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Lunar and Planetary Science XXIX 1322.pdf

CHARACTERIZATION OF MAJOR VOLCANIC EDIFICES ON USING MARS ORBITER LASER ALTIME- TER DATA. James W. Head1, N. Seibert1, S. Pratt1, D. Smith2, M. Zuber3, S. C. Solomon4, P. J. McGovern4, J. B. Garvin2, and the MOLA Science Team, 1Dept. of Geological Sciences, Brown University, Providence, RI 02912; 2NASA GSFC, 20771; 3MIT, Cambridge, MA 02139; 4DTM, CIW, Washington, DC 20015. [email protected].

Introduction: Mars is known to have a wide range of vol- large-scale slide deposit northwest of and we canic deposits and associated edifices [1] and although the suggest that the topography may be related to earlier slope morphology and broad morphometric characteristics of many failure at the base of the growing edifice. of these have been documented [2], most regional and global Alba Patera (40.6N, 110W) is the largest volcano on morphometric and slope data are tentative because of their low Mars (~2700 km diameter) and is characterized by a distinc- resolution, relatively low accuracy, or limited coverage. The tive radiating pattern of lava flows, a low profile relative Mars Orbiter Laser Altimeter (MOLA) instrument [3] was to the Montes and , and flank charac- designed to provide very high resolution altimetric data along teristics that suggest the presence of pyroclastic flows [9]. A individaul profiles and to provide global high-density altimet- MOLA profile across the western part of Alba shows that the ric data to characterize volcanic deposits and edifices and to structure is very asymmetric in a to north direction. The address outstanding questions related to these features [4]. rise toward the summit begins about 1030 km south of the Here we report on initial MOLA results for the characteriza- highest point in the profile (22N, 117.1W), and slopes upward tion of several volcanic edifices and structures on Mars. at about 0.2° for the next 780 km. At this point, the flank of MOLA Observations: Cross-sectional profiles across the volcano increases its slope to about 0.66°, and rolls over to several major edifices (Fig. 1) illustrate the nature and variety about 0.34° near the high point of the profile at about 5.7 km among volcanic landforms. altitude. The part of the edifice within the arcuate graben Olympus Mons (18N, 133W) was crossed during orbit 24 structure is relatively flat (0.26°) but slightly convex upward, along its middle western flank about 150 km west of the sum- reflecting the radial array of flows away from the true center mit caldera and MOLA data show the contrasting slopes of the of the edifice. The northern flank slopes are steeper (~1.73°) northern and southern scarps. To the north, there is a very than those to the south and at the base, the lower flanks extend steep 8.5 km high scarp (average ~14° slope, up to 26° at the for 1800 km to the north at about 0.27°, similar to the slopes to uppermost part of the scarp) the top of which defines the the south. Thus, on the basis of this profile which is not di- faulted margin of the volcano from which the aureole deposits rectly across the summit and thus does not represent maximum were derived. A narrow apron of -aged flows is possible values, the height of the edifice may be in excess of seen at the base of the scarp (~1.7° slope). To the south, the ten km above the base of its outer slope, and the diameter break in slope at the scarp is less distinct, occurring at about (measured from the edge of the descending slopes) may be as 9.5 km above the base, and the slope is more clearly two com- much as 3200 km. ponent, an upper part representing the flow-veneered scarp (24.5N, 213.3W) is part of the Elysium face (~7.6° slope) and the lower part the apron of flows (~2.4° volcanic province and is a distinctive symmetrical shield with slope). These remarkable scarps demonstrate clearly the ma- 12.5 km relief, concentric graben, radial channels and fracture jor loss of material from the base of the volcano relatively late systems, and long digitate radial flows with channels and in its history; the rise to the south and one further north (at ridged tubes [1]. These deposits overlie a distal flanking ) may reflect early degraded aureole deposits. spongy-looking terrain which, together with early topographic (9.5S, 121.2W), one of the , is data, led to the interpretation that Elysium was a composite 17450 m high along the highest point of this profile near its volcano [10]. MOLA obtained three profiles across the summit, and is characterized by a flat caldera floor (about 1 Elysium rise along the distal flanks of Elysium Mons. Orbit 3, km below the summit, one-half the depth previously estimated 250 km west of the summit of Elysium Mons (Fig. 1), shows [5]), steep upper flanks representing the slope of the major that the rise extends up to 5 km in elevation from the relatively part of the edifice itself (~4.1-5.3° slopes), shallower interme- flat northern plains, crosses the rise in the vicitnity of Elysium diate flanks representing the fans of flows emerging from the Fossae, and then descends about 4.5 km to the south. Orbit 29 flank rift zones (~1.1-1.2° slopes), and <1.0° slopes, repre- passes about 400 km to the east of the summit and shows a senting the lowermost flow aprons and intervolcano flow de- much lower and more symmetrical profile at a place where the posits. The asymmetry in the upper flanks is due to the fact Elysium plains deposits embay farthest into the northern plains that the profile intersects the lava fan closest to its origin on [11]. Orbit 20 passes about 600 km west of the summit, and the southern slope (at about 12 km altitude, 5.5 km below the the topography is relatively subdued, with two broad highs summit; about 2 km higher than previously thought [5]) and at where the profile crosses distal Elysium volcanics [11]. about 9.5 km altitude on the northern slope, where the fan Syrtis Major is a broad volcanic plain of age laterally intersects the upper flanks. About 1050 km north of that contains two calderas (Nili and Meroe Paterae); Earth- Arsia Mons along the profile and northwest of Pavonis Mons based radar altimetry first showed that this broad plain was a is a prominent north-facing scarp about 1400 m high. Previ- very low-relief shield about 1100 km in diameter with Nili and ous topographic data [6] do not detect the distinctive scarp Meroe in a summit depression about 280 km in diameter, sur- revealed by the MOLA data. The scarp represents the exten- rounded by gentle slopes, flows, and radial and concentric sion of a -Hesperian-aged highly tectonized unit mare ridges [12]. MOLA data from orbit 25 pass generally NS (undivided terra materials [6]) exposed just to the northwest, across the shield structure about 150 km west of Nili Patera and now covered in the area of the profile by flows emanating (Fig. 1) in a direction orthogonal to the radar data and support from Tharsis Montes. The detection of this previously un- the presence of a very low shield structure about 1100 km documented prominent scarp shows the importance of the wide with its summit near the position of the calderas. The N- MOLA data and helps to explain the distribution of lava flow S profile shows that Syrtis topography is high toward the units in this region [6]. This terrain occurs just west of the south, rises about 450 m to a high point at about 8.8N (~0.05° Lunar and Planetary Science XXIX 1322.pdf

MAJOR VOLCANIC EDIFICES ON MARS: J. W. Head et al.

slope), and then descends to a low point on its northern margin volcano [14,15] that in Viking images is characterized by ra- (~0.14° slope), about 1 km below the southern margin. The dial channels, a 75 km central depression, and a mountain highest relief from a line connecting the margins of the deposit internal to the central depression [1]; this structure has been is about 750 m. There is a central depression about 130 km interpreted as a highlands-type caldera with an incipient resur- wide, centered on 7.7N, which may be related to the summit gent dome possibly representing silicic volcanism [1]. This caldera containing the two paterae [12]. Detailed correlation feature is highly dissected by graben of Tempe Fossae and is of topography with images shows that the topographic rough- probably of Noachian age [14]. Where the MOLA profile ness of the edifice surface is due to both wrinkle ridges and crosses the edifice, a relatively prominent structure 120 km in lava flows. Imaging spectroscopy data of Syrtis Major has diameter and about 1200 m in elevation is observed (~1.2-2.3° been interpreted to mean that the surface units here are similar flank slopes), and evidence for the graben structures are seen to shergottites [13]; the low topography of Syrtis compared to in the 'saw-toothed' nature of the summit in the profile. others (Fig. 1, 2) could thus be related to composition, low Jovis : A series of smaller volcanoes in the Tharsis viscosity, and broad dispersal of lavas forming the edifice. region are known as tholi [1], and a MOLA profile across the Uranius Patera (26N, 93W) is classified as a minor shield summit of the smallest of these, (18.3N, 117.5W; volcano in the Tharsis region and is characterized by a low 77 x 62 km), shows that the height of the edifice along the height-to base ratio, a large summit caldera, an asymmmetric profile is about 950 m, less than half the 2 km previously profile, subsidiary vents, and superposed impact craters [1]. thought [7,8]. Flank slopes, previously estimated to be 3-8°, Uranius is embayed by early Tharsis Montes volcanic flows of are shown by the MOLA data to be 2.95°on the southern flank Amazonian age to the south, and older Hesperian-aged flows and 3.33° on the north flank. Jovis is completely surrounded to the north [15]. Uranius was crossed by orbit 26 about 15-20 by younger volcanic plains [7] which slope about 0.1°. km east of the summit caldera and MOLA data show that the Summary: These examples show that MOLA data pro- structure here is 255 km in width, with the southern younger vide important new insight in determining 1) absolute edifice Amazonian-aged flows 550 m higher than the northern Hes- heights; 2) nature and variations in flank slopes; 3) actual perian-aged flows. At the highest point on the profile, the margins of edifice deposits, and thus diameters; 4) more rigor- edifice is 1350 m above the northern plains and the flanks ous values for edifice volumes, and thus flux; 5) the signifi- have low slopes (N ~0.66° slope; S ~0.67° slope). Flank cance of shapes in volcanic style and possible composition topographic roughness includes two superposed impact cra- (e.g., Syrtis Major-Arsia-Jovis comparisons); 6) the signifi- ters, flows and the margins of some parasitic cones. A major cance of post-formation burial in edifice size (e.g., Jovis question about the minor Tharsis shields is how deeply they Tholus); and 7) the relative significance of edifice slope fail- are buried [16]; do they represent a) heavily buried edifices ure (both early and later; Pavonis and Olympus), post- with shield-like caldera-basal diameter ratios, or b) are they a emplacement tilting (Alba), and regional faulting (Tempe Fos- distinct class of feature that is less heavily buried than implied sae Structure). by a)? Extrapolation of a line connecting the highest points in References: 1. C. Hodges and H. Moore, USGS Prof. Paper 1534, 1994. 2. R. the pre-Uranius, Noachian-aged hilly terrain about 250 km to Pike, PLPSC 9, 3239, 1978. 3. M. Zuber et al., JGR, 97, 7781, 1992. 4. L. the north shows that it intersects Uranius Patera within a hun- Wilson and J. Head, Rev. Geophys., 32, 221, 1994. 5. USGS, Map I-1711, dred meters of the present northern base of the volcano. As- 1986. 6. D. Scott et al. USGS Misc. Inv. Series, Map I-2113, 1991. 7. J. suming that the Uranius Patera is built on this substrate, these Plescia, Icarus, 111, 246, 1994. 8. USGS Map I-1684, 1986. 9. P. Mouginis- Mark et al., Bull. Volc., 50, 361, 1988. 10. M. Malin, GSA Bull., 88, 908, 1977. data would suggest that the presently exposed edifice topog- 11. R. and J. Guest, USGS Misc. Inv. Series, Map I-1802-B, 1987. 12. rapy is close to that of the original edifice, and that the cal- G. Schaber, JGR, 87, 9852, 1982. 13. J. Mustard et al., JGR, 102, 25605, 1997. dera-basal diameter ratios represent landforms different than 14. D. Scott and K. Tanaka, USGS Misc. Inv. Series, Map I-1802-A, 1986. 15. the larger shield volcanoes [16]. D. Scott, JGR, 87, 9839, 1982. 16. J. Whitford-Stark, JGR, 87, 9829, 1982. Tempe Fossae Structure: MOLA orbit 35 crosses Tempe Figure 1. Comparative MOLA altimetry profiles of volcanic edifices; Terra volcanic province and an unnamed edifice mapped as a north to right of diagram.