Provenance of block fields along lunar wrinkle ridges

Abstract

Block fields and associated relatively high reflectance material along wrinkle ridge summits are revealed in meter scale Lunar Reconnaissance Orbiter Camera (LROC) images. Wrinkle ridges with block fields in Mare Australe, Crisium, Fecunditatis, Frigoris, Nubium, the northern half of Oceanus Procellarum, Serenitatis, and Tranquillitatis are evenly distributed, and block density generally increases with wrinkle ridge slope. The median cross-sectional area (proxy for diameter) of the 1,368 blocks measured in this study is 7 m2 and 75% of these blocks are = 14 m2. We propose that the largest variation in cross-sectional area of the blocks is related to substrate physical properties of the mare basalt and not a function of the slope on which they occur, suggesting that physical properties rather than slope is a stronger control on block size. The maximum length of blocks may constrain the minimum basalt flow thickness or joint width; our block measurements suggest basalt flows 2 – 14 m thick, agreeing with previous estimates. The data suggest that blocks originate from mare basalt layers that buckle and break as a result of movement along ridge-forming thrust faults. High reflectance material associated with wrinkle ridge blocks likely represents freshly exposed rock and soil. Meter-scale blocks may erode relatively quickly due to collisional disruption, indicating recent down-slope movement of regolith exposing preexisting blocks, or blocks formed and exposed by recent activity on ridge-forming faults.