Thursday, February 14, 2013

Numerov's Graben

Normal faults in regolith formed remarkably small graben in Nectarian age Numerov crater (70.7°S, 160.7°W). Only a handful of small craters superpose the faults, indicating a young age. LROC NAC M171619370RE, image width is 600 m [NASA/GSFC/Arizona State University].
Drew Enns
LROC News System

Graben on the Moon come in a variety of sizes. Some of the larger rilles in the maria stretch for several tens of kilometers and can be a few kilometers in width. These linear rilles are thought to be the result of extensional stresses near the edges of the maria and are thus graben.

Since the mare basalts are dense, they weigh down the crust in the center of the deposit, pulling rock near the margins inward.

However, the Featured Image today shows much smaller graben that span only hundreds of meters in length and tens of meters in width. To complicate matters, these graben are not in mare basalts, they are inside a crater!

Context image for today's Featured Image. The graben are pointed to by the arrow. A nearby lobate scarp extends from A to A', its low relief enhanced by the low Sun mosaic. Image width is 100 km [NASA/GSFC/Arizona State University].
The LROC Wide Angle Camera (WAC) context image (above) helps us decipher the origin of these graben, as a nearby lobate scarp can be seen at this scale. Lobate scarps form in compressional stress environments as layers of rock or regolith fold and thrust upwards. The thrusting might cause nearby crust or regolith to uplift and bend.

The graben and scarp are only hundreds of meters apart which argues for a compressional interpretation.Thus the interplay between compressional and extensional stresses is reflected in the distribution of tectonic features within Numerov crater. The end result is that we see small graben situated very near to lunar lobate scarps!

Numerov show its great Nectarian age at minimal shadowing in this LROC QuickMap 125 meter resolution orthographic projection assembled from LROC WAC photography and the LROC WAC-based digital terrain model (DTM). By contrast, its larger neighbor shouldered against it's western edge is Antoniadi, an uncharacteristically youthful (Upper Imbrium) impact crater for this part of the lunar surface, deep within South Pole-Aitken basin, and home of the Moon's deepest elevation. The smaller stress affects discussed in the post by Drew Enns are not as apparent at this scale, though other stress affects, scarps in particular, are easier to pick out [NASA/GSFC/ASU/DLR].
Explore more of the lobate scarp and graben in the full LROC NAC, HERE.

Related Posts:
Watch That First Step!
Its the Moon's Fault
Pull Apart - Grabens

LROC WAC mosaic presented using the Virtual Moon Atlas 6 shows Numerov in context with Antoniadi and Minnaert, a triple astrobleme that is easy to spot on maps of the farside and South Pole-Aitken basin [NASA/GSFC/ASU/VMA6].

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