GEOLOGIC MAP OF THE VICTORIA (H-2) QUADRANGLE OF MERCURY By George E. McGill and Elbert A. King 1983 Prepared for the National Aeronautics and Space Administration by U.S. Department of the Interior, U.S. Geological Survey (Published in hardcopy as USGS Miscellaneous Investigations Series Map IĐ1409, as part of the Atlas of Mercury, 1:5,000,000 Geologic Series. Hardcopy is available for sale from U.S. Geological Survey, Information Services, Box 25286, Federal Center, Denver, CO 80225) Please direct questions or comments about the digital version to: Richard Kozak U.S. Geological Survey 2255 N. Gemini Drive Flagstaff, AZ 86001 e-mail: rkozak@flagmail.wr.usgs.gov DESCRIPTION OF MAP UNITS PLAINS MATERIALS ps SMOOTH PLAINS MATERIALŃForms mostly small areas of plains that appear smooth at available image resolution. Albedo similar to or higher than that of intermediate plains unit. Superposed craters all smaller than 20 km in diameter and of low density. Characteristically occupies floors of craters, including large c4 craters. Reference area: lat 36Ą N., long 29Ą. Interpretation: A mixture of ejecta from small craters and material mass wasted from walls of large craters. May include some volcanic materials psi INTERMEDIATE PLAINS MATERIALŃForms smooth to moderately rough plains characterized by many bright patches and streaks on a background generally slightly darker than intercrater plains material. Commonly associated with elongate symmetric and asymmetric ridges and rounded scarps, all of which appear younger. superposed craters are of moderate density, are of c2 age or younger, and are mostly less than 50 km in diameter. Also superposed are secondary craters from several c4 and fresh c3 craters 100 to 150 km in diameter. Reference area: lat 30Ą N., long 25Ą. Interpretation: Probably basaltic lava flows, but no unequivocal supporting evidence has been found in the Victoria quadrangle pc CRATERED PLAINS MATERIALŃForms moderately rough to rough surfaces very similar to intercrater plains material. Unit defined to correlate with similar unit in Kuiper quadrangle (De Hon and others, 1981) to the south; elsewhere on Victoria quadrangle cratered plains material is included in intercrater plains material. Reference area: lat 22Ą N., long 42Ą. Interpretation: Volcanic rock, clastic blanket generated by secondary impact events, or both pi INTERCRATER PLAINS MATERIALŃForms moderately rough to very rough surfaces with an albedo generally slightly higher than that of the intermediate plains unit. Contacts with intermediate plains and cratered plains units locally gradational. Density of superposed craters high. Reference area: lat 27Ą N., long 40Ą. Interpretation: Undivided rim and ejecta materials of overlapping, very old craters and basins. Probably also includes material similar to that forming intermediate plains unit. CRATER AND BASIN MATERIALS Subscripts refer to degradation sequence from cl (most degraded) to c5 (least degraded). Degree of degradation is inferred to represent relative age (cl oldest to c5 youngest) of craters of closely similar diameters (see McCauley and others, 1981, for discussion of this age sequence). Craters less than about 20 km in diameter are not mapped. Interpretation: Impact craters. A very few irregular, small craters may be endogenic c5 CRATER MATERIALŃUndivided floor, rim, wall, and ejecta material of craters with complete and well-defined rims, central peaks, textured ejecta blankets, and high-albedo halos and rays. All are <50 km in diameter. Density of superposed craters low cp5 CENTRAL PEAK MATERIALŃSingle or multiple rugged peaks near center of crater cf5 FLOOR MATERIALŃRough material on crater floor cr5 RADIAL EJECTA MATERIALŃGrooved and ridged material extending outward from raised rim as lineated apron with many secondary craters c4 CRATER MATERIALŃUndivided rim, wall, and ejecta material of craters with complete and well-defined rims, central peaks, and textured ejecta blankets. Density of superposed craters low, but higher than on smooth plains material. Unit includes floor material for most craters less than 35 km in diameter cp4 CENTRAL PEAK MATERIALŃSame as cp5 except occurs within c4 craters cf4 FLOOR MATERIALŃSame as cf5 except occurs within c4 craters cr4 RADIAL EJECTA MATERIALŃRadially textured annulus around unit c4; includes secondary craters that are sufficiently well preserved to be clearly visible around most of the periphery of larger craters cs4 SECONDARY CHAIN MATERIALŃAligned crater chains related to c4 craters c3 CRATER MATERIALŃundivided rim, wall, and ejecta material of craters with complete and well-defined rims. Most have central peaks. Density of superposed craters low to moderate. Unit includes floor material for most craters less than 35 km in diameter cp3 CENTRAL PEAK MATERIALŃSame as cp5 except occurs within c3 craters cf3 FLOOR MATERIALŃSame as cf5 except occurs within c3 craters cr3 RADIAL EJECTA MATERIALŃAround largest craters, includes moderately well preserved textured ejecta blankets and incompletely preserved fields of secondary craters c2 CRATER MATERIALŃUndivided floor, rim, wall, and ejecta material of craters with complete but subdued rims. Only largest craters have remnants of textured ejecta blankets. Secondary craters not recognizable. Density of superposed craters moderate cp CENTRAL PEAK MATERIALŃSame as cp5 except seen in very few c2 craters c1 CRATER MATERIALŃUndivided material of craters with subdued and incomplete rims and no recognizable central peaks, ejecta blankets, or secondary craters. Density of superposed craters moderate to high csu CRATER CHAIN MATERIAL, UNDIVIDEDŃ Crater chains not associated with any visible crater Contact ScarpŃLine marks top; hachures point downslope Crest of elongate ridge Crater rim crest Crater rimŃGreatly subdued or buried Area of bright crater-ray materialŃInterpreted as ejecta from relatively recent impacts INTRODUCTION Most of the Victoria quadrangle lies within an area that appears bright on telescopic images of the planet, the bright-albedo feature Aurora, which approximately coincides with the east half of the quadrangle (Davies and others, 1978, fig. 11). As is common with most of the imaged portions of Mercury, the Victoria quadrangle is dominated by basins and large craters, with plains materials occupying the areas between them. Almost all the pictures acquired by Mariner 10 that were used for mapping were obtained during the first encounter: those covering the southeast half of the quadrangle are incoming close-encounter images, and those covering the north-west corner are outgoing close-encounter images. At the time the pictures were obtained, the terminator was at about long 7Ą to 8Ą, within the eastern part of the quadrangle. A large gap in coverage between the incoming and outgoing images appears as a northeast-trending diagonal blank strip on the base map. A small part of this gap was filled in the southwestern part of the quadrangle by very poor second-encounter images. No images provide a vertical view; in fact, the smallest angle between the planetary surface normal and the camera axis is about 50Ą. The high obliquity of the images, the wide range in sun-elevation angles, and the complete transection of the quadrangle by the gap in coverage greatly hamper geologic mapping. Only in about 15 percent of the quadrangle, near the southeast corner, do data permit separation of units with the confidence possible in other quadrangles on Mercury. STRATIGRAPHY Three widespread units are recognized within the Victoria quadrangle. These are, from oldest to youngest, intercrater plains material, intermediate plains material, and smooth plains material. In addition, central peak, floor, rim, and ejecta materials related to the numerous craters and basins larger than about 20 km in diameter are mapped. The simplicity of the stratigraphic scheme is at least in part due to deficiencies in the data base; the history of plains formation almost certainly is more complex than our threefold division indicates, but we were not able to define consistent criteria of albedo, texture, and cratering for more than three plains units because of the highly variable quality of available pictures. PLAINS MATERIALS Intercrater Plains Material About half of the intercrater area consists of material characterized by a very high density of small, mostly degraded craters, and an irregular to rough surface. Superposition relations suggest that this unit is about the same age as, or older than, all mappable craters and basins. The origin of intercrater plains material is enigmatic; some may be primitive crust, as implied by Trask and Guest (1975), but more likely it is of mixed origin, dominated by breccias formed by now-unrecognizable ancient craters. Some of the more plainslike areas included within this unit may well have an origin similar to that of intermediate plains material. Cratered Plains Material Within the 5Ą overlap area with the Kuiper quadrangle to the south, an area has been mapped that displays moderately rough to rough terrain and a high density of mostly degraded craters. This unit is very similar to intercrater plains material, and cannot be distinguished from it anywhere else in the Victoria quadrangle. Most of the cratered plains material is probably volcanic in origin, but some of it may consist of impact breccias. Intermediate Plains Material Smooth to moderately irregular plains occupy most of the area between large craters not underlain by intercrater plains material. These plains superficially resemble the plains of the lunar maria; they generally have a relatively low albedo (Hapke and others, 1975) and are characterized by numerous elongate ridges. Like the lunar maria, the two younger mercurian plains units have been ascribed to volcanic activity (Trask and Guest, 1975; Strom and others, 1975; Trask and Strom, 1976), although this interpretation has been questioned (Wilhelms, 1976). A volcanic origin seems most probable, but no compelling evidence exists in the Victoria quadrangle to support this opinion. The elongate ridges, though clearly associated with intermediate plains material, are not restricted to it. Locally, ridges extend into intercrater plains material adjacent to intermediate plains material, and large young (c4 and c3) craters superposed on the intermediate plains material commonly are transected by these ridges . Smooth Plains Material Partly filling most craters is plains material that is smoother and less densely cratered than intermediate plains material. Because most areas underlain by this unit are enclosed within craters, contacts between smooth plains and older plains units are rare. Smooth plains material thus is defined almost entirely by texture and apparent crater density. Few superposition data directly support the inferred age sequence, but the relative youth of the smooth plains unit is indicated by its presence on the floors of craters that are superposed on intermediate plains material. The smooth plains unit probably includes materials of a wide range in age, but the exposed areas are too small to test this possibility quantitatively. Although a volcanic origin cannot be ruled out for all or part of the smooth plains material, it is more probably a mixture of ejecta from very small craters and colluvium mass wasted from crater walls. STRUCTURE The ridges associated with the intermediate plains unit are best interpreted as tectonic in origin because they extend into adjacent exposures of intercrater plains material and, more significantly, because they transect ejecta, rims, and floors of craters. The ridges range in length from about 50 km to many hundreds of kilometers, are sinuous to lobate in plan, and generally trend about north-south. Most are asymmetric, with one slope steeper than the other, and at places they can be more logically referred to as rounded scarps. Commonly, an individual ridge changes along trend from symmetric ridge to asymmetric ridge to rounded scarp. Strom and others (1975) interpreted most of these features to be surface expressions of thrust faults, and we can find no evidence within the Victoria quadrangle not already considered in their discussion. Because of their globally systematic orientations, these ridges and scarps have been associated with stresses developed by tidal despinning of Mercury (Melosh, 1977). However, most trend approximately north-south and thus do not fit the pattern expected in the midlatitude belt, unless stresses from overall contraction were superposed on the stresses due to despinning (Melosh, 1977, figs. 3 and 5). GEOLOGIC HISTORY The oldest material and features in the Victoria quadrangle are the intercrater plains material and areally associated, severely degraded basins. No craters are clearly older than intercrater plains material, and the relative ages of the cl basins are ambiguous. Numerous large craters are superposed on intercrater plains material; by analogy with lunar and martian history (Hartmann, 1966, 1973; Neukum and others, 1975), these craters most likely date from more than about 4 b.y. ago. The available evidence suggests a relatively long history of plains formation. Some of the material included in the intercrater plains unit appears to have been plainslike before the intense cratering characteristic of the unit. In addition, the younger plains units exhibit densities of superposed craters ranging from moderate to very sparse. The intermediate plains material is older than the freshest (c4 and some c3) large craters (100-150 km in diameter) but younger than all basins, and younger than all large craters that are more than moderately degraded (some c3, and all c2 and c1). Thus, the material mapped as the intermediate plains unit overlaps in time of origin the tail end of the primordial bombardment. The stresses responsible for the elongate ridges and scarps must have occurred after the end of the primordial bombardment and after emplacement of the intermediate plains unit. Where smooth plains material abuts ridges and scarps, the evidence is mostly ambiguous because we cannot tell if ridge formation involved smooth plains material or if the ridges are upwarped intermediate plains material with smooth plains material ponded against them. On the floors of some craters, such as Gluck, scarps apparently offset material mapped as smooth plains, but the exposures are so small that this interpretation could easily be challenged. Ridges appear to be both older and younger than medium-size craters (30Đ60 km in diameter) on the intermediate plains unit, but intersections of ridges with craters in this size range are too rare to constrain the time of ridge formation. Thus, ridge formation obviously occurred after emplacement of the intermediate plains unit, but how long after remains uncertain in this quadrangle. Smooth plains material is apparently younger than all large craters, and hence is the youngest material in the quadrangle with the exception of the local material related to some very small craters (<20 km in diameter). CRATER DISTRIBUTIONS Figures 1 to 3 are plots of cumulative diameter versus frequency of craters on the three plains units. Figures 1 and 2 include craters ł3 km in diameter on the large areas of intercrater and intermediate plains units in the southeastern portion of the quadrangle. The two counting areas together are approximately bounded by lats 20Ą and 32Ą and longs 15Ą and 42Ą and are separated by an irregular but approximately east-west-trending contact; the intercrater plains material lies to the south and the intermediate plains material to the north. Figure 3 includes craters ł1.2 km in diameter on smooth plains material that covers the floors of six large craters. Because of the varied and generally poor quality of the imagery, detailed cratering history cannot be inferred from these plots. However, three observations seem valid: (1) the density of large craters is distinctly higher for intercrater than for intermediate plains material; (2) for the intercrater and intermediate plains material, curves for craters with diameters between 3 and 15 km nearly coincide (the abundant, mostly degraded small craters characteristic of intercrater plains material but not characteristic of intermediate plains material are less than 3 km in diameter); and (3) craters of all sizes on the smooth plains unit are much less abundant than on the other units, although the smooth-plains plot is unreliable in detail because of the small total number of craters counted and the need to combine counts from isolated exposures. A serious sampling problem exists for counting craters on the intermediate plains unit, because those in the diameter range of 50 to 150 km commonly occur in clusters, and it is very difficult to determine which craters of a cluster are younger than the surrounding plains unit and which are older. The area counted for figure 2 does not include any of these clusters. Immediately to the north, however, intermediate plains material surrounds three clusters of large craters, including Holbein, the c4 crater ~100 km in diameter centered at lat 36Ą N., long 29Ą. This large crater clearly is younger than the surrounding intermediate plains material, and superposition relations suggest that two or three of the 50- to 60-km-diameter craters east of it also may be younger. The ages of other craters in the clusters, relative to the age of intermediate plains, are ambiguous. A second count of intermediate plains material was made that included the area of clusters of large craters, and that was based on the extreme assumption that all these large craters are younger than intermediate plains material. The resulting plot (not shown) differs from figure 2 at the large-crater end, as would be expected, but it still shows a lower density of large craters than does the plot of craters from intercrater plains material. Because not all the large craters occurring in scattered clusters are likely to be younger than intermediate plains material, the true plot cumulative diameter versus frequency should not differ much from that shown in figure 2. REFERENCES Davies, M. E., Dwornik, S. E., Gault, D. E., and Strom, R. G., 1978, Atlas of Mercury: National Aeronautics and Space Administration, Special Publication SP-423, 128 p. De Hon, R. A., Scott, D. H., and Underwood, J. R., Jr., 1981, Geologic map of the Kuiper quadrangle of Mercury: U.S. Geological Survey Miscellaneous Investigations Series Map I-1233, scale 1:5,000,000. Hapke, B. W., Danielson, G. E., Jr., Klaasen, K. P., and Wilson, Lionel, 1975, Photometric observations of Mercury from Mariner 10, 1975: Journal of Geophysical Research, v. 80, no. 17, p. 2431Đ2443. Hartmann, W. K., 1966, Early lunar cratering: Icarus, v. 5, no. 4, p. 406Đ418. _____1973, Martian cratering, 4, Mariner 9 initial analysis of cratering chronology: Journal of Geophysical Research, v. 78, no. 20, p. 4096Đ4116. McCauley, J. F., Guest, J. E., Schaber, G. G., Trask, N. J., and Greeley, Ronald, 1981, Stratigraphy of the Caloris Basin, Mercury: Icarus, v.47, no.2, p.184Đ202. Melosh, H. J., 1977, Global tectonics of a despun planet: Icarus, v. 31, no. 2, p. 221Đ243. Neukum, Gerhard, Konig, Beate, Fechtig, H., and Storzer, D., 1975, Cratering in the Earth-Moon system: Consequences for age determination by crater counting: Lunar Science Conference, 6th Proceedings, p. 2597Đ2620. Strom, R. G., Trask, N. J., and Guest, J. E., 1975, Tectonism and volcanism on Mercury: Journal of Geophysical Research, v. 80, no. 17, p. 2478Đ2507. Trask, N. J., and Guest, J. E., 1975, Preliminary geologic terrain map of Mercury: Journal of Geophysical Research, v. 80, no. 17, p. 2461Đ2477. Trask, N. J., and Strom R. G., 1976, Additional evidence of mercurian volcanism: Icarus, v. 28, no. 4, p. 559Đ563. Wilhelms, D. E., 1976, Mercurian volcanism questioned: Icarus, v. 28, no. 4, p. 551Đ558. NOTES ON BASE This map sheet is one of a series covering that part of the surface of Mercury that was illuminated during the Mariner 10 encounters (Davies and Batson, 1975). The source of map data was the Mariner 10 television experiment (Murray, 1975). ADOPTED FIGURE The map projections are based on a sphere with a radius of 2439 km. PROJECTION The Lambert conformal conic projection is used for this sheet, with a scale of 1:4,623,000 at lat 22.5Ą. Latitudes are based on the assumption that the spin axis of Mercury is perpendicular to the plane of the orbit. Longitudes are positive westward in accordance with the usage of the International Astronomical Union (IAU, 1971). Meridians are numbered so that a reference crater named Hun Kal (lat 0.6ĄS) is centered on long 20Ą (Murray and others, 1974; Davies and Batson, 1975). CONTROL Planimetric control is provided by photogrammetric triangulation using Mariner 10 pictures (Davies and Batson, 1975). Discrepancies between images in the base mosaic and computed control point positions appear to be less than 2 km, except for the area in the northwest corner of the quadrangle. Pictures of this area are so foreshortened that accurate map transformations were not possible. Since the base mosaic was controlled by a later iteration of the control net, large discrepancies exist between this sheet and the Kuiper (H-6) sheet to the south. These discrepancies were not adjusted, so that features in the zone of overlap on this sheet appear at different latitudes and longitudes than they do on the Kuiper quadrangle. MAPPING TECHNIQUES Mapping techniques are similar to those described by Batson (1973a, 1973b). A mosaic was made with pictures that had been digitally transformed to the Lambert conformal conic projection. Shaded relief was copied from the mosaics and portrayed with uniform illumination with the sun to the west. Many Mariner 10 pictures besides those in the base mosaic were examined to improve the portrayal. The shading is not generalized and may be interpreted with nearly photographic reliability (Inge, 1972; Inge and Bridges, 1976). Shaded relief analysis and representation were made by Susan L. Davis. NOMENCLATURE All names on this sheet are approved by the International Astronomical Union (IAU, 1977). H-2 Abbreviation for Mercury (Hermes) sheet number 2. H 5M 45/45 G: Abbreviation for Mercury (Hermes) 1:5,000,000 series; center of sheet, 45Ą N lat, 45Ą long; geologic map, G. REFERENCES Batson, R. M., l973a, Cartographic products from the Mariner 9 mission: Journal Geophysical Research, v. 78, no. 20, p. 4424Đ4435. ______1973b, Television cartography: U.S. Geological Survey open-file report, Astrogeology 58, 35 p. Davies, M. E., and Batson, R. M., 1975, Surface coordinates and cartography of Mercury: Journal Geophysical Research, v. 80, no. 17, p. 2417Đ2430. Inge, J. L., 1972, Principles of lunar illustration: Aeronautics Chart and Information Center Reference Publications RP-72-1, 60 p. Inge, J. L., and Bridges, P. M., 1976, Applied photointerpretation for airbrush cartography: Photogrammetric Engineering and Remote Sensing, v. 42, no. 6, p. 749Đ760. International Astronomical Union, 1971, Commission 16: Physical study of planets and satellites, in 14th General Assembly, Brighton, 1970, Proceedings: International Astronomical Union Transactions, v. 14B, p. 105Đ108. _______977, Working Group for Planetary System Nomenclature, in 16th General Assembly, Grenoble, 1976, Proceedings: International Astronomical Union Transactions, v. 16b, p. 321Đ325, 330Đ332, 351Đ355. Murray, B. C., 1975, The Mariner 10 pictures of MercuryŃan overview: Journal Geophysical Research, v. 80, no. 17, p. 2342Đ2344. Murray, B. C., Belton, M. J. S., Danielson, G. E., Davies, M. E., Gault, D. E., Hapke, Bruce, OŐLeary, Brian, Strom, R. G., Soumi, Verner, and Trask, Newell, 1974, MercuryŐs surface: Preliminary description and interpretation from Mariner 10 pictures: Science, v. 185, no. 4146, p. 169Đ179.