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

Abstract

Large-scale expressions of lunar tectonics-contractional wrinkle ridges and extensional rilles or graben-are directly related to stresses induced by mare basalt-filled basins 1,2 . Basin-related extensional tectonic activity ceased about 3.6 Gyr ago, whereas contractional tectonics continued until about 1.2 Gyr ago 2 . In the lunar highlands, relatively young contractional lobate scarps, less than 1 Gyr in age, were first identified in Apollo-era photographs 3 . 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 stress [4][5][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 scarps 2,3 . These small-scale scarps are thought to be the surface expression of thrust faults 3 . Crosscutting relations with Copernican-age, small-diameter impact craters indicate the lobate scarps are relatively young, less than 1 Gyr old (ref. 3).Small-scale graben revealed in 0.5-2.0 m/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 scarp 3,7 (∼20.3• N, 30.5• E) and are spatially correlated with a narrow rise along the crest of the scarp face and the elevated back-limb terrain 3 . 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 ∼40 m wide and ∼500 m long. Graben in the back-limb area of the Pasteur scarp (∼8.6• S, 100.6 • E; Supplementary Fig. S1) are ∼1.2 km from the scarp face (Fig. 1b). Unlike the Madler graben, the orientation of the Pasteur graben are subparallel to the scarp and extend for ∼1.5 km, with the largest ∼300 m in length and 2...

Geologic map of the northern hemisphere of Vesta based on Dawn Framing Camera (FC) images

“…The small-scale lineations represented by E-W grooves and pit crater chains could have a tectonic origin related to the Saturnalia Fossae system. Similar, although smaller, lineations associated with extensional tectonics have been observed on the Moon (e.g., Watters et al, 2012).…”

Tectonic evolution of northwestern Imbrium of the Moon that lasted in the Copernican Period

“…The formation of the young graben near Promontorium Laplace may have been allowed by the relative uplift of the region by the thermal subsidence of the surrounding regions. The hot interior of the terrain may have mitigated the horizontal compression by the global cooling, as was suggested for the fresh grabens by Watters et al (2012).…”

“…Howard and Muehlberger (1973) found fissures along the crest of a mare ridge in Serenitatis. Watters et al (2012) and French et al (2015) found the swarms of fissures and grabens in the areas where the surface layers have been subjected to buckling along the crests of ridge-like rises. French et al (2015) also reported globally distributed small-scale grabens that are not associated with compressional features.…”

“…Since the graben that we found is ∼500 m wide and ∼30 km long, the graben is morphologically different from those found by previous researchers. The grabens along positive topographic features swarm, and individual ones are ∼40 m wide and ∼500 m long (Watters et al 2012;French et al 2015). The grabens that are not associated with positive topographic features have a mean width and length of 19 and 146 m, respectively (French et al 2015).…”

“…8, 10). Their crisp morphology suggests that they are formed in the Copernican Period (Watters et al 2012). Some lobate scarps are thought to be even younger than 50 Ma from the crosscut relationships between the scarps and small ruptured craters, whose formation ages were estimated to be younger than 50 Ma .…”