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 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, 5, 6 that might be expected to completely suppress the formation of graben.
