyke processes. (A) Pit crater formation
is linked to lateral propagation of the dyke tip, perhaps driven by
fluid escape into and/or collapse of the tip cavity, or eruption (Hughes
et al., 2018; Scott \& Wilson, 2002). Pit crater age decreases further
from the dyke source. (B) Pit crater formation occurs as the dyke widens
behind the propagating tip, perhaps linked to tensile fracturing (Tanaka
\& Golombek, 1989) as dyke widening induces extension of the overlying
rock, fluid escape into and/or collapse of the tip cavity, or eruption
(Hughes et al., 2018; Scott \& Wilson, 2002). Pit crater age decreases
further from the dyke source. (C) Pit crater formation occurs as the
magma driving pressure or supply wanes, causing magma to flow backwards
(e.g., Philpotts \& Asher, 1994) and/or solidify and contract (i.e. a
volume reduction) (e.g., Caricchi et al., 2014). Pit crater age
increases further from the dyke source.}

Pit crater depth

Where pit craters are observed on the surface of Earth and other
planetary bodies, we are typically restricted to measuring their
plan-view geometry (e.g., axial dimensions) and crater depth (e.g.,
Whitten \& Martin, 2019; Wyrick et al., 2004). Numerous studies have
demonstrated that pit crater depths of most populations likely related
to dyke or fault activity, positively correlates with their long axis
length via power-law relations (R\textsuperscript{2} = 0.45--0.90) with
similar slopes (Fig. 12C) (e.g., Ferrill et al., 2011; Gwinner et al.,
2012; Kling et al., 2021; Okubo \& Martel, 1998; Scott \& Wilson, 2002;
Whitten \& Martin, 2019; Wyrick et al., 2004); all these data can be fit
by a power-law relationship (R\textsuperscript{2} = 0.94; Fig. 12C).
Some pit craters within the Golan volcanic province, southern Levant,
are an exception to this power-law trend as they show no correlation
between pit crater depth and long axis, which has been attributed to
recent modification by erosion and sedimentation (R\textsuperscript{2} =
0.05; Fig. 12C) (Frumkin \& Naor, 2019).

From our seismic reflection data, we can measure pit crater morphometric
properties, but we first need to establish whether the height of the
inverted cone sections we describe, or that of the deflected reflections
they contain, are equivalent to pit crater depth (Fig. 3B). For pit
craters above dykes, dyke-induced faults, and tectonic faults, we find
that cone height and pit long axis length positively correlate
(R\textsuperscript{2} = 0.61, 0.24, and 0.77, respectively) and overlaps
with data from many other locations (Fig. 12C). However, there is no
meaningful correlation (R\textsuperscript{2} = \textless{}0.1) for
deflection height and pit long axis length (Fig. 12C). The deflection
height measurements also fall an order of magnitude below the power-law
trend fit to the compiled published pit crater data; this is similar to
pit craters in the Golan volcanic province and Noctis Labyrinthus
(Mars), both of which display evidence for post-formation modification
of their initial geometries (Fig. 12C) (Frumkin \& Naor, 2019; Kling et
al., 2021). Based on these comparisons, we suggest that the walls of the
inverted cone sections represent the original pit crater morphology
(Fig. 14). The reflections within the inverted cone sections are
interpreted to correspond to material that infilled the pit crater (Fig.
14). Our results support recent suggestions that relationships between
pit crater depths and long axis lengths may provide some indication as
to whether post-formation erosional and infilling processes have
modified pit geometry (Figs 12C and 14) (Frumkin \& Naor, 2019; Kling et
al., 2021).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image14/image14}

\end{center}
\end{figure}

\emph{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; M\selectlanguage{ngerman}ège
\& 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 \textasciitilde{}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).

\textbf{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.

\textbf{Acknowledgements}

We are grateful to Geoscience Australia for making the data we use
freely available, and to Schlumberger for providing use of Petrel
seismic interpretation software. Funding for this work was acquired by
Magee as part of a NERC Independent Research Fellowship
(NE\textbackslash{}R014086\textbackslash{}1) and a Royal Society
International Exchange Grant
(IES\textbackslash{}R3\textbackslash{}193221).

\textbf{Data availability statement}

Seismic reflection and well data used in this study are freely available
from the Geoscience Australia NOPIMS
(https://nopims.dmp.wa.gov.au/nopims) data repository. The NOPIMS data
access centre allows `Wells' (i.e. for this study Chandon 1, Chandon 2,
Chandon 3, Mercury 1, and Yellowglen 1) and `Surveys' (i.e. for this
study the Chandon 3D MSS seismic reflection survey) from offshore
Australia to be searched for. From these search results, well data can
be downloaded by highlighting the correct well and using `view details
for selected rows'; due to the file size of seismic reflection segy data
like the Chandon 3D MSS, few can be downloaded but all can be added to a
basket and freely requested from Geoscience Australia. All measurements
and calculations acquired during this research are provided in
Supporting Table 1 and copies will be hosted in the National Geoscience
Data Centre of the British Geological Survey
(https://www.bgs.ac.uk/geological-data/national-geoscience-data-centre/)
and Figshare (https://figshare.com/).

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