A new map of graben on the lunar nearside : initial observations and classification

Introduction: Faults and fractures are visible records of the stresses operating on and in planetary bodies. On the Moon, tectonic structures are concentrated on the nearside, are spatially associated with the maria [e.g., 1], and are divided into 3 main types: wrinkle ridges, lobate scarps, and graben. Lunar graben (or rilles) are long, narrow troughs up to several hundreds of kilometers long and up to 5 km in width. The occurrence of graben on material that formed after basin formation and mare emplacement indicate that graben formed relatively recently [2]. Current model ages suggest that graben formation ceased around 3.6 Ga [1-3] and contractional structures (wrinkle ridges and lobate scarps) formed ~1 Ga [1, 4]. Crosscutting relationships, however, indicate that some wrinkle ridges pre-date the graben [5] and recent work on Rupes Recta, a large normal fault in Mare Nubium, indicates that it is younger than 3.2 Ga [6]. These contradictory observations indicate that a detailed, systematic study of the distribution and timing of graben formation is needed. Here, I present the results from the first part of this systematic study: the mapping campaign. Mapping: Mapping of graben on the lunar nearside (270° to 90° E, 70°N to 70°S) at a scale of 1:500,000 has been completed. The Lunar Reconnaissance Orbiter (LRO) Wide Angle Camera (WAC) global morphologic map was used as the base map (100 m/px, available at http://wms.lroc.asu.edu/lroc/view_rdr/WAC_GLOB AL). The WAC base map is preferred over other available datasets because it has a generally uniform incidence angle for consistent identification of scarps and it is already complete. LRO WAC digital terrain maps (DTMs), produced at DLR (100 m/px; [7]), and LRO Lunar Orbiter Laser Altimeter (LOLA) [8] Gridded Data Records (GDR) (at 512 and 1024 px/deg) were co-registered with the WAC base map in ArcGIS. Graben and other fractures were identified primarily based on map view morphology. The primary characteristic was the presence of a pair of abrupt scarps with the portion between them down-dropped relative to the surroundings. In general, the scarps are subtle, with heights up to several hundred meters. Statistics for fault lengths and graben widths and depths have not yet been compiled, but are forthcoming. The map presented here is a significant improvement over earlier maps produced using low resolution pre-LRO data at scales of 1:5 million and 1:1 million [e.g., 1, 2, 9]. Classification: Based on morphology and map view orientation, the mapped graben have been divided into 4 categories (Fig. 1): linear graben, arcuate graben, floor fractured crater fractures, and lineaments. Linear graben (Fig. 1A) are structures that are linear or nearly linear, have a relatively flat floor, roughly parallel walls, and approximately constant widths. In contrast, arcuate graben (Fig. 1B) meet the criteria of linear graben, but have orientations that vary from linear by more than several degrees and are typically concentric about maria. Floor fractured crater (FFC) fractures (Fig. 1C) are undifferentiated fractures or graben that are contained within a FFC identified by [10]. Lineaments (Fig. 1D) may have similar trends to identified graben in the area and are shallow curvilinear depressions. These structures may not be graben, but may have formed under similar stress fields and are thus useful for understanding the tectonic history of the nearside.