Thrust faults on the Moon and Mercury: The tectonics of one-plate rocky bodies

Plate tectonics is the primary driving force responsible for thrust fault belts on Earth. Exploration of the solar system has revealed that Earth may be the only member in a spectrum of rocky bodies with current plate tectonics and perhaps is the only planet to have developed multiple lithospheric plates. With one-plate lithospheres the rule rather than the exception, one might conclude that thrust faults are rare elsewhere in the solar system. In fact, positive relief landforms that show evidence of significant contractional strain are common on the Moon, Mercury, and Mars. These landforms, described as lobate scarps, are interpreted to be the surface expression of thrust faults. They are distinguishable from the other contractional landforms by their asymmetric cross-section and evidence of surface-breaking faults. Lobate thrust fault scarps exhibit a dramatic range in scale. Mercury's largest thrust fault scarp is ~1,000 km in length and has over 3 km of relief, while the smallest scarps are only a few kilometers long with only tens of meters of relief. In contrast to Mercury, only small-scale scarps are found on the Moon. Displacement-length relations of thrust faults on Mercury, the Moon, and Mars are similar to terrestrial thrust faults.

The driving mechanism for contractional deformation of one-plate lithospheres is, as in plate tectonics on Earth, primarily heat loss from the interior. Thermal evolution of rocky bodies proceeds from an early stage of net thermal expansion that is followed by a transition to net thermal contraction. Global radial contraction of a one-plate lithosphere results in the formation of a broadly distributed array of thrust faults. The amount of radial contraction recorded by Mercury's thrust fault scarps is ~1 to 2 km since late heavy bombardment (~4 Ga). The radial contraction of the Moon expressed by its global array of small thrust fault scarps is <100 m.

Small-scale thrust fault scarps on the Moon and Mercury are susceptible to erosion from impacts. Their pristine appearance, lack of superposed craters, and crosscut small craters indicate young thrust faults and, thus, recent tectonic activity. The relatively small amount of global contraction of the Moon and Mercury suggests an evolutionary path for these one-plate bodies where interior heat is retained and tectonic activity is sustained.