Dilational Fault Slip and Pit Chain Formation on Mars

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Dilational Fault Slip and Pit Chain Formation on
Mars
GSA Today (2004) Ferrill, Wyrick, Morris, Sims,
Franklin.
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Dilational Fault Slip and Pit Chain Formation on
Mars

• The purpose of this paper is to explain the
  formation of pit crater chains on Mars, by
  compiling information from previous research, and
  also using data from analog modeling conducted by
  the authors.

• Martian Pit Crater Chains
• What are they?
• How are they formed?
• Why are they important?
• 4. Conclusions

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1. What are Pit Chains?

• Lines of pit craters observed on the ground
  surface
• Form chains (alignments), parallel with normal
  faults
• Common and widespread on Mars, rarely observed on
  Earth
• Martian pit craters are very large (can be gt4km
  wide)
• Do not have crater rims or ejecta deposits -
  conical shape.
• - Near horizontal stratigraphy - alternating
  layers with relatively high (steep slopes) and
  relatively low (gentle slopes) mechanical
  strength.

• Interpretations -
• Result from the collapse of loose surface
  material into a subsurface void they are
  collapse structures.
• Close association with faulting.
• Presence of rock layers with different strengths.

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2. How are Pit Crater Chains formed?

• Methods used in investigation-
• Physical Analog models
• High-Resolution images of recent pit chain
  formation in Iceland.
• Interpretation of pit chain fault relationship.

Laboratory Physical Analog Modeling

• Simulated normal fault slip on Mars
• Unconsolidated dry sand surface materials
• Mobile rigid plates cohesive powders used to
  produce dilating void spaces at depth
• The plates are then slowly separated.
• Reproduced the pit chain morphologies observed on
  Mars.
• Developmental sequence.

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High-Resolution Images
- Aerial photographs of modern pit chains in
Iceland were compared with high-res. images of
pit chains on Mars geological analogs for Mars.
Interpretation of Pit Chain Fault Relationship
A Pit chain and trough localized along the
surface traces of dilational faults. B Pit chain
occurring within a graben (between two normal
fault scarps).
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Interpretation of Pit Chain Fault Relationship

• Pit chains form in response to fault slip and
  dilation..
• Alternating layers of strong and weak rock
  layers
• - strong rock layers -gt steep slopes
• - weak rock layers -gt gentle slopes

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3. Why are Martian Pit Chains important?

• 2 main reasons -
• i.) Supporting evidence of active tectonics on
  Mars
• It is thought that some craters are actively
  forming (no sediment accumulation or surface
  subsidence, etc.).
• ii.) Pit chains mark subsurface voids
  potential conduits for groundwater flow.
• Potentially important sites for storage of water
  or ice sites of interest in the search for
  extraterrestrial life.

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4. Conclusions