Imaging the subsurface structure of pit craters Craig Magee¹, Corbin L. Kling2, Paul K. Byrne2 ¹Institute of Geophysics and Tectonics, School of Earth Science and Environment, University of Leeds, Leeds, LS2 9JT, UK ([email protected]) ²Planetary Research Group, Department of Marine, Earth, and Atmospheric Sciences, North Carolina State Uni-versity, Raleigh, NC 27695, USA Rationale Pit crater distribution & 3D structure - preliminary results Pit craters (chains) enigmatic features on Earth, (A) Top pit craters Pit craters aligned along & Mars, Mercury, Phobos, Eros, etc... above multiple dikes (F4A) Can only view surface morphology Most pit craters within dike- driven graben (F2, 4B) Collapse of subsurface voids created by: some pit craters solely related Dilational faulting → extension, mechanical layering to tectonic faults (4B) Tensile fracturing → extension, hydrologic cycle(?) some pit craters above steep Dike intrusion → volcanic products fault (dilational?) sections (F4C) (C) Aim: (1) Quantify surface & subsurface structure (B) Top Athol Formation 3.0 3.0 of pit craters; (2) compare to examples from Mars (& elsewhere): (3) test formation models 3.2 3.2 Study areas 3.4 3.4 Exmouth Plateau, offshore NW AustraliaAUSTRALIA Exmouth Australia (F1) 3.6 3.6 Bathymetry (km) Plateau -6 0 N Time (s TWT) Dike Elevation (km) 20° - + 0 1 Dike swarm imaged in 3D (F2) F -2.69 -3.20 Variance F4: (A) Map of top pit crater plan-view geometries overlain on dyke traces. Pit crater tops occur at subtly different structural levels. (B) Two-way time structure map, overlain with Dike-related pit crater variance (measure of difference in signal traces), showing pit crater structure at Top Athol 3.8 3.8 - + Dike-driven faults & graben (F2) Formation horizon. Bright red-yellow areas = faults. (C) Seismic sections, with one overlain 100 km 110° 115° 5 km Tectonic fault-related pit crater 0.5 km Variance 0.5 km by variance showing pit crater extending down to steep section of dike-driven fault. F1: Exmouth Plateau location map. (A) Seabed (B) Chester-1 ST1 (C) Seabed N Pit crater Pit crater Time (s TWT) Dazi NW SE NW SE S α -2.96 -3.04 azi Fd L Dike trace Lazi (NW) 2 2 TM Fd - + Sp (SE) Dip Ch TJ α Ph 3 TJ TM TJ -3.00 FD 3 TA TM DD Pph 4 TA Basalt Dike tip 4 P-Pph 4 TM Plan-view Cross-section L - Long axis Ch - Crater height -3.25 S - Short axis Ph - Pit height TWT) Sill Sp - Spacing Pph - Pipe height 5 Dike Dike Dike Fd (NW) - Distance to NW fault P-Pph - Pit-pipe height B D E Fd (SE) - Distance to SE fault DD - Dike depth Time (s FD - Fault depth Dike Dike L - Long axis azimuth D E azi α - Pit slope Dazi - Dike trace azimuth 5 αazi - Angular difference between Lazi and Dazi Time (s TWT) Time TWT) TWT) -3.50 Pit craters typically funnel- 5 1 km 2 km 2.5 km Time (s Time (s shaped (F5) TJ = Top Jurassic unconformity TM = Top Mungaroo Fmn TA = Top Athol Fmn Vertical Tectonic normal faults Dike-induced normal faults Dike Pit craters (D) N exaggeration ≈3 Was surface expression at (C) formation characterized by 5 km ~10 km N Pit craters Ch or Ph? (F5) Time (s TWT) + F3 Dyke -2.7 -3.7 F Have pits been infilled (Ch < Ph) - + + Dip F5: 3D view of pit crater structure extending down to underlying dike. See F2D for location. Inset: Schematic showing structural measurements. or not (Ch = Ph) (E) R² = 0.95 R² = (A) Previous data 1 (B) Our data Literature shows correlation A B C D E F G 0.92 RMS Amplitude 10 R² = R² = R² = - + 0.30 of L to “crater depth” (F6A) 0.95 h (km) 0.95 t p 0.01 e D ] R² = 0.80 N 1 Craters infilled Ch Standard Ph similar trend with L (F6B) H R² = 0.74 (Ch < Ph) + error 0.0001 25 km (A) R² = 0.02 R² = 0.64 Ch unrelated to L (F6B) 0.1 0000.1 0.01 1 100 I ] (km) Major axis (km) + R² = Ph = “crater depth” of other work Ph R² = 0.80 0.77 R² = 0.51 0.01 Noctis pits scattered (F6B) + Borehole R² = 0.41 h” / Crater height [ R² = : (A-C) Seismic sections showing dikes, faults, and pit craters. See (D) and (E) for line locations. t F2 0.07 + p R² = 0.49 (D) Top Mungaroo structure map (Chandon 3D survey) shwoing dike-induced faults. (E) Time-slice at e (B) Gwinner et al., 2012 - Mercury 4 s TWT showing dikes as long, linear zones of low reflection brightness amplitude. Inset: Glencoe 3D / Pit height [ 0.001 R² = Cut-off marks where later survey dike map. R² = 0.79 0.76 Okubo & Martel, 1998 - Earth (Hawaii) Kling - Earth (Hawaii) Dike-related pit Ph - Earth (NW Australia) Kling - Earth (Idaho) Dike-related pit Ch - Earth (NW Australia) “Crater d Ferrill et al., 2011 - Earth (Iceland) Tectonic fault-related pit Ph - Earth (NW Australia) infill restricts “crater depth” 0.0001 Whitten et al., 2019 - Earth (Iceland - sedi. host) Tectonic fault-related pit Ch - Earth (NW Australia) 25 km Whitten et al., 2019 - Earth (Iceland - basalt host) (C) Noctic Labyrinthus - Mars measurement (F6B) (A) °W W 0° Wyrick et al., 2004 - Mars 0 ° fault 2 0 pit Scott & Wilson, 2002 - Mars Previous data (see (A)) 60° N 1 6 0.00001 0000.1 0.001 0.01 0.1 1 10 100 0000.1 0.001 0.01 0.1 1 10 100 Pits above dikes / faults not Long axis [L] (km) Long axis [L] (km) 14°S F6: Plots of pit long axis vs “crater depth” measured from previous studies (A), as well as Ch and Ph measured from 3D seismic reflection data (B). separated by L vs Ph (F6B) 0° B Elevation (km) -8 21 4000 km Preliminary conclusions 15°S (B) 0° e a s s 95° W 105° W 100° W o 98° 97° 96° (1) Seismic reflection data reveal pit crater subsurface structure F is t c o N 5° S Many graben, pit Noctis (2) Support systematic relationship between pit long axis (L) and crater depth (Ph) Lab yri nth us craters, & canyons (A) (B) infilling material can obscure crater depth 10° S S yria Pla num Pit crater chains we define a possible cut-off in L vs Ph space c Some pits in graben, to identify infilled craters Elevation (m) 15° S some not -725 9000 Possible dike swarm 250 km (3) Supports pit crater formation via dike- &/or : (A) Global topographic map of Mars in Robinson Projection induced faulting, and F3 F7: (A) Volatile release, contraction, magma removal from dyke tip can produce a cavity. (B) Steep section of a showing location of Noctis Labyrinthus. (B) Regional map of Noctis dilational fault-related cavity collapse (F7) fault can dilate during slip, producing a cavity. Labyrinthus showing the complex landforms present. (C) Examples pit craters of pit craters within Noctis Labyrinthus, Mars. @DrCraigMagee.