The most well known tectonic landforms on the Moon are found in and around the nearside mare basins. Wrinkle ridges, formed by contraction, and rilles or troughs, formed by extension, deform the mare basalt-filled impact basins and the adjacent highlands. The wrinkle ridges and extensional troughs are the result of loading from mare basalts that causes downward flexure of the lunar lithosphere, resulting in contraction in the interior of the basin and extension near the margins.
Tectonic landforms on the Moon not directly associated with the mare basins are lobate scarps. Lobate scarps look like stair-steps in the landscape; they are one-sided and often have lobate fronts. These landforms are the surface express of thrust faults. Thrust faults are a break in the near-surface materials formed when crustal materials are contracted or pushed together. Crustal material is thrust upward along the fault forming a scarp.
Lobate scarps were first found in the highest resolution images and photographs taken by the Lunar Orbiters and the Panoramic Cameras flow on the Apollo 15, 16, and 17 missions. Because these high resolution images and photographs covered only a small area of the surface confined mostly to the lunar equatorial zone, it was not known how widely distributed lobate scarps were on the Moon.
The Lunar Reconnaissance Orbiter, launch in June, 2009, is returning the highest resolution images of the Moon ever obtained from orbit. These images, taken by the Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Cameras (NACs), have a resolution of 0.5 to 2 meter per pixel. The new NAC images are being used to search for previously unknown lobate scarps.
Newly discovered lobate scarps are being found in the LROC images. Many of the previously undetected fault scarps are located at high lunar latitudes and some have been found near the lunar poles.1 Lunar scarps found well outside the equatorial zone indicate that they are globally distributed.
A plot of the locations of newly detected and previously known lobate scarps shows that the faults are globally distributed. The newly discovered lobate scarps are shown by the white dots and the previously known scarps are shown by black dots. Most of the previously known lobate scarps were found in Apollo Panoramic Camera photographs that covered only part of the lunar equatorial region. The locations of the lobate scarps are plotted on the Lunar Orbiter Laser Altimeter (LOLA) global topographic model of the Moon. Credit: NASA/GSFC/Arizona State University/Smithsonian
Credit: Smithsonian Institution
The lobate scarps were formed when the lunar crust was pushed together as the Moon contracted. This causes the near-surface materials to break forming a thrust fault. The thrust fault carries crustal materials up and sometimes over adjacent crustal materials.
Credit: Smithsonian Institution
Over recent geologic time, as the lunar interior cooled and contracted the entire Moon shrank by about 100 meters (328 feet). As a result its brittle crust ruptured and thrust faults (compression) formed distinctive landforms known as lobate scarps. In a particularly dramatic example, a thrust fault pushed crustal materials (arrows) up the side of the farside impact crater named Gregory (2.1°N, 128.1°E). By mapping the distribution and determining the size of all lobate scarps, the tectonic and thermal history of the Moon can be reconstructed over the past billion years. Credit: NASA/GSFC/Arizona State University/Smithsonian
The most likely reason for the formation of the lobate scarps is global contraction caused by interior cooling. The loss of heat from the Moon’s interior results in contraction. Although the lobate scarps indicate contraction, the Moon has not contracted by much in the recent past. The total radial contraction or decrease in the Moon’s radius is estimated to be only about 100 meters.
The age of lobate scarps is also being investigated. Examining the crosscutting relations between the fault scarps and small diameter impact craters, their age is estimated to be no more than 1 billion year old. An even younger age for the scarps is suggested by the lack of superimposed, large-diameter impact craters. Also, the scarps are very pristine and undegraded. The young age of the fault scarps indicates that the Moon has contracted very recently.
1. Watters, Thomas R., Robinson, Mark S., Beyer, Ross A., Banks, Maria E., Bell, James F. III, Pritchard, Matthew E., Hiesinger, Harald, van der Bogert, Carolyn H., Thomas, Peter C., Turtle, Elizabeth P., Williams, Nathan R. 2010. "Evidence of Recent Thrust Faulting on the Moon Revealed by the Lunar Reconnaissance Orbiter Camera." Science 329.5994. 936 - 940.