Thursday, October 10, 2013

Wrinkles in Mare Frigoris

M181102837R LROC Featured Image, October 10, 2013
A complex wrinkle ridge deforms Mare Frigoris (52.935°N; 11.131°E) Two kilometer-wide field of view from LROC NAC observation M181102837R, LRO orbit 11804, January 13, 2013; 76.04°evening illumination angle of incidence from the west (left), resolution 1.65 meters per pixel from 168.24 km [NASA/GSFC/Arizona State University].
James Ashley
LROC News System

Lunar mare present many excellent examples of wrinkle ridges, where tectonic activity caused the foreshortening of near-surface rocks. The loading of large basins by dense mare basalts is thought to have resulted in isostatic adjustment of the underlying anorthositic crust, leading to buckling and overriding of surface rock units one atop another as compression occurred. The same stresses may also produce extensional (rather than compressional) deformation in adjacent areas. A variety of complex landforms can thus result.

Area shown at high resolution in the LROC Featured Image is designated with a small arrow in this 34.4 km-wide field of view from LROC Wide Angle Camera (WAC) monochrome (604 nm)observation M146911901CE, LRO orbit 6784, December 13, 2010; early morning 78.67° angle of incidence, resolution 59.3 meters per pixel from 43 km. The area of interest is in south central Mare Frigoris. [NASA/GSFC/Arizona State University].
Most of the ridges we see in today's Featured Image are produced by thrust faulting in Mare Frigoris. Just below center in the Featured Image frame, however, is a left-lateral strike-slip fault (also called a sinistral fault). Wrinkle ridges can be lumpy and ropey-looking, not exactly what comes to mind when one thinks of a textbook compressional fault. But in cross section the faulting would be readily apparent (refer again to the links above). Just south of the strike-slip fault are zones of extension where tension cracks have formed (small white arrow in Featured Image).

Further context for the wrinkle ridge in this field of view 107 km wide south central Mare Frigoris, scared by secondary crater streams from Aristoteles crater to the southeast. LROC WAC monochrome mosaic (604 nm) from five sequential orbits captured under local sunrise (emphasizing topography over albedo), LRO orbits 6782-6786, December 14, 2010 averaging a 77° angle of incidence from 43 km [NASA/GSFC/Arizona State University].
Morphologic nuances can be explored elsewhere in the NAC frame. Note the ropey appearance of some of these ridges, again showing that motions within the rock were complex indeed. Other examples of strike-slip faults have been found in association with lobate scarps on the Moon. Recent evidence suggests that shrinkage of the Moon from deeply seated internal cooling may have contributed to the occurrence of some lobate scarps and wrinkle ridges.

More examples of wrinkle ridges from NAC frame M181102837R [NASA/GSFC/Arizona State University].
Explore the full NAC frame HERE.

Additional examples of wrinkle ridges can be found in LROC Featured Image posts, "Really Wrinkled," "Wrinkle Ridge in Mare Crisium," and "Bulging Wrinkle."

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