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CHANNELS and BASIN TERRACES of the MOON. D.W. Leverington, Department of Geosciences, Texas Tech University, Lubbock, TX 79409-1053

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CHANNELS and BASIN TERRACES of the MOON. D.W. Leverington, Department of Geosciences, Texas Tech University, Lubbock, TX 79409-1053 NLSI Lunar Science Conference (2008) 2110.pdf CHANNELS AND BASIN TERRACES OF THE MOON. D.W. Leverington, Department of Geosciences, Texas Tech University, Lubbock, TX 79409-1053. Introduction: The characteristics of lunar channels with some of this variation likely related to the degree and basin terraces are of great relevance to our under- of degradation of inner flanks after channel formation standing of the basic processes that have shaped the [17]. surface of the Moon. The largest lunar channels head Many lunar channels commence abruptly at large at volcanic source regions that are likely to have topographic depressions that mark volcanic sources played key roles in the development of adjacent vol- [e.g., 1,13,14]. It is typical, however, for sinuous lunar canic plains. Smaller (narrower) lunar channels are rilles to lack distinct raised landforms at their mouths commonly found in association with mare deposits, [7], a characteristic that can be attributed to processes and in some cases distributed flows over distances of such as burial of channel mouths by subsequent lava hundreds of kilometers. Some lunar channels show flows. It is also likely that the large volumes and low evidence for substantial erosion of upland materials, viscosity of lunar lava flows [e.g., 18,19] inhibited suggesting that lunar lavas had a capacity for erosion development and preservation of morphologically- that is generally deficient in modern terrestrial flows. distinct accumulations of terminal deposits. The characteristics of lunar channel systems have great Many lunar channels appear to have developed potential to be used to better constrain the physical constructively by acting as distributaries for flows de- properties of associated flows, and to illuminate basic posited on older materials. However, some lunar channels also appear to have developed at least in part processes involved in the emplacement of lunar vol- through substantial incision into underlying materials. canic terrains. Because the incision processes of some Examples of Lunar Channels of Special Signifi- lunar channels seem likely to have operated in con- cance: The largest lunar channels are found in the junction with gradual decreases in the surface levels of Aristarchus-Prinz region of Oceanus Procellarum. large magma bodies, improved knowledge of the for- Located at Aristarchus Plateau, an upland possibly mation mechanisms of these channels may yield im- composed partly of large volumes of ballistically- portant new information regarding the nature of past emplaced volcanic fines [e.g., 14], Vallis Schröteri is subsidence of lunar magma bodies. Related insights the largest lunar channel, with a length of ~175 km and are also likely to be produced through investigation of maximum width of ~10 km [e.g., 11]. Vallis Schröteri basin terraces, which appear to mark the maximum is characterized by a prominent terrace and interior lateral and vertical extents of adjacent volcanic units channel, and heads at and crosscuts Aristarchus Pla- Characteristics of Lunar Channels: Lunar sinu- teau, ultimately extending onto adjacent mare plains. ous rilles share numerous morphological and contex- Smaller but similarly-terraced channels are found in tual characteristics with their generally smaller terres- regions including the vicinity of crater Plato, north of trial volcanic counterparts, and it is partly on this basis Mare Imbrium [e.g.., 12]. that they are interpreted as volcanic landforms [e.g., 1- Rima Beethoven, one of four large channels com- 4]. The absence of hydrous minerals in lunar samples prising the Rimae Prinz system [13,14], is over 60 km [e.g., 5], including those collected at Rima Hadley by in length and up to ~3 km in width. The channel is Apollo 15 astronauts [6], suggests that sinuous lunar especially notable because it cross-cuts uplands in a rilles did not form in the presence of water and manner suggestive of vertical incision on a scale of strengthens volcanic interpretations of these land- hundreds of meters [e.g., 11,13,20]. Lunar channels forms. such as Beethoven may have developed by the flow of Morphological characteristics of lunar channels can lava during the terminations of volcanic highstands, include inner channels [7], channel levees [8-11], with gradual channel incision taking place during the channel terraces [11,12], typical gradients of less than lowering of magma levels, possibly resulting from one degree [13,14], longitudinal profiles intermittently magma devolitization and phase change [11,21]. characterized by reverse gradients [2,13], and exposure Smaller lunar rilles that otherwise have the appear- of channel floors as a result of partial or complete ance of constructive mare lava conduits cut across up- drainage of late-stage lava pulses [6]. The sinuosity lands in a manner similar to that seen at Rima Beetho- characteristics of lunar rilles vary considerably ven. For example, a channel of the Rimae Herigonius [15,16], as do the width-to-depth ratios of lunar rilles system cross-cuts highland terrain near crater Gassendi (ratios of ~4:1 to 11:1 approximately describe common in a manner consistent with incision during the lower- ranges). Cross-sectional profiles of lunar channels can ing of mare magma levels [11,21,22]. vary from “v-shaped” profiles to flat floored profiles, NLSI Lunar Science Conference (2008) 2110.pdf Formation of volcanic channels on the Moon led in morphologies of lunar channels and channel systems; some cases to development of streamlined landforms and 2) an improved basis for identification of terrace that may be erosional residuals. Although uncertain- systems and characterization of their collective geome- ties remain, a lunar rille that heads at the rim of crater tries. Documentation of the high-resolution geometry Krieger contains streamlined landforms (“islands” up of upland-crossing channels will provide new informa- to ~2 km in length) that appear to have formed at least tion regarding the nature of subsidence of mare depos- in part by the erosive properties of associated lava its and the capacity of lunar lavas to substantially flows [12]. Streamlined island-like features are asso- erode substrates. Determination of the collective ge- ciated with a complex anastamosing channel system at ometry of lunar terraces will allow for investigation of Mare Imbrium, near crater Euler [e.g., 10,12]. the nature of basin-wide subsidence of ancient magma A proportion of lunar channels developed roofs bodies. Identification of deviations of terrace geome- through solidification of flow surfaces, and it is not tries from expected forms may potentially be used to uncommon for rilles to be discontinuously expressed at quantify later deformation of the lunar surface. the surface as a result of incomplete collapse of these References: [1] Greeley, R. (1971a) The Moon, 3, roofs. Visited by the Apollo 15 astronauts [17], Rima 289-314. [2] Greeley, R. (1971b) Science, 172, 722- Hadley is a large and partly roofed lunar channel that 725. [3] Cruikshank, D.P., C.A. Wood (1972) The extends at least 135 km across the volcanic plains of Moon, 3, 412-447. [4] Hulme, G. (1973) Mod. Geol., Palus Putredinis in southeastern Mare Imbrium, aver- 4, 107-117. [5] Papike, J., L. Taylor, S. Simon (1991) aging ~1.2 km in width and ~370 m in depth [1]. Ex- Lunar minerals, in Lunar Sourcebook, Cambridge, pp. posed reaches of Rima Hadley are in places character- 121-181. [6] Swann, G.A., and 19 others, (1972) Pre- ized by scalloped rims that appear to outline former liminary geologic investigations of the Apollo 15 land- collapse pits [e.g., 22]. Smaller examples of roofed ing site, in Apollo 15 Preliminary Science Report, channels, with typical widths of ~25-200 m, are found NASA SP-289, pp.5-1 to 5-112. [7] El-Baz, F., A.M. in such regions as the Marius Hills area of Oceanus Worden, V. D. Brand (1972) Proc. Lunar Sci. Conf., Procellarum [e.g., 2,22]. The roofed channels of the 3rd, 85-104. [8] El-Baz, F., S.A. Roosa (1972) Proc. Moon are natural shelters of potential value to future Lunar Sci. Conf., 3rd, 63-83. [9] Hulme, G. (1974) long-term manned missions. Geophys. J. R. Astr. Soc., 39, 361-383. [10] Schaber, Basin Terraces: Terraces found on the inner rims G.G. (1973) Proc. Lunar Planet. Sci. Conf., 4th, 73-92. of some lunar basins appear to mark the maximum [11] Schultz, P.H. (1976) Moon Morphology, UT lateral and vertical extents of past magma bodies [e.g., Press, Austin. [12] Leverington, D.W. (2004) J. Geo- 11,21,23,24]. Basin terraces are in some cases rela- phys. Res., 109, E10011, doi:10.1029/2004JE002311. tively restricted in extent, having formed in association [13] Strain, P. L., F. El-Baz (1977) The Moon, 16, with small impact craters or caldera-like landforms. 221-229. [14] Zisk, S.H., C.A. Hodges, H.J. Moore, For example, terraces are present at crater Bowditch, a R.W. Shorthill, T.W. Thompson, E.A. Whitaker, D.E. ~35 km long caldera-like basin located northeast of Wilhelms (1977) Moon, 17, 59-99. [15] Young, R.A., Mare Australe. The Bowditch terraces are up to ~3.5 W. J. Brennan, R.W. Wolfe, D.J. Nichols (1973) Proc. km wide and range in elevation between ~50 and 200 Lunar Sci. Conf., 4th, 57-71. [16] Wilhelms, D.E. m, and are hypothesized to mark the high level of a (1987) The Geologic History of the Moon, USGS Prof. lava lake that later breached the crater rim, forming the Paper 1348, Washington, D.C. [17] Howard, K.A., 120-km-long flow field that comprises Lacus Solitudi- J.W. Head, G.A. Swann (1972) Proc. Lunar Sci. nus [7,24-26]. Conf., 3rd, 1-14. [18] Head, J.W. (1976) Rev. Geophys.
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