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Ancient Martian Volcanoes in the Aeolis Region' New Evidence from MOLA Data

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Ancient Martian Volcanoes in the Aeolis Region' New Evidence from MOLA Data JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. E8, PAGES 17,505-17,513,AUGUST 25, 2001 Ancient Martian volcanoes in the Aeolis region' New evidence from MOLA data Emily M. Stewart and JamesW. Head Departmentof Geological Sciences,Brown University, Providence,Rhode Island Abstract. Mars Orbiter Laser Altimeter (MOLA) altimetricdata confirm the volcanicorigin of a Noachian-agedhilly feature(Zephyria Tholus) in the Aeolis areaof Mars. Similar characteristics sharedby at least one nearbytopographic high suggestthat volcanicactivity representedby such featuresmay have beenmore widespreadthan previously thought. Morphologicalstructures and morphometricdata for Zephyriaare consistentwith a stratovolcanoorigin, in which the edifice formedby mixed explosiveand effusiveeruptions, and was alteredby subsequentflank erosion,to producea distinctivetruncated cone with concaveupward flanks. Criteriaare outlined to help detectand distinguishthe origin of suchfeatures in otherancient regions of Mars. 1. Introduction As on Earth, the topographic and slope characteristics of Feature A could provide information about its eruptive history. An unusually symmetricalcone located in the Aeolis region In this paper we use Mars Orbiter Laser Altimeter (MOLA) to- of Mars (20øS, 187øW) with a flat-floored summit crater and dis- pographicdata [Smith•t al., 1998, 1999; Zuber et al., 2000] to tinctive radial dissection of its flanks was first recognized by study the shapesand slope characteristicsof this feature and to Greeley and Spudis [1978] and interpreted to have a volcanic assessits origin. Furthermore, we examine the characteristics origin (Feature A, Plate 1). In their global survey of Martian of other, more degradedtopographic highs nearby to assessthe volcanoes,Hodges and Moore [1994] describedthe featureas a likelihood that they represent the same class of feature, and single, symmetric, crateredcone in the hilly Noachian high- thus have the same origin, as Feature A. lands with a basal diameter of-30 km and a circular summit cra- ter 8 km across. On the basis of its shape and its striking mor- 2. Stratovolcanoes on Earth phological similarity to Earth stratovolcanoes (e.g. Tongariro in New Zealand;Figure 4lB of Hodgesand Moore [1994]), they On Earth, compositevolcanoes or stratovolcanoesare rela- interpreted this feature to be the best candidatein their global tively large constructionalvolcanic featuresbuilt of layers of survey for a composite cone or stratovolcano. Tongariro vol- small-volume lava flows and pyroclastic deposits [Bates and cano is a truncated composite cone composedof andesitic ash Jackson, 1984], unlike shield volcanoes, which are built pre- and lava flows which has subsequentlyundergone flank erosion dominantly of lava flows with a widerrange of lengths. The by glaciers and streams[Gregg, 1960]. greaterexplosiveness of stratovolcanoeruptions relative to FeatureA (Zephyria Tholus) is formed within the hilly unit shield volcano effusions is usually attributed to higher viscos- of the lower Noachian highlandplateau sequence (Nplh), one of ity and/orvolatile contentof stratovolcanomagmas [Cas and the oldest recognized stratigraphic units on Mars [Scott and Wright, 1987, pp. 386-390]. The pyroclastic deposits and Tanaka, 1986]. Nplh consists of rough, hilly, and fractured lava flows associated with stratovolcanoes tend to accumulate material with locally high relief and is inferred to be ancient in the near-vent area, contributing to smaller basal diameter highlandrocks and impact brecciaproduced during the period of (30-100 km) and steeper flank slopes (as high as 35ø) than heavy bombardment. In the Aeolis region, occurrencesof Nplh thosetypical of shieldvolcanoes [Walker, 2000; Davidsonand are surroundedby highly cratered, fractured, and channelized de Silva, 2000]. Terrestrial arc stratovolcanoesare typically 2- material mapped as Npl•, which is thought to consist of a mix- 2.5 km in height, while intraplate structurescan be as tall as ture of volcanic materials,erosional products, and impact brec- 3.7 km [Walker, 2000]. In profile, young stratovolcanoes cias of middle Noachian age [Scott and Tanaka, 1986; Greeley havea simpleconical shape. Older stratovolcanoescommonly and Guest,1987]. FeatureA is thusinterpreted to be a very an- haveconcave upward flank profilesas a result of fluvial dissec- cient structure, remarkably pristine for its age (Plate 1). tion or sector collapse [e.g., Reid et al., 2000]; truncation of Zephyria Tholus is located at the head of Durius Valles, which the upperslopes by calderaformation (by explosion or col- consists of several flat-floored tributaries incised into the lapse) also occurs. Stratovolcanoesare typically found in crateredhighlands of Terra Cimmeria. These tributaries con- chains on the overriding plate of convergent plate margins, verge into a single channel before emptying into De with intervolcanospacings comparable to their diameters(30- Vaucolueurs, a-300 km diameter degraded impact basin 100 km). Exceptionsinclude some edifices found on Iceland [Nelsonet al., 2001a]o Zephyria Tholus may be relatedto the and some associated with the East African Rift. sourceand origin of Durius Valles [Nelson et al., 200lb]. Copyright2001 by the AmericanGeophysical Union. 3. Volcanism and Stratovolcanoes on Mars Papernumber 2000JE001322. Volcanic edifices on Mars include the giant central vent vol- 0148-0227/01/2000JE001322509.00 canoes (e.g. Olympus and Elysium Montes), paterae, tholi, 17,505 17,506 STEWART AND HEAD: ANCIENT MARTIAN VOLCANOES 3000 ,, 10949.1.]•. b. 2000- ? looo.j Verticalexaggeration 10x 0 I • ! ß 3000 11515.1 12433.1 2 000. 1000- ...... 0 ! -'- I I I 3000 12942.2 2000- 1000- 0 '1 ....... I I 3000 .•2000- 11691.1 • 1000- 0 I I ! ! 3000 ........... -•' 11848.2 10452.2 • 2000- o • 1000- 0 • • "/..... 186.75 .r '•::?•:': :71::-.:. ", ..... 184 ........... •::•.•.......•.•4•::•....... '":<.."::::... • ' ........ .: .:-..•-m..•:.':••"•.•.•:•::•:•............ •. '".%:•...: •. .. ......•...• •..:•.... ...... • ß"• ......... '"• -":....' ....... L'4..':...'...:;2 •: ......'...-' :::•..-.-'..--........... ............. ß......... '•4•:•:• ..... o ---- ...............:.. o •::••"::'• .......•::g&:.•...? •-"•'•:•/•-•g• F "' T" :.:'-::.'.'.:-"..•.......:.:."."i'•............. •;•:•::....:;::::..::?:.?•:•:•:•:.::.::..:::..:.:'.......... ...:•:•:..•:• 185 -.- - 19.5 •fitude (de• -20.5 -17.25 Latitude (deg) - 18.25 Figure 1. MOLA altimetry data for several candidatefeatures: (a)MOLA profiles acrossFeature A and (b) MOLA profiles acrossFeature B. Here and in all otherfigures, zero referenceis MOLA-derived mean radius(see Smith et al. [1998, 1999] and Zuber et al. [2000] for details). Points are individual MOLA data points, and numbersrefer to profile number. North is to the left in both images. cones, and small shields [Hodges and Moore, 1994; (e.g., see summaryby Wilson and Head [1994]). There is also Zimbelman, 2000; Greeley et al., 2000]. On the basis of flow extensive evidence for explosive or pyroclastic activity [e.g., morphology and other considerations,most of these features Mouginis-Mark et al., 1982; Greeley and Crown, 1990; Crown are interpretedto be the result of effusive basaltic volcanism and Greeley, 1993; Head and Wilson, 1998; Plescia, 2000]. STEWART AND HEAD: ANCIENT MARTIAN VOLCANOES 17,507 F ature A - Feature B ß ß t .% 3O km ",,•.• -1000 0 rn 1000 2000 3000 4000 "•" ' Plate1. RegionalViking image mosaic overlain on MarsOrbiter Laser Altimeter (MOLA) topography, with superposedimages of thefeatures mapped as Nplh in theAeolis region of Mars.Feature A is ZephyriaTholus. Thisis Vikingimage F430S23 of thecandidate volcano located at 20øS,187øW. Image width is 60 km. Note the flat-flooredsummit crater and the lobatefan depositin the impactcrater to the northeast. FeatureB is a Vikingimage of materialmapped as Nplh located at 18øS,184øW. Note the several domical features with to- pographysimilar to FeaturesA andB locatedto theirsouth (annotated as C1-C5). 17,508 STEWART AND HEAD: ANCIENT MARTIAN VOLCANOES Although Francis and Wood [1982] arguedthat few Martian edi- 4. Topographic Characteristics of Candidate fices could have a pyroclastic origin on the basis of their low Volcanic Features in the Aeolis Region topographicslopes, Wilson and Head [1994] demonstratedthat becauseof the difference in atmospheric pressure and gravity The recent acquisition of accuratetopography information on Mars relative to Earth, pyroclastic deposits should be very for Mars using the Mars Orbiter Laser Altimeter (MOLA) [Smith common on Mars. In addition, volcanic deposits of all kinds et al., 1998, 1999; Zuber et al., 2000] permits the accurate are expectedto travel farther from the source vent than they measurementof the slope and shape of the Aeolis features would on Earth. This would, in turn, result in volcanic con- (Plate 1, Figures 1-3). Here we describe their characteristics, structs on Mars a factor of 2 broader and a factor of 4 shorter similarities, and differences and assess evidence relevant to a than Earth constructs, calling into question the slope argu- possible volcanic origin. ments of Francis and Wood [1982]. Wilson and Head[1994] demonstrated that volatile nuclea- 4.1. Feature A (Zephyria Tholus) tion and the resultant disruption of magma can occur in liquids MOLA data show that FeatureA (Zephyria Tholus; Plate 1, of basaltic composition on Mars because of the low atmos- FeatureA)is a truncatedcone, characterizedby asymmetrical pheric pressure. They noted that elevation would have
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