Lunar and Planetary Science XXXII (2001) 1613.pdf

TOPOGRAPHY OF LARGE-SCALE LOBATE SCARPS IN THE SOUTHERN HEMISPHERE OF MERCURY. T. R. Watters1, M. S. Robinson2, and A. C. Cook1, 1Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, Washington, D.C. 20560 ([email protected]); 2Department of Geological Sciences, Northwestern University, Evanston, Illinois 60208 ([email protected]).

Summary: A regional-scale digital elevation arc formed by the structures. Our DEM shows that model of an area in the southern hemisphere of the topographic expression of the scarp face of Mercury (Discovery quadrangle), derived from Adventure and Resolution Rupes is continuous digital stereo-analysis of Mariner 10 images, is used except where it is interrupted by the presence of a to examine the topography of three large-scale lobate prominent high-relief ridge that appears to crosscut scarps, Adventure Rupes, Resolution Rupes, and the Adventure-Resolution Rupes trend (Figure 1). Discovery Rupes. The thrust faults that formed these The displacement-length relationship of the thrust landforms occur along a rough arc that extends for faults associated with Adventure and Resolution over 1000 km. The new topography shows that Rupes also supports the interpretation that the two vertical uplift occurred on the same side of the three features are segments of a single structure. The ratio structures, suggesting that the fault-planes all dip to of maximum displacement to fault length g for the the concave side of the arc. These data also indicate Adventure and Resolution Rupes thrust faults are that Adventure and Resolution Rupes are both ~1.2´10-2, where the displacement is given by D topographically continuous, suggesting the two = h/sinq and where h is the measured relief of the features were formed by a single thrust fault. scarp and q is the dip of the fault-plane [5]. These Analysis of the inferred stresses that formed these are almost a factor of 2 higher than g for other thrust faults suggest that they were influenced by -3 mercurian thrust faults (6.5 ± 3.2´10 , n = 10) using regional, nonhydrostatic horizontal stresses or by estimates of D based on q = 25° [5]. If Adventure mechanical discontinuities in the crust, possibly and Resolution Rupes were formed by a single thrust caused by buried impact basins. The new fault, the combined fault has a value of g of ~6.3´10- topographic data indicate a broad topographic low in 3, consistent with that of the Discovery Rupes thrust this area. Thus the Discovery and Adventure- fault and other mercurian thrust faults. This would Resolution Rupes thrust faults may have been make the Adventure-Resolution Rupes thrust fault localized by mechanical discontinuities in the second only to the Discovery Rupes thrust fault in mercurian crust introduced by one or more ancient scale with lengths of ~500 km and ~550 km, buried impact basins. respectively. In an effort to determine the geometry Introduction: One of the remarkable of the compressional stresses that formed the three characteristics of the surface of Mercury imaged by rupes, a beta analysis [see 9] was performed. The Mariner 10 is the presence of hundreds of landforms orientation of the scarps was approximated by 20 described as lobate scarps and interpreted to be the digitized segments. The beta analysis indicates a surface expression of thrust faulting [1, 2, 3, 4, 5]. The distribution, orientation, and geometry of thrust maximum concentration of 23.2% per 1% area (a = faults on Mercury is important in constraining models 190) located at approximately 45°S, 55°W, roughly for the origin of the compressional stresses that 600 km northwest of Discovery Rupes. The results formed these structures. New topographic data for of this analysis are consistent with the hypothesis that Mercury is being derived from digital stereoanalysis the formation of the thrust faults associated with [4, 6] using updated Mariner 10 camera orientations Discovery, Resolution, and Adventure Rupes were [7, 8]. influenced by either regional nonhydrostatic Topography and Analysis: Discovery Rupes, horizontal stresses or preexisting mechanical Resolution Rupes, and Adventure Rupes occur along discontinuities in the crust. Examination of the a rough arc that extends for over 1000 km (Figure 1). topography near the stress center suggested by the The topographic data indicates that Discovery Rupes beta analysis indicates the presence of a broad, has the greatest relief (~1.5 km), followed by shallow depression. If the thrust faults were Adventure Rupes (~1.3 km) and Resolution Rupes localized by a preexisting mechanical discontinuity in (~0.9 km). The topography of Discovery, Resolution, the crust, the most likely source is a buried impact and Adventure Rupes indicates that the scarp faces of basin. Spudis and Guest [10] mapped 20 pre- the three landforms occur on their southern-side Tolstojan multiring basins randomly distributed over (Figure 1). This suggests that the fault-planes of the the mercurian hemisphere imaged by Mariner 10. three thrust faults all dip to the concave side of the One of these basins, the Andal-Coleridge basin, is Lunar and Planetary Science XXXII (2001) 1613.pdf

LARGE-SCALE LOBATE SCARPS ON MERCURY: T. R. Watters et al.

centered at 43°S, 49°W, just east of Schubert crater. Discovery Rupes. The center of the Andal-Coleridge basin is located References: [1] Strom R.G., Trask N.J. and Guest near the stress center indicated in the beta analysis of J.E. (1975) J. Geophys. Res., 80, 2,478-2,507. [2] the inferred stresses (45°S, 55°W). The broad Cordell, B.M. and Strom R.G. (1977) Phys. Earth topographic low in this area may be explained by the Planet. Inter., 15, 146-155. [3] Melosh H.J. and presence of an ancient impact basin. However, the McKinnon W.B. (1988) in Mercury, 374-400. [4] proposed outer ring of the Andal-Coleridge basin, Watters T.R., Robinson M.S. and Cook A.C. (1998) about 875 km from the basin center, is not parallel to Geology, 26, 991-994. [5] Watters, T.R., R.A. the arcuate trend of Adventure and Resolution Rupes. Schultz, and M.S. Robinson, (2000) Geophys. Res. The proposed inner rings of the Andal-Coleridge Lett., 27, 3659-3662. [6] Cook, A.C. and Robinson, basin are subparallel to northern segments of M.S. (2000) J. Geophys. Res., 105, 9429-9443. [7] Discovery Rupes, but not the southern segments. Robinson, M.S., Davies, M.E., Colvin, T.R. and This may reflect the influence of more than one Edwards, K.E. (1999) J. Geophys. Res., 104, 30847- ancient basin. Thus it is possible that mechanical 30852. [8] Robinson, M.S. and Lucey, P.G. (1997) discontinuities introduced by one or more ancient Science, 275, 197-200. [9] Watters, T.R. (1993) J. buried multiring impact basins localized the thrust Geophys. Res., 98, 17049-17060. [10] Spudis, P.D. faults that formed Adventure, Resolution, and and Guest, J.E. (1988) in Mercury, 118-164.

Figure 1. Color-coded regional-scale digital elevation model (DEM) in the Discovery quadrangle. This subsection (25°W to 80°W, 50°S to 75°S) was extracted from a larger DEM mosaic generated using over 260 individual stereo pairs and is overlaid on an image mosaic. Shades of blue to black are lows, and shades of red to white are highs. White arrows indicate the location of Adventure Rupes, Resolution Rupes, and Discovery Rupes. Elevations are relative to the 2439.0 km Mercury radius reference sphere.