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| A mare wrinkle ridge transitions to a highland lobate scarp at the edge of Oceanus Procellarum. Illumination is from the lower-left in this 2.9 km wide mosaic of LROC Narrow Angle Camera (NAC) frames M107069913LE and M107069913RE, LRO orbit 918, September 8, 2009. View the full size LROC Featured Image HERE [NASA/GSFC/Arizona State University]. |
Nathan Williams
LROC News System
A mare wrinkle ridge transitions to a highland lobate scarp at the edge of Oceanus Procellarum. Illumination is from the lower-left in this 2.9 km wide mosaic of LROC NAC images M107069913LE and M107069913RE [NASA/GSFC/Arizona State University].
Two types of compressional tectonic landforms are commonly observed on the Moon: wrinkle ridges and lobate scarps. Wrinkle ridges are long, often sinuous hills in mare basalts and are thought to be folded rock layers overlying deeper faults. Lobate scarps usually occur in the highlands and are interpreted as rocks lifted up by faults very near to or even breaking the surface. There are a few locations where a wrinkle ridge transitions into a lobate scarp or vice-versa, such as here at the northern edge of Oceanus Procellarum (60.5°N, 331.4°E). In this LROC NAC mosaic, the lobate scarp in the highlands massif to the northeast meets a wrinkle ridge to the southwest when it reaches the otherwise flat-lying Procellarum basalts. You can also see lots of boulders eroding out of the wrinkle ridge.
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| Reduced resolution NAC mosaic of images M107069913L and M107069913R showing the wrinkle ridge - lobate scarp transition. Illumination is from the bottom-left in this 16 km wide mosaic. [NASA/GSFC/Arizona State University]. |
Wrinkle ridges are thought to have formed after the basaltic lavas erupted, filling in the basins on the nearside, and weighing down the crust. All that extra weight probably made the ground sag and bend, causing the basalt to buckle and fold in some areas. On the other hand, lobate scarps are thought to form from radial contraction or shrinking of the entire Moon. The global radial contraction built up compressional stresses in the crust until the stress was great enough to fracture all the way to the surface. The transition from wrinkle ridge to lobate scarp may be due to the contrast in materials, especially if the basaltic lavas are layered and the highland massif lacks layering. However, the relationships between wrinkle ridges and lobate scarps at transitions like this are still being studied.
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| LROC Wide Angle Camera mosaic (604 nm band) showing the Featured Image (yellow box) in context around 7 kilometers west of craterlet Fontenelle X, and the system of wrinkle ridges and scarps, admittedly easier to see at higher resolutions. From a larger LROC WAC mosaic swept up through 14 orbital observations from an average 36 kilometers altitude, January 26 and 27, 2011; Field of view roughly 62 kilometers (phase angle averaged 70° - see image following) NASA/GSFC/Arizona State University]. |
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| Highly reduced original LROC WAC mosaic (January 26, and 27, 2011) showing the location of the Featured Image and its environs within Mare Frigoris, north of Sinus Iridum and Mare Imbrium, possibly superimposed on the older (even with younger volcanic fill) and irregularly-shaped basin known as Oceanus Procellarum. Some investigators propose most of the nearside may be an ancient basin, responsible, perhaps at least in part, for the near and far side elevation discontinuity, and a basin that is perhaps larger than the nearside hemisphere itself, originally centered northwest of Mare Tranquillitatis. This so-called "Gargantuan" impact basin is far from without it's detractors, however. Without definitive proof, the South Pole-Aitken basin remains the largest and oldest recognized impact basin. |
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| Then there is the laser altimetry building up from the LOLA instrument on-board LRO, from hemisphere-views released earlier this year, here showing the location of the Featured Image (blue arrow) from over far north, and the farside highlands beyond 90° west. Without the nearside's familiar albedo features, the near and farside dichotomy is easier to sense [NASA/GSFC/LOLA/SVS]. |
Related Posts:
Scarps in Schrödinger
Lunar Lobate Scarp
Slipher Crater Fractured Moon in 3-D
Forked Wrinkle Ridge
Stress and Pull
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