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

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Journal of Volcanology and Geothermal Research

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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, USA b Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, 85721, USA c Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA d Jet Propulsion Laboratory, Pasadena, CA, 91109, USA e Centro de Biología Molecular, CSIC-Universidad Autónoma de Madrid, 28049 Cantoblanco, Madrid, Spain f McDonnell Center for the Space Sciences, Department of Earth and Planetary Sciences, Washington University, St. Louis, 63130, USA g McDonnell Center for the Space Sciences, Department of Physics, Washington University, St. Louis, 63130, USA h International Research School of Planetary Sciences, Università d’Annunzio, 65421 Pescara, Italy i Space Sciences and Astrobiology Division, NASA Ames Research Center, Moffett Field, CA 94035, USA j U.S. Geological Survey, Sacramento, California, 95819, USA k School of Earth and Environmental Sciences, Washington State University, Pullman, WA, 99164, USA l U.S. Geological Survey, Flagstaff, Arizona, 86001, USA m University Museum, University of Tokyo, Tokyo 113-0033, Japan article info abstract

Article history: In this paper, we show that the complex geological evolution of Valles Marineris, Mars, has been highly Received 22 May 2008 influenced by the manifestation of magmatism (e.g., possible plume activity). This is based on a diversity of Accepted 21 November 2008 evidence, reported here, for the central part, Melas Chasma, and nearby regions, including uplift, loss of huge Available online 3 December 2008 volumes of material, flexure, volcanism, and possible hydrothermal and endogenic-induced outflow channel activity. Observations include: (1) the identification of a new N50 km-diameter caldera/vent-like feature on Keywords: fl fi Mars the southwest ank of Melas, which is spatially associated with a previously identi ed center of tectonic Valles Marineris activity using Viking data; (2) a prominent topographic rise at the central part of Valles Marineris, which Tharsis includes Melas Chasma, interpreted to mark an uplift, consistent with faults that are radial and concentric plume about it; (3) HiRISE-identified landforms along the floor of the southeast part of Melas Chasma that are superplume interpreted to reveal a volcanic field; (4) CRISM identification of sulfate-rich outcrops, which could be magma indicative of hydrothermal deposits; (5) GRS K/Th signature interpreted as water–magma interactions and/ water or variations in rock composition; and (6) geophysical evidence that may indicate partial compensation of canyon system the canyon and/or higher density intrusives beneath it. Long-term magma, tectonic, and water interactions life (Late Noachian into the Amazonian), albeit intermittent, point to an elevated life potential, and thus Valles Marineris is considered a prime target for future life detection missions. © 2008 Elsevier B.V. All rights reserved.

1. Introduction resulted in the formation of a magmatic complex, encompassing a total surface area of approximately 2×107 km2 (Dohm et al., 2001a, 2007). Stratigraphic, paleotectonic, spectroscopic, and geophysical infor- Tharsis is an order of magnitude larger than the largest terrestrial mation of Mars, compiled through geologic mapping investigations at igneous plateau, Ontong Java, which covers a total surface area of global to local scales, demonstrate that magmatic-driven processes approximately 2×106 km2 (Maruyama, 1994; Richardson et al., 2000). significantly contributed to a dynamic geologic history. This may be Tharsis evolution reportedly includes plume-driven activity, possibly best exemplified at Tharsis and its surrounding regions (e.g., Scott and ranging from a mantle plume (e.g., Raitala, 1987; Mège and Masson, Tanaka, 1986), where five major stages of pulse-like geologic activity 1996) to a superplume (Maruyama et al., 2001; Baker et al., 2001, 2007; Dohm et al., 2001b, 2007), as well as dike emplacement (McKenzie and Nimmo, 1999; Ernst et al., 2001; Wilson and Head, 2002), and crustal/ ⁎ Corresponding author. Department of Hydrology and Water Resources, University fl of Arizona, Tucson, AZ, 85721, USA. Tel.: +1 520 626 8454. lithospheric exure related to its growth (e.g., Golombek, 1989; E-mail address: [email protected] (J.M. Dohm). Banerdt et al., 1992; Phillips et al., 2001). Such activity resulted in

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

J.M. Dohm et al. / Journal of Volcanology and Geothermal Research 185 (2009) 12–27 13 volcanic constructs of diverse sizes and shapes and extensive lava flow of tension fractures at depth (Tanaka and Golombek, 1989); fields (Scott and Tanaka, 1986), large igneous plateaus (Dohm and (3) development of keystone grabens at the crest of a bulge (e.g., Tanaka, 1999; Dohm et al., 2001c), catastrophic outflow channels Lucchitta et al., 1992 and references therein); or (4) rotation of the (Scott and Tanaka, 1986; Dohm et al., 2001c, 2007), episodic Thaumasia plateau (as a lithospheric block) during the Late Noachian inundations on the northern lowlands (Fairén et al., 2003), putative or Early Hesperian (Anguita et al., 2001) (there are three periods ash-flow and air-fall deposits (Malin, 1979; Hynek et al., 2003), and defined for the history of Mars, as follows from oldest to youngest: systems of radial faults and circumferential systems of wrinkle ridges Noachian, Hesperian, and Amazonian). and fold belts (Schultz and Tanaka, 1994) centered about local and To further investigate the origin of Valles Marineris, we mainly regional centers of magmatic-driven activity (Anderson et al., 2001). focus on its central region, which includes Melas Chasma through One of the most conspicuous features of Tharsis is Valles Marineris, application of a multidisciplinary approach. Our approach, which a vast and deep canyon system with Melas Chasma near its center includes tier-scalable reconnaissance, is likened to the geologist who (Figs. 1–3). A variety of processes have been proposed to explain the compiles the diverse layers of information at various scales for com- development of Valles Marineris, including: (1) rifting (Schultz, parative analysis and ultimate interpretation. Through this approach, 1991); (2) collapse of near-surface materials due to the withdrawal of tectonic, volcanic, topographic, fluvial, spectral, geophysical, and underlying material such as magma (McCauley et al., 1972) or opening elemental 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, Tharsis Montes, the vast complex canyon system, Valles Marineris, Warrego rise (W.R.), Warrego Valles (W.V.), the Noachian mountain ranges with complex structure and magnetic signatures, 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 of which 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, and the 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.) Author's personal copy

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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, which are 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 the central 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 inferred rim 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 proposed Noachian 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 reader is referred to the web version of this article.)

Fig. 3. (A) Shuttle Radar Topography Mission (SRTM) digital elevation model (DEM) (Farr and Kobrick, 2000) of the Himalayas and Tibetan Plateau. Also shown is the transect line for topographic 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 profile across the Tibetan plateau and (D) topographic profile across Valles Marineris centered on Melas Chasma. The zero elevation of the profile for Melas has been shifted to coincide with the chasma floor and the vertical exaggeration is 80× for both profiles. The comparison shows the massive void in the Martian crust. Author's personal copy

J.M. Dohm et al. / Journal of Volcanology and Geothermal Research 185 (2009) 12–27 15 collectively point to magmatic-driven activity such as possible plume which is the canyon system of Valles Marineris (Figs. 1–3), extending manifestation along a lithospheric zone of weakness playing a several thousand kilometers across the equatorial region. The system significant role in the development of Valles Marineris. Consequential of canyons is interpreted to occur along a lithospheric zone of weak- to this activity was the interaction of tectonomagmatic processes and ness, resulting from magmatic activity during the Late Noachian–Early water (e.g., Weitz et al., 2003; Komatsu et al., 2004a; Quantin et al., Hesperian, including uplift, faulting, possible volcanic and hydro- 2005a) that could have resulted in a diversity of environments con- thermal activity (Dohm et al., 1998, 2001a,c, 2007; Komatsu et al., ducive to the origin and evolution of life. 2004a; Baker et al., 2007), and possible rotation of a lithospheric block (transtensive, dextral movement of the Thaumasia plateau resulting 2. Magmatic–tectonic setting from E–W compression, Anguita et al., 2001). The Late Noachian and Early Hesperian development of the Tharsis Tharsis-related activity has structurally dominated the western magmatic complex was a major stage in its evolution (stage 2 activity— equatorial region of Mars since the Noachian Period (Scott and Dohm, Dohm et al., 2001a). In addition to magmatic-driven tectonism and 1997; Anderson et al., 2001). Associated with Tharsis are enormous possible associated volcanic eruptions and hydrothermal activity radiating systems of tectonic structures, the most conspicuous of related 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 newly identified caldera/vent-like structure (B; white arrows) are highlighted. A merged MOLA and daytime THEMIS map (C) further highlights the newly identified caldera/vent-like structure. 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 green highlighted 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. Also shown 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 for location 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). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Author's personal copy

16 J.M. Dohm et al. / Journal of Volcanology and Geothermal Research 185 (2009) 12–27 activity included continued growth of Syria Planum (including Noctis The newly interpreted feature is temporally associated with the Labyrinthus) and Warrego rise. Similar to the central part of Valles Late Noachian–Early Hesperian faults that are radial and concentric Marineris, Syria Planum is also a site of long-lived (Noachian through about Melas Chasma (Figs. 4–6). Similar to the radial and concentric at least Late Hesperian) magmatic/tectonic activity, which includes faults, the feature is embayed by Early Hesperian (and possibly uplift and associated radial and concentric faulting, but at a much younger) lavas of Sinai and Solis Planae. The faults of the western larger scale than is recognized at the central part of Valles Marineris. margin of the Thaumasia Planum tectonic province are overlain by Magmatic-related activity at Syria Planum and central Valles Marineris lavas from the west in Sinai Planum, where there is very little to no may be genetically associated with the early development of the evidence of extensional deformation (Fig. 5). These stratigraphic and circum-Chryse outflow channel systems (e.g., Dohm et al.,1998, 2001a, cross-cutting relations among lavas and faults delineate a distinct d; McKenzie and Nimmo, 1999). The source region of Warrego Valles geologic contact extending from the central part of Valles Marineris to (Warrego rise) has been interpreted to be a site of intrusive-related the Thaumasia highlands mountain range, thousands of kilometers to doming and tectonic and hydrothermal activity also during the Late the south. The Early Hesperian lavas of Sinai Planum constrain Noachian/Early Hesperian. This activity is interpreted to have resulted incipient Late Noachian Valles Marineris-related deformation (Dohm in the formation of the well-defined valley networks of Warrego Valles and Tanaka, 1999; Dohm et al., 2001c). (Gulick, 1993; Dohm and Tanaka, 1999). Precipitation-related dissec- The paleotectonic information recorded in the Thaumasia Planum tion may have also contributed to its development (Dohm et al., 2001c; tectonic province points to possible manifestation of magmatic-driven Ansam and Mangold, 2006). activity (Scott and Dohm, 1990a; Dohm et al., 2001c), which includes The Late Noachian/Early Hesperian development of the Tharsis uplift and associated tectonism, volcanism, and possible hydrothermal magmatic complex included the uplift of the Thaumasia plateau and and flood-related activity (Dohm et al., 2001a,d, 2007). Other related contractional deformation and the formation of complex rift evidence for magmatic activity includes layering in the canyon systems along its eastern and southern margins (Dohm and Tanaka, walls, which is interpreted to indicate voluminous volcanism 1999). Collectively, the central part of Valles Marineris (including Melas (McEwen et al., 1999) and intrusive activity (Williams et al., 2003), Chasma), Syria Planum, Warrego rise, and the Thaumasia plateau point and the gravity/topography admittance observed in the region to major activity during the stage 2 growth of Tharsis (Dohm et al., (McGovern et al., 2002, 2004). A distinct topographic rise, identified 2001a). This may have included the manifestation of magma bodies with topographic data from MOLA (Fig. 2) further supports the pre- such as mantle plumes (e.g., Mège and Masson, 1996; Mège and Ernst, MOLA interpretation of magmatic-driven uplift and related deforma- 2001), dike emplacement (e.g., e.g., McKenzie and Nimmo, 1999; Ernst tion for the central part of the canyon system (Dohm et al., 2001a,c,d), et al., 2001), and crustal underplating (Dohm and Tanaka, 1999). as well as landforms observed along the floor of Melas Chasma A paleotectonic investigation of Tharsis and its surroundings using interpreted to be volcanic from geologic investigation using Mars Viking data identified specific centers of tectonic activity based on the Orbiter Camera (MOC) (e.g., Lucchitta, 2002) and the High Resolution orientations of more than 25,000 radial grabens and concentric Imaging Science Experiment (HiRISE) images (example provided wrinkle 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 by Anderson et al. (2001) included those located near the central part of Valles Marineris, Syria Planum, and Warrego rise, consistent with other geologic information described above highlighting major magmatic activity during the Late Noachian/Early Hesperian. The stage 2 center of tectonic activity of Anderson et al. (2001) located near the central part of Valles Marineris spatially registers with a N50 km-diameter feature interpreted to be magmatic in origin using the Thermal Emission Imaging System (THEMIS) and the Mars Orbiter Laser Altimeter (MOLA) data, approximating the width of the caldera complex of Olympus Mons (Figs. 1 and 4). The feature, originally interpreted to be an impact crater using Viking data (Witbeck et al., 1991), is re-interpreted here to be of magmatic origin using the new THEMIS and MOLA data. The morphology of the feature is clearly distinct from an impact feature or from pit crater chains or other depressions commonly interpreted to mark collapse-dominated processes in the Tharsis region, not necessarily related to volcanism (Wyrick et al., 2004; Ferrill et al., 2004). Instead, the newly identified feature displays a distinct annulus and structures concentric about its central part, reminiscent of other Fig. 5. Part of the paleotectonic map of the Thaumasia region by Dohm et al. (2001c) magma-related vent structures in the Tharsis region, which include using Viking data. The map records the distribution and ages of faults (Noachian—stage 1 shown in red; Late Noachian/Early Hesperian—stage 2 shown in green; for stage Halex Fossae (located to the northeast of Olympus Mons) and Fortuna information see Dohm and Tanaka, 1999 and Dohm et al., 2001a,c), as well as wrinkle Fossae (located to the southeast of Ascraeus Mons) (Figs. 1 and 4). ridge systems, geologic contacts, impact craters (white lines), and channels (blue lines). Halex Fossae has been interpreted to be a volcano–tectonic structure Note that the tectonic province of Thaumasia Planum records stage 2 tectonism, which due to its arcuate grabens that become more closely spaced toward its includes incipient development of the central part of Valles Marineris, interpreted to center and lavas that appear to have issued from some of the arcuate mark magmatic-driven uplift and associated tectonism. Faults are clearly radial and fl concentric about Melas Chasma (white arrow). In Sinai Planum, undeformed, stage 4, fractures, owing radially away from the feature's center (Morris, older flows of lower member of the Syria Planum Formation (unit Hsl1) bury stage 3, 1984). It was also mapped and interpreted by Scott and Tanaka (1986) younger ridged plains material (unit Hr) that in turn buries stage 2, rock surfaces and as a volcano. Similar to Halex Fossae, Fortuna Fossae has been associated structures of older ridged plains material (unit HNr) along the western part interpreted to mark magmatic activity such as the emplacement of of the Thaumasia Planum province (also see Fig. 1 for regional setting). The younger Early Hesperian (stage 3) lavas of Sinai Planum constrain the Late Noachian–Early dike swarms (Mège and Ernst, 2001; Ernst et al., 2001). It displays Hesperian (stage 2) development of the central part of the canyon system. (For arcuate grabens and lavas that appear to originate from some of the interpretation of the references to color in this figure legend, the reader is referred to fractures, similar to that described for Halex Fossae. the web version of this article.) Author's personal copy

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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 and concentric about Melas Chasma (see index map of Fig.1 for regional setting), marking its incipient development. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) below) obtained from the Mars Global Surveyor (MGS) and Mars associated with hydrothermal activity, such as sulfates, sulfides, and Reconnaissance Orbiter (MRO) spacecraft, respectively. metal hydroxides/oxides; and (7) identification of deposits indicative of water alterations such as hydrated phyllosilicates, the minerals 3. Geologic, mineralogic, and elemental information of central jarosite and hematite, or relatively high concentrations of specific Valles Marineris elements such as chlorine (Cl), potassium (K), and thorium (Th). Preliminary investigation of the central part of Valles Marineris using Volcanic landforms and various tectonic structures record a a high-resolution, HiRISE image (spatial resolution approximating dynamic internally-driven (endogenic) history, especially in and 0.25 m/pixel, McEwen et al., 2007) reveals hills with summit depressions surrounding the Tharsis magmatic complex (Scott and Tanaka 1986; (possible volcanic vent structures), lineaments (possible faults), flow Scott and Dohm,1990a,b; Dohm et al.,1998, 2001a, 2007; Tanaka et al., features (possible lava flows), and ridges (possible dikes exposed through 1998, 2005; Anderson et al., 2001; Komatsu et al., 2004a). Abundant differential erosion) in the southeast part of Melas Chasma (Figs. 7 and 8). volcanism, in conjunction with evidence for water/ice-related This assemblage of features collectively point to a history of magmatic features on the Martian surface (e.g., Baker, 2001) points to elevated activity, which includes the development of a volcanic field. This is potential for hydrothermal environments (Carr, 1979; Newsom, 1980; consistent with the magmatic-driven activity previously hypothesized Farmer and Des Marais, 1997; Dohm et al., 1998, 2000, 2001a–c, 2004; for this and other parts of the vast canyon system (Lucchitta et al., 1990, Hagerty and Newsom, 2003). Based on diverse criteria, the central 1992; Scott and Dohm, 1990a; Dohm et al., 1998, 2001a,c; Anderson et al., part of Valles Marineris, Melas Chasma, has been identified as a prime 2001; Williams et al., 2003). These features display characteristics similar candidate site of hydrothermal activity (Schulze-Makuch et al., 2007). to those of Mid Miocene/Pliocene volcanic fields on Earth where These criteria included: (1) evidence of the action of liquid water; basement structural control influenced the subterranean migration of (2) evidence of volcanic constructs and/or lava flows; (3) evidence of magma, as evidenced through differentially eroded vent alignments, a center of magmatic-driven tectonism; (4) topographic depressions plugs, dikes that display trends similar to those of fault systems (Dohm, and/or valleys hypothesized to be the result of structurally controlled 1995), and where differential erosion has resulted in inverted topography collapse and/or rifting, respectively; (5) geomorphologic evidence of (e.g., ponding of lavas in topographic lows that are subsequently impact craters in ice-rich regions; (6) identification of deposits usually differentially eroded to form mesas, e.g., Holm, 2001). Author's personal copy

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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 is the 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 of August 31, 2007). Several of the authors of this work provided the HiRISE and CRISM Science Teams with 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 CRISM data. Also shown are footprints of CRISM Multispectral strip observation-MSP at 200 m/pixel resolution (black rectangular outlines), CRISM targeted image (green circle which is larger 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 be magmatic 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, the dorsa of the Tharsis magmatic complex have been interpreted to be associated with magmatic-driven activity, which includes mantle plume doming and associated compressional tectonism (Raitala, 1987). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

An example of basement structural control on volcanism is clearly landscape. The landscape includes exposed plugs and dikes, degraded evident in the S.P. Crater area of the San Francisco volcanic field, volcanic constructs, faults, fault/dike transitions, alignments of northern Arizona, where elongated volcano and vent alignments volcanic constructs, and the spatial associations of volcanoes and occur along faults and/or approximate the trends of faults (Fig. 8). In dikes, as well as the emplacement of lava flows into topographic lows such cases, magmas used basement structures as conduits to reach the such as valleys. These valleys subsequently became mesas through surface (Shoemaker et al., 1978a,b; Dohm, 1995). Unlike the southeast differential erosion and topographic inversion (Fig. 8) (e.g., also see part of Melas Chasma, however, there are no visible plugs or dikes Holm (2001) for details on topographic inversion of the Mormon and observed using the Landsat Satellite image of the San Francisco San Francisco volcanic fields, northern Arizona). volcanic field. This is due to the geologically young age of the San Located only a few kilometers to the northeast from this proposed Francisco volcanic field. Basaltic volcanism in the volcanic field, for differentially eroded volcanic field, sulfate-containing layered deposits example, began in the Miocene/Pliocene and continued into the late have been identified using Compact Reconnaissance Imaging Spectro- Pleistocene such as in the case of the distinct cylindrical volcano, SP meter for Mars (CRISM) data (with spatial resolution approximating Crater, and its associated pristine lava flow (Fig. 8). The volcanoes have 18 m/pixel using 544 spectral channels and multispectral mapping not been eroded down to their plumbing system (e.g., dikes and strips with lower spatial resolution (~200 m/pixel) using 72 spectral plugs), as is typical of the older, Mid Miocene and Pliocene volcanic channels, Murchie et al., 2007a,b)(Fig. 9; also see Pelkey et al. (2007)). fields of the Colorado Plateau (e.g., Ulrich et al., 1984; Ulrich and A hydrothermal origin for the sulfate-containing outcrops seems Bailey, 1987; Wolfe et al., 1987) where plugs and dikes have been reasonable among other possible interpretations described below exposed though differential erosion, often simulating the trends of the considering the close proximity of the layered sediments to the regional fault systems (Dohm, 1995). proposed volcanic field detailed in Fig. 8. A similar scenario can be envisioned for the example presented in The hypothesis of widespread hydrothermal activity within Melas the southeast part of Melas Chasma where differential erosion and Chasma is further reinforced by the general distribution of sulfates and basement structural control has resulted in a magmatic-influenced iron-oxide minerals associated with the layered deposits that are Author's personal copy

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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 Melas Chasma). 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 summit depressions (possible volcanic vent structures; pink arrows), lineaments (possible faults; black arrows), flow features (possible lava flows; red arrows), and ridges (possible dikes exposed through differential erosion; violet arrows) in the southeast part of Melas, all of which are consistent with hypothesized magmatic-driven activity of the canyon and surrounding 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) display characteristics similar to features on Earth where basement structural control influenced the subterranean migration of magma observed using Landsat at 15 m/pixel. The only difference 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.) Author's personal copy

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Fig. 9. (Top left) CRISM image HRS0000285D corrected using volcano scan atmospheric correction and Lambertian-reflectance photometric correction. The locations of the mineralogical spectra (blue square=12.330S, 69.309W; red square=12.327S, 69.273W; green square=69.247W) and reference spectra (oval symbols), shown in the middle plot as thick 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 spectral summary 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 be due 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 high and geographically clustered. The ratios of the mineralogical spectra to their reference spectra are shown in the bottom plot. The mineralogical spectra were chosen because they have high SINDEX values. The reference spectra were chosen to be in the same column as the mineralogical spectra. These spectra, when compared to their reference, show a distinct drop off 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 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; (4) groundwater circulation of sulfate-rich waters associated with Lucchitta et al., 1992; Komatsu et al., 1993), consistent with the hydrothermal activity (Warren, 1999; Andrews-Hanna et al., 2007; outcropping shown in Fig. 9. Spectroscopic evaluations using data Mangold et al., 2007); and (5) erosion and migration of previously from Viking, the Phobos II infrared spectrometer (ISM), the Mars Express layered sulfate evaporites (Fan et al., 2008). Some of the sulfate-rich Observatoire pour la Minéralogie, l'Eau, les Glaces, et l'Activité (OMEGA) layered deposits in Melas Chasma are dissected by dense and highly imaging spectrometer, and the MRO CRISM show that Kieserite organized valley networks as revealed by THEMIS (Quantin et al.,

(MgSO4·H2O), polyhydrated sulfates, and iron oxides occur in associa- 2005a), suggesting that an efficient water cycle existed within Valles tion with the layered deposits (Geissler et al., 1993; Christensen et al., Marineris during the Late Hesperian (Hartmann and Neukum, 2001), 2001; Gendrin et al., 2005; Bibring et al., 2005; Roach et al., 2007, 2008; which possibly included atmospheric precipitation driving a fluvio- Quantin et al., 2005a,b; Murchie et al., 2007a,b). Sulfate-rich deposits lacustrine system (Quantin et al., 2005a). appear over a wide range of elevations (from −4 km in Ius Chasma to The source of the sulfur that lead to the formation of the massive +3 km in Candor Chasma) (Gendrin et al., 2005) and provide direct sulfate deposits within Valles Marineris may be related to the presence of evidence of long-lasting and widespread aqueous activity within the crustal sulfide deposits (i.e. pyrite) (Burns and Fisher, 1993; Chevrier et canyons, and possibly prior to their formation. al., 2006). While metal-sulfide deposits have not been directly detected By analogy with terrestrial geology, five (non-mutually exclusive) on the surface of Mars, their presence has been suggested for a long time hypotheses for the deposition of sulfates are: (1) evaporation of (Clark et al., 1982; Burns and Fisher, 1990, 1993; Baker et al., 1991; standing bodies of water on the surface (Squyres et al., 2004); Chevrier et al., 2006). Like on Earth, pyrite could have formed on Mars in a (2) deposition of volcanic ash followed by reaction with condensed number of environments, typically volcanic and sedimentary. On Earth, sulfur dioxide- and water-bearing vapors emitted from fumaroles pyrite deposits associated with volcanic activity often form Volcanogenic (McCollom and Hynek, 2005); (3) deposition from a turbulent flow Massive Sulfide Deposits (VMSD), which can reach thicknesses of tens to of rock fragments, salts, sulfides, brines, and ice produced by me- hundreds of meters and extend over thousands of square kilometers teorite impact with subsequent weathering by intergranular water within the crust (Barrie and Hannington, 1999). Anoxic groundwater films without invoking near-surface aquifers (Knauth et al., 2005); percolating through the VMSD results in iron and sulfate-rich solutions, Author's personal copy

J.M. Dohm et al. / Journal of Volcanology and Geothermal Research 185 (2009) 12–27 21 which upon evaporation precipitate mineralogical assemblages that along basement structures, the presence of thick sulfate deposits, and include Fe- and Ca-sulfates and Fe-oxides. These terrestrial mineralogical salt diapirism collectively play a key role in its geologic history (Chong assemblages are compatible with the mineralogy of the layered deposits Diaz et al., 1999; Riquelme et al., 2002). within Valles Marineris (Burns and Fisher, 1990, 1993; Baker et al., 1991; Yet another important clue to the history of Melas Chasma and its Chevrier et al., 2004, 2006; Davila et al., 2008). surroundings is elemental information acquired through the Mars Therefore, the hypothesis that magmatic activity played a central role Odyssey Gamma Ray Spectrometer (GRS) instrument. GRS detects in the evolution of Valles Marineris, not only fits well with the diversity gamma rays from the surface without any instrument collimation, of tectonic and morphologic features identified within the Chasma, but providing a spatial resolution approximately equal to the altitude of the also provides a plausible origin for the sulfur required to form massive spacecraft, ~450 km (Boynton et al., 2004, 2007). GRS is providing sulfate deposits within the canyon system. Due to the vast scale of the evidence of substantial elemental differences across the surface of Mars Valles Marineris system, it is possible that magmatic-driven processes, (Boynton et al., 2002, 2004, 2007). Unlike other orbital-based spectro- which includes tectonism and hydrothermal activity, as well as salt- meters that can detect the mineralogy of the upper micrometer, related manifestations (e.g., Beyer et al., 2000; Milliken et al., 2007; perspectives of which are often obscured by fine-grained materials Montgomery et al., in press), may have operated simultaneously. A such as dust, the GRS instrument is sensitive to elemental compositions terrestrial example of this integration of processes is observed in the within a few tens of centimeters below the Martian surface and is thus Atacama Desert, Chile, where magmatism, tectonism, fluid migration capable 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) 3D perspective image shows the K/Th map of Taylor et al. (2006) over MOLA topography (25× exaggeration) looking obliquely to the northwest across Tharsis, which includes the central part of Valles Marineris (yellow arrow). Also highlighted, a geologic contact that separates the tectonic province of Thaumasia Planum (Dohm and Tanaka,1999; Dohm et al., 2001c) from the lava plains of Sinai Planum (black arrows; also see Figs.1 and 5) is approximately defined by the GRS K/Th signature. In addition, northwestward from the tectonic province, K/Th increases towards Melas (yellow arrow), the caldera/vent-like structure (pink arrow), and the source regions for the outflow channel systems, Kasei (K) and Maja (M) Valles. The K/Th is distinctly low east–southeast of Coprates rise mountain range (CR). The GRS patterns appear consistent with geologic and geomorphic information (Dohm et al., 2001a,c). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Author's personal copy

22 J.M. Dohm et al. / Journal of Volcanology and Geothermal Research 185 (2009) 12–27 may be present. GRS data reveal that near-surface materials are far from processes are solely responsible. Other considerations that add to the homogenous, with distinct elemental signatures across all regions of complexity of interpreting the K/Th signature in the Valles Marineris Mars (with respect to both variations in a single element and establish- region are also petrologic fractionation issues such as the mineral ing correlations or differences between elements; Boynton et al., 2007). assemblage of the source region, different degrees of partial melting, Taylor et al. (2006) investigated GRS-based potassium(K)/thorium and the potential for assimilation of pre-Tharsis crustal materials. (Th) since K and Th are potentially useful for assessing the extent of For example, paleotopographic reconstructions based on a synth- aqueous alteration, as they and their host minerals fractionate esis of published geologic information and high-resolution topogra- differently in aqueous solutions. For example, a neutral pH environ- phy, including topographic profiles, revealed the potential existence of ment results in a decreased K/Th ratio for residual materials since K is an Europe-sized drainage basin/aquifer system in the eastern part of more mobile than Th, while an acidic pH environment mobilizes both the Tharsis region (Fig. 2) during the incipient development of Tharsis K and Th. Therefore, under neutral aqueous conditions, K will be (pre-Valles Marineris formation) (Dohm et al., 2001a). As such, the reduced in the source region, since some of it was mobilized, resulting basin would have been a catchment for materials (including sediment in a lower relative K/Th. Consequentially, wherever there has been and water) that originated from diverse provenances such as the ponding of this water, there will be an enrichment of K, yielding a Thausmasia highlands mountain range to the south–southwest, later higher ratio. On the other hand, under acidic conditions, both K and Th to be exposed by the formation of Valles Marineris (Fig. 2). In addition, will be partly removed from the source rocks and concentrated in the if the location of Valles Marineris was the central part of a basin prior to region of deposition. the formation of the canyon system (Dohm et al., 2001a), then it would The GRS-based map information highlights distinctly higher and have been a catchment of water enriched in both ions. This is due to the lower than the average K/Th regions for Mars (5330±220; see Taylor leaching of rock materials in the various geologic provenances that et al., 2006 for more details), including the central part of Valles partly contributed to the basin (Dohm et al., 2001a,c) Marineris and Thaumasia Planum, respectively (Fig. 10). The former Any interpretation, be it water–magma interactions, igneous rocks, has been interpreted as either water–magma interactions and/or lava and/or older sedimentary rocks exposed by canyon formation (e.g., flow materials, which intrinsically have a high K/Th, (Taylor et al., pre-Tharsis sedimentary rocks overlain by sheet lavas and interfinger- 2006). If considered separate of the geomorphic evidence, the K/Th ing sedimentary deposits related to the growth of Tharsis; Dohm et al., fractionation ratios are unremarkable. Taylor et al. (2006) are explicit 2001a), all comprising relatively high K and Th, are consistent with a in stating that the range of observed values could be ascribed to developmental history of Melas Chasma and surroundings influenced igneous processes. However, when the K/Th ratios are viewed within by magmatic-driven activity, including magma–water/water–ice the context of the geomorphic evidence for aqueous processes interactions (also see Chapman and Tanaka, 2001, 2002; Komatsu occurring throughout parts of Martian history (Scott et al., 1995; et al., 2004b). For example, even with its regional spatial resolution as Fairén et al., 2003), it becomes probabilistically unlikely that igneous noted 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 observed topography and gravity in C and D, respectively. (B) Portion of the MOLA map with a stretched color scale to emphasize the region of flank uplift around Melas 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 km depth 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 for

Te =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 positive

Bouguer anomaly in the center of the chasma. The model gravities also demonstrate a positive increase in gravity toward the center of the chasma with decreasing Te values due to the upward 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.) Author's personal copy

J.M. Dohm et al. / Journal of Volcanology and Geothermal Research 185 (2009) 12–27 23 materials of the Thaumasia Planum, a Late Noachian tectonic province solution for the Mars gravity field derived from the tracking data of (described in Section 2), from onlapping Early Hesperian lavas from MGS, Odyssey, Pathfinder, and Viking Lander 1. The modeled gravity Sinai and Solis Planae along the province's western margin (Dohm and has been low pass filtered to reflect the resolution of the observed Tanaka, 1999; Dohm et al., 2001c). This could be explained both by gravity field. The flexure model results in elevated topography and differences in the geochemistry of the rock materials and/or the gravity on the canyon flanks, and within the canyon, a positive increase degree of weathering. K/Th elevates towards Melas Chasma, the newly in topography and gravity at the center resulting from the upward identified caldera/vent-like structure, and the source regions for the deflection of the surface and Moho with compensation. Within Melas outflow channel systems, Kasei and Maja Valles (Fig. 10), north- Chasma, the amplitude of the negative gravity predicted from westward from the Thaumasia Planum tectonic province, consistent topography is ~300 mGal larger than observed. This is reflected in with the geological and paleohydrological histories described above. the positive ~300 mGal Bouguer anomaly within the chasma and implies that some upward flexural compensation of the chasma has 4. Geophysical setting occurred and/or that material of higher density is present within the crust. There is a trade-off between the initial depth of the modeled

Using geological, topographic, and geophysical information, we cavity and the Te that result in a post-flexural surface and gravity performed geophysical modeling to further assess the developmental similar to that observed for Melas Chasma. This is shown in Fig. 12 history of the central part of Valles Marineris and its surroundings, where the differences between the final modeled topographies and the including Melas Chasma. In particular, we evaluated possible mech- observed topography at the center of the chasma (represented by anisms that could have resulted in the observed distinct topographic distance 0 in Fig. 11) are plotted for values of Te =100–300 km at rise (Fig. 2). increments of 50 km. A small Te requires a deeper initial cavity as the The formation of the Valles Marineris canyon system would induce a resulting deflection of lithosphere is more prominent. Conversely,

flexural response (Anderson et al., 1999). Flexural flanks are observed increasing Te requires a smaller initial topographic amplitude as the around Melas Chasma (Fig. 11) and provide information on the com- greater elastic strength subdues the lithospheric response to the cavity. pensation state and formation of the chasma. We modeled the flexural Low values of Te are not consistent with the observed free-air gravity uplift of a canyon given the removal of an initial volume of crustal (Fig. 11B). For example in Fig. 11,atTe =100 km, no values for initial material that results in a chasma with approximately the same volume depth provide a good fit to the data as the deflection of the Moho is represented by Melas Chasma for comparison with the observed rim significant. The model topography and gravity are both similar to the topography. A range of initial canyon depths were employed with the observed values for Te ~200 km only and an initial cavity depth ~25– initial cavity having a bowl shape with a constant half width of 200 km at 30 km. This results in flank uplift of ~8 km. zero elevation chosen to approximate the dimensions of Melas. The If the current configuration of the canyon is due to the flexural trade-off between the effective elastic thickness, Te, and the initial depth response of the lithosphere to a chasm, the initial cavity, requiring a of the cavity was then explored. The equation describing the elastic depth of 25–30 km, would require the removal of material that flexure of the lithosphere is (e.g. Watts, 2001): exceeds the volume of the present-day canyon. Its current depth "# – − ranges from 8 10 km, thus the erosion of ~5 km of additional material ρ 4 1 ðÞ − ðÞ c Dk ; from the rim to subdue the topography to its current observed form Wk = Hk ρ 1+ ρ m mg would be required. If Melas was the site of focused magmatic activity such as the possible surface manifestation of a mantle plume, then fl where W(k) is the wavenumber domain representation of the exure intrusive igneous material in the underlying crust may represent a and H(k) is the wavenumber domain representation of the initial region of elevated density within the chasma. This would diminish the ρ ρ topography prior to any compensation, c and m are the crustal and magnitude of the flexural response. mantle densities taken to be 2900 kg m−3 and 3400 kg m− 3, respectively, g is the gravitational acceleration, and D is the flexural rigidity:

ET3 D = e ; 12 1 − m2 which is a function of the elastic thickness, Te, Young's modulus, E=1011 Nm−2, and Poisson's ratio, ν=0.25. The resulting deflection for Te =100 km, 200 km, and 300 km are shown in Fig. 11.Thelower value used for the effective elastic thickness is representative of the results obtained by McGovern et al. (2004) for various parts of the Valles Marineris region; the higher value is taken here as a conservative upper limit because the majority of values presented by McGovern et al. (2004) are lower limits. The resulting free-air gravity is determined by:

ðÞsurface π ρ ðÞðÞ ðÞ; Gk =2 G c Hk + Wk

ðÞMoho π ðÞρ − ρ ðÞ ðÞ− ; Gk =2 G m c Wkexp ktcr where G(k)surface and G(k)Moho are the wavenumber domain repre- sentation of the gravity at the surface and Moho, respectively, and the total G(k)=G(k)surface +G(k)Moho. G is the gravitational constant, and tcr is the crustal thickness assumed here to be 50 km. fl The results are shown in Fig.11 along with the observed free-air and Fig. 12. Differences between A) modeled surface topography (post- exure) and observed topography, and B) modeled and observed free-air gravity at the center of the Bouguer gravity derived from the MGS95J spherical harmonic model of Chasma. Models with Te ~200 km (arrows) provide similar values to both the observed the Mars gravity field (Konopliv et al., 2006); the model is the global topography and gravity. Author's personal copy

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Valles Marineris represents a massive void (canyon-forming Dohm 1990a; Dohm et al., 2001a). Geophysical analysis indicates that a removal of materials) in the Martian crust. For comparison, Earth's positive Bouguer anomaly within the chasma is likely the result of highest mountain system, the Himalaya Range, forms a 2400 km arc upward compensation of the chasma. Yet bulging of the crust prior to which varies in width from 150 km to 400 km and contains over 100 formation of the chasma due to elevated heat flow and dynamic forcing mountains exceeding 7200 m (Qinye and Du, 2004). A large part of by a possible mantle plume cannot be ruled out. The tremendous void this mountain range would fit within Valles Marineris (Fig. 3). Bulging in the crust makes it exceedingly difficult to assess the canyon's of the crust may have been present prior to formation of the chasma incipient development (Kiefer, 2006). Other geophysical considera- due to elevated heat flow and dynamic forcing by magmatic-driven tions are consistent with intrusive activity centered in central Valles activity (e.g., mantle plume as it impinged on the bottom of the Marineris (McGovern et al., 2002, 2004). lithosphere). The volume of Melas Chasma however represents a But again, where did all the materials from the formation of such a significant deficit of mass in the crust. The lithostatic pressure at 8 km vast canyon system go? Was there a removal of tremendous volumes depth is ~85 MPa and the removal of such overburden would likely of subterranean volatiles (e.g., water) and magma? Several works dominate the subsequent flexural signal. point to Tharsis-driven floods of enormous proportions, floodwaters There is some geophysical evidence favoring the existence of of which inundated the northern plains to form water bodies ranging intrusive bodies beneath Valles Marineris. The study of McGovern et al. in size from lakes to oceans (Baker et al., 1991; Dohm et al., 1998, (2002, 2004) of Hebes, Candor, and Capri chasma found that the 2001a,d; McKenzie and Nimmo, 1999; Fairén et al., 2003), transferring gravity/topography admittance signal of these chasma can be fitted for deep crustal rock materials to the northern plains (Baker et al., 2007; models with high effective elastic thickness (higher than 60, 120, and Dohm et al., 2008). So is the release of tremendous amounts of water 110 km for Hebes, Candor and Capri chasma, respectively) and a sig- from a Europe-sized basin (Dohm et al., 2001a) an explanation for the nificant component of subsurface loading emplaced at the crust– tremendous void? In addition, during periods of long-term Tharsis mantle boundary. A plausible explanation for this subsurface loading quiescence, were evaporite deposits (Dohm et al., 2001a) sufficient would be the presence of high density intrusives in the lower crust enough to allow for the manifestation of salt diapirism, at least as a (McGovern et al., 2002), in accordance with some scenarios proposed part of the geological evolution of Valles Marineris (Milliken et al., for the formation of Valles Marineris (e.g., Mège and Masson, 1996; 2007)? These and many more questions may be only addressed Dohm et al., 1998, 2001a; McKenzie and Nimmo, 1999). Alternatively, through eventual in-situ investigation and tier-scalable, smart robotic models without subsurface loading would involve a substantially reconnaissance (Fink et al., 2005). lower crustal density in this region than the average value for the Importantly, no single observation provides a definitive answer to Martian crust (McKenzie et al., 2002; McGovern et al., 2002). However, whether the geological history of Valles Marineris is dominated by McGovern et al. (2002) consider this possibility as unlikely due to the magmatic-driven processes. Collectively, however, the diverse observa- extensive presence of basaltic materials (which are relatively dense) in tions presented here suggest this to be a possibility. These include the this region (McEwen et al., 1999; Williams et al., 2003). geologic, paleotectonic, and topographic signatures of magmatic-driven The best fit component of bottom loading is 0.4–0.7 for Hebes uplift (Scott and Dohm,1990a; Dohm et al.,1998, 2001a; Anderson et al., Chasma, 0.5 for Candor Chasma, and 0.2–0.3 for Capri Chasma 2001;), promontories interpreted to be associated with volcanism (McGovern et al., 2004). McGovern et al. (2002, 2004) consider these within Valles Marineris (Lucchitta, 1990; Lucchitta et al., 1992), and the values as consistent with decreasing dike amount with increasing identified N50 km-diameter structure shown in Figs.1 and 4. Magmatic- distance from central Tharsis. Significantly, they are also consistent with driven activity as a major influence on the development of the Valles intrusive activity centered in central Valles Marineris. In a similar sense, Marineris, which includes possible hydrothermal activity, is further modeling of the residual gravity results in a weak positive anomaly in supported by both the spectral identification of water-related miner- Melas Chasma, which could be caused by intrusive materials (Kiefer, alogies, such as hydrated sulfates, in the same region that displays 2006). However, anomalies are not observed in Ius and Coprates chasma landforms interpreted to be volcanic in origin (Figs. 7 and 8), as well as (west and east of Melas Chasma, respectively). the relatively high GRS-based K/Th signature when compared to the Magmatic activity may have been related to elevated heat flow in regional average (Fig. 10). the Valles Marineris region. The high effective elastic thickness of the lithosphere obtained for this region indicate relatively low heat flows 6. Summary and implications for the Late Hesperian/Early Amazonian (McGovern et al., 2004), the time when the present-day topography was established. However, Rifting, magma withdrawal, and tension fracturing have been local high flows are not disproved by these results, and geological or proposed as possible processes involved in the initiation and develop- geophysical indicators of heat flow for epochs before the Late ment of the vast canyon system (Lucchitta et al., 1992). Lucchitta et al. Hesperian/Early Amazonian could have been obscured or eliminated (1992) noted that the depth of the large troughs may have been caused by the further development of Valles Marineris. by: (1) collapse of near-surface materials due to withdrawal of under- On the other hand, there is a certain degree of overlap between the lying material or opening of tension fractures at depth; (2) development heat flows deduced (from the effective elastic thickness) for the of keystone grabens at the crest of a bulge; or (3) failure and subsequent central Valles Marineris region with those obtained for other parts of drifting of plates. Many of the faults associated with Valles Marineris Tharsis. Alternatively, central Tharsis-related intrusive activity (e.g., may have been associated with volcanic activity (Lucchitta,1990). Many dike emplacement sourcing from the Tharsis Montes or Syria Planum of these hypotheses regarding the formation of Valles Marineris may not regions) is also consistent with high effective elastic thickness, since, be mutually exclusive, as indicated by key pieces of information such as: in this case, the generation of melt would occur further away from the (1) the tectonic center of activity identified by Anderson et al. (2001); central part of Valles Marineris. (2) the central Valles Marineris rise shown by Dohm et al. (2001a);(3) the faults that are radial and concentric about Melas Chasma (Scott and 5. Discussion Dohm, 1990a; Dohm et al., 2001c); (4) the landforms interpreted to be volcanic in origin in the canyons (Lucchitta, 1990; Lucchitta et al., 1992), Valles Marineris represents a massive void in the Martian crust, including the features identified in the southeast part of Melas Chasma enough to include a large part of the Himalaya Range. What was the using HiRISE data that collectively point to a volcanic field; (5) sulfate- cause of such a massive void? Certainly, a distinct topographic rise enriched interior deposits, which were observed in the southeast part of occurs where Viking-era geologic investigations indicated potential Melas Chasma (in close proximity to the possible volcanic field of (4)); uplift in and surrounding the central part of Valles Marineris (Scott and (6) the newly identified structure interpreted to be volcanic that is Author's personal copy

J.M. Dohm et al. / Journal of Volcanology and Geothermal Research 185 (2009) 12–27 25 spatially associated with the Late Noachian–Early Hesperian center Boynton, W.V., Feldman, W.C., Mitrofanov, I.G., Evans, L.G., Reedy, R.C., Squyres, S.W., fi Starr, R., Trombka, J.I., d'Uston, C., Arnold, J.R., Englert, P.A.J., Metzger, A.E., Wänke, de ned by Anderson et al. (2001); and (7) a distinctly higher than H., Brückner, J., Drake, D.M., Shinohara, C., Fellows, C., Hamara, D.K., Harshman, K., average K/Th signature (Taylor et al., 2006). Magmatic activity along Kerry, K., Turner, C., Ward, M., Barthe, H., Fuller, K.R., Storms, S.A., Thornton, G.W., basement structures appears to be a significant contributor to the Longmire, J.L., Litvak, M.L., Ton' Chev, A.K., 2004. The Mars Odyssey Gamma-Ray Spectrometer Instrument Suite. Space Sci. Rev. 110, 37–83. development of Valles Marineris, as evidenced by geological investiga- Boynton, W.V., et al., 2007. 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