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New evidence for a magmatic influence on the origin of Valles Marineris, Mars

James M. Dohm a,b,⁎, Jean-Pierre Williams c, Robert C. Anderson c,d, Javier Ruiz e, Patrick C. McGuire f,g,Goro Komatsu h, Alfonso F. Davila i, Justin C. Ferris j, Dirk Schulze-Makuch k, Victor R. Baker a,b,William V. Boynton b, Alberto G. Fairén i, Trent M. Hare l, Hirdy Miyamoto m,Kennth L. Tanaka l, Shawn J. Wheelock a

a Department of Hydrology and Water Resources, University of Arizona, Tucson, AZ85721, USAb Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, 85721, USAc Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USAd Jet Propulsion Laboratory, Pasadena, CA, 91109, USAe Centro de Biología Molecular, CSIC-Universidad Autónoma de Madrid, 28049 Cantoblanco, Madrid, Spainf McDonnell Center for the Space Sciences, Department of Earth and Planetary Sciences, Washington University, St. Louis, 63130, USAg McDonnell Center for the Space Sciences, Department of Physics, Washington University, St. Louis, 63130, USAh International Research School of Planetary Sciences, Università d’Annunzio, 65421 Pescara, Italyi Space Sciences and Astrobiology Division, NASA Ames Research Center, Moffett Field, CA 94035, USAj U.S. Geological Survey, Sacramento, California, 95819, USAk School of Earth and Environmental Sciences, Washington State University, Pullman, WA, 99164, USAl U.S. Geological Survey, Flagstaff, Arizona, 86001, USAm University Museum, University of Tokyo, Tokyo 113-0033, Japan

a b s t r a c ta r t i c l e i n f o

Article history:Received 22 May 2008Accepted 21 November 2008Available online 3 December 2008

Keywords:MarsValles MarinerisTharsisplumesuperplumemagmawatercanyon systemlife

In this paper, we show that the complex geological evolution of Valles Marineris, Mars, has been highlyinfluenced by the manifestation of magmatism (e.g., possible plume activity). This is based on a diversity ofevidence, reported here, for the central part, Melas Chasma, and nearby regions, including uplift, loss of hugevolumes of material, flexure, volcanism, and possible hydrothermal and endogenic-induced outflow channelactivity. Observations include: (1) the identification of a new N50 km-diameter caldera/vent-like feature onthe southwest flank of Melas, which is spatially associated with a previously identified center of tectonicactivity using Viking data; (2) a prominent topographic rise at the central part of Valles Marineris, whichincludes Melas Chasma, interpreted to mark an uplift, consistent with faults that are radial and concentricabout it; (3) HiRISE-identified landforms along the floor of the southeast part of Melas Chasma that areinterpreted to reveal a volcanic field; (4) CRISM identification of sulfate-rich outcrops, which could beindicative of hydrothermal deposits; (5) GRS K/Th signature interpreted as water–magma interactions and/or variations in rock composition; and (6) geophysical evidence that may indicate partial compensation ofthe canyon and/or higher density intrusives beneath it. Long-term magma, tectonic, and water interactions(Late Noachian into the Amazonian), albeit intermittent, point to an elevated life potential, and thus VallesMarineris is considered a prime target for future life detection missions.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Stratigraphic, paleotectonic, spectroscopic, and geophysical infor-mation of Mars, compiled through geologic mapping investigations atglobal to local scales, demonstrate that magmatic-driven processessignificantly contributed to a dynamic geologic history. This may bebest exemplified at Tharsis and its surrounding regions (e.g., Scott andTanaka, 1986), where five major stages of pulse-like geologic activity

resulted in the formation of amagmatic complex, encompassing a totalsurface area of approximately 2×107 km2 (Dohm et al., 2001a, 2007).Tharsis is an order of magnitude larger than the largest terrestrialigneous plateau, Ontong Java, which covers a total surface area ofapproximately 2×106 km2 (Maruyama,1994; Richardson et al., 2000).Tharsis evolution reportedly includes plume-driven activity, possiblyranging from a mantle plume (e.g., Raitala, 1987; Mège and Masson,1996) to a superplume (Maruyamaet al., 2001; Baker et al., 2001, 2007;Dohmet al., 2001b, 2007), aswell as dike emplacement (McKenzie andNimmo,1999; Ernst et al., 2001;Wilson and Head, 2002), and crustal/lithospheric flexure related to its growth (e.g., Golombek, 1989;Banerdt et al., 1992; Phillips et al., 2001). Such activity resulted in

Journal of Volcanology and Geothermal Research 185 (2009) 12–27

⁎ Corresponding author. Department of Hydrology and Water Resources, Universityof Arizona, Tucson, AZ, 85721, USA. Tel.: +1 520 626 8454.

E-mail address: [email protected] (J.M. Dohm).

0377-0273/$ – see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.jvolgeores.2008.11.029

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volcanic constructs of diverse sizes and shapes and extensive lava flowfields (Scott and Tanaka, 1986), large igneous plateaus (Dohm andTanaka, 1999; Dohm et al., 2001c), catastrophic outflow channels(Scott and Tanaka, 1986; Dohm et al., 2001c, 2007), episodicinundations on the northern lowlands (Fairén et al., 2003), putativeash-flow and air-fall deposits (Malin, 1979; Hynek et al., 2003), andsystems of radial faults and circumferential systems of wrinkle ridgesand fold belts (Schultz and Tanaka, 1994) centered about local andregional centers of magmatic-driven activity (Anderson et al., 2001).

One of themost conspicuous features of Tharsis is VallesMarineris,a vast and deep canyon system with Melas Chasma near its center(Figs. 1–3). A variety of processes have been proposed to explain thedevelopment of Valles Marineris, including: (1) rifting (Schultz,1991); (2) collapse of near-surface materials due to the withdrawal ofunderlyingmaterial such as magma (McCauley et al., 1972) or opening

of tension fractures at depth (Tanaka and Golombek, 1989);(3) development of keystone grabens at the crest of a bulge (e.g.,Lucchitta et al., 1992 and references therein); or (4) rotation of theThaumasia plateau (as a lithospheric block) during the Late Noachianor Early Hesperian (Anguita et al., 2001) (there are three periodsdefined for the history of Mars, as follows from oldest to youngest:Noachian, Hesperian, and Amazonian).

To further investigate the origin of Valles Marineris, we mainlyfocus on its central region, which includes Melas Chasma throughapplication of a multidisciplinary approach. Our approach, whichincludes tier-scalable reconnaissance, is likened to the geologist whocompiles the diverse layers of information at various scales for com-parative analysis and ultimate interpretation. Through this approach,tectonic, volcanic, topographic, fluvial, spectral, geophysical, andelemental information in and near Melas Chasma (detailed below)

Fig. 1.MOLA shaded relief map showing that the diameter of the newly identified vent structure (short black arrow) located to the southwest of the central part of Valles Marineris,Melas Chasma (white arrow), approximates the diameter of the caldera complex of Olympus Mons. Also shown are the northeast-trending chain of giant shield volcanoes, TharsisMontes, the vast complex canyon system, Valles Marineris, Warrego rise (W.R.), Warrego Valles (W.V.), the Noachian mountain ranges with complex structure and magneticsignatures, Thaumasia highlands and Coprates rise (C.R.) (Dohm et al., 2001a,c, 2007), a Late Noachian–Early Hesperian tectonic province, Thaumasia Planum (Dohm et al., 2001c),Early Hesperian–Late Hesperian lava plains of Solis and Sinai Planae, and the locations of Noctis Labyrinthus (N.L.), Halex Fossae (H.F.), and Fortuna Fossae (F.F.), the latter two ofwhich are interpreted to mark caldera/vent-like structures. Also shown is the regional context map showing the location of central Valles Marineris (long arrow), Elysium rise, andthe impact basins, Hellas and Argyre in the eastern and western hemispheres, respectively. (For figure legend, the reader is referred to the web version of this article.)

13J.M. Dohm et al. / Journal of Volcanology and Geothermal Research 185 (2009) 12–27


Fig. 2. Modified from Dohm et al. (2001a): (A) MOLA profile (Transect A–A′) across the west-central part of the Thaumasia highlands (hachured pink lines represent scarps, whichare interpreted to be thrust faults), the southeast part of a proposed Noachian drainage basin (queried blue line represents uncertain vertical extent of the basin), including thecentral Valles Marineris rise (center of tectonic activity, interpreted to be the result of magmatic-driven uplift (Dohm et al., 2001a; Anderson et al., 2001), and materials of inferredrim of Chryse impact basin, (B) MOLA shaded relief map showing features of interest, including the central Valles Marineris rise and the approximate boundary of the proposedNoachian drainage basin (dashed blue line), and (C) part of the geologic map of Scott and Tanaka (1986). (For interpretation of the references to color in this figure legend, the readeris referred to the web version of this article.)

Fig. 3. (A) Shuttle Radar TopographyMission (SRTM) digital elevationmodel (DEM) (Farr and Kobrick, 2000) of the Himalayas and Tibetan Plateau. Also shown is the transect line fortopographic profile of (C). (B) MOLA DEM of the Valles Marineris canyon system at the same scale. The transect line for topographic profile of (D) is shown. (C) Topographic profileacross the Tibetan plateau and (D) topographic profile across Valles Marineris centered onMelas Chasma. The zero elevation of the profile for Melas has been shifted to coincide withthe chasma floor and the vertical exaggeration is 80× for both profiles. The comparison shows the massive void in the Martian crust.

14 J.M. Dohm et al. / Journal of Volcanology and Geothermal Research 185 (2009) 12–27


collectively point to magmatic-driven activity such as possible plumemanifestation along a lithospheric zone of weakness playing asignificant role in the development of Valles Marineris. Consequentialto this activity was the interaction of tectonomagmatic processes andwater (e.g., Weitz et al., 2003; Komatsu et al., 2004a; Quantin et al.,2005a) that could have resulted in a diversity of environments con-ducive to the origin and evolution of life.

2. Magmatic–tectonic setting

Tharsis-related activity has structurally dominated the westernequatorial region of Mars since the Noachian Period (Scott and Dohm,1997; Anderson et al., 2001). Associated with Tharsis are enormousradiating systems of tectonic structures, the most conspicuous of

which is the canyon system of Valles Marineris (Figs. 1–3), extendingseveral thousand kilometers across the equatorial region. The systemof canyons is interpreted to occur along a lithospheric zone of weak-ness, resulting frommagmatic activity during the Late Noachian–EarlyHesperian, including uplift, faulting, possible volcanic and hydro-thermal activity (Dohm et al., 1998, 2001a,c, 2007; Komatsu et al.,2004a; Baker et al., 2007), and possible rotation of a lithospheric block(transtensive, dextral movement of the Thaumasia plateau resultingfrom E–W compression, Anguita et al., 2001).

The Late Noachian and Early Hesperian development of the Tharsismagmatic complexwas amajor stage in its evolution (stage 2 activity—Dohm et al., 2001a). In addition to magmatic-driven tectonism andpossible associated volcanic eruptions and hydrothermal activityrelated to the development of the central part of Valles Marineris,

Fig. 4. Using THEMIS day time mosaic (A), radial and concentric faults about Melas Chasma (blue lines), wrinkle ridges (violet lines), narrow ridges (orange lines), and the newlyidentified caldera/vent-like structure (B; white arrows) are highlighted. A merged MOLA and daytime THEMIS map (C) further highlights the newly identified caldera/vent-likestructure. Note that the faults (black arrows) mark magmatic-driven activity associated in space and time with the development of the central Valles Marineris rise (see greenhighlighted faults located along the north-central margin of Fig. 5), which includes a distinct curvilinear annulus with structures concentric about the feature's central part. Alsoshown are other caldera/vent-like structures, Halex Fossae (D; H.F.) and Fortuna Fossae (E; F.F.) for comparison with the newly identified caldera/vent-like structure (see Fig. 1 forlocation with respect shield volcanoes, Olympus Mons and Tharsis Montes, and Valles Marineris), as well as the approximate locations of Figs. 8 and 9 (yellow arrow). (Forinterpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)



activity included continued growth of Syria Planum (including NoctisLabyrinthus) and Warrego rise. Similar to the central part of VallesMarineris, Syria Planum is also a site of long-lived (Noachian throughat least Late Hesperian) magmatic/tectonic activity, which includesuplift and associated radial and concentric faulting, but at a muchlarger scale than is recognized at the central part of Valles Marineris.Magmatic-related activity at Syria Planumand central VallesMarinerismay be genetically associated with the early development of thecircum-Chryse outflowchannel systems (e.g., Dohmet al.,1998, 2001a,d; McKenzie and Nimmo, 1999). The source region of Warrego Valles(Warrego rise) has been interpreted to be a site of intrusive-relateddoming and tectonic and hydrothermal activity also during the LateNoachian/Early Hesperian. This activity is interpreted to have resultedin the formation of thewell-defined valley networks ofWarrego Valles(Gulick, 1993; Dohm and Tanaka, 1999). Precipitation-related dissec-tionmayhave also contributed to its development (Dohmet al., 2001c;Ansam and Mangold, 2006).

The Late Noachian/Early Hesperian development of the Tharsismagmatic complex included the uplift of the Thaumasia plateau andrelated contractional deformation and the formation of complex riftsystems along its eastern and southern margins (Dohm and Tanaka,1999). Collectively, the central part of VallesMarineris (includingMelasChasma), Syria Planum, Warrego rise, and the Thaumasia plateau pointto major activity during the stage 2 growth of Tharsis (Dohm et al.,2001a). This may have included the manifestation of magma bodiessuch as mantle plumes (e.g., Mège and Masson, 1996; Mège and Ernst,2001), dike emplacement (e.g., e.g., McKenzie and Nimmo, 1999; Ernstet al., 2001), and crustal underplating (Dohm and Tanaka, 1999).

A paleotectonic investigation of Tharsis and its surroundings usingViking data identified specific centers of tectonic activity based on theorientations of more than 25,000 radial grabens and concentricwrinkle ridges in time and space using digital structural mapping,quantification, and statistical methodologies (Anderson et al., 2001).The Late Noachian–Early Hesperian (stage 2) centers identified byAnderson et al. (2001) included those located near the central part ofValles Marineris, Syria Planum, and Warrego rise, consistent withother geologic information described above highlighting majormagmatic activity during the Late Noachian/Early Hesperian. Thestage 2 center of tectonic activity of Anderson et al. (2001) locatednear the central part of Valles Marineris spatially registers with aN50 km-diameter feature interpreted to be magmatic in origin usingthe Thermal Emission Imaging System (THEMIS) and the Mars OrbiterLaser Altimeter (MOLA) data, approximating the width of the calderacomplex of Olympus Mons (Figs. 1 and 4).

The feature, originally interpreted to be an impact crater usingViking data (Witbeck et al., 1991), is re-interpreted here to be ofmagmatic origin using the new THEMIS and MOLA data. Themorphology of the feature is clearly distinct from an impact featureor frompit crater chains or other depressions commonly interpreted tomark collapse-dominated processes in the Tharsis region, notnecessarily related to volcanism (Wyrick et al., 2004; Ferrill et al.,2004). Instead, the newly identified feature displays a distinct annulusand structures concentric about its central part, reminiscent of othermagma-related vent structures in the Tharsis region, which includeHalex Fossae (located to the northeast of Olympus Mons) and FortunaFossae (located to the southeast of Ascraeus Mons) (Figs. 1 and 4).Halex Fossae has been interpreted to be a volcano–tectonic structuredue to its arcuate grabens that become more closely spaced toward itscenter and lavas that appear to have issued from some of the arcuatefractures, flowing radially away from the feature's center (Morris,1984). It was also mapped and interpreted by Scott and Tanaka (1986)as a volcano. Similar to Halex Fossae, Fortuna Fossae has beeninterpreted to mark magmatic activity such as the emplacement ofdike swarms (Mège and Ernst, 2001; Ernst et al., 2001). It displaysarcuate grabens and lavas that appear to originate from some of thefractures, similar to that described for Halex Fossae.

The newly interpreted feature is temporally associated with theLate Noachian–Early Hesperian faults that are radial and concentricabout Melas Chasma (Figs. 4–6). Similar to the radial and concentricfaults, the feature is embayed by Early Hesperian (and possiblyyounger) lavas of Sinai and Solis Planae. The faults of the westernmargin of the Thaumasia Planum tectonic province are overlain bylavas from the west in Sinai Planum, where there is very little to noevidence of extensional deformation (Fig. 5). These stratigraphic andcross-cutting relations among lavas and faults delineate a distinctgeologic contact extending from the central part of Valles Marineris tothe Thaumasia highlands mountain range, thousands of kilometers tothe south. The Early Hesperian lavas of Sinai Planum constrainincipient Late Noachian Valles Marineris-related deformation (Dohmand Tanaka, 1999; Dohm et al., 2001c).

The paleotectonic information recorded in the Thaumasia Planumtectonic province points to possiblemanifestation of magmatic-drivenactivity (Scott and Dohm, 1990a; Dohm et al., 2001c), which includesuplift and associated tectonism, volcanism, and possible hydrothermaland flood-related activity (Dohm et al., 2001a,d, 2007). Otherevidence for magmatic activity includes layering in the canyonwalls, which is interpreted to indicate voluminous volcanism(McEwen et al., 1999) and intrusive activity (Williams et al., 2003),and the gravity/topography admittance observed in the region(McGovern et al., 2002, 2004). A distinct topographic rise, identifiedwith topographic data from MOLA (Fig. 2) further supports the pre-MOLA interpretation of magmatic-driven uplift and related deforma-tion for the central part of the canyon system (Dohm et al., 2001a,c,d),as well as landforms observed along the floor of Melas Chasmainterpreted to be volcanic from geologic investigation using MarsOrbiter Camera (MOC) (e.g., Lucchitta, 2002) and the High ResolutionImaging Science Experiment (HiRISE) images (example provided

Fig. 5. Part of the paleotectonic map of the Thaumasia region by Dohm et al. (2001c)using Viking data. The map records the distribution and ages of faults (Noachian—stage1 shown in red; Late Noachian/Early Hesperian—stage 2 shown in green; for stageinformation see Dohm and Tanaka, 1999 and Dohm et al., 2001a,c), as well as wrinkleridge systems, geologic contacts, impact craters (white lines), and channels (blue lines).Note that the tectonic province of Thaumasia Planum records stage 2 tectonism, whichincludes incipient development of the central part of Valles Marineris, interpreted tomark magmatic-driven uplift and associated tectonism. Faults are clearly radial andconcentric about Melas Chasma (white arrow). In Sinai Planum, undeformed, stage 4,older flows of lower member of the Syria Planum Formation (unit Hsl1) bury stage 3,younger ridged plains material (unit Hr) that in turn buries stage 2, rock surfaces andassociated structures of older ridged plains material (unit HNr) along the western partof the Thaumasia Planum province (also see Fig. 1 for regional setting). The youngerEarly Hesperian (stage 3) lavas of Sinai Planum constrain the Late Noachian–EarlyHesperian (stage 2) development of the central part of the canyon system. (Forinterpretation of the references to color in this figure legend, the reader is referred tothe web version of this article.)



below) obtained from the Mars Global Surveyor (MGS) and MarsReconnaissance Orbiter (MRO) spacecraft, respectively.

3. Geologic, mineralogic, and elemental information of centralValles Marineris

Volcanic landforms and various tectonic structures record adynamic internally-driven (endogenic) history, especially in andsurrounding the Tharsis magmatic complex (Scott and Tanaka 1986;Scott and Dohm,1990a,b; Dohm et al., 1998, 2001a, 2007; Tanaka et al.,1998, 2005; Anderson et al., 2001; Komatsu et al., 2004a). Abundantvolcanism, in conjunction with evidence for water/ice-relatedfeatures on the Martian surface (e.g., Baker, 2001) points to elevatedpotential for hydrothermal environments (Carr, 1979; Newsom, 1980;Farmer and Des Marais, 1997; Dohm et al., 1998, 2000, 2001a–c, 2004;Hagerty and Newsom, 2003). Based on diverse criteria, the centralpart of Valles Marineris, Melas Chasma, has been identified as a primecandidate site of hydrothermal activity (Schulze-Makuch et al., 2007).These criteria included: (1) evidence of the action of liquid water;(2) evidence of volcanic constructs and/or lava flows; (3) evidence ofa center of magmatic-driven tectonism; (4) topographic depressionsand/or valleys hypothesized to be the result of structurally controlledcollapse and/or rifting, respectively; (5) geomorphologic evidence ofimpact craters in ice-rich regions; (6) identification of deposits usually

associated with hydrothermal activity, such as sulfates, sulfides, andmetal hydroxides/oxides; and (7) identification of deposits indicativeof water alterations such as hydrated phyllosilicates, the mineralsjarosite and hematite, or relatively high concentrations of specificelements such as chlorine (Cl), potassium (K), and thorium (Th).

Preliminary investigation of the central part of Valles Marineris usinga high-resolution, HiRISE image (spatial resolution approximating0.25m/pixel,McEwen et al., 2007) reveals hillswith summit depressions(possible volcanic vent structures), lineaments (possible faults), flowfeatures (possible lavaflows), and ridges (possibledikes exposed throughdifferential erosion) in the southeast part ofMelas Chasma (Figs. 7 and8).This assemblage of features collectively point to a history of magmaticactivity, which includes the development of a volcanic field. This isconsistent with the magmatic-driven activity previously hypothesizedfor this and other parts of the vast canyon system (Lucchitta et al., 1990,1992; Scott andDohm,1990a; Dohmet al.,1998, 2001a,c; Anderson et al.,2001;Williams et al., 2003). These features display characteristics similarto those of Mid Miocene/Pliocene volcanic fields on Earth wherebasement structural control influenced the subterranean migration ofmagma, as evidenced through differentially eroded vent alignments,plugs, dikes that display trends similar to those of fault systems (Dohm,1995), andwheredifferential erosionhas resulted in inverted topography(e.g., ponding of lavas in topographic lows that are subsequentlydifferentially eroded to form mesas, e.g., Holm, 2001).

Fig. 6. Stage 2 (Late Noachian–Early Hesperian) faults (red lines) on a Viking-based spherical projection (based from Anderson et al., 2001). Note the faults that are radial andconcentric aboutMelas Chasma (see indexmap of Fig.1 for regional setting), marking its incipient development. (For interpretation of the references to color in this figure legend, thereader is referred to the web version of this article.)



An example of basement structural control on volcanism is clearlyevident in the S.P. Crater area of the San Francisco volcanic field,northern Arizona, where elongated volcano and vent alignmentsoccur along faults and/or approximate the trends of faults (Fig. 8). Insuch cases, magmas used basement structures as conduits to reach thesurface (Shoemaker et al., 1978a,b; Dohm, 1995). Unlike the southeastpart of Melas Chasma, however, there are no visible plugs or dikesobserved using the Landsat Satellite image of the San Franciscovolcanic field. This is due to the geologically young age of the SanFrancisco volcanic field. Basaltic volcanism in the volcanic field, forexample, began in the Miocene/Pliocene and continued into the latePleistocene such as in the case of the distinct cylindrical volcano, SPCrater, and its associated pristine lava flow (Fig. 8). The volcanoes havenot been eroded down to their plumbing system (e.g., dikes andplugs), as is typical of the older, Mid Miocene and Pliocene volcanicfields of the Colorado Plateau (e.g., Ulrich et al., 1984; Ulrich andBailey, 1987; Wolfe et al., 1987) where plugs and dikes have beenexposed though differential erosion, often simulating the trends of theregional fault systems (Dohm, 1995).

A similar scenario can be envisioned for the example presented inthe southeast part of Melas Chasma where differential erosion andbasement structural control has resulted in a magmatic-influenced

landscape. The landscape includes exposed plugs and dikes, degradedvolcanic constructs, faults, fault/dike transitions, alignments ofvolcanic constructs, and the spatial associations of volcanoes anddikes, as well as the emplacement of lava flows into topographic lowssuch as valleys. These valleys subsequently became mesas throughdifferential erosion and topographic inversion (Fig. 8) (e.g., also seeHolm (2001) for details on topographic inversion of the Mormon andSan Francisco volcanic fields, northern Arizona).

Located only a few kilometers to the northeast from this proposeddifferentially eroded volcanic field, sulfate-containing layered depositshave been identified using Compact Reconnaissance Imaging Spectro-meter for Mars (CRISM) data (with spatial resolution approximating18 m/pixel using 544 spectral channels and multispectral mappingstrips with lower spatial resolution (~200 m/pixel) using 72 spectralchannels, Murchie et al., 2007a,b) (Fig. 9; also see Pelkey et al. (2007)).A hydrothermal origin for the sulfate-containing outcrops seemsreasonable among other possible interpretations described belowconsidering the close proximity of the layered sediments to theproposed volcanic field detailed in Fig. 8.

The hypothesis of widespread hydrothermal activity within MelasChasma is further reinforced by the general distribution of sulfates andiron-oxide minerals associated with the layered deposits that are

Fig. 7. Using part of the MC-18 (Coprates) Mars 1:5 million-scale map with Gazetteer of Planetary Nomenclature (http://planetarynames.wr.usgs.gov/mgrid_mola.html), shown isthe prime candidate target of hydrothermal activity, Melas Chasma (Schulze-Makuch et al., 2007), where HiRISE and CRISM data in PDS and soon to be PDS archived exist (as ofAugust 31, 2007). Several of the authors of this work provided the HiRISE and CRISM Science Teamswith targeted areas (red, yellow, and violet boxes and corresponding lat. and long.lines of similar color) for the top ten candidate targets identified by Schulze-Makuch et al. (2007), including the Melas Chasma for detailed investigation through HiRISE and CRISMdata. Also shown are footprints of CRISM Multispectral strip observation-MSP at 200 m/pixel resolution (black rectangular outlines), CRISM targeted image (green circle which islarger than image footprint; i.e., FRT = full resolution observation and HRL = half resolution observation), and HiRISE image (red circles, which are larger than image footprint).Green circle also marks the general location of Figs. 8 and 9 and blue box encloses acquired data of the Melas Chasma region. Also note that the feature of Fig. 4, interpreted to bemagmatic in origin, occurs along the southeast margin of Sinai Dorsa (near the southwest corner of the blue box), a distinct system of ridges in the Sinai Planum region. In general, thedorsa of the Tharsis magmatic complex have been interpreted to be associated with magmatic-driven activity, which includes mantle plume doming and associated compressionaltectonism (Raitala, 1987). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)



Fig. 8. (A) Using HiRISE image TRA_000849_1675, features possibly mark magmatic-driven activity (see Figs. 4 and 7 for regional setting information for location within MelasChasma). Also shown are the area highlighted in (B) (black arrow) and some of the features of Fig. 9 (red triangles) covered by the CRISM data. (B) Highlighted are hills with summitdepressions (possible volcanic vent structures; pink arrows), lineaments (possible faults; black arrows), flow features (possible lava flows; red arrows), and ridges (possible dikesexposed through differential erosion; violet arrows) in the southeast part of Melas, all of which are consistent with hypothesized magmatic-driven activity of the canyon andsurrounding regions (Dohm et al., 2001a–d; Anderson et al., 2001; Williams et al., 2003; Scott and Dohm, 1990a,b; Lucchitta et al., 1990, 1992). (C) The features of (B) displaycharacteristics similar to features on Earth where basement structural control influenced the subterranean migration of magma observed using Landsat at 15 m/pixel. The onlydifference here in the SP Mountain area of the San Francisco volcanic field is that there are no dikes visible due to its geologically youthful age (Upper Pliocene to Pleistocene; e.g.,Ulrich and Bailey, 1987; Wolfe et al., 1987; Holm, 2001). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)



exposed in various parts of the Valles Marineris (e.g., Nedell et al., 1987;Lucchitta et al., 1992; Komatsu et al., 1993), consistent with theoutcropping shown in Fig. 9. Spectroscopic evaluations using datafromViking, thePhobos II infrared spectrometer (ISM), theMars ExpressObservatoire pour laMinéralogie, l'Eau, les Glaces, et l'Activité (OMEGA)imaging spectrometer, and the MRO CRISM show that Kieserite(MgSO4·H2O), polyhydrated sulfates, and iron oxides occur in associa-tion with the layered deposits (Geissler et al., 1993; Christensen et al.,2001; Gendrin et al., 2005; Bibring et al., 2005; Roach et al., 2007, 2008;Quantin et al., 2005a,b; Murchie et al., 2007a,b). Sulfate-rich depositsappear over a wide range of elevations (from −4 km in Ius Chasma to+3 km in Candor Chasma) (Gendrin et al., 2005) and provide directevidence of long-lasting and widespread aqueous activity within thecanyons, and possibly prior to their formation.

By analogy with terrestrial geology, five (non-mutually exclusive)hypotheses for the deposition of sulfates are: (1) evaporation ofstanding bodies of water on the surface (Squyres et al., 2004);(2) deposition of volcanic ash followed by reaction with condensedsulfur dioxide- and water-bearing vapors emitted from fumaroles(McCollom and Hynek, 2005); (3) deposition from a turbulent flowof rock fragments, salts, sulfides, brines, and ice produced by me-teorite impact with subsequent weathering by intergranular waterfilms without invoking near-surface aquifers (Knauth et al., 2005);

(4) groundwater circulation of sulfate-rich waters associated withhydrothermal activity (Warren, 1999; Andrews-Hanna et al., 2007;Mangold et al., 2007); and (5) erosion and migration of previouslylayered sulfate evaporites (Fan et al., 2008). Some of the sulfate-richlayered deposits in Melas Chasma are dissected by dense and highlyorganized valley networks as revealed by THEMIS (Quantin et al.,2005a), suggesting that an efficient water cycle existed within VallesMarineris during the Late Hesperian (Hartmann and Neukum, 2001),which possibly included atmospheric precipitation driving a fluvio-lacustrine system (Quantin et al., 2005a).

The source of the sulfur that lead to the formation of the massivesulfate depositswithinVallesMarinerismay be related to the presence ofcrustal sulfide deposits (i.e. pyrite) (Burns and Fisher, 1993; Chevrier etal., 2006). While metal-sulfide deposits have not been directly detectedon the surface of Mars, their presence has been suggested for a long time(Clark et al., 1982; Burns and Fisher, 1990, 1993; Baker et al., 1991;Chevrier et al., 2006). LikeonEarth, pyrite couldhave formedonMars in anumber of environments, typically volcanic and sedimentary. On Earth,pyrite deposits associatedwith volcanic activity often formVolcanogenicMassive Sulfide Deposits (VMSD), which can reach thicknesses of tens tohundreds of meters and extend over thousands of square kilometerswithin the crust (Barrie and Hannington, 1999). Anoxic groundwaterpercolating through the VMSD results in iron and sulfate-rich solutions,

Fig. 9. (Top left) CRISM image HRS0000285D corrected using volcano scan atmospheric correction and Lambertian-reflectance photometric correction. The locations of themineralogical spectra (blue square=12.330S, 69.309W; red square=12.327S, 69.273W; green square=69.247W) and reference spectra (oval symbols), shown in themiddle plot asthick and thin lines, respectively, are marked on the CRISM image. [Color stretch: blue=1.428 µm, stretch=0.23 to 0.33; green=2.298 µm, stretch=0.20 to 0.30; red=2.404 µm,stretch=0.20 to 0.30]. For comparison, red triangles on both Figs. 8 and 9 mark the same features. (Top right) Image of Spectral Summary Parameter (SINDEX) with the spectralsummary flattening routine applied [linear stretch from 0.025 to 0.045]. SINDEX detects convexity at 2.29 µm due to absorptions at 1.9/2.1 µm and 2.4 µm. Importantly, this can bedue to monohydrated or polyhydrated sulfates. The green line in the SINDEX image is for guiding the eye, in order to indicate roughly where the SINDEX value is both relatively highand geographically clustered. The ratios of themineralogical spectra to their reference spectra are shown in the bottomplot. Themineralogical spectrawere chosen because they havehigh SINDEX values. The reference spectrawere chosen to be in the same column as the mineralogical spectra. These spectra, when compared to their reference, show a distinct dropoff at ~2.4 µm and an absorption band at either 1.9 µm or 2.1 µm, which is a hallmark of hydrated sulfates. The dip at ~1 µm may be a calibration artifact. (For interpretation of thereferences to color in this figure legend, the reader is referred to the web version of this article.)



which upon evaporation precipitate mineralogical assemblages thatinclude Fe- and Ca-sulfates and Fe-oxides. These terrestrialmineralogicalassemblages are compatible with the mineralogy of the layered depositswithin Valles Marineris (Burns and Fisher, 1990, 1993; Baker et al., 1991;Chevrier et al., 2004, 2006; Davila et al., 2008).

Therefore, thehypothesis thatmagmatic activityplayeda central rolein the evolution of Valles Marineris, not only fits well with the diversityof tectonic and morphologic features identified within the Chasma, butalso provides a plausible origin for the sulfur required to form massivesulfate deposits within the canyon system. Due to the vast scale of theValles Marineris system, it is possible that magmatic-driven processes,which includes tectonism and hydrothermal activity, as well as salt-related manifestations (e.g., Beyer et al., 2000; Milliken et al., 2007;Montgomery et al., in press), may have operated simultaneously. Aterrestrial example of this integration of processes is observed in theAtacama Desert, Chile, where magmatism, tectonism, fluid migration

along basement structures, the presence of thick sulfate deposits, andsalt diapirism collectively play a key role in its geologic history (ChongDiaz et al., 1999; Riquelme et al., 2002).

Yet another important clue to the history of Melas Chasma and itssurroundings is elemental information acquired through the MarsOdyssey Gamma Ray Spectrometer (GRS) instrument. GRS detectsgamma rays from the surface without any instrument collimation,providing a spatial resolution approximately equal to the altitude of thespacecraft, ~450 km (Boynton et al., 2004, 2007). GRS is providingevidence of substantial elemental differences across the surface of Mars(Boynton et al., 2002, 2004, 2007). Unlike other orbital-based spectro-meters that can detect the mineralogy of the upper micrometer,perspectives of which are often obscured by fine-grained materialssuch as dust, the GRS instrument is sensitive to elemental compositionswithin a few tens of centimeters below the Martian surface and is thuscapable of seeing through any thin layers of fine-grained materials that

Fig. 10. (A) Modified from Taylor et al. (2006). Map of the variation of K/Th on Mars with the central part of VM (black arrow) being one of several regions that record elevated K/Th,interpreted to reflect distinct igneous and/or aqueous alteration signatures (Taylor et al., 2006). (B) 3Dperspective image shows theK/Thmap of Taylor et al. (2006) overMOLA topography(25× exaggeration) looking obliquely to the northwest across Tharsis, which includes the central part of VallesMarineris (yellow arrow). Also highlighted, a geologic contact that separatesthe tectonic province of Thaumasia Planum (Dohmand Tanaka,1999; Dohmet al., 2001c) from the lava plains of Sinai Planum (black arrows; also see Figs.1 and 5) is approximately definedby theGRSK/Thsignature. Inaddition, northwestward fromthe tectonicprovince,K/Th increases towardsMelas (yellowarrow), thecaldera/vent-like structure (pink arrow), andthesourceregions for the outflow channel systems, Kasei (K) andMaja (M) Valles. The K/Th is distinctly low east–southeast of Coprates risemountain range (CR). TheGRS patterns appear consistentwith geologic and geomorphic information (Dohmet al., 2001a,c). (For interpretation of the references to color in this figure legend, the reader is referred to theweb version of this article.)



may be present. GRS data reveal that near-surfacematerials are far fromhomogenous, with distinct elemental signatures across all regions ofMars (with respect to both variations in a single element and establish-ing correlations or differences between elements; Boynton et al., 2007).

Taylor et al. (2006) investigated GRS-based potassium(K)/thorium(Th) since K and Th are potentially useful for assessing the extent ofaqueous alteration, as they and their host minerals fractionatedifferently in aqueous solutions. For example, a neutral pH environ-ment results in a decreased K/Th ratio for residual materials since K ismore mobile than Th, while an acidic pH environment mobilizes bothK and Th. Therefore, under neutral aqueous conditions, K will bereduced in the source region, since some of it was mobilized, resultingin a lower relative K/Th. Consequentially, wherever there has beenponding of this water, there will be an enrichment of K, yielding ahigher ratio. On the other hand, under acidic conditions, both K and Thwill be partly removed from the source rocks and concentrated in theregion of deposition.

The GRS-based map information highlights distinctly higher andlower than the average K/Th regions for Mars (5330±220; see Tayloret al., 2006 for more details), including the central part of VallesMarineris and Thaumasia Planum, respectively (Fig. 10). The formerhas been interpreted as either water–magma interactions and/or lavaflow materials, which intrinsically have a high K/Th, (Taylor et al.,2006). If considered separate of the geomorphic evidence, the K/Thfractionation ratios are unremarkable. Taylor et al. (2006) are explicitin stating that the range of observed values could be ascribed toigneous processes. However, when the K/Th ratios are viewed withinthe context of the geomorphic evidence for aqueous processesoccurring throughout parts of Martian history (Scott et al., 1995;Fairén et al., 2003), it becomes probabilistically unlikely that igneous

processes are solely responsible. Other considerations that add to thecomplexity of interpreting the K/Th signature in the Valles Marinerisregion are also petrologic fractionation issues such as the mineralassemblage of the source region, different degrees of partial melting,and the potential for assimilation of pre-Tharsis crustal materials.

For example, paleotopographic reconstructions based on a synth-esis of published geologic information and high-resolution topogra-phy, including topographic profiles, revealed the potential existence ofan Europe-sized drainage basin/aquifer system in the eastern part ofthe Tharsis region (Fig. 2) during the incipient development of Tharsis(pre-Valles Marineris formation) (Dohm et al., 2001a). As such, thebasinwould have been a catchment for materials (including sedimentand water) that originated from diverse provenances such as theThausmasia highlands mountain range to the south–southwest, laterto be exposed by the formation of VallesMarineris (Fig. 2). In addition,if the location of VallesMarineriswas the central part of a basinprior tothe formation of the canyon system(Dohmet al., 2001a), then it wouldhave been a catchment ofwater enriched in both ions. This is due to theleaching of rock materials in the various geologic provenances thatpartly contributed to the basin (Dohm et al., 2001a,c)

Any interpretation, be it water–magma interactions, igneous rocks,and/or older sedimentary rocks exposed by canyon formation (e.g.,pre-Tharsis sedimentary rocks overlain by sheet lavas and interfinger-ing sedimentary deposits related to the growth of Tharsis; Dohm et al.,2001a), all comprising relatively high K and Th, are consistent with adevelopmental history of Melas Chasma and surroundings influencedby magmatic-driven activity, including magma–water/water–iceinteractions (also see Chapman and Tanaka, 2001, 2002; Komatsuet al., 2004b). For example, even with its regional spatial resolution asnoted above, GRS defines a geological contact that separates rock

Fig. 11. (A) MOLA topography of Valles Marineris and Thaumasia region of Tharsis. The white box shows the region in B and the solid black line is the ground track of observedtopography and gravity in C and D, respectively. (B) Portion of theMOLAmapwith a stretched color scale to emphasize the region of flank uplift aroundMelas Chasma (black arrows).(C) Topographic profile (solid black) transecting the south rim of Melas Chasma. The results of the flexural model are shown assuming initial canyon topography (solid gray) of 25 kmdepth and a diameter of 400 km. The resulting deflection and surface topography for a lithosphere with elastic thicknesses (Te) of 100 km (dot), 200 km (dot-dash), and 300 km(dash) are shown. (D) The observed free-air gravity (solid black) and the bouguer gravity (solid gray) for the same ground track and the resulting gravity from the flexural model forTe=100 km (dot), 200 km (dot-dash), and 300 km (dash). The interior of Melas has a smaller negative gravity signature than predicted from the topography resulting in the positiveBouguer anomaly in the center of the chasma. Themodel gravities also demonstrate a positive increase in gravity toward the center of the chasmawith decreasing Te values due to theupward deflection of the Moho. (For interpretation of the references to color in this figure legend, the reader is referred to theweb version of this article.)



materials of the Thaumasia Planum, a Late Noachian tectonic province(described in Section 2), from onlapping Early Hesperian lavas fromSinai and Solis Planae along the province's westernmargin (Dohm andTanaka, 1999; Dohm et al., 2001c). This could be explained both bydifferences in the geochemistry of the rock materials and/or thedegree ofweathering. K/Th elevates towardsMelas Chasma, the newlyidentified caldera/vent-like structure, and the source regions for theoutflow channel systems, Kasei and Maja Valles (Fig. 10), north-westward from the Thaumasia Planum tectonic province, consistentwith the geological and paleohydrological histories described above.

4. Geophysical setting

Using geological, topographic, and geophysical information, weperformed geophysical modeling to further assess the developmentalhistory of the central part of Valles Marineris and its surroundings,including Melas Chasma. In particular, we evaluated possible mech-anisms that could have resulted in the observed distinct topographicrise (Fig. 2).

The formation of the VallesMarineris canyon systemwould induce aflexural response (Anderson et al., 1999). Flexural flanks are observedaround Melas Chasma (Fig. 11) and provide information on the com-pensation state and formation of the chasma. We modeled the flexuraluplift of a canyon given the removal of an initial volume of crustalmaterial that results in a chasma with approximately the same volumerepresented by Melas Chasma for comparison with the observed rimtopography. A range of initial canyon depths were employed with theinitial cavityhavinga bowl shapewith a constant halfwidth of 200 kmatzero elevation chosen to approximate the dimensions of Melas. Thetrade-off between the effective elastic thickness, Te, and the initial depthof the cavity was then explored. The equation describing the elasticflexure of the lithosphere is (e.g. Watts, 2001):

W kð Þ = − H kð Þ ρc

ρm1 +

Dk4

ρmg

" #−1

;

where W(k) is the wavenumber domain representation of the flexureand H(k) is the wavenumber domain representation of the initialtopography prior to any compensation, ρc and ρm are the crustal andmantle densities taken to be 2900 kg m−3 and 3400 kg m−3,respectively, g is the gravitational acceleration, and D is the flexuralrigidity:

D =ET3

e

12 1− m2� � ;

which is a function of the elastic thickness, Te, Young's modulus,E=1011 N m−2, and Poisson's ratio, ν=0.25. The resulting deflectionfor Te=100 km, 200 km, and 300 km are shown in Fig. 11. The lowervalue used for the effective elastic thickness is representative of theresults obtained byMcGovern et al. (2004) for various parts of theVallesMarineris region; the higher value is taken here as a conservative upperlimit because themajorityof values presentedbyMcGovernet al. (2004)are lower limits.

The resulting free-air gravity is determined by:

G kð Þsurface = 2πGρc H kð Þ + W kð Þð Þ;

G kð ÞMoho = 2πG ρm − ρcð ÞW kð Þ exp −ktcrð Þ;

where G(k)surface and G(k)Moho are the wavenumber domain repre-sentation of the gravity at the surface and Moho, respectively, and thetotal G(k)=G(k)surface+G(k)Moho. G is the gravitational constant, andtcr is the crustal thickness assumed here to be 50 km.

The results are shown in Fig.11 alongwith the observed free-air andBouguer gravity derived from theMGS95J spherical harmonicmodel ofthe Mars gravity field (Konopliv et al., 2006); the model is the global

solution for the Mars gravity field derived from the tracking data ofMGS, Odyssey, Pathfinder, and Viking Lander 1. The modeled gravityhas been low pass filtered to reflect the resolution of the observedgravity field. The flexure model results in elevated topography andgravity on the canyon flanks, andwithin the canyon, a positive increasein topography and gravity at the center resulting from the upwarddeflection of the surface and Moho with compensation. Within MelasChasma, the amplitude of the negative gravity predicted fromtopography is ~300 mGal larger than observed. This is reflected inthe positive ~300 mGal Bouguer anomaly within the chasma andimplies that some upward flexural compensation of the chasma hasoccurred and/or that material of higher density is present within thecrust. There is a trade-off between the initial depth of the modeledcavity and the Te that result in a post-flexural surface and gravitysimilar to that observed for Melas Chasma. This is shown in Fig. 12where the differences between thefinalmodeled topographies and theobserved topography at the center of the chasma (represented bydistance 0 in Fig. 11) are plotted for values of Te=100–300 km atincrements of 50 km. A small Te requires a deeper initial cavity as theresulting deflection of lithosphere is more prominent. Conversely,increasing Te requires a smaller initial topographic amplitude as thegreater elastic strength subdues the lithospheric response to the cavity.Low values of Te are not consistent with the observed free-air gravity(Fig. 11B). For example in Fig. 11, at Te=100 km, no values for initialdepth provide a good fit to the data as the deflection of the Moho issignificant. The model topography and gravity are both similar to theobserved values for Te~200 km only and an initial cavity depth ~25–30 km. This results in flank uplift of ~8 km.

If the current configuration of the canyon is due to the flexuralresponse of the lithosphere to a chasm, the initial cavity, requiring adepth of 25–30 km, would require the removal of material thatexceeds the volume of the present-day canyon. Its current depthranges from 8–10 km, thus the erosion of ~5 km of additional materialfrom the rim to subdue the topography to its current observed formwould be required. If Melas was the site of focused magmatic activitysuch as the possible surface manifestation of a mantle plume, thenintrusive igneous material in the underlying crust may represent aregion of elevated density within the chasma. This would diminish themagnitude of the flexural response.

Fig. 12. Differences between A) modeled surface topography (post-flexure) andobserved topography, and B) modeled and observed free-air gravity at the center of theChasma. Models with Te~200 km (arrows) provide similar values to both the observedtopography and gravity.



Valles Marineris represents a massive void (canyon-formingremoval of materials) in the Martian crust. For comparison, Earth'shighest mountain system, the Himalaya Range, forms a 2400 km arcwhich varies in width from 150 km to 400 km and contains over 100mountains exceeding 7200 m (Qinye and Du, 2004). A large part ofthis mountain rangewould fit within Valles Marineris (Fig. 3). Bulgingof the crust may have been present prior to formation of the chasmadue to elevated heat flow and dynamic forcing by magmatic-drivenactivity (e.g., mantle plume as it impinged on the bottom of thelithosphere). The volume of Melas Chasma however represents asignificant deficit of mass in the crust. The lithostatic pressure at 8 kmdepth is ~85 MPa and the removal of such overburden would likelydominate the subsequent flexural signal.

There is some geophysical evidence favoring the existence ofintrusive bodies beneathVallesMarineris. The study ofMcGovern et al.(2002, 2004) of Hebes, Candor, and Capri chasma found that thegravity/topography admittance signal of these chasma can befitted formodels with high effective elastic thickness (higher than 60, 120, and110 km for Hebes, Candor and Capri chasma, respectively) and a sig-nificant component of subsurface loading emplaced at the crust–mantle boundary. A plausible explanation for this subsurface loadingwould be the presence of high density intrusives in the lower crust(McGovern et al., 2002), in accordance with some scenarios proposedfor the formation of Valles Marineris (e.g., Mège and Masson, 1996;Dohm et al., 1998, 2001a; McKenzie and Nimmo, 1999). Alternatively,models without subsurface loading would involve a substantiallylower crustal density in this region than the average value for theMartian crust (McKenzie et al., 2002;McGovern et al., 2002). However,McGovern et al. (2002) consider this possibility as unlikely due to theextensive presence of basalticmaterials (which are relatively dense) inthis region (McEwen et al., 1999; Williams et al., 2003).

The best fit component of bottom loading is 0.4–0.7 for HebesChasma, 0.5 for Candor Chasma, and 0.2–0.3 for Capri Chasma(McGovern et al., 2004). McGovern et al. (2002, 2004) consider thesevalues as consistent with decreasing dike amount with increasingdistance from central Tharsis. Significantly, they are also consistentwithintrusive activity centered in central Valles Marineris. In a similar sense,modeling of the residual gravity results in a weak positive anomaly inMelas Chasma, which could be caused by intrusive materials (Kiefer,2006).However, anomalies are not observed in Ius and Coprates chasma(west and east of Melas Chasma, respectively).

Magmatic activity may have been related to elevated heat flow inthe Valles Marineris region. The high effective elastic thickness of thelithosphere obtained for this region indicate relatively low heat flowsfor the Late Hesperian/Early Amazonian (McGovern et al., 2004), thetime when the present-day topography was established. However,local high flows are not disproved by these results, and geological orgeophysical indicators of heat flow for epochs before the LateHesperian/Early Amazonian could have been obscured or eliminatedby the further development of Valles Marineris.

On the other hand, there is a certain degree of overlap between theheat flows deduced (from the effective elastic thickness) for thecentral Valles Marineris region with those obtained for other parts ofTharsis. Alternatively, central Tharsis-related intrusive activity (e.g.,dike emplacement sourcing from the Tharsis Montes or Syria Planumregions) is also consistent with high effective elastic thickness, since,in this case, the generation of melt would occur further away from thecentral part of Valles Marineris.

5. Discussion

Valles Marineris represents a massive void in the Martian crust,enough to include a large part of the Himalaya Range. What was thecause of such a massive void? Certainly, a distinct topographic riseoccurs where Viking-era geologic investigations indicated potentialuplift in and surrounding the central part of VallesMarineris (Scott and

Dohm1990a; Dohmet al., 2001a). Geophysical analysis indicates that apositive Bouguer anomaly within the chasma is likely the result ofupward compensation of the chasma. Yet bulging of the crust prior toformationof the chasmadue to elevated heatflowanddynamic forcingby a possible mantle plume cannot be ruled out. The tremendous voidin the crust makes it exceedingly difficult to assess the canyon'sincipient development (Kiefer, 2006). Other geophysical considera-tions are consistent with intrusive activity centered in central VallesMarineris (McGovern et al., 2002, 2004).

But again, where did all the materials from the formation of such avast canyon system go? Was there a removal of tremendous volumesof subterranean volatiles (e.g., water) and magma? Several workspoint to Tharsis-driven floods of enormous proportions, floodwatersof which inundated the northern plains to form water bodies rangingin size from lakes to oceans (Baker et al., 1991; Dohm et al., 1998,2001a,d; McKenzie and Nimmo,1999; Fairén et al., 2003), transferringdeep crustal rock materials to the northern plains (Baker et al., 2007;Dohm et al., 2008). So is the release of tremendous amounts of waterfrom a Europe-sized basin (Dohm et al., 2001a) an explanation for thetremendous void? In addition, during periods of long-term Tharsisquiescence, were evaporite deposits (Dohm et al., 2001a) sufficientenough to allow for the manifestation of salt diapirism, at least as apart of the geological evolution of Valles Marineris (Milliken et al.,2007)? These and many more questions may be only addressedthrough eventual in-situ investigation and tier-scalable, smart roboticreconnaissance (Fink et al., 2005).

Importantly, no single observation provides a definitive answer towhether the geological history of Valles Marineris is dominated bymagmatic-driven processes. Collectively, however, the diverse observa-tions presented here suggest this to be a possibility. These include thegeologic, paleotectonic, and topographic signatures of magmatic-drivenuplift (Scott andDohm,1990a; Dohmet al.,1998, 2001a; Anderson et al.,2001;), promontories interpreted to be associated with volcanismwithin Valles Marineris (Lucchitta, 1990; Lucchitta et al., 1992), and theidentified N50 km-diameter structure shown in Figs.1 and 4.Magmatic-driven activity as a major influence on the development of the VallesMarineris, which includes possible hydrothermal activity, is furthersupported by both the spectral identification of water-related miner-alogies, such as hydrated sulfates, in the same region that displayslandforms interpreted to be volcanic in origin (Figs. 7 and 8), as well asthe relatively high GRS-based K/Th signature when compared to theregional average (Fig. 10).

6. Summary and implications

Rifting, magma withdrawal, and tension fracturing have beenproposed as possible processes involved in the initiation and develop-ment of the vast canyon system (Lucchitta et al., 1992). Lucchitta et al.(1992) noted that the depth of the large troughs may have been causedby: (1) collapse of near-surface materials due to withdrawal of under-lyingmaterial or openingof tension fractures at depth; (2) developmentof keystone grabens at the crest of a bulge; or (3) failure and subsequentdrifting of plates. Many of the faults associated with Valles Marinerismay have been associatedwith volcanic activity (Lucchitta,1990).Manyof these hypotheses regarding the formation of VallesMarinerismay notbemutually exclusive, as indicated by key pieces of information such as:(1) the tectonic center of activity identified by Anderson et al. (2001);(2) the central Valles Marineris rise shown by Dohm et al. (2001a); (3)the faults that are radial and concentric about Melas Chasma (Scott andDohm, 1990a; Dohm et al., 2001c); (4) the landforms interpreted to bevolcanic in origin in the canyons (Lucchitta, 1990; Lucchitta et al., 1992),including the features identified in the southeast part of Melas Chasmausing HiRISE data that collectively point to a volcanic field; (5) sulfate-enriched interior deposits, whichwere observed in the southeast part ofMelas Chasma (in close proximity to the possible volcanic field of (4));(6) the newly identified structure interpreted to be volcanic that is



spatially associated with the Late Noachian–Early Hesperian centerdefined by Anderson et al. (2001); and (7) a distinctly higher thanaverage K/Th signature (Taylor et al., 2006). Magmatic activity alongbasement structures appears to be a significant contributor to thedevelopment of Valles Marineris, as evidenced by geological investiga-tion, which includes geophysical analysis. This is particularly likelyduring its incipient development, which may have included Tharsis-driven activity, including the manifestations of possible plumes. Thecoexistence of relatively abundant liquid water and intense andwidespread magmatic activity highlighted in and near its central partalso implies a high potential for life, which ought to be regarded as aprime target for future life detection missions.

Acknowledgements

The authors are indebted to two technical reviewers, AlvaroMarquezand Patrick McGovern, as well as two U.S. Geological Survey internalreviewers, James Skinner and an anonymous reviewer, for theirthoughtful reviews, which have resulted in a significantly improvedmanuscript. James Dohm was supported by the NASA Mars DataAnalysis Program, JavierRuizbya contract I3Pwith theCSIC, co-financedfrom the European Social Fund, Jean-Pierre Williams by the CaliforniaInstitute of Technology through the O. K. Earl Postdoctoral Fellowshipand the National Science Foundations Astronomy and AstrophysicsResearch Grants program (AST-0709151), Patrick McGuire by a RobertM.Walker senior research fellowship from theMcDonnell Center for theSpace Sciences, and by NASA funds through the Applied PhysicsLaboratory, under subcontract from the Jet Propulsion Laboratory (JPLContract #1277793), and both Alberto G. Fairén and Alfonso F. Davilawere supported by the Oak Ridge Associated Universities and the NASAPost-Doctoral Program. We would also express our gratitude to theGamma Ray Spectrometer Team whose diligent efforts have yieldedtremendous fruit.

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