Recent extensional tectonics on the Moon revealed by the Lunar Reconnaissance Orbiter Camera

Large-scale expressions of lunar tectonics—contractional wrinkle ridges and extensional rilles or graben—are directly related to stresses induced by mare basalt-filled basins1,2. Basin-related extensional tectonic activity ceased about 3.6 Gyr ago, whereas contractional tectonics continued until about 1.2 Gyr ago2. In the lunar highlands, relatively young contractional lobate scarps, less than 1 Gyr in age, were first identified in Apollo-era photographs3. However, no evidence of extensional landforms was found beyond the influence of mare basalt-filled basins and floor-fractured craters. Here we identify previously undetected small-scale graben in the farside highlands and in the mare basalts in images from the Lunar Reconnaissance Orbiter Camera. Crosscut impact craters with diameters as small as about 10 m, a lack of superposed craters, and graben depths as shallow as ∼1 m suggest these pristine-appearing graben are less than 50 Myr old. Thus, the young graben indicate recent extensional tectonic activity on the Moon where extensional stresses locally exceeded compressional stresses. We propose that these findings may be inconsistent with a totally molten early Moon, given that thermal history models for this scenario predict a high level of late-stage compressional stress4–6 that might be expected to completely suppress the formation of graben. Basin-localized lunar tectonics resulted in both basin-radial and basin-concentric graben and wrinkle ridges. Typically, basinlocalized graben are found near basin margins and in the adjacent highlands whereas wrinkle ridges are restricted to the basin interior (ref. 2, plate 6) (Supplementary Note S1). The dominant contractional tectonic landform found outside of mare basins are lobate scarps2,3. These small-scale scarps are thought to be the surface expression of thrust faults3. Crosscutting relations with Copernican-age, small-diameter impact craters indicate the lobate scarps are relatively young, less than 1Gyr old (ref. 3). Small-scale graben revealed in 0.5–2.0m/pixel Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC) images are often associated with lobate scarps (Supplementary Note S2). These graben were first found in the back-limb area of the Lee–Lincoln scarp3,7 (∼20.3N, 30.5 E) and are spatially correlated with a narrow rise along the crest of the scarp face and the elevated back-limb terrain3. Newly detected graben found in the back-limb terrain of the Madler scarp (∼10.8 S, 31.8 E; Supplementary Fig. S1) are located ∼2.5 km from the scarp face (Fig. 1a). The orientation of these graben is roughly perpendicular to the trend of the scarp. The dimensions of the graben vary, with the largest being∼40m wide and∼500m long. Graben in the back-limb area