U.S. DEPARTMENT OF THE INTERIOR TO ACCOMPANY MAP 1-2727 U.S. GEOLOGICAL SURVEY GEOLOGIC MAP OF THE TEMPE-MAREOTIS REGION OF MARS By Henry J. Moore INTRODUCTION GEOGRAPHIC SETTING A principal motivation for mapping the Tempe- The Tempe-Mareotis region lies along the northeast Mareotis region is to examine the low volcanic shields trend of the Tharsis Montes volcanic chain, which and lava plains that represent a style of volcanism unlike includes four large, superposed volcanoes: Arsia, Pavo­ the styles of volcanism of the vast ridged plains and the nis, and Ascraeus Montes and Uranius Patera (fig. 1). large Tharsis Montes volcanoes. The goal is to place the The region also includes the elevated western margin volcanism in a spatial and temporal context with other of Tempe Terra and adjacent plains to the west (fig. volcanism on Mars, especially the volcanism that pro­ 2; U.S. Geological Survey, 1985, 1989). In the north- duced the Tharsis Montes volcanic chain (Arsia, Pavonis, central part of the region, terraced highlands are tran­ and Ascraeus Montes). The Tempe-Mareotis region lies sected by the deep, north-south canyons of Tanais Fossae along a great circle that passes through the Tharsis shield along the western margin of Tempe Terra. Farther west, volcanoes and Uranius Patera (fig. 1, see map sheet for all isolated, irregular- to northeast-trending mountains (Tana- figures). Also of interest is the predominantly extensional ica Montes and Gonnus Mons) rise above the western tectonism that preceded, accompanied, and followed the plains, and Enipeus Vallis, a channel system, stretches 480 volcanism. The geologic record of the region is compli­ km from south to north across the western plains. Near cated by the interplay of volcanic, tectonic, and fluvial the east-central part of the region, Baphyras Catena com­ processes. prises three contiguous elongate abysses aligned north­ The Tempe-Mareotis region has been the subject of east; the center depression, about 38 km long and 14 km volcanic (Plescia, 1980, 1981; Hodges, 1980) and tec­ wide, is nearly 1.7 km deep. In the southeast part of the tonic (Scott and Dohm, 1990; Davis and others, 1995; region, the elevated, flat-topped ridges and intervening Golombek and others, 1996) studies. The geology of this valleys of Tempe Fossae trend northeast and attain lengths region has been mapped at 1:25,000,000 (Scott and Carr, that exceed 200 km. The plains of Ascuris Planum sepa­ 1978), 1:5,000,000 (Wise, 1979), and 1:15,000,000 (Scott rate the terraced highlands to the north from the Tempe andTanaka, 1986) scales. Fossae ridge-valley system to the south. Small, conical Six l:500,000-scale controlled photomosaics (U.S. hills and low, broad ridges, having elliptical bases and, Geological Survey, 1991a-f) were combined and then in places, summit depressions, are common features on reduced to 1:1,000,000 scale to form the base for this map. southwestern Ascuris Planum, on patches of plains within The 1:500,000-scale photomosaics and Viking Orbiter the Tempe Fossae ridge-valley system, and on the western images were used extensively to prepare this map. Image plains. These small hills and low ridges are interpreted to quality varies because of resolution and, perhaps, atmo­ be shield volcanoes. The shield volcanoes are typically spheric conditions (in parts of the northwest corner of the 2-10 km across, but Issedon Tholus (near the west-cen­ region). Two sets of images (448B and 299A) that are tral edge of the region at lat 36.3° N., long 94.7° W.) not included in the photomosaic have resolutions of 8-12 is 50-60 km across. Several large volcanoes surrounding m/pixel. The 448B images provide useful, but limited, the map area include (1) an unnamed volcano described coverage from about lat 34.5° N., long 82.2° W. to about by Hodges and Moore (1994) about 190 km to the east lat 36.9° N., long 81.8° W., but the 299A images are not in central Tempe Terra (near lat 37.6° N., long 75.9° W.); useful because of hazy atmosphere. Problems with the (2) Alba Patera, a large shield, about 600 km to the west photomosaic base that do not seriously affect the results (at lat 40.5° N., long 110.0° W.); (3) Uranius Patera, a of this study include: (1) mismatches of images from dif­ large shield, about 400 km to the south (at lat 26.2° N., ferent orbits, which result in offsets or even missing topo­ long 92.9° W.); and (4) older volcanoes to the east (Wise, graphic features, and (2) adjoining images with differing 1979; Plescia and Saunders, 1979; Scott, 1982; Hodges illumination directions. and Moore, 1994). The volcano mapped by Scott and Tanaka (1986) near lat 44° N., long 79° W. is question­ Nplt) is demonstrated by a fluvial channel that cuts into able. the southeast flank of crater Reykholt at lat 40.4° N., long Topographic information (U.S. Geological Survey, 85.3° W. Furthermore, cumulative frequencies of craters 1989) indicates elevations along the south edge of the greater than 2 km and 4 km that are superposed on the flu­ map area rise from 2,000 m in the east to 4,000 m in the vial erosion surface of the terraced plateau (unit Nplt) and west and elevations along the north edge are near 3,000 lineated plateau (Npll) materials indicate resurfacing near m. Photoclinometric estimates indicate the west edge middle Hesperian (fig. 3#). Reykholt crater material (unit of Tempe Terra rises up to 1,780 m above the floor of Ncr) is probably Noachian because Reykholt is a highly Tanais Fossae to the west (Davis and others, 1995). Relief degraded impact crater having two or more superposed of shield volcanoes about 5 km across have been esti­ degraded craters, its flanks were eroded during the Hes­ mated using photoclinometry (table 1; Davis and Soderb- perian, and the large diameter of Reykholt favors (but lom, 1984; Tanaka and Davis, 1988) to average about 90 does not prove) an older age. Lineated plateau material meters and to range from a few tens of meters to about (Npll) is interpreted to be ejecta from Reykholt mixed 200 meters. The relief of larger shield volcanoes, such as with local materials. Issedon Tholus, may be near 1,000 meters. Undivided material (unit Nu) includes outliers that rise above the flat surfaces of the upper member of the Tempe Terra Formation (unit Htu) and mesas, hills, STRATIGRAPHY and ridges that rise above fractured plateau material The three rock-stratigraphic systems (Scott and Carr, (unit HNplf). Degraded crater material (unit Nc) appears 1978; Tanaka, 1986; Tanaka and others, 1992a) are rep­ degraded and (or) fractured and deeply embayed by resented in the Tempe-Mareotis map area, but the age younger material; smooth, flat floors are common and assignments of some units are uncertain because resur­ extensive. facing processes have altered crater size-frequency distri­ butions and because the limited areal extents of the units HESPERIAN-NOACHIAN SYSTEMS counted result in large statistical uncertainties. Relative Fractured plateau material (unit HNplf) is essentially ages from crater size-frequency distributions are in gen­ the areal equivalent of older fractured material (unit eral agreement with those from superposition relations Nf) of Scott and Tanaka (1986). In contrast with and some previous assignments to systems and series. their description of the unit, impact crater outlines are Ejecta from some impact craters are intercalated with vol­ largely well preserved rather than "largely destroyed." canic deposits. Ages of significant units within the map Fractured plateau material (unit HNplf) is considered to area range from Late Noachian to Early Amazonian. be Hesperian-Noachian, but its stratigraphic position is uncertain. Previous workers have (1) suggested it may be NOACHIAN SYSTEM the fractured equivalent of the material of Lunae Planum The Noachian System comprises the oldest map (Wise, 1979), which is Hesperian, (2) included it with units. Five material units are mapped: (1) terraced plateau Hesperian ridged plains material (unit Hprg of Scott material (unit Nplt), (2) Reykholt crater material (unit and Carr, 1978), (3) described it as younger than the Ncr), (3) lineated plateau material (unit Npll), (4) undi­ material of Lunae Planum (Plescia, 1980, 1981), and vided material (unit Nu), and (5) degraded crater mate­ (4) placed it in the Noachian (Scott and Tanaka, 1986). rial (unit Nc). On this map, terraced plateau material (unit Cumulative frequencies of craters greater than 2 and 4 Nplt) includes the basement complex (unit Nb), hilly and km place fractured plateau material near the middle of the cratered units of the plateau sequence (units Nplh and Hesperian (fig. 3C, D), but craters greater than 8 km (fig. Npli), and part of the lower member of the Tempe Terra 3D) suggest that Upper Noachian is possible. Adjacent to Formation (unit Htl) of Scott and Tanaka (1986) because the east edge of the map area, faulted lava flows and tube the four units cannot be separated using stratigraphic evi­ ridges are part of fractured plateau material (unit HNplf; dence. For example, surface morphologies are the same see Viking frames 621 All and 18). These faulted lava on either side of the contact between hilly plateau mate­ flows and tube ridges extend 170 km to the northwest rial and cratered plateau material (units Nplh and Npli) as of an unnamed volcano caldera across units mapped mapped by Scott and Tanaka (1986) between lat 40.0° N., as Noachian and Hesperian by Scott and Tanaka (their long 85.5° W. and lat 37.5° N., long 85.5° W.