Lunar Lobate Scarp

A North-South trending lobate scarp lies within Xenophanes crater. LROC NAC M118031613LE, image width is 600 m [NASA/GSFC/Arizona State University].

Lobate scarps are long, curvilinear structures found on some planetary bodies. They are interpreted to be tectonic in nature, the result of a thrust fault developed in rocks that are otherwise structurally sound. Faults (planar fractures) come in several styles: Normal faults occur when one slab of rock (the hanging wall) slides below the level of a neighboring slab of rock (the foot wall). Thrust faults (also called reverse faults when the fault angle is greater than 45 degrees to the horizontal) are the opposite of a normal fault. Instead of the hanging wall slipping below the foot wall, the hanging wall is thrust above the foot wall. In general, normal faults are the result of regional or local extension, while thrust and reverse faults give evidence of compression.

LROC NAC nadir image of a degraded crater inside the larger Xenophanes crater on the Moon with an arrow pointing to the location of the featured lobate scarp.
Context image of today's featured lobate scarp.  The scarp runs along the wall of a degraded crater within the larger Xenophanes crater. Image width is 100 km [NASA/GSFC/Arizona State University).

So what mechanism produces lobate scarps? On Mercury, lobate scarps are large and globally distributed, and have been interpreted to be the result of global contraction of the planet. What could make an entire planet contract? How about the slow cooling of its interior.

Data from LROC indicates that the lobate scarps on the Moon are also distributed globally. However, they are also small and deform the regolith locally. Because these faults are located in relatively unconsolidated material, they must be very young because the regolith is constantly churned by small asteroid and comet impacts. Scientists have thus interpreted the lunar scarps to be the result of late-stage, not early-stage, global contraction of the Moon!

Can you trace the entire length of the lobate scarp in the full NAC frame?

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Published by Drew Enns on 14 September 2011