Prepared for the National Aeronautics and Space Administration Geologic Map of the MTM –20272 and –25272 Quadrangles, Tyrrhena Terra Region of Mars By Scott C. Mest and David A. Crown Pamphlet to accompany Scientific Investigations Map 2934 65° 65° MC–1 MC–5 MC–7 MC–6 30° 30° 300° 240° MC–12 MC–13 MC–14 MC–15 360° 315° 270° 225° 180° 0° 0° MC–20 MC–21 MC–22 MC–23 300° 240° –30° –30° MC–28 MC–27 MC–29 MC–30 –65° –65° 2006 U.S. Department of the Interior U.S. Geological Survey INTRODUCTION occurred in the Noachian Period. Erosion of impact craters and highland terrains in Tyrrhena Terra, as well as other equato- Mars Transverse Mercator (MTM) –20272 and –25272 rial (±30° lat) highland regions, resulted primarily from fluvial quadrangles (lat 7.5°–27.5° S., long 270°–275° W.) cover processes that were most likely precipitation driven (Craddock part of the highlands of Tyrrhena Terra north of Hellas Planitia and Maxwell, 993; Craddock and Howard, 2002). (fig. ). The surface of the Tyrrhena Terra region records Central-vent volcanism in the highlands surrounding a complex history of impact cratering and modification by Hellas Planitia occurred in the Late Noachian and Early fluvial and eolian activity (Schaber, 977; Greeley and Guest, Hesperian Epochs during formation of the highland paterae 987; Craddock and Maxwell, 993; Maxwell and Craddock, (Greeley and Crown, 990; Crown and others, 992; Crown 995; Tanaka and Leonard, 995; Leonard and Tanaka, 200; and Greeley, 993; Tanaka and Leonard, 995; Gregg and Mest and Crown, 2002a, 2005; Mest, 2004). The map area others, 998). Hadriaca and Tyrrhena Paterae, southeast and consists primarily of intercrater plains, impact crater material, east of the map area, respectively, are interpreted to consist of and crater floor material (Mest and Crown, 2002a, 2005). An pyroclastic deposits, possibly resulting from phreatomagmatic extensive valley network, Vichada Valles, as well as several eruptions as magma rose through surface material containing smaller networks, dissects the northern part of the map area. ground ice or water (Greeley and Spudis, 98; Greeley The abundance and widespread nature of fluvial features within and Crown, 990; Crown and Greeley, 993). The presence the map area have significant implications for past Martian of lava flow lobes within the approximately ,000-km-long environmental conditions. The degraded terrains surrounding Tyrrhena Patera flowfield and a smooth deposit filling the Hellas Planitia provide constraints on the role and timing of caldera of Hadriaca Patera suggest that some effusive activity volatile-driven activity in the evolution of the highlands (Mest occurred late in the volcanic history, presumably during the and Crown, 200a). The geologic history of this area may have Late Hesperian to Early Amazonian (Crown and others, 992; been influenced not only by the presence of Hellas Planitia but Gregg and others, 998; Mest and Crown, 200b). also by other buried impact basins as described by Frey and others (2000). STRATIGRAPHY REGIONAL GEOLOGY Geologic units in MTM –20272 and –25272 quadran- gles were identified and characterized using a combination The highlands surrounding Hellas Planitia include of Viking Orbiter (VO), Mars Global Surveyor (MGS), Mars Noachian material that represents some of the oldest rocks on Orbiter Camera (MOC), and Mars Odyssey Thermal Emis- Mars (Murray and others, 97; Tanaka, 986; Tanaka and sion Imaging System (THEMIS) images and Mars Orbiter others, 988, 992). MOLA data show that the map area slopes Laser Altimeter (MOLA) and Thermal Emission Spectrometer approximately 0.23° S. toward Hellas Planitia and is surrounded (TES) data. We determined relative ages of geologic units using by large impact craters and rugged highland material at higher observed stratigraphic and crosscutting relations in combina- elevations (fig. 2). Erosion of highland material character- tion with crater size-frequency distributions (table ). ized the Late Noachian Epoch and continued throughout the Hesperian Period, resulting in deposition of extensive plains MAPPING METHODOLOGY materials in and around Hellas Planitia (Greeley and Guest, 987; Crown and others, 992; Leonard and Tanaka, 200; We analyzed the geologic units exposed in the Tyrrhena Mest and Crown, 200a). The geologic units along the north Terra region using VO, MOC, and THEMIS images, TES rim of Hellas Planitia were initially grouped into the plateau data, and MOLA topography. VO image coverage of the and high-plains assemblage, which consists of several rugged map area generally ranges from 50–70 m/pixel near crater and cratered highland units and various younger plains units, Isil (south-central MTM –25272) to approximately 230 m/ some of which contain channels, ridges, scarps, and mesas pixel; however, the west-central part of the map area, which (Greeley and Guest, 987). Previous mapping at :5,000,000 includes Millochau crater, is covered by high-resolution VO (Schaber, 977) and :5,000,000 (Greeley and Guest, 987) images (~6 m/pixel). High-resolution MOC coverage (~2–6 scales each describe two main units in the highlands north of m/pixel) is sparse in the map area but has proven useful in Hellas Planitia shown in this map: the “hilly and cratered high- characterizing geologic material in detail, as well as analyzing land material” and “dark mottled plains material” (Schaber, the spatial extents of various units identified in the region. 977) and the “dissected plateau material” and “ridged plains THEMIS daytime infrared (~95–00 m/pixel) and visible material” (Greeley and Guest, 987). These researchers (~8–20 m/pixel) images cover much of the map area and interpreted this area to be a mixture of volcanic material and provide adequate intermediate image resolutions to correlate ancient impact breccias and ejecta that represents a heavily units observed in both VO and MOC images. TES data (3 km/ dissected part of the highlands (Schaber, 977; Greeley and pixel) were used specifically to characterize the deposits on Guest, 987). Previous Viking-based analyses by Maxwell the floor of Millochau crater. MOLA topographic data were and Craddock (995) in eastern Tyrrhena Terra (lat 5°–30° used to identify relations between valleys and impact craters, S., long 260°–270° W.) showed that formation of intercrater as well as to assess the distributions and stratigraphic posi- plains units, emplaced following extensive highland erosion, tions of geologic units. The unit color scheme used in this map follows that of with the related complicating effects of secondary craters. Tanaka and others (2005). Color is used to provide relative age We found significant variation in the areal exposures of map information for most units; a violet to red spectrum represents units and the presence of large-diameter impact craters with oldest to youngest units, respectively. However, the three units distinct, widespread ejecta blankets that may superpose these designated as other crater deposits (units c3, c2, c1) do not units. Several units have small areas, specifically valley floor follow this scheme because they are widespread both spatially material (unit HNvf), talus material (unit AHt), mountainous and temporally within and outside of the map area, and they material (unit Nm), and the Millochau crater floor deposits have no definitive Martian age. (units Amd, HNme, HNmr, Nmp). Valley floor material forms narrow linear deposits that typically do not show clear relative CRATER COUNTING METHODOLOGY age relations to impact crater ejecta; however, an example of We compiled crater size-frequency distributions to clear age relations occurs where the trunk valley of Vichada constrain the relative ages of geologic units and to determine Valles appears to be truncated and partly buried by the ejecta the timing and duration of inferred geologic processes. Rela- from several large (>25 km diam) craters near lat 24° S. These tive age information for the geologic units in MTM –20272 craters have ejecta blankets that cover areas extending out to and –25272 quadrangles (:500,000 scale; lat 7.5°–27.5° S., distances twice as large as their diameters. Where age rela- long 270°–275° W.) was partly derived by determining the tions are clear, craters with widespread eject blankets were not number and size distribution of superposed impact craters for included in counts for units of small areal extent. To include each geologic unit mapped (Tanaka, 986). We digitized unit these craters in the counts for valley floor material would also boundaries and determined unit areas using NIH Image .6 require including the areas of their ejecta blankets in the area software. Individual craters were identified and counted using of valley floor material. This would imply that large quantities the MTM photomosaics and a Mars Digital Image Mosaic of valley floor material presumably extend to some distance of VO images (MDIM 2; 23.4 m/pixel; lat 3°–30° S., long beneath the ejecta deposits; whereas, in reality, valley material 265°–280° W.), as well as individual VO frames. Crater diam- would only account for a minor fraction of the surface buried eters were measured in centimeters using Adobe Illustrator by the ejecta. Inclusion of the area of the ejecta deposits with 9.0 software and were converted to kilometers; diameters for the area of limited-exposure units would greatly increase the larger craters were checked for accuracy using GRIDVIEW areas of those units and provide inaccurate crater statistics and software (Roark and others, 2000; Roark and Frey, 200). relative ages. The highland terrain of the map area contains impact For units with large areas, such as intercrater plains (unit craters of a wide range in size to a maximum of approximately Npi), the method of counting all craters superposed on the 4 km in diameter. Based on VO images, the minimum crater surface does not present a problem. The plains are mapped as a widespread unit that has been superposed by a number diameter counted in valley floor material (unitHNvf ), highland of large-diameter impact craters.