Ancient Drainage Basin of the Tharsis Region, Mars: Potential Source for Outflow Channel Systems and Putative Oceans Or Paleolakes

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Ancient drainage basin of the Tharsis region, Mars: Potential source for outflow channel systems and putative oceans or paleolakes

J. M. Dohm

J. C. Ferris

V. R. Baker

R. C. Anderson

T. M. Hare

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Recommended Citation Dohm, J. M.; Ferris, J. C.; Baker, V. R.; Anderson, R. C.; Hare, T. M.; Strom, R. G.; Barlow, N. G.; Tanaka, K. L.; Klemaszewski, J. E.; and Scott, D. H., "Ancient drainage basin of the Tharsis region, Mars: Potential source for outflow channel systems and putative oceans or paleolakes" (2001). Faculty Bibliography 2000s. 7973. https://stars.library.ucf.edu/facultybib2000/7973 Authors J. M. Dohm, J. C. Ferris, V. R. Baker, R. C. Anderson, T. M. Hare, R. G. Strom, N. G. Barlow, K. L. Tanaka, J. E. Klemaszewski, and D. H. Scott

This article is available at STARS: https://stars.library.ucf.edu/facultybib2000/7973 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. El2, PAGES 32,943-32,958, DECEMBER 25, 2001

Ancient drainage basin of the Tharsis region, Mars: Potential source for outflow channel systems and putative oceans or paleolakes

J. M. Dohm, • J. C. Ferris, • V. R. Baker, •,2 R. C. Anderson,3,4 T. M. Hare? R. G. Strom,2 N. G. Barlow, 6 K. L. Tanaka? J. E. Klemaszewski,? and D. H. Scott8

Abstract. Paleotopographicreconstructions based on a synthesisof publishedgeologic informationand high-resolutiontopography, including topographic profiles, reveal the potential existenceof an enormousdrainage basin/aquifer system in the easternpart of the Tharsisregion duringthe NoachianPeriod. Large topographichighs formed the margin of the giganticdrainage basin. Subsequently, lavas, sediments, and volatilespartly infilled the basin,resulting in an enormousand productiveregional aquifer. The stacked sequencesof water-bearingstrata were then deformedlocally and, in places,exposed by magmatic-drivenuplifts, tectonicdeformation, and erosion.This basin model providesa potential sourceof water necessaryto carve the large outflow channelsystems of the Tharsisand surroundingregions and to contributeto the formation of putative northern- plainsocean(s) and/or paleolakes.

1. Introduction development(Plates 1 and 2), includingvolcanic constructs of varyingsizes and extensive lava flow fields, large igneous plateaus, Stratigraphic, tectonic, and erosional records, compiled fault and rift systemsof varyingextent and relativeage of forma- throughgeological mapping investigations at regional and lo- tion, giganticoutflow channel systems, vast canyon systems, and cal scales,demonstrate a significantcontribution of magmatic- local and regionalcenters of tectonicactivity. Many of the local driven processesto the dynamicgeologic history of Mars [for and regionalcenters of tectonicactivity [Anderson et al., 1998, example,Mouginis-Mark, 1985; Scott and Tanaka, 1986; Gree- 2001;Anderson and Dohm, 2000]are interpretedto be the result ley and Guest,1987; Baker et al., 1991; Crownet al., 1992;Scott of magmatic-relatedactivity, including uplift, faulting,dike em- et al., 1993; Robinsonet al., 1993; Crown and Greeley,1993; placement,volcanism, and local hydrothermalactivity [Dohm et Gregget al., 1998].These processes are perhapsbest exempli- al., 1998,2000b, 2000d, 2001a; Dohm and Tanaka,1999]. The fied at Tharsis and the surroundingregions in the western geologichistory of the numerouscomponents of the Tharsismag- hemisphere[Frey, 1979; Wiseet al., 1979;Plescia and Saunders, matic complex,which has been analyzedin detail by numerous 1982; Solomon and Head, 1982; Scottand Tanaka, 1986; Morris investigators(see Plate 1), is incorporatedinto the analysesand and Tanaka, 1994; Scott and Zimbelman, 1995;Andersonet al., interpretationsthat follow. 1998, 2001; Scott et al., 1998; Golombek, 2000; Solomon and Inasmuch as the relative age of formation of the primary Head, 2000], where pulsesof magmaticactivity associated with componentsof the complexare generallyunderstood, the gen- the developmentof the Tharsis Magmatic Complex (TMC) eral stratigraphyestablished, and the high-resolutiontopogra- [e.g., Dohm et al., 2000a, 2000b] may have triggered cata- phy of present-dayMars known,the featuresand materialscan strophic floods and short-lived climatologicalperturbations be "backstripped"systematically from their presentconfigura- [Bakeret al., 1991, 2000;Baker, 2001]. tion. This provides an approximatesequential three-dimen- The Tharsis magmaticcomplex is composedof numerous sional(3-D) view of the paleotopographyat the cessationof componentsthat formed duringspecific stages of the complex's each of the five stagesof the complex'sdevelopment (Plates 3-7). Stageinformation correlates with the schemedevised by •Departmentof Hydrologyand Water Resources, University of Ar- Dohm and Tanaka [1999] (see Platesi and 2). izona, Tucson, Arizona, USA. The purposeof the 3-D visualizationand quasi-quantitative 2Lunarand Planetary Laboratory, Department of PlanetaryScience, Universityof Arizona, Tucson,Arizona, USA. backstrippingis not to generatea quantitativelyaccurate re- 3jet PropulsionLaboratory, Pasadena, California, USA. constructionof Martian paleotopographyat discrete time 4Lunarand PlanetaryLaboratory, University of Arizona,Tucson, steps.Such a reconstructionmay well be possibleat a future Arizona, USA. date when more data become available. However, the more SU.S.Geological Survey, Flagstaff, Arizona, USA. 6RobinsonObservatory, Department of Physics,University of Cen- qualitativevisualizations employed in this studyserve to sum- tral Florida, Orlando, Florida, USA. marize inferencesmade from geologicalmapping and analyses ?Departmentof Geology,Arizona State University, Tempe, Ari- and synthesisof publishedmap information.This summary, zona, USA. really an illustratedworking hypothesis,leads to the identifi- 8DeceasedAugust 9, 2000. cation of an ancient, gigantic drainage basin that persists Copyright2001 by the American GeophysicalUnion. through much of the history of the region. Moreover, the Paper number2000JE001468. drainagebasin, once identified, is found to be consistentwith 0148- 0227/01/2000JE001468509.00 a diverse series of other observations. The overall coherence of

32,943 32 944 DOHM ET AL.: ANCIENT DRAINAGE BASIN OF THE THARSIS REGION MARS

Plan urn ustrale

Stage Oer•od

0 5 N

Plate 1. Mars Orbiter Laser Altimeter (MOLA) shadedrelief map of the western hemisphereof Mars (courtesyof the MOLA ScienceTeam). Shown are the major geologicfeatures of the TharsisMagmatic Complex (TMC) and their stagesof development(stage assignments 1-5 of Dohm and Tanaka [1999] and Dohm et al. [2001a]correlated with the time-stratigraphicage assignment:Noachian, Hesperian, and Ama- zonian periodsof Tanaka [1986]), basedon numerousdetailed geologicinvestigations [e.g., Milton, 1974; Baker and Milton, 1974; Plesciaet al., 1980; Plesciaand Saunders,1982; Scott and Tanaka, 1986; Tanaka, 1986; Tanaka and Davis, 1988;Frey and Grant, 1990; Tanaka, 1990;Scott and Dohm, 1990a, 1990b;Baker et al., 1991; I45'tbecket al., 1991;Chapman et al., 1991;Morals et al., 1991;Lucchitta et al., 1992;DeHon, 1992;Scott, 1993; Gulick, 1993; Morals and Tanaka, 1994; Schultz and Tanaka, 1994; Rotto and Tanaka, 1995; Scott and Zimbelman,1995; Scott et al., 1995;Rice and DeHon, 1996;Chapman and Tanaka, 1996;Scott and Dohm, 1997; Scottet al., 1998;Dohm et al., 1998;Anderson et al., 1998;Dohm and Tanaka, 1999;Nelson and Greeley,1999; McKenzieand Nimmo, 1999;Anderson and Dohm, 2000; Dohm et al., 2000b; Chapmanand Lucchitta, 2000; Head et al., 2000;Baker et al., 2001;Dohm et al., 2001a,200lb;Anderson et al., 2001]. Also shownare Solisand ThaumasiaPlanae (S.P. and T.P., respectively),approximated margins of the TMC (black dashedoutline), northwesternslope valleys (NSVs), and the Noachiandrainage basin (blue dashedoutline).

these observations can be considered to be a tentative confir- ing the above referencedtopographic, stratigraphic, paleoero- mation of utility for the basin hypothesis.While the quantita- sional, and paleotectonic information. This information in- tive confirmation of detailed validity for various reconstruc- cludesspatial and temporal relations amongshield volcanoes, tions is beyond the scope of this preliminary study, such igneousplateaus, magmatic-driven centers of tectonic activity, reconstructionswould be among the several possiblefollow- fault and rift systems,and lava flow fields. In this process, ons to the present study. featuresand materialsof a known relative age were systematically backstrippedfrom their presentMOLA-based configuration, providingapproximate sequential views of the paleoto- 2. Three-Dimensional Portrayal of the pography at the cessationof each of the five stagesof the Evolution of the Tharsis Magmatic Complex complex'sdevelopment. A 3-D visualizationprogram (Bryce 4 by the MetaCreations We emphasizethat theseillustrations are qualitativeat this Corporation) providesa heuristicrepresentation of the geo- point. They representa working hypothesis,based on our in- logicalevolution of the TharsisMagmatic Complex.Although terpretationof the geologicmapping and analysesand synthe- this program cannot modify present-dayMars Orbiter Laser sis of publishedmap information, that shouldbe tested with Altimeter (MOLA)-based topographyin absoluteelevation quantitativedata as these become more availablein formats values, it can mold surfaces and render and animate 3-D that can be related to the geology.We alsoemphasize that the scenes.Visual appearancesfor each of the five major stagesof thicknessesand areal extent of features/materialsare approx- activityof the Tharsis Magmatic Complexwere generatedus- imated, becausegeologic histories for Mars are established DOHM ET AL.: ANCIENT DRAINAGE BASIN OF THE THARSIS REGION, MARS 32,945

STAGES FEATURES

•; m

•q) •Q. 'E:o o (n

u • Z •0 • e• • =• 5 • e

_ o '; ß •

-• 2 • vV

...... Plate 2. Chart comparingthe stagesof geologicactivity in the TharsisMagmatic Complex region with major geologicfeatures, including the Noachian drainagebasin (correlatesto Plate 1). Size of the solid areas is roughlyproportional to the degreeof exposeddeformation. Violet, centersof tectonicactivity interpreted to be the result of magmatic-drivenuplift and local volcanism,dike emplacement,and hydrothermalactivity; orange,mountain building; blue, water; red, primarily emplacementof shield-formingand lava-field-forming lavas.Note that the commencementand/or end of activityof the componentsof the complexare not absolutely constrainedand that featuressuch as the shieldvolcanoes of TharsisMontes and OlympusMons could be presentlyactive.

through photogeologicmapping and relative-agedetermina- to be the resultof magmatic-drivenuplifts and local volcanism tion of surfacesand structures.Even on Earth, paleotopo- and hydrothermalactivity along large structural discontinuities in graphicreconstructions are difficultwhere a muchricher set of the Martian crust[Dohm et al., 1998;Anderson and Grimm, 1998; tools for analyzingthe geologicrecord can be used,including Dohm et al., 2001a]; the Late Noachian-Hesperiancenters of field mappingand absolutedating of rocks. tectonicactivity postdate the proposeddrainage basin. In addition to utilizing publishedgeologic information and We startedfrom Stage5 (present day, Amazonian Period; crosssections (e.g., Plates 8a-8c), our reconstructionsmake Plate 3) by importing the MOLA Experiment Gridded Data useof MOLA-derivedtopographic profiles (e.g., Plates 9-11) to Record (EGDR) and ended with Stage 1 (Noachian Period; best estimate thicknesses and areal extents of features and mate- Plate 7). For example, for the Late Hesperian/Amazonian rials. Plates9 and 10, for example,show substantial rises in the (Stages4-5; Plates 4 and 3, respectively),gigantic volcanic centraland westernparts of Valles Marineris,which are located constructsof Tharsis Montes and Olympus Mons and their near the centralpart of the proposeddrainage basin. These rises, associated lavas flow fields were removed from the Noa- whichhave been identified as centersof tectonicactivity [Ander- chian/Early Hesperian (Stages l-3) reconstructionsusing sonet al., 1998,2001; Anderson and Dohm, 2000],are interpreted Bryce surface editing tools (Plates 5-7, respectively). In 32,946 DOHM ET AL.: ANCIENT DRAINAGE BASIN OF THE THARSIS REGION, MARS

Alba Patera

Tempe Terra Ceraunius Fossae plateau rise • Chryse ß..--Uranius Fossae impact? Ulysses rise rise Ascraeus -•. rise Olympus'"'"'"' circum-Chryse Mons . Pavonis outflow channel systems rise NSVs central Vailes

. . Marine r rise .'.: Arsia ...... S.yria '...Planurn ß rise .

Thaumasia Claritas plateau •Coprates Uzboi rise rise /Vailis

Argyre Warrego impact rise

Plate 3. Three-dimensionalportrayal of the major geologicfeatures of the TharsisMagmatic Complex using MOLA data (Stage5, Amazonian[Dohm and Tanaka, 1999;Dohm et al., 2001a]).

addition, materials of regional extent with no obviouspoint knowledge of Mars increases,the major features and se- source (e.g., volcanic construct) were removed from the quences of events presented here provide an improved topographic reconstructionsof earlier stages of the com- understandingof the evolution the Tharsis Magmatic Com- plex's development using published geologic map informa- plex as well as its influence on global geology and paleo- tion coupled with MOLA-based geologiccross sections. For climate. example, Early Hesperian (Stage 3) ridged material was removed from the Noachian reconstruction(Stage 1 and early part of Stage 2) usingthe geologicmap of the western 3. Geologic Summary of the Evolution hemisphere [Scott and Tanaka, 1986] and MOLA-based of the Tharsis Magmatic Complex geologic crosssections (e.g., Plates 8a-8c). In other situa- tions, materials were added to approximate the original This summaryprovides an interpretivegeologic history of topography that may have been subsequentlyworn down by the Tharsis Magmatic Complexbased on topographic,strati- erosional processesfor each stage of the TMC's develop- graphic, paleoerosional,and paleotectonicinformation com- ment. For example, becauseClaritas rise developedduring piled from the work of numerousinvestigators. Key informa- the Noachian Period [Andersonet al., 1998, 2001] and was tion usedin this constructionincludes the spatialand temporal subsequentlymodified by erosional and tectonic processes, relationsamong shield volcanoes, igneous plateaus, magmatic- material was added to the rise to approximate its original driven centersof tectonic activity, fault and rift systems,and configuration during Stage 1 (Plate 7). Again, we wish to lavaflow fields(see Plates 1 and 2). Stageinformation is based reemphasize that this methodologyis qualitative, based on on Dohm and Tanaka [1999] and Dohm et al. [2001a] and experience with terrestrial and Martian geology. Although roughly correspondsto the Martian stratigraphic scheme the details (e.g., unit thicknesses)will be refined as our [Tanaka, 1986]. DOHM ET AL.: ANCIENT DRAINAGE BASIN OF THE THARSIS REGION, MARS 32,947

Alba Patera • TempeTerra CerauniusFossae plateau Chryse impact? ris• • •UraniusFossae Ulysses rise Olympus Mons rise Ascraeusrise circum-Chryse Pavonis o tflow channel systems

NSVs rise central Vailes Marineris rise Arsia Syria Planum rise

Thaumasia

Claritasrise .., plateau •Coprates rise .•,•Uzboi .,,•,•6½ "" Vailis '•?• •O6•' Argyre impact Warrego rise

Plate 4. Three-dimensionalportrayal of the major geologicfeatures of the Tharsis Magmatic Complex during Stage 4 (Late Hesperian-EarlyAmazonian [Dohm and Tanaka, 1999;Dohm et al., 2001a]). When comparedto Stage5 (Plate 3), significanthighlights include (1) lessdeveloped shield volcanoes, including Olympus Mons, Tharsis Montes, and Alba Patera and their associatedlava flows, (2) recessionof the highland-lowlandboundary, (3) lessmodified Thaumasia plateau (cessationof plateauuplift is mappedas Early Hesperian[Dohm and Tanaka, 1999;Dohm et al., 2001a],and (4) lessinfilled and more definedbasins (e.g., Chryseand Argyre) and canyons(e.g., Valles Marineris) as well as deeperoutflow channels.

3.1. Early to Middle Noachian, Part of Stage 1 may represent large dislocations in the Martian crust/ (Plates 1, 2, and 7) lithosphere.Such dislocationsmay be the result of the TMC The greatest percentageof faults preserved in Noachian developmentand (or) represent plate or block boundaries materialsof the westernhemisphere originate near the central formedduring the periodof highheat flow [e.g.,Schubert et al., part of the Claritasrise. This regionis markedby an enormous 1992;Sleep, 1994]. rift systemand highly deformed promontoriesinterpreted to Broad rises,rugged mountain ranges,and a ridge of mate- be basementcomplex. The Claritas rise is a center of activity rials, which may representthe remainsof a highly eroded rim representinga region of broad magmatic-drivenuplift and of Chryseimpact basin, partly form the marginof the proposed associatedvolcanism and tectonism. In addition to Claritas, enormousNoachian drainagebasin. The developmentof the magmatic-driventectonic activity is also identified for the highland-lowlandboundary sometime during the Middle to Tempe plateau and pre-TharsisMontes rises:Uranius, Cerau- Late Noachian may have resultedin a substantiallydifferent nius, and Arsia SW. Uncertaintyexists in the commencement paleohydrologicregime in ChrysePlanitia region, including an of ancientlocal and regional centersof magmatic-relatedac- enhancedhydraulic gradient. tivity. Syria Planurn and Arsia rise, for example,most likely 3.2. Late Noachian to Early Hesperian, Stage 2 began as local centersof activityduring the Early to Middle (Plates 1, 2, and 6) Noachian with substantial growth, perhaps episodically, through the Hesperian. The Noachian magmatic activity In addition to continued growth of the Arsia rise (pre- mostlyoccurs along large fracture/faultzones, many of which TharsisMontes), centersof magmatic-driventectonic activity 32,948 DOHM ET AL.: ANCIENT DRAINAGE BASIN OF THE THARSIS REGION, MARS

A!ba Patera • TempeTerra CerauniusFossae plateau Chryse ise • impact? %UraniusFossae

rise Uly.sses rise circum-Chryse" ,,,./' Pavonis outflow channel systems rise

NSVs Arsia Syria " ..... centraValle rise Planu Marinerisrise

Thaumasia Claritasplateau •Coprates rise ,•Xx•\306•½•;½• rise -•UzboiVailis

Warrego• Argyreimpact rise

Plate 5. Three-dimensionalportrayal of the major geologicfeatures of the Tharsis Magmatic Complex duringStage 3 (Early Hesperian[Dohm and Tanaka, 1999;Dohm et al., 2001a]).When comparedto Stage4 (Plate 4), significanthighlights include (1) absenceof OlympusMons and TharsisMontes shieldvolcanoes, includingassociated lava flows,(2) incipientPavonis rise, (3) lessdeveloped Alba Pateraand associatedlava flows,(3) more prominentSyria Planum, Ceraunius Fossae, Uranius Fossae, central Valles Marineris,Arsia, Claritas,and Warrego risesand the NSVs, (4) recessionof the highland-lowlandboundary, (5) lessmodified Thaumasia(cessation of plateauuplift is mappedas Early Hesperian[Dohm and Tanaka, 1999;Dohm et al., 2001a])and Tempe Plateaus, and (6) lessinfilled and more defined basins (e.g., Solis, Chryse, and Argyre) and smaller and less defined outflow channels. and possibleassociated volcanic eruptions and hydrothermal sandsof kilometersto the west. In addition,the sourceregion activityare identifiednear the centralpart of Valles Marineris, of Warrego Vailes has been interpreted to be a site of intru- Syria Planum, and the sourceregion of Warrego Vailes. Nu- sive-relateddoming and tectonic and hydrothermalactivity merousfaults, for example,are radial to or concentricabout the resultingin the formation of well-definedvalley networksof centralpart of Valles Marineris,representing a broad center of WarregoVailes [Gulick, 1993;Dohm and Tanaka, 1999;Dohm magmatic-drivenuplift alonga large crustal/lithosphericdisloca- et al., 2001a]. tion (Plate9). Similarto the centralpart of VallesMarineris, Syria The Thaumasia plateau uplift also occurred during this Planumis alsoa siteof long-lived(Noachian through at leastLate time [Dohm and Tanaka, 1999; Dohm et al., 2001a]. The Hesperian)magmatic/tectonic activity, which includes domal up- plateau uplift and local/regionalcenters of magmaticactivity lift, volcanism,and associatedradial and concentricfaulting, but largely modified the paleotopographyof the TMC region. at a muchlarger scaleand longerduration than is recognizedat These activities resulted in the modification and deforma- the centralpart of Valles Marineris. tion of the Tharsis basin, releasing catastrophicfloods that Magmatic-relatedactivity such as doming underlyingArsia led to the early formation of the circum-Chrysesystem of Mons and at SyriaPlanurn and centralVailes Marinerismay be outflow channels[e.g., Rotto and Tanaka, 1995; Nelson and geneticallyassociated with the early developmentof the cir- Greeley,1999] and the northwestslope valleys. This probably cum-Chryseoutflow channel systems[e.g., Dohm et al., 1998; drove volatiles (e.g., groundwater) from uplifted regions McKenzieand Nimmo, 1999] as well as the formation of the such as the central part of Valles Marineris and Syria Pla- newly defined northwest slope valleys [Dohm et al., 2000c, num into nearby topographiclows [e.g., Dohm et al., 2000b; 200lb], located on the oppositeside of the Tharsis rise from Barlow et al., 2001]. The time of initial formation of these the circum-Chrysesystem of outflow channels,several thou- outflowchannel systems is uncertain(Plate 2). For example, DOHM ET AL.: ANCIENT DRAINAGE BASIN OF THE THARSIS REGION, MARS 32,949

TempeTerra Chryse CerauniusFossae plateau rise % impact?

•Uraniusossae rise

circum-Chryse outflow channel systems

.. ,, central Vailes ' NSVs .At • Marineris rise .r"' Syria •. • , Planum

ßolarita ,, Thaumasia•'"•;oprates •rise plateau rise Uzboi Vailis

Warrego ....ß Argyre rise ....• impact

ß ,

Plate 6. Three-dimensionalportrayal of the major geologicfeatures of the Tharsis Magmatic Complex during Stage2 (Late Noachian-EarlyHesperian [Dohm and Tanaka, 1999;Dohm et al., 2001a]). When comparedto Stage3 (Plate 5), significanthighlights include (1) absenceof Alba Patera and associatedlava flows,(2) lessprominent Warrego and Arsia rises,(3) incipienttrough development at Valles Marineris,(3) more prominent mountain ranges of Coprates rise and Thaumasia highlands,Tempe Terra and Thua- masiaigneous plateaus, Syria Planum, CeraunisFossae and Claritas rises,(3) recessionof the highland- lowland boundary, (4) less modified Thaumasia and Tempe plateaus, and (6) less infilled and more definedbasins (e.g., Solis,Chryse, and Argyre), the NSVs, and Uzboi Vallis and smaller and lessdefined outflow channels.

lava flows sourcing from Stage 1 and Stage 2 centers of water-water ice interactionshave been proposed[Chapman activity may have partly infilled and followed paleovalleys and Tanaka, 2001]. associatedwith early developmentof the large outflow channel systems,resulting in an inversion of topography(lava 3.3. Early Hesperian, Stage 3 (Plates 1, 2, and 5) ridges/mesasresulting from subsequenterosion of lesscom- petent brecciatedsurrounding materials). Later episodesof Continued magmatic/tectonicactivity is identified at Ar- magmatic-triggeredflooding may have formed new valleys sia-SW dome (continuedgrowth of Tharsis rise drainagedi- or may have followed paleovalleys.Also during this time, vide), Syria Planum, and Tempe plateau. Incipient activity extensiveolder ridged plains materials [Dohm and Tanaka, (pre-TharsisMontes activity) is alsorecognized in the Pavonis 1999; Dohm et al., 2001a] and intercrater materials were Mons [Plesciaand Saunders,1982; Anderson et al., 1998,2001] emplaced in topographically low areas. Some intercrater and Alba and UlyssesPaterae regions.Also during the Early materials may be the result of phreatomagmaticexplosions, Hesperian Period, additionaldevelopment is recordedfor the such as in the Valles Marineris region, where magma- circum-Chrysesystem of outflow channelsand at the north- 32 950 DOHM ET AL.: ANCIENT DRAINAGE BASIN OF THE THARSIS REGION, MARS

TempeT rra CerauniusFossae hryse rise ' plateau impact '""•Uranius Fossae rise

Drainage ..• Arsia Basin rise Syria P!anum

Uzboi Clarita rise _. . omo5•ø Argyre impact

Plate 7. Three-dimensionalportrayal of the major geologicfeatures of the Tharsis Magmatic Complex [Solomonand Head, 2000;Dohm et al., 2000b;Anderson et al., 2001] during the Early to Middle Noachian Period (part of Stage1 [Dohmand Tanaka,1999; Dohm et al., 2001a]),including the drainagebasin. When comparedto Stage2 (Plate6), significanthighlights include (1) drainagebasin, (2) narrowerand sharper mountain rangesof Copratesrise, Thaumasia highlands, and multiringed structures of the Argyreimpact basin (the Charitum andNereidum Montes), (3) incipientCeraunis Fossae, Claritas, and Arsia rises, Syria Planum, and Tempe plateau, (4) absenceof highland-lowlandboundary, Valles Marineris, circum-Chryse outflow channel systems, and Thau- masiaplateau, (5) more distinctArgyre and putativeChryse impact basins, and (6) lessdistinct Uzboi Vallis. westernslope valleys, possibly related to anotherpulse of mag- notablyat SyriaPlanum [Dohmand Tanaka, 1999;Dohm et aL, matic activityat Arsia-SW dome,Syria Planum, and the central 2001a]. The final appearanceof large-scaletectonism for the part of Valles Marineris.Extensive younger ridged plains ma- westernhemisphere is associatedwith the dominantcenter of terialswere emplacedin topographiclows. tectonic activity, Alba Patera [Andersonet al., 1998, 2001]. Significantoutflow channel formation also occurred in the 3.4. Late Hesperian to Early Amazonian, Stage 4 circum-Chrysesystem of outflow channelsduring the Late (Plates 1, 2, and 4) Hesperianand into the Early Amazonian.This may be related Significantvolcanic activity is alsorecorded during the Late to yet another pulse of magmaticactivity at Tharsis Montes, Hesperian and Early Amazonian, including the development SyriaPlanurn, and the central part of Vailes Marineris, which of Olympus Mons and the Tharsis Montes shield volcanoes. is consistentwith the driving mechanismin the MEGAOUT- Also emplacedwere voluminoussheet lavas centered at the FLO hypothesis[Baker et al., 1991, 2000]. large shieldvolcanoes and at Syria Planum. Lava flows centered at Arsia Mons, for example,may extendto the northwest 3.5. Amazonian, Stage 5 (Plates 1, 2, and 3) as far as the northwesternslope valleys embaying the gigantic Evidencefor continuedgrowth of OlympusMons and the northwesttrending promontoriesand partly infilling the sys- TharsisMontes shieldvolcanoes is identifiedduring this geo- tem of valleys[Zimbelman et al., 2000]. logicperiod. Isolated occurrences of magmatic/tectonicactivity A significanttransition from magmatic-tectonicactivity to appear to be associatedwith continuedconstruction of the volcanic activity is observedat the major centers of activity, Tharsis Montes shield volcanoesand Olympus Mons. Other DOHM ET AL.: ANCIENT DRAINAGE BASIN OF THE THARSIS REGION, MARS 32,951

O!y,mpus Chryse

, ,4 d5 Amazonis d2 d3 Mons UD e• t dl O•--•, ASD Circurri-Cttry•e , • ,. •NSVs•.' Outflowchannel Ssten• z0,•,

Syria\ t, (SD) Mangala Thaumasia Vailest •2•}.ASWD Plateau 50• 0 500 1000 K m

Verticalexaggeration. 150 X a dl d2 d3 d4 d5 a' 7147 i ! - -

4300 Mangala t624 Nplh Shalbatana

-t15t \ I !IU ' ?'"•Npil'•_...•..• .... suite of potential materials (MF, Kas' •926 i NSVs---Tharsi•-related volcanic; fluvial, eolian•glacial?•

-67{x0 ßPSS P,BS PBS c 0 1516780 3033560 4550340 (•167120 7583{)00 91½)543 Distance(m)

Plate 8. (a) MOLA shadedrelief map showingfeatures of interest,including NSVs, Tharsisrise, circum- Chryseoutflow channelsystems, and centersof tectonicactivity [Andersonet al., 1998;Dohm et al., 1998] interpretedto representmagmatic-driven doming events (UD, Ulysses;ASD, Ascaeussouth; ASWD, Arsia southwest;CVD, centralValles; SD, Syria), (b) part of the geologicmap of the westernequatorial region of Mars (representativemap unitsare shown[Scott and Tanaka, 1986]), (c) generalizedgeologic cross section (a-a', transectof A and B; PBS, potential locationof basementstructures; TM1, TharsisMontes lavas;MF, MedusaeFossae materials) [Dohm et al., 200lb, Figure 2]. 32,952 DOHM ET AL.: ANCIENT DRAINAGE BASIN OF THE THARSIS REGION, MARS

A Vertical exaggeration75 X A' 5652 A centralVailes 388 TH?;t•ar•• Marinerisrise_ 713 , ..,.,...,__,...... ,-•: . -1762

4237 0 893 1787 2680 3574 4467 5361 Distance(km) B

A' ..• Chryse

.,,

Mari Tha•m sa ß , ' Plate ' '

1000 0 1000 Kilometers

Plate 9. (a) Present-dayMOLA profile (transectA-A') acrossthe west central part of the Thaumasia highlands,the southeastpart of the Noachiandrainage basin (queried blue line representsuncertain basin extent),including the centralValles Marinerisrise (centerof tectonicactivity, interpreted to be the resultof magmatic-drivenuplift [Dohmet al., 1998;Anderson et al., 1998,2001; Dohm et al., 2000b]),and materialsof inferredrim of Chryseimpact basin, (b) MOLA shadedrelief map showingfeatures of interest,including the approximatedboundary of the Noachiandrainage basin (dashed blue line) and the centralValles Marineris rise, and (c) part of the geologicmap of the westernequatorial region of Mars (representativemap unitsare shown[Scott and Tanaka, 1986]).

than minor grabenformation associated with the final stagesof rates of planetwide surface degradation than for post- Alba Patera, these local volcanicsources may representlate- Noachiantime [Masurskyet al., 1977;Pieri, 1980;Barlow, 1990; stagepulses of magmatic/tectonicactivity in the Tharsisregion. Craddock and Maxwell, 1993; Scott et al., 1995; Cart and Chuang,1997; Tanaka et al., 1998]. The basin/aquifersystem was subsequentlyobscured and 4. Basin Model: Physiographic Setting deformed by middle Noachian and younger geologicactivity and Stratigraphic Summary (Stages1-5), including(1) the Thaumasiaplateau uplift, (2) During the Early to Middle Noachian(early part of Stage1), continuedgrowth of the Arsia rise and the developmentof topographichighs associatedwith centersof tectonic activity other centers of tectonic activity, which also correspondto (Uranius Fossae,Ceranius Fossae, Arsia and Claritas rises), topographicrises (including central Valles Marineris rise, Syria ruggedmountain rangesof the east trendingThaumasia high- Planum, and pre-TharsisMontes Pavonis),(3) the emplace- lands and north trending Coprates rise, high-standingterrain ment of the intercrater plains materials,older and younger locatedsouthwest of ChrysePlanitia (interpretedhere to be a ridgedplains materials, and lavasassociated with SyriaPlanum highlyeroded impact crater rim), and Tempe plateau formed and Tharsis Montes and local sourcessuch as fissurefed lavas, the marginsof the proposeddrainage basin (Plate 7). During and (4) the constructionof the TharsisMontes shieldvolca- this same time period, lavas, sediments,and volatiles partly noes. The interaction of magmatic intrusions with water- infilled the basin,resulting in a large aquifer system.The par- bearing strata may have resulted in lateral migration of sub- tial infillingof the basinperhaps coincides with reportedhigher surface volatiles away from magmatic-driven heating and DOHM ET AL.: ANCIENT DRAINAGE BASIN OF THE THARSIS REGION, MARS 32,953

Profile 6

Vertical exaggeration75 X Warrego A' 7655A Ri.se(MTLC) I •E4964

0c: 2264 . :• Thaumasia (0 -436 > Highlands iTi-3136 MarinerisRise(MTLC) Plateau

- 5836 ...... 0 1022 2044 3066: 4088 5110 6131 Distance (km)

1000 0 1000 Kilometers

Plate 10. (a) Present-dayMOLA profile(transect A-A') acrossthe Warregorise (centerof tectonicactivity, interpretedto be the result of magmatic-drivenuplift [Andersonet al., 1998, 2001;Dohm et al., 1998;Dohm and Tanaka,1999; Dohm et al., 2000b,2001a]), west central part of Thaumasiahighlands, central part of the Noachiandrainage basin (queriedblue line representsuncertain basin extent), includingwest central Valles Marinerisrise (centerof tectonicactivity, interpreted to be the resultof magmatic-drivenuplift [Andersonet al., 1998, 2001;Dohm et al., 1998;Dohm and Tanaka, 1999;Dohm et al., 2000b,2001a]), and Tempe Terra plateau,(b) MOLA shadedrelief map showingfeatures of interest,including the approximatedboundary of the Noachiandrainage basin (dashed blue line) and the westcentral Valles Marinerisrise, and (c) part of the geologicmap of the westernequatorial region of Mars (representativemap units are shown[Scott and Tanaka, 1986]).

uplift. Such occurrenceswere probably common throughout wasmodified by the Thaumasiaplateau uplift, which mayhave the evolutionof the drainagebasin [Anderson et al., 1998,2001; been the result of magma plume head. Such activitymay be Dohm et al., 1998, 2000e; Solomonand Head, 2000] and may likened to topographicinversions observed on Earth. Large explainthe concentrationsof near-surfacevolatiles in the Solis igneousplateaus occur on both continentaland oceaniccrust Planum region expressedby an anomalousconcentration of in purely intraplate settings,on present and former plate layeredejecta craters [Barlow et al., 2001]. In addition,stacked boundaries (large lithosphericweaknesses), and along the sequencesof basin materialswere locally exposedin the can- edgesof continentsand are interpreted to be the result of yon walls of Valles Marineris by magmatic-drivendoming, plume activity[Coffin and Eldholm, 1994]. The largestterres- tectonic deformation, and erosion. trial exampleof plume-derivedigneous plateau growth among Whether the proposeddrainage basin is strictlythe result of topographicbasins is Ontong-JavaPlateau in the west central surroundingtopographic highs or whether there is an endo- Pacific [Richardsonet al., 2000]. geniccomponent is difficult to answer.As noted in the previ- ous section,the central part of the proposedbasin, Valles Marineris, is the locationof centersof tectonicactivity. These 5. Aquifer Characteristics centersare interpreted to be the result of magmatic-driven The sourcesof the largestMartian outflow channelsystems uplifts and local volcanism,dike emplacement,and hydrother- (circum-Chryseand the proposednorthwestern slope valleys mal activity.In addition,more than half of the proposedbasin (NSVs)) suggestthat the infill materialsof the giganticdrain- 32 954 DOHM ET AL.: ANCIENT DRAINAGE BASIN OF THE THARSIS REGION, MARS

A Vertical exaggeration75 X A A' 16789

g 12837

o ,,.. > 8787 Coprates

4737

687 rlse• ...... ••-.• pSFjintm?•rAi;;i a• 0 876 1753 2629 3506 4382 5259

B Distance(km) Alba Patera

1000 0 1000 2000 Kilometers

Plate 11. (a) Present-dayMOLA profile (transectA-A') acrossthe Copratesrise [Schultzand Tanaka, 1994; Dohm and Tanaka,1999; Dohm et al., 2001a],southern part of the Noachiandrainage basin (queried blue line representsuncertain basin extent), Syria Planum (centerof tectonicactivity, interpreted to be the resultof magmatic-drivenuplift andvolcanism [Tanaka and Davis,1988;Anderson et al., 1998,2001; Dohm et al., 1998; Dohm and Tanaka, 1999;Dohm et al., 2000b,2001a]), Arsia rise (centerof tectonicactivity, interpreted to be the resultof magmatic-drivenuplift andvolcanism [Anderson et al., 1998,2001; Dohm et al., 2000b]),and Arsia Mons [Scottand Tanaka, 1986;Scott and Zimbelman, 1995], (b) MOLA shadedrelief map showingfeatures of interest,including the approximatedboundary of the Noachiandrainage basin (dashedblue line), and (c) part of the geologicmap of the westernequatorial region of Mars (representativemap unitsare shown[Scott and Tanaka, 1986]).

age basin may have produced a highly productive aquifer. terrestrialaquifers. Preliminary analyses of the equationsthat Large pulsesof magmaticactivity related to the development govern subsurfacehydrological behavior imply not only that of the Tharsismagmatic complex [Dohm et al., 2000a, 2000b] this relationshipremains true for a Martian aquifer but that probablyresulted in the partial infilling of the basin by em- the valueof hydrologicconductivity may be amplifiedowing to placingstacked sequences of lavas.Evidence of this maybe the the loweredfrictional resistanceafforded by the lessergravity layeredwalls of Vailes Marineris,which have been interpreted of Mars [Dohm et al., 2000c, 200lb]. Additionally, any inter- to consistof flood lavas[e.g., Witbecket al., 1991;Lucchitta et fingered sedimentarydeposits would serve as local reservoirs al., 1992]. Depending on Noachian climate conditions,these for subsurfacewater capable of being tapped by the highly sequencesmay be interfingeredwith lacustrineand/or eolian fractured basalts. deposits.Furthermore, the basalticsequences are highlyfrac- We estimate the average depth of the Tharsis basin to be tured by magmatic and tectonic activities [e.g., Scott and between2 and 7 km (for example,Plates 9-11). The measured Tanaka, 1986; Tanaka and Davis, 1988; Tanaka, 1990; Scott areaof the basinis approximately9 x 106 km2 (Plates1, 7, and Dohm, 1990a, 1990b, 1997; Anderson et al., 1998, 2001; 9-1 1), and as such,the fill volumefor an averagedepth of 5 km Dohm et al., 1998, 2000b; Dohm and Tanaka, 1999; Solomon is approximately4.5 x 107 km3. If the fill werelargely com- and Head, 2000; Dohm et aL, 2001a]. Highly fractured basalt posedof highly fractured vesicularextrusive basalts, then the can have unusually high values of hydraulic conductivityin porosityand permeabilitywould be very high. On Earth the DOHM ET AL.: ANCIENT DRAINAGE BASIN OF THE THARSIS REGION, MARS 32,955 porosityof unfracturedvesicular basalts ranges from roughly and the recently proposedstructurally controlled system of 20% for low vesicularityto roughly75% for highlyvesicular valleysthat routed Noachianand Early Hesperianfloods from scoriaceousbasalts [Saar and Manga, 1999]. The porosityand the Arsia Mons regionto the northwestinto AmazonisPlanitia especiallythe permeabilitycould be evenhigher in the Tharsis [Dohmet al., 2000c,200lb]), (3) the thicksequences of layered region, as they are highly fractured from tectonicprocesses, materials exposedin the walls of Valles Marineris by mag- including faulting. Unlike the highly fractured surfacesthat matic, tectonic,and erosionalactivity [Scott and Tanaka, 1986; prevail throughoutthe Tharsis Magmatic Complex and thus Witbecket al., 1991; Lucchitta et al., 1992; Dohm et al., 1998; high potential permeabilities,impact crater eventsmay lower McKenzie and Nimmo, 1999; McEwen et al., 1999; Dohm et al., porosityand permeabilitybecause they will producefines that 2001a],(4) an anomalousconcentration of layeredejecta cra- infill pore spaces[MacKinnon and Tanaka, 1989]. When com- ters in the Solisand ThaumasiaPlanae regions [Barlow et al., pared to highly cratered surfacesof the southernhighlands, 2001], (5) the volatile-richnature of Valles Marineris [Scott however,the proposedbasin region (also of the southernhigh- and Tanaka, 1986; Witbecket al., 1991;Lucchitta et al., 1992], lands) is significantlyless cratered. and (6) the observedhematite depositsdetected in Valles If the terrestrialporosities are characteristicof the Tharsis Marineris [Christensenet al., 1998;Noreen et al., 2000]. basin fill, then the potentialvolume of water containedin the The thick sequenceof layered materials observedin the aquifer would be more than equivalentto the volume of water canyonwalls of Valles Marineris, for example,may represent required to create the putative ocean in the northern plains the accumulationof volcanicand other typesof materialsshed estimatedat 1.4 x 107 km3 [Headet al., 1999].For example,if into the basin. In addition, the hematite depositsdetected in the porosityof basaltswas 44%, and the removal of total water the Valles Marineris region by the Thermal EmissionSpec- from the aquifer was 60%, the minimum fill volume of the trometer (TES) instrumentmay be the result of phreatomag- northern plainsocean would be achieved.The inferred poros- matic mixingbetween magma and groundwateror surfacewa- ity is well within the values measuredfor unfractured basalt ter within the basin [Noreen et al., 2000; Chapman and flowsof moderateporosity [Saar and Manga, 1999]. We realize, Lucchitta,2000]. Although the true hydrogeologiccomplexity however, that it is hard to imagine that 60% of the entire of the basin and aquifer systemundoubtedly eludes us, the aquifer could have been dischargedin one event. In addition, circum-Chrysecatastrophic outflow channelsand the NSVs one can onlyguess at the effectivenessof the proposedaquifer demonstrate the tremendous productivity of this Noachian system.Thus we posethe followingconsiderations. Other hy- aquifer. Likewise,the hypothesisthat the circum-Chrysecata- drogeologicactivities associated with catastrophicflooding and strophicoutflow channels and northwesternslope valleys re- relatedshort-lived (-•10 4 to 105 year)episodes of quasi-stablesulted from catastrophicflood eventsis strengthenedby the climatic conditions[Baker et al., 1991, 2000] may have also identificationof the giganticdrainage basin/aquifer system, a contributedwater to the hypothesizedoceans by mechanisms potentialsource for the tremendousamount of water required suchas spring-fedactivity along areasof the highland-lowland to sculptthe surface.The sourceregion for catastrophicflood- boundary. Certainly, other aquifersmay have contributedto ing generallyhas been thought to be the cratered highlands, the putativenorthern plains ocean as well, especiallyfollowing which are marked by canyonsystems of Valles Marineris, lava catastrophicflood eventsand related short-livedclimatic per- flows,and chaoticterrain [Scottand Tanaka, 1986; Witbecket turbationsdescribed by Baker et al. In addition,a large quan- al., 1991;Lucchitta et al., 1992].However, the magnitudeand tity of groundice (e.g., stagnantice sheets)may have already location of the most significantMartian flood features in the been in place in the northernplains during the period(s)of Tharsis region can be best explainedby the existenceof a catastrophicflooding [Dohm et al., 2000c, 200lb]. This large Tharsisbasin that facilitatedthe collectionof large amountsof quantity of ice would be the likely consequenceof an earlier volatilesearly in Martian history[Masursky et al., 1977;Pieri, warm and wet phaseof Mars, possiblyinduced by catastrophic 1980; Craddockand Maxwell, 1993; Scottet al., 1995; Can' and flooding[Baker et al., 1991,2000]. As new floodwaterswashed Chaung,1997; Tanaka et al., 1998].In addition,the magmatic- over the northern plains, the additional heat would melt the triggeredfloodwaters that originatedfrom the basinprobably upper layersof ice, and the gradientscreated would allow the representa major sourcefor bodies of water in the northern melt water to cycleinto the hydrologicsystem. Alternatively, a plains,including oceans and paleolakesthat may have resulted proposed massivedebris-flow ocean [Tanaka et al., 2001] in short-livedclimatic perturbations [Baker et al., 1991, 2000]. and/or a mud ocean[JOns, 1986] require lesssource water then Magmatic-drivenuplifts associated with the developmentof a putativevast northern ocean [Parkeret al., 1987, 1993;Head the Tharsis magmatic complex subsequentlytransferred et al., 1999]. In addition,potential paleolakes,which also re- groundwaterlaterally along structurallycontrolled conduits quire less sourcewater, have been mapped in the northern within and outsideof the basin.This magmatic-drivenactivity plains [Scottet al., 1995]. eventuallyforced some of the volatiles out of the basin by catastrophicflooding [Bakeret al., 1991, 2000]. Such lateral movementsof volatilesthroughout geologic history would re- 6. Discussion sult in parts of the TMC beingvolatile-rich and volatile-poor. There are severalsignificant observations/characteristics in Impact crater studieshave revealeda region in the Solis and the Tharsisand surroundingregions that might be collectively ThaumasiaPlanae (located southof Valles Marineris in the explainedby an Early to Middle Noachian drainage basin. southcentral part of the basin;Plate 1) where an anomalous These include (1) the paucityof Noachianoutcrops (Plates concentrationof cratersdisplaying multiple-layer ejecta mor- 8-11) [e.g.,Scott and Tanaka, 1986;Rotto and Tanaka, 1995; phologiesis correlatedwith a large area of smaller onset di- Dohm and Tanaka, 1999;Nelson and Greeley,1999; Dohm et ametersfor cratersdisplaying single-layer morphologies [Bar- al., 2001a] and crustalmagnetic anomalies in the proposed low et al., 2001]. Barlow et al. [2001] proposed that the basinregion [Acuna et al., 1999],(2) a sufficientsource of water anomalousconcentration marks a regionthat wasparticularly necessaryto carvethe circum-Chryseoutflow channel systems rich in volatiles (ice near the surfaceand liquid at greater 32,956 DOHM ET AL.: ANCIENT DRAINAGE BASIN OF THE THARSIS REGION, MARS depths)at the time of crateringcompared to otherparts of the have formed owing to local and regional volcanicconstructs. Tharsismagmatic complex and the rest of the equatorialre- Further assumingthat the topographicirregularities formed gion. Thus the impactcratering record may be consistentwith early enoughin the developmentof the Tharsisrise that they a scenariowhere magmatic-drivenuplifts locally deformed the existedwithin an early warmer and wetter Martian environ- basin,driving some of the water from the basinby catastrophic ment [e.g.,Masursky et at., 1977;Pieri, 1980; Gotombeket at., floodingand transferringvolatiles from the uplifted localities 2000], the topographicirregularities would have allowed for into portionsof the existingsubsurface reservoir. This, in turn, the trappingof rainfalland runoff within localized topographic may have resulted in a local increaseof the hydraulichead. depressions.As the pondedwater infiltratedinto the ground,it Sucha linkageof processesis a predictableconsequence of the may have become trapped near the surfaceas the climate interaction of magmatic intrusions and the water-bearing cooled. If it remained frozen, it would remain in situ as Tharsis strata, which we envisioned to have been a common occur- continuedto rise,only to be releasedupon meteorite impact to rence throughoutthe long and complexevolution of the TMC producethe anomalousfield of layeredejecta craters. A prob- and the enormousNoachian drainagebasin. lem with thishypothesis, however, is that geologicmapping has not revealedany Early to Middle Noachian-agevolcanic edifices 7. Alternative Models within the basinregion, with the exceptionof incipientSyria Planum[e.g., Dohm and Tanaka,1999; Dohm et at., 2001a]. The basinmodel is not the only model that offersexplana- Finally, another model to consideris one presentedby tions for some of the significantcharacteristics of the Thatsis Tanaka et at. [2000] in which Syria-Planum-centeredfold and and surroundingregions, including the putativ½local presence thrust structuresand a cryosphereperhaps a kilometer or so of near-surfacewater in the Thatsisregion. A modelproposed thick resultedin a huge, largely sealedaquifer systemwithin by Ctifford [1993] also predicts near-surfacewater. Clifford the Thaumasiaplateau. Although a belt of suchstructures has proposedthat if the planetary inventory of outgassedH:O been recognizedcentered about Syria Planum [Schuttzand exceededthe pore volume of the cryosphcr½by more than a Tanaka,1994], especially in the Thuamasiaregion [Dohm and few percent, a subpermafrostgroundwater system of global Tanaka,1999; Dohm et at., 2001a],a continuousbelt within the extent would necessarilyresult. This interconnectedglobal Thaumasiaplateau is not observedin the stratigraphicand aquifer allowsthe downwardmigration of polar basalmalt to paleotectonicrecords, especially on the northernand western result in the upward migration of water at temperate and marginsof the plateau [Dohmand Tanaka, 1999;Dohm et at., equatoriallatitudes. Theoretically, this would result in the wa- 2001a;Anderson et at., 2001]. In addition,numerous basement ter that was lost from the crust at these latitudesby sublima- tion, impact, or catastrophicfloods to be replenished.Al- structures,which are radial about the Tharsisrise, form po- thoughClifford's model can adequatelyexplain the occurrence tential conduitsfor the migrationof groundwateraway from of near-surfacewater in the Thatsis and surroundingregions, the rise. Thus the model presentedby Tanakaet at. [2000] is it cannotby itself explainthe suite of significantobservations not by itself sufficientto explainthe suiteof significantobser- and characteristicspreviously discussed. vations/characteristicsdiscussed earlier. Thrust structures, The putativ½presence of local near-surfacewater in the such as those potentiallyobserved in the Copratesrise and Thatsis region, as identified by anomalousconcentrations of Thaumasia highlands regions [Schuttz and Tanaka, 1994; layered ½j½ctacraters in the Solis and Thaumasia Plana½re- Dohm and Tanaka,1999], however, could have played a role in gions,can be explainedby other means.A warmer and wetter forming a highlyproductive Noachian drainage basin/aquifer climatepossibly induced from the carly (e.g., carly to Middle system. Noachian) developmentof the Thatsis rise [Golombeket al., 2000],for example,may haveresulted in the pondingof water 8. Conclusion into local depressionsand infiltration of water into the subsurface as it flowed acrossthe planct's surface. As this water Paleotopographicreconstructions based on analysesof syn- movedthrough the subsurface,it may have encounteredlenses thesizedstratigraphic, paleotectonic, erosional, and MOLA- of material that preventedits downwardmigration, or it may derivedtopographic map data revealthe potentialexistence of have migrated in the subsurfacealong the natural gradient an enormousdrainage basin in the Tharsisregion during the formed by the Thatsis rise. In the former casethe aquitards Early to Middle Noachianperiod. Lavas and sedimentspartly would have led to the creationof perchedaquifers. Over time infilled the basin, resultingin a highly productiveregional these perchedaquifers may have risen to the near surfaceor aquifer. The stackedsequences of basinmaterials were subse- surface. As the Martian climate cooled, these near-surface quently exposedin the canyonwalls of Valles Marineris by aquiferswould have been frozen as interstitialice. As long as magmatic-drivendoming, tectonic deformation, and erosion. they remained frozen, they would remain trapped there, re- The basin model may collectivelyexplain (1) the paucityof gardlessof the changinggradients causedby the continued Noachianoutcrops and crustalmagnetic anomalies in the pro- developmentof the Tharsis bulge. This water would be re- posedbasin region, (2) the thick sequencesof layeredmateri- leased only by the energy associatedwith meteoric impact als exposedin the wallsof Valles Marineris,(3) an anomalous [Newsom,1980], resultingin the anomalouscharacteristics of concentrationof layered ejecta cratersin the Solisand Thau- impact craters in Solis and Thaumasia Planae. However, this masia Planae regions,(4) the volatile-rich nature of Valles model cannotexplain the giganticquantities of water required Marineris,and (5) the observedhematite deposits detected in to carve the outflow channels. Valles Marineris. The basin may also provide a sufficient There is yet another model that might explain the near- sourceof water necessaryto carve the circum-Chryseoutflow surfacepresence of water amongother characteristics.During channelsystems and the recentlyproposed northwestern slope the early rise of Tharsis[e.g., Banerdt et at., 1992; Gotombeket valleysand to form Hesperianand youngerputative ocean(s) at., 2000], for example,certain topographic irregularities may and (or) paleolakes. DOHM ET AL.: ANCIENT DRAINAGE BASIN OF THE THARSIS REGION, MARS 32,957

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