Constraints on the Fault Dip Angles of Lunar Graben and Their Significance for Lunar Thermal Evolution

Lunar grabens are the largest tensional linear structures on the Moon. In this paper, 17 grabens were selected to investigate the dips and displacement-length ratios (gamma) of graben-bounding faults. Several topographic profiles were generated from selected grabens to measure their rim elevation, width and depth through SLDEM2015 (+LOLA) data. The differences in rim elevation ( increment h) and width ( increment W) between two topographic profiles on each graben were calculated, yielding 146 sets of data. We plotted increment h vs. increment W for each and calculated the dip angle (alpha) of graben-bounding faults. A dip of 39.9 degrees was obtained using the standard linear regression method. In order to improve accuracy, large error data were removed based on error analysis. The results, 49.4 degrees and 52.5 degrees, were derived by the standard linear regression and average methods, respectively. Based on the depth and length of grabens, the gamma value of the graben-bounding normal fault is also studied in this paper. The gamma value is 3.6 x 10-3 for lunar normal faults according to the study of grabens and the Rupes Recta normal fault. After obtaining the values of alpha and gamma, the increase in lunar radius indicated by the formation of grabens was estimated. We suggest that the lunar radius has increased by approximately 130 m after the formation of grabens. This study could aid in the understanding of normal fault growth and provide important constraints on the thermal evolution of the Moon.