the origin of fluvial valleys and early geologic history...

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 95, NO. Bll, PAGES 17,289-17,308, OCTOBER 10, 1990

The Origin Of Fluvial Valleys And Early Geologic History, Aeolis Quadrangle, Mars

G. ROBERT BRAKENRIDGE

Su•ficial Processes Laboratory, Department of Geography, Dartmouth College, Hanover, New Hampshire

In southern Aeolis Quadrangle in eastern Mars, parallel slope valleys, flat-floored branching valleys, V- shaped branching valleys, and flat-floored straight canyons dissect the heavily cratered plateau sequence. Associated knife-like ridges are interpreted as fissure eruption vents, and thin, dark, stratiform outcrops are interpreted as exhumed igneous sills or lava flows. Ridged lava plains are also common but are not themselves modified by fluvial processes. I mapped 56 asymmetric scarps or ridges that are probable thrust faults. These faults exhibit an orientation vector mean of N63ow + 11 o (95% confidence interval), and they transect the lava plains and the older plateau sequence units. By comparison, the vector mean for the 264 valleys mapped is N48ow + 12 o, with a larger dispersion about the mean. The similar orientations displayed by thrust fault and valley axes suggest that valley locations are partly controlled by preexisting thrust faults and related fracture systems. Most valleys are also arranged orthogonally to, and along the perimeter of, the ridged plains. A possible model for valley development is: (1) freshly outgassed water became entombed as frost, snow, and ice within the cratered terrains during heavy bombardment and the accompanying deposition of impact ejecta, volcanic ash, and eolian materials, (2) effusive volcanism and lava sill emplacement heated subsurface ice in the vicinity of the ridged plains, and faults and fractures provided zones of increased permeability for water transport to the surface, and (3) headward sapping at thermal springs, thermokarst subsidence, and limited downvalley fluid flows then carved and modified the valleys.

INTRODUCTION

Ever since their discovery during the 1972 Mariner 9 planetary mission, the ancient fluvial valley networks of Mars have been described as relict from an earlier warmer and denser atmosphere [Sharp and Malin, 1975; Masursky et al., 1977; Chapman and Jones, 1977; Pollack, 1979; Cess et al., 1980; Pollack and Yung, 1980; Mars Channel Working Group, 1983; Kahn, 1985; Pollack et al., 1987]. If this inference is true, then these dry valleys constitute spectacular evidence for planetary-scale climatic change. Liquid water is not presently stable anywhere on the planer's surface, and the operation of an Earth-like hydrological cycle on Mars requires a much warmer atmosphere and one very much denser than the 7-mbar atmosphere of today [Pollack et al., 1987]. Also, the valleys are nearly restricted to heavily cratered landscapes dating from the Heavy Bombardment period of early solar system history [Pieri, 1976; Cart and Clow, 1981]. During this period, the Sun's luminosity may have been only 70% of its present value [Gough, 1981]. It is therefore unlikely that the valleys are the direct result of an earlier, more favorable climate associated with solar evolution. Large amounts of climatic change are most easily explained by depletion of an early dense

atmosphere rich in CO 2, H20, or some other greenhouse gas [e.g. Cart, 1987]. This dense atmosphere might have temporarily kept the planet warm, despite a fainter sun [Pollack et aI., 1987].

Calculations for ice-covered river flows on Mars [Cart, 1983]

suggest that fluvial features could form at present if sustained water discharge at the surface were to somehow occur. Heat loss

Copyright 1990 by the American Geophysical Union.

Paper number 90JB00540. 0148-0227/90/90JB-00540505.00

by conduction through modeled ice-covered rivers is slow, and latent heat is added to the system by water freezing (for a terrestrial example, see Corbin and Benson [1983]). The limiting factor for fluvial activity on Mars is water release, and not water persistence as an erosive fluid once discharged [Wallace and Sagan, 1978; Cart, 1983]. Water releases could be related to a

hydrological cycle and a dense atmosphere, but other alternatives are (1) solar heating and melting of dust-rich snow and ice deposited under high obliquity orbital conditions [Jakosky and Cart, 1987; Clow, 1987], or (2) geothermally heated waters reaching the surface through fractures and faults [Brakem'idge et al., 1985; Wilhelms, 1986; Brakenridge, 1987, 1988; Gulick et aI., 1988; Wilhelms and BaMwin, 1989]. Given these alternatives,

the inference of an early dense paleoatmosphere may be in error. Do the ancient valley networks compel inference of a large

amount of climatic change, or may other genetic models, without climatic change, suffice? The present report demonstrates that Martian valleys in Aeolis Quadrangle exhibit spatial patterns, stratigraphic relationships, and morphologies that are compatible with genesis through volcanism-induced hot spring discharges. The following sections (1) summarize known geologic events that occurred during the time period of valley evolution (2) document detailed stratigraphic and spatial relationships of Aeolis valleys to tectonic features and volcanic landforms; and (3) describe local

evidence for one valley network's episodic growth by subsidence, headward sapping, and downvalley fluid flows.

GEOLOGIC UNITS IN AEOLIS QUADRANGLE

Two disparate landscapes exist on Mars. One is heavily cratered and is dissected by relict valleys, and the other is lightly cratered or uncratered and is undissected. Aeolis Quadrangle is astride the planet-wide boundary between these two landscapes (Figure 1). The southern, heavily cratered landscape was created

17,289

17,290 BRAKENRIDGE, AEOLIS QUADRANGLE, MARS

North

I I ,] I • I • • !. ] . [ ,,I ]

_ - 60

50

40

Elysium Planitia I• .Qlympus 30 lvlons

, ,• ,• Amazonis Planitia ;•:• • z:• • • :'-:• • 10 20

'.:-:. • ..:.:':-:':':':': '. •P;:•2>;' • •:;• ;, • • ':

................... : ....... :e'.•e• '.•m• :•:• ß '-x•S••••P• •/•.-• --[

•... . • •,•.•:• .

240 210 180 150 120

Fig. 1. Map of major landscapes near Aeolis Quadrangle and their inte•ed ages, as redrawn from Barlow [1988]. The dark shading indicates surfaces Formed during h•avy bombardment, the lighter shading indicates surfaces formed n•ar the end ot heavy bombardmere (similar crater size

&equency distribution, but lower crater densities), and the white areas are lighfiy craigred or uncrat•red surlhccs Formed after the end ot th• h•avy bombardmere flux, approximately 3.2 Ga. The ages ot Olympus Mons and •hree volcanos in Elysium Planitia are also showfl.

during the final stage of planetary accretion (the late heavy bombardment), and local examples occur of densely cratered surfaces exhibiting lunar-like preservation of small craters [e.g., Cart, 1981, p. 69]. In contrast, the northern plains landscape is post-heavy bombardment in age, and may consist of sedimentary plains and/or lava flows. The cause of the planetary dichotomy represented by these two landscapes remains controversial [Wilhelms and Squyres 1984; Wise et al., 1979].

Martian time stratigraphy is divided into the Noachian System, the Hesperian System, and the Amazonian System [Tanaka, 1986] (see also Table 1). Each system is further subdivided into series, each with mapped reference units. The heavily cratered landscape in Aeolis is, mainly, of Noachian age and includes the dissected unit and the cratered unit (both middle Noachian) and the subdued unit (upper Noachian) of the plateau sequence [Greeley and Guest, 1987]. Scattered within the plateau sequence landscape are ridged plain units of inferred Lower Hesperian age (Figure 2).

The plateau sequence may include interstratified impact breccia and ejecta, reworked aeolian debris, fluvial or lacustrine deposits, lava flows or sills, and, possibly, ice [Tanaka, 1986; Wilhelms and Baldwin, 1989]. These deposits are locally dissected by valleys of probable fluvial origin (,Figure 2). In contrast, the lower Hesperian ridged plains are mapped as lava flow plains [e.g. Tanaka, 1986; Greeley and Guest, 1987], and they are not commonly dissected by fluvial valleys [Tanaka, 1986]. The size distribution of the superposed impact craters

indicates that most preserved ridged plains formed near the end o! heavy bombardment [Barlow, 1988].

An early Hesperian age for most undissected ridged plains has been used to constrain valley genesis in time. The following chronology is inferred by 'Fanaka [1986] in his global summary: (1) Noachian deposition of plateau sequence strata, (,2) late Nochian fluvial dissection, and (3) early Hesperian embayment

and partial burial of the plateau sequence materials by extruded lavas. This places a discrete interval of plains volcanism subsequent to extensive fluvial valley development [Tanaka, 1986] and also implies that any climate favorable to valley development had ended by Hesperian time: most landforms of this and younger age are not dissected. However, any chronology must be reconstructed from the preserved geologic record, and preservation factors should also be considered. Wilheims [1987, p. 279] models lava plains on the Moon as the visible results of increased preservation, not increased extrusion, as large-impact rates declined at the end of heavy bombardment. A similar genetic model for ridged lava plain preservation may apply to Mars.

In this respect, the maps of Scott and Tanaka [1986] and Greeley and Guest [1987] also include widely scattered, older ridged lava plains of Noachian age. These authors infer, as well, that interbedded flow volcanics are a common internal component

of the plateau sequence. Extensive plains volcanism may, therefore, have been underway during Noachian time, but such volcanism was not widely preserved before the early Hesperian. Table I gives this alternative process history reconstruction. The reconstruction agrees with the preserved stratigraphy described by Tanaka [1986] and with the crater statistic results of Gurnis [1981] and Barlow [1988]. It implies that fluvial valley development and ridged plain volcanism overlapped in time. Direct crater dates on valley networks by Baker and Partridge [1986] also indicate that valley networks range from Noachian through early Hesperian in age and thus independently support plains volcanism and valley development as coeval processes.

VALLEY CLASSIFICATION

Valleys developed on plateau sequence units exhibit semicircular theater-shaped headwalls and steep valley sides, relatively few and short tributaries, and aligned straight segments suggestive of fault or fracture control [Sharp and Malin, 1975]. In contrast to runoff-created valleys on the Earth, drainage densities are very low, and tributary junction angles do not progressively increase in the downstream direction. Apparently, structural controls on Martian valley location "randomize the junction angle systematics, while regularizing the link orientations" (D. Pieri, written communication, 1989). Such morphologic observations

suggest headward erosion from initial spring locations, instead of surficial runoff and progressive downstream integration into increasingly larger, topography following channels [Pieri, 1976, 1980].

An earlier classification of Martian valleys is based on

planimetric and cross-section morphology [Brakenridge eta!., 1985]. Figure 3 is a modified version of that classification and also incorporates commonly observed geologic contexts. The number of valley classes has been increased from five to six, and class numbers are revised to establish a size trend from smaller

BRAKENRIDGE, AEOLlS QUADRANGLE, MARS 17,291

TABLE 1. Geological Context of Valley Development in Aeolis Quadrangle

System a Series a Age Preserved

Ga b Geologic Units a

Processes t'

Hesperian

Noachian

Noachian

Lower 3.2 ridged plains; smooth unit

of the plateau sequence

Upper 3.5 subdued unit of the plateau sequence

Middle 3.85 hilly, dissected, and and cratered units of the

Lower 3.92 plateau sequence

heavy bombardment ends,

ejecta and ice deposition slow, effusive volcanism continues,

lava plains are widely preserved, widespread thrust faulting

continued heavy bombardment,

ejecta and ice deposition, effusive volcanism, formation of Ai-qahira and Ma'adim Vailes

solidification of the crust,

heavy bombardment, ejecta and ice deposition, effusive volcanism

aModified from Tanaka [ 19861, Greeley and Gleest [ 19871, and Barlow [ 1988].

bSpeculative (crater statistics-based) ages refer to upper boundary of series. cValley development n-my have occurred during all three time intervals.

North

I...• II 200km I mp12/, • Ap'ollinariseatera I 6 I ............. • • I ?"::::!::i•.

---:- ;- .......

220 210 200 190

-10

-2O

-30

180

Fig. 2. Geologic map of Aeolis Quadrangle, illustrating the older geologic units of Greeley a,d G,est [ 1987] and the fluvial valleys of Cart a,d Ciow [19811. Dark shading illustrates lower Hesperian ridged plains units, "Npld" is the dissected unit, "Npll" the cratered unit, "Np12" is the subdued unit, and "Hp13" is the smooth unit of the (middle Noachian to early Hesperian) plateau sequence. The dashed border separating the dissected unit from the cratered unit indicates the gradational contact between these two map units. The valleys are shown as thin solid lines, and the wrinkle ridges are shown as thick dashed lines; thick solid lines are large chasms or scarps. At feature 1, "r" marks smooth plains material, and "e" marks ejecta associated with this crater. All numbered features are described in the text.


III

PARALLEL

SLOPE VALLEYS

FLAT-FLOORED STRAIGHT

CANYONS

II

.3-5 km

IV

5 km

1 km

V-SHAPED

BRANCHING VALLEYS

(Upstream) 5 km

(Downstream) 2 km

FLAT-FLOORED BRANCHING

VALLEYS

I ' I i I

-1o km , .7S km V-SHAPED TRIBUTARY

CHASMS FRETTED CHANNELS

V VI

10 km 5-30 km

Fig. 3. Combined planimetric/cross-sectional classification of valley networks on Mars, revised from B,'aken,'idge et al. [ 1985]. Shown for each class are representative map views (on the left) and valley cross sections (on the right); dimensions are approximate. Typical relationships to surfitce geology are also shown, "RP" symbolizes ridged plains flow volcanics, "PS" units are plateau sequence materials, "Cf' represent.,/.' modified crater floor deposits, and "Avf" indicates large chasm interior deposits.

(classes I and II) to larger (classes III-VI) valleys. Valley widths given in the figure are typical values; depths are estimated and are not well established. Note that the enormous, hundreds of

kilometer-wide, "outflow channels" [Mars Channel Working

Group, 1983] are not included in Figure 3. As determined from crater counting, the outflow channels are of a wide variety of ages and have, therefore, never been used as evidence for or against an early dense Mars atmosphere.

10 km). The straight canyons are relatively short and commonly debouch at ridged plain/plateau sequence boundaries. Most exhibit smooth floors that appear to be continuous with the plains (Figure 3). The headward terminations of these canyons are steep, and the straightness of canyon wall orientations suggest fault or fracture controls. In contrast to the canyons, the flat-floored

branching valleys traverse hundreds of kilometers of complex plateau sequence landscapes (Figure 3). Large-scale circular

Class I valleys (parallel slope valleys) and class II valleys (V- patterns may suggest that their locations are partly controlled by shaped branching valleys) include the finest scale valleys visible on Viking imagery. The parallel slope valleys occur on the flanks of large modified crater landforms or on other steep, relatively uniform slopes that are adjacent to low-albedo ridged plains. The V-shaped branching valleys are of similar dimensions and geologic setting, but branch upstream, and their upstream reaches exhibit narrow, V-shaped cross sections. Both valley classes commonly dissect the plateau sequence, and terminate at the margins of adjacent ridged plains (Figure 3).

Class III (flat-floored straight canyons) and class IV (flat- floored branching valleys) exhibit flat valley floors, and widths comparable to terrestrial river valleys of moderate to large size (5-

impact-related faults or fractures [Brakenridge et al., 1985; Gulick, 1986; Schultz et al., 1982]. Gaps between segments of these dry valleys may be locally caused by post-valley faulting or by piecemeal valley genesis through headward sapping (Figure 3; see also following discussion).

The last two classes occur only at a few locations within Aeolis. Many class V (V-shaped tributary chasm) and class VI (fretted channel) valleys are young uncratered landforms and are, therefore, not evidence for changed conditions. The V-shaped chasms are characterized by steep gradients, and by large widths and depths compared to lengths. They occur as tributaries to the Valles Marineris and other large chasms and to some large

BRAKENRIDGE, AEOLIS QUADRANGLE, MARS 17,293

outflow channels. The V shape may result from intersecting only; and Figure 5c, undifferentiated faults and fractures only. debris slopes that are still active. The fretted channels exhibit The measured orientations are illustrated by rose diagrams in locally sinuous valley reaches, suggesting that downstream fluid these maps, and the relevant descriptive statistics are summarized flows may have occupied the entire valley widths (see discussion in Table 2. In computing the statistics, no weighting is used for by Baker [1982]. The floors of some fretted channels may consist feature lengths (short linear features are included on an equal basis of debris mantles whose movement is facilitated by interstitial ice with long ones) and, for gently curvilinear features, the two end~ [Squyres, 1979]. This report is concerned with the origins of points of the feature define the orientation. valley classes I-IV: those valleys that are relict from early Mars The 264 measured valleys (Figure 5a) in Aeolis commonly history and thus indicative of changed conditions. occur in north to northwest orientations; the vector mean is

N48ow + 12o (95% confidence interval). The 56 inferred thrust

GEOMORPHOLOGICAL MAPPING faults exhibit similar orientations (Figure 5b), with a vector mean

Heavily cratered southern and central Aeolis is a land area of of N63ow + 1 lO (95% confidence interval). The uniform 4.3 x 106 km2, or approximately the size of the United States east of the Rocky Mountains. The abundant Mariner 9 and Viking orbiter imagery available ranges in scale from single frames including all of this land area to frames with a resolution of 33

m/pixel and land area "footprints" of approximately 70 km 2. The Aeolis portions of two previously published maps (the 1:15,000,000 geologic map of eastern Mars [Greeley and Guest, 1987] and the global map of valleys [Cart and Clow, 1981] are superimposed in Figure 2. The figure thereby illustrates an apparent association of valleys with Noachian plateau sequence terrains, and a lack of valleys on the Hesperian ridged plains. Valleys appear to be most common in the general vicinity of the ridged plains, and also near the large chasms or scarps.

Figure 4 illustrates a new geomorphological map of Aeolis Quadrangle prepared using, as base maps, the 1:2,000,000 Viking photomosaics [U.S. Geological Survey, 1979a, b, c, 1982]. This map emphasizes (1) fluvial valley features, (2) other linear features of possible tectonic origin, and (3) ridged plains and individual volcanic constructs. Impact craters are not illustrated except where they form the origination or terminus of a mapped valley. For geomorphic mapping purposes, "fluvial valleys" are narrow linear or curvilinear troughs with dimensions similar to those given in Figure 3. "Undifferentiated faults or fractures" are linear or slightly curvilinear ridges or (ill-defined) lineations. On high-resolution imagery, several of these lineations appear to be highly elongated strips of knobby or hilly topography. Finally, "thrust faults or wrinkle ridges" are linear or curvilinear asymmetric ridges that exhibit steep scarps and relatively gently sloping land on opposing sides. These features are the probable complex surface expressions of deep-seated faulting within a compressional stress field [Plescia and Golombek, 1986; AubeIe, 1988]. They commonly form complex wrinkle ridges in the lower Hesperian ridged plains, but relatively simple scarps in the surrounding, apparently weaker, plateau sequence deposits.

VALLEY, FAULT, AND FP,.ACTURE ORIENTATIONS

Most sapping models for valley origin predict that faults and fractures, where present, should be important in localizing groundwater discharge at the surface. However, preferred valley erosion along faults and fractures may obscure the underlying structural features, or such features may be covered by undisturbed deposits. In order to test for the presence of structural

distribution hypothesis for both valleys and thrust faults can be rejected at the 0.05 significance level, and the confidence intervals about both means overlap. This, as well as visual comparison of the rose diagrams (Figures 5a and 5b), suggest that valley growth may have been affected by preexisting, northwest oriented faults and/or fractures.

The strengths of the vector means (R/n; Table 2) measure the amount of dispersion in each data set. These statistics can range from zero for very high orientation dispersions to 1 for very low dispersions. The valleys (R/n = 0.39) are more dispersed than the thrust faults (R/n = 0.75). This difference suggests that other variables also influenced valley orientations.

In regard to the other photolineations, the calculated vector mean for the 83 undifferentiated faults and fractures is N49OE, but

visual inspection of the rose diagram suggests a bimodal

distribution and two mean orientations, at approximately N10ow and N65OE (Figure 5c). No confidence intervals can be calculated without first subdividing these data, and the number of observations is not sufficient for a reliable subdivision. Despite these limitations, the undifferentiated faults and fractures exhibit

different dominant orientations, as well as differing morphologies, than the thrust faults. Valley genesis may have been affected by these faults and fractures, also, as some overlap of orientations is evident (compare Figures 5a and 5c).

GEOLOGICAL CONTEXTS OF AEOLIS VALLEYS

The orientation analysis suggests that structural features did influence the location of valley development in Aeolis. However, other geological factors should be important as well [e.g., Kochel and Phillips, 1987]. Preexisting scarps of non-tectonic origin may also be favorable sites for spring sapping to be initiated. If geothermal heating is important, spatial controls may be exerted by locally high geothermal gradients related to volcanic or tectonic activity. Additional information concerning the geological contexts of Aeolis valley development is present in the 1:2,000,000 Viking photomosaics. The photomosaics are widely available [U.S. Geological Survey, 1979a, b, c, 1982] and are not reproduced here. The following large-scale features are important to the question of Aeolis valley morphogenesis, are visible in the photomosaics, and are located in Figures 2 and 4.

Feature Descriptions

Feature 1. Superimposed on the flat floor of this large crater is

controls on valley orientations, Figure 4 is abstracted into three a preserved interior remnant ("r" in Figures 2 and 4) of smooth component maps: Figure 5a, valleys only; Figure 5b, thrust faults plains material similar to that mapped immediately to the north


z

z

'* I

.: z,.. :7' .- z

/'/,z


I

I

I

220 o

I i

I

I

2oo ø 190 • 18(P

B

N=56

...... '-•

, $

220 ø 210 ø 200 ø 190 ø 180 ø

Fig. 5. Thematic maps obtained from Figure 4, (a) Small valleys only. (b) Thrust faults only. (c) Undifferentiated faults and fractures only. Also shown are rose diagrams of orientation data for each class of features.


i

l

220 ø 210 ø 200 ø 190 ø 180 ø

Fig. 5. (continued)

[Greeley and Guest, 1987]. Light-dark banding along the southeastern margins of the remnant indicates that this material is internally stratified. A V-shaped valley with four straight segments breaches the raised southwest rim of the crater (Figures 2 and 4), and the drainage direction was toward the crater floor.

How old is the valley? Greeley and Guest [1987] map the crater and its ejecta as superposed on the upper Noachian subdued cratered unit of the plateau sequence (Figure 2). However, the ejecta ("e" in Figure 4) and the associated strings of secondary craters are prominent and sharply defined to the south of •he subdued unit boundary and become abruptly diffuse or absent at

the boundary and closer to the crater . The ejecta and the secondary craters thus appear to be partially buried by the subdued cratered unit. The actual chronology of events may be (1) large impact into older plateau sequence units, (2) partial burial of the crater and ejecta by the subdued cratered unit, and (3) subsequent valley erosion and removal of much of the crater fill. Single, short valleys radial to old, flat-floored modified craters are common in

Aeolis (Figure 4). Valley cutting may extend into Hesperian time, and may be coeval to crater modification processes.

Feature 2. A strip of plateau sequence terrain separates two lower Hesperian ridged plains at this location, and it is heavily

TABLE 2. Tectonic And Fluvial Landform Orientations

Landforms n Vector

Mean a Strength Standard Raleigh Test of Vector Error for

Mean b Uniformity c

Small valleys 264 N48øW + 12.0 ø 0.39 6.1 0.00

Undifferentiated 83 N49øE 0.19 23.1 0.05

faults and fractures

Thrust faults 56 N63øW + 11.2 ø 0.75 5.7 0.00

a Vector mean is arctan [X/Y]; X = Y• cos Oi; Y = • sin Oi; shown with 95% confidence intervals. bStrength of vector mean is R/n, where R = [X 2 + y2] 1/2. CRaleigh statistic is exp -[R2/n] ß for Raleigh values <0.05, the uniform vector distribution hypothesis is rejected at the 0.05 level of significance.


dissected by V-shaped branching valleys. Based on the branching pattern, the most prominent valleys shown on the Viking photomosaics drain toward the northwest and terminate at the western ridged plain border. The location of this heavily dissected land is typical for Aeolis.

Approximately 70 km to the southwest, a NNE trending flat- floored straight canyon extends nearly to an intersection with a

NW trending scarp interpreted as a thrust fault. The SW-facing scarp is mapped as a valley by Cart and Clow [1981 ], but there is no opposing wall. Near to the scarp, the valley tums abruptly NW and is located along it; the valley then becomes indistinct in a

complex area near the margin of the western plain. A permissive inference is that valley cutting here was post thrust faulting.

4). Two unusually wide east and northeast oriented (adjoining) reaches exhibit dark floors and scalloped margins and are transitional northeastward into an irregular closed depression with abundant knobs that are probable collapse blocks. Genetic processes could include sill volcanism and overburden collapse, perhaps along a fault or fracture zone. However the chasms themselves originated, numerous post chasm valleys extend headward from the chasm scarps into the surrounding plateau sequence materials. One such valley, at the chasm's southern terminus, appears to follow a secondary scarp mapped as a thrust fault (Figure 4).

Approximately 100 km to the east of feature 6 is an isolated 40-km-long plateau or mesa (Figure 4). At least 14 light and dark

Feature 3. At this location, dark-floored V-shaped branching strata crop out along its hillslopes and are visible in the valleys dissect lighter plateau sequence material. Drainage photomosaic [U.S. GeologicalSurvey, 1979c]. The banding can direction is westward and toward a ridged plain that is mapped in be traced continuously along the perimeter of the plateau, and the Figu•re 4 but not in the smaller scale Greeley and Guest [1987] concentric outcrop pattern suggests that the plateau is an erosional map (Figure 2). Immediately to the southwest of the valleys, an remnant formed by approximately horizontal strata. Similar interdigitate contact is visible between a dark surficial unit (to the regularly bedded internal stratification is noted also at feature 1 west) and a much lighter unit (to the east). The simplest and has been previously described for the plateau sequence units explanation for this contact is differential stripping of the lighter, [Malin, 1976]. The origin and nature of such stratification are an superposed stratum from an underlying darker stratum. Thus the unresolved question: possibilities include interbedded sediment terrain immediately surrounding the valleys appears to be and lava; interbedded sediments of varying clast lithology, mean underlain by several stratiform units of contrasting albedo. The size, or matrix; or interbedded sediment and ice similar to that

..

dark valley floors have tapped an underlying darker stratum.

Feature 4. Valley classes I-III densely dissect this plateau sequence terrain, and many of the valleys exhibit dark floors. There is disagreement regarding the nature of two adjacent plains: (1) The plain to the west of the valleys is not illustrated in Greeley and Guest's smaller-scale map (compare Figures 2 and 4). Valley

occurring in the polar layered terrains.

Feature 7. Faulting or fracturing was probably involved in the origin of this 180-km-long, north northeast oriented chasm, but wall slumping, floor collapse, or fluid flows along the chasm may also have played important roles. Headward erosion then carved the smaller tributary valleys that debouch into the canyon along its

branching directions, however, indicate that the surface of this southeastern margin. relatively smooth plain is lower than the surrounding heavily Feature 8. Two en echelon, northeast trending ridges are here dissected and higher-albedo landscape. (2) Greeley and Guest mapped as undifferentiated faults or fractures (Figure 4). The [1987] map the eastern plain as a superposed smooth unit of the western ridge, approximately 60 km in length, forms a straight plateau sequence, instead of as a ridged plain (compare Figures 2 segment along the northern rim of the large crater Molesworth and 4). I could find no evidence for such superposition: the (not shown), and at least seven small class I valleys originate plain's western margin is below the adjacent surface of the plateau along this ridge and debouch southward at the smooth crater floor sequence, and numerous dark-floored valleys debouch from the (also not shown). The eastern ridge, approximately 20 km in plateau sequence materials at the margin of the plain. These length, is less well-marked but also is coincident with the heads valleys are arranged othogonally to the boundary, and the of four similar valleys that exit northward onto a ridged plain. complex, interdigitate nature of the boundary itself is similar to Suggestive evidence of further tectonic complexity is present in that southwest of Feature 3. the form of a 25-km-diameter crater bisected by the eastern

As noted, the simplest interpretation of the outcrop pattern at terminus of the western fault (Figure 4) and showing an apparent this and other interdigitate boundaries is that a dark, stratiform unit, entombed within the plateau sequence, extends from relatively interior positions (where it is exposed by the deep valley floors) to the subaerial p!ains surface itself. The dark valley floors may represent lava sills or buried lava flows that are now exhumed by erosion (see discussion of such stratigraphy by Wilhelms and Baldwin [ 1989]).

offset of approximately 10 km in the right lateral sense (examine U.S. Geological Survey [1979b]). Whatever the exact nature of these faults, the associated valleys appear to be syntectonic or post-tectonic features: they are not transected by the faults.

Feature 9. This 40-km-wide, 150-km-long belt of dissected plateau sequence is adjacent to an unusually well defined ridged plain. The valleys debouch onto this plain. At least 20 class I and

Feature 5. At this typical location, southeastward draining II valleys can be counted on the photomosaic; 10 of the more subparallel slope valleys and branching V-shaped valleys dissect the plateau sequence, and are located about the periphery. of an unambiguous ridged plain (compare Figures 2 and 4).

Feature 6. Immediately to the south of a northeast oriented chasm also mapped by Greeley and Guest [1987] is a complex and interconnecting system of broad, flat-floored chasms (Figure

prominent ones are shown on Figure 4. Feature 10. Al-qahira Vallis, a 600 km long, 25 km wide

chasm, is discussed by Sharp and Malin [1975] as an an outflow channel (Figure 4). However, unlike many outflow channels, no channel bedforms are visible (see discussion of such criteria by

Baker, 1982). The exceptionally straight wall segments suggest


structural control over sapping or, perhaps, an active tectonic (rift?) origin. In support of such inferences, other faults or fractures are abundant in this vicinity, and many are parallel to the NNE and WNW trends of Al-qahira's walls (Figure 4). In detail, the chasm walls are scalloped, and a variety of much smaller tributary valleys are eroded into the surrounding, mostly undissected, plateau sequence. If faulting was involved in the origin of Al-qahira, then these small valleys also postdate, or are coeval to, this period of faulting.

No ridged plains are mapped in this area, but a 60-km-wide, approximately circular, positive topographic feature is present along the chasm's west side (Figure 4). Associated radial reentrants separating sloping, plateau-like surfaces and a visible central crater suggest that this landform is a "hydromagmatic" (Tyrrhena Patera-like) volcanic complex. Such volcanic piles on Mars are inferred to result from basaltic eruptions in ice-rich terrains [Greeley and Spudis, 1987; Crown et al., 1988]; the growth of this one may also be coupled to tectonism along A1- qahira.

Feature 11. This north and northwest trending chasm may be the erosionally and/or slumped surface expression of a low-angle thrust fault. Thus the eastern portion of a 20-km-diameter crater on the northeastern block has, apparently, been thrust over the southwestern block, where no visible counterpart crater fragment exists (Figure 4; see also U.S. Geological Survey [1979a]). An alternative suggested by one reviewer (that the miss!ng southwestern crater half has been eroded) is possible, but there is no accompanying mechanism for erosional removal of one crater half and not of the other. If thrusting indeed initiated this landform's genesis, then the implied compressional stresses are congruent with those that produced the many other northwest oriented thrust faults in this region.

A 22-km-wide volcano is present along the extension of this chasm to the south and within the inferred over-riding block, east of the surface trace of the fault (Figure 4). This volcano exhibits clear diagnostic topography, such as an apica! caldera, steep symmetrical flanks, and radial reentrants (see also Figure 7.15 by Greeley [1987, p. 165]). The plateau sequence here is not heavily dissected, but the chasm itself may have been modified by downstream fluid flows.

Feature 12. Finally, Ma'adim Vailis is a 700-km-long, 15-km- wide, gently winding chasm about 1 km deep [Sharp and Malin, 1975]. Numerous flat-floored straight canyons and other valleys form scattered tributaries to it. Fluid flows may have occurred along this chasm: an inner channel exists near its downstream terminus where the chasm transects a modified crater (location

shown in Figure 4), and medial, stream-lined ridges occur along several reaches of the flat chasm floor. However, tectonic

plateau sequence deposits to be stratified and, perhaps, composi.tionally heterogeneous. All of the mapped valleys transect the plateau sequence, and most are arranged orthogonal to, and along the perimeter of, ridged volcanic plains. However, some valleys occur as tributaries to large chasms that are surrounded by otherwise lightly dissected or undissected plateau sequence landscapes. Four specific examples are also noted of inferred syn-faulting or post-faulting valley development, and these examples support the orientation statistics-based conclusion that faults and fractures are important controls over valley locations.

TABLE 3. Summary of Viking Photomosaic-Based Observations

Observation Feature Numbers

Valleys transecting plateau sequence deposits Valleys orthogonal to ridged plains Valleys tributary to chasm walls Low albedo valley floors

Syn-tectonic or post-tectonic valleys Evidence for down-valley flows

Interstratification of plateau sequence deposits

! ,2,3,4,5,6,7,8,9,1 O, 12

2,3,4,5,8,9

6,7,10, 12

3,4,6,7

2,6,8,10, 12

7,11,12

1,6

Complex interdigitate boundaries occur between the relatively smooth, low-lying, darker, ridged plains and the adjacent higher, dissected, lighter, plateau sequence terrains. If the ridged plains are floored by extruded volcanic units., then two alternatives could explain such boundaries: (1) Surface lava flows, originating within the plains, partially fill the downstream reaches of valleys cut into the surrounding terrain but do not cover the interfluves. In this case, the valleys are embayed by, and are older than, the

,

lavas (as inferred by Tanaka [1986]). (2) Stratiform sill lavas, injected into the plate•tu sequence from below, exit onto the surfac'e at the bases of slopes bounding th, e plains. In this event, the volcanic strata forming the plains extend in.to the surrounding

,

plateau sequence units, and are locally exposed at the surface by valley incision there (as inferred by Wilhelms and Baldwin [1989]). If this is the actual stratigraphy, then valley ei'osion postdates or is coeval to phi.ns volcanism. I ,favor the second alternative for the examples cited, because the relatively dark

,

valley floors extend continuously to the valley headwalls (see class III valley in Figure 3 for an example of the resulting outcrop pattem).

processes may also be involved in chasm genesis: (1) the north There also exist in Aeolis small, commonly unmapped, ridged and northwest orientations of two major segments are congruent plains confined within flat-floored craters. One larger than normal with the preferred orientation of the regional thrust faults, and (2) example 'is the rid. ged plain within a modified crater immediately a 30-km-diameter flat-floored crater transected by Ma'adim west of feature 11 in Figure 4. A hypot. hesis to explain such (south of the "12" symb. ol in Figure 4) appears to be left-laterally associations is that the deep-seated ring fractures associated with displaced approximately 8 km. impact structures provided conduits for crater-interior lava flows

and/or for lava sill injections into remnants. of the stratified, Summaw possibly ice-rich post-crater deposits [Costard and Dollfus, '1987].

Table 3 lists the qualitative photogeological observations as Crater modification processes may, in this event, be genetically they relate to valley genesis. Local banded outcrops indicate the related to valley development.


FLUVIAL VALLEYS AT HIGHER RESOLUTION

Valleys in Plateau Sequence/Ridged Plain Borderlands Additional observations of valley geological contexts at higher

surface resolution are useful in analyzing valley origins. Figure 6

is a geomorphological map of a portion of southern Aeolis

constructed from a mosaic of Viking orbiter images; see Figure 4 for the location within Aeolis. Illustrated are ridged plain and plateau sequence terrains, inferred thrust faults (wrinkle ridges), a prominent lava flow front within one of the ridged plains, other

faults or fractures, flat-floored straight canyons, and small V- shaped branching valleys. In agreement with the observations

Ejecta

.:........• Unmodified, Superposed Craters

Modified Craters

'-'- Buried t • I I , , Craters

i

i

i i

Frames A-D, Figure 7

Npld

Npld i

ii$ ,

k

Ridges Interpreted as Thrust Faults

ß Possible Volcanic Constructs

20 km

Ridged Plains (arrows indicate flow margin)

Modified Crater Floor Sediments

Valleys or Large Collapse Depressions

Undifferentiated Faults or Fractures

J Npld , ]

Dissected Unit of the Plateau Sequence s: Smooth to rough, largely intact r: Isolated smooth remnants

d' Intricately dissected k: Karst-like

Fig. 6. Geomorphological map of a portion of Aeolis Quadrangle, illustrating valley development along plateau sequence/ridged plain borderlands. The map is based on a mosaic of 11 Viking frames (425S27-31' 426S26-31); map location is illustrated in Figure 4.


made above, the fluvial landforms (1) are incised into the plateau sequence deposits, (2) are oriented orthogonal!y to the ridged plain, and (3) terminate at the the ridged plain. Figures 7a, 7b, 7c, and 7d are four of the Viking frames used in producing the map.

In Figure 7a, the small branching valley below and to the right of the "A" symbol could be interpreted, on lower-resolution imagery, as embayed by the lava plains (to the left). However, this frame demonstrates that the materials incised by the valley are at a considerable altitude above the ridged plain (note the scarp near the widest portion of the valley). Instead of having undergone embayment, the valley must have developed during or after plains emplacement. In the lower right quarter of the image, abundant irregular closed depressions cause a scab-like appearance of this marginal plateau sequence area. The boundary between the plains and the plateau sequence is not sharp, as expected for embayment, but is irregular and marked by apparent collapse of the plateau sequence material at some locations.

In Figure 7b, a visible lava flow front (arrows; also see examples from Theilig and Greeley [1986]) is approximately

sequence is modified by a complex, closely spaced network of closed depressions: the entire surface is pitted and also is gouged by troughs (the "karst-like" plateau sequence of Figure 6). Several flat-floored straight canyons also occur, but regional collapse here dominated over fluvial erosion. Clear embayment relations with the plains are absent. Instead, the ridged plain material interfingers in a very complex manner with the collapsed plateau sequence. This detail is missing on the 1:2,000,000 Viking photomosaics [U. S. Geological Survey 1979b], which misleadingly show the boundary to be relatively sharp and congruent with an embayment interpretation.

The lava flow front mapped in Figure 6 and illustrated in Figure 7b suggests that lava venting occurred from a source area to the west, but vents are not visible. It is possible that the vents lie buried within the collapsed plateau sequence that forms the western margin of the plain. Movement of effusive lavas to the east could have occurred as one or more lava sills localized along subsurface lithological discontinuities. These sills may then have emerged as subaerial lava flows in the area now mapped as a

parallel to the ridged plain/plateau sequence contact, but the ridged plain. If plateau sequence strata include ice-cemented eastward flowing lava did not reach that boundary. Several flat- clastic material, sill volcanism could explain plateau sequence floored straight canyons and modified craters are floored by collapse (see also Wilhelms and Baldwin [1989] and Squyres et al. similar appearing, smooth material that is continuous with the lava [1987]).

plain (center of image). In the lower right corner of the frame, An alternative stratigraphy could be locally important at ridged three V-shaped valleys extend headward into the plateau sequence plain/plateau sequence borders. Studies of the stratigraphy of from scarps produced by local collapse, again at the ridged Columbia Plateau (U.S.)basalts document "invasive" behavior of plains/plateau sequence boundary (see also Figure 6). Valley those lavas where they encountered much less dense marine erosion was here preceded by scarp production, and the collapse sediments. There, 120 m thick surface lava flows deeply intruded, features themselves may be related to the nearby volcanism. An at their margins, siliclastic sediment piles [Wells and Niem, 1987; alternative hypothesis (that plains volcanism simply embays older, Byefly and Swanson, 1987; Pfaffand Beeson, 1987]. If sediment already dissected and locally collapsed terrain) lacks the needed densities of the plateau sequence strata are relatively low, then supporting evidence of clear lava flow fronts along the complex invasive lava behavior may have occurred. This could also and highly irregular plains/plateau sequence contact. explain some interfingering of plateau sequence and the ridged

Given the presence of subaerial lava flows, some examples plain materials. should exist of lava embayment contacts with older landforms. In Figure 7c, the eastern margin of the plateau sequence highland is densely dissected by numerous small valleys and ravines (the "intricately dissected" unit of Figure 6). The fluvial landforms are rectilinear, parallel, or digitate near the right center of the frame, and such detailed modification of the plateau sequence borderland is accompanied by two relatively large, flat-floored branching canyons. The plains/plateau sequence contact here could reasonably be interpreted as one of embayment: it is relatively

sharp and regular, and the intricately dissected hillslopes appear to dip, at various angles, into and below the plains material.

Despite the possibility of embayment, it is not clear that fluvial

Valleys Within The Plateau Sequence

Several ancient Aeolis flat-floored branching valleys coalesce into integrated systems extending hundreds of kilometers; they follow regional topographic gradients, exhibit valley widths of 5- 10 km or more, and are not proximal to large ridged plains units. A geological map and image of a portion of such a valley system comprise Figures 8a and 8b; see Figure 4 for the location in Aeolis. This and similar integrated valley systems appear to constitute the clearest evidence for greatly changed atmospheric conditions on Mars. However, Brakenridge et al. [ 1985] propose an alternative hypothesis: that the valley system developed in a

landscape modification here predates plains volcanism. The piecemeal fashion, through headward sapping and fluid flows floors of the relatively large straight canyons are continuous with, and not embayed by, the plains. Several cone-shaped, radially rilled mountains occur on the plateau sequence near its northwestern border (marked by arrows, top of Figure 7c; see also Figure 6). These mountains appear to be volcanic constructs (using criteria given in Greeley and Spudis [1987]) and they are accompanied by extensive modification of the plateau sequence.

caused by impact melts and thermal springs. Part of the Brakenridge et al. [1985] hypothesis may be

unnecessarily restrictive. Thus, endogenetic volcanism as well as impact melt could be an important heat source for thermal springs. As illustrated in the earlier report, two branches of the Figure 8 system form a 95-kin-wide circular pattern. However, the terrain interior to these branches is complex and itself cratered: no direct

Volcanism and valley dissection in this example are, at the least, association of a large crater with the valleys is now visible. Better spatially related; they may also be causally related (see other candidate sites exist for direct impact melt heating as a factor in examples in Wilhelms and Baldwin, 1989). valley genesis (e.g. see Mouginis-Mark [1987, p. 282]. Although

In Figure 7d, and to the west of the ridged plain, the plateau preferential excavation of the Figure 8 flat-floored branching

BRAKENRIDGE, AEOLIS QUADRANGLE, MARS 17 303

A

N

Fig. 8. (a) Geomorphological map and (b) image (Viking frame 596A26) of a portion of the extensive flat-floored branching valley system located in Figure 4. See Figure 6 for symbology key to the map. Letters mark locations discussed in the text, and the position of Figure 9 is also illustrated.

valleys may indeed be related to an old, buried, impact-associated "c"). The plains may represent either subaerial lava flows or ring fracture system, valley development could have greatly post- exhumed sills associated with fissure ridge emplacement. The dated cooling of the impact melt. combined igneous activity certainly could have provided local

The internally complex nature of the plateau sequence is heat sources for ice melting and thermal springs. visible in Figure 8 because some strata have been partially The entire valley network is locally interrupted at numerous removed. At locations "a" and "b" (Figure 8a), straight knife-like locations and especially in the headwater regions {,see detailed ridges exhibit much lower albedos than surrounding lithologies, map in Brake/,'idge et al. [1985]). It is not certain whether (1) a and are similar to features interpreted as fissure eruption ridges by continuous, integrated valley system once existed and was then Wilhelms [1986]. Their cross sections are exposed in the wall of a modified by post-valley resurfacing processes such as cratering, prominent erosional scarp (compare Figures 8a and 8b to locate volcanism, or eolian deposition; or (2) the valley links developed this exposure; note clear outcrop at letter "b"). Two plains independently, at different times. In the latter case, the system adjacent to the scarp are underlain by similar, dark material, and was never more integrated than at present. an igneous origin is supported by their physical continuity with Some evidence supports the latter possibility. Although post- the knife-like ridges. A third plain lies to the north (near letter valley modifications are obvious at some locations (e.g., fresh


B

Fig. 8. (continued)

t

superposed craters interrupt a valley segment near "c"), other valley interruptions appear to be primary. For example, northwest trending ridges interpreted as thrust faults are common in Figure 8. Isolated valley links of the valley system terminate at them (at "c" in Figure 8a). In agreement with the regional orientation analysis, a major northwest oriented tributary valley (near location "e", Figure 8a) is aligned along a thrust fault: this local episode of valley development postdates the faulting. Also, a valley segment breaches the crater rim at "f", instead of being transected by it. Similar primary drainage net gaps occur along other branching valley networks, and they are suggestive evidence for valley erosion controlled mainly by local fault and fracture systems instead of by topography.

Individual, continuous downstream reaches along this valley

system do reach considerable lengths. At "f" in Figure 8a, the

continuous to an abrupt terminus approximately 150 km to the northeast, at an elevation approximately 500 m lower (see also Brakenridge et al. [1985]). The valley is no wider at the terminus than it is far upstream, and its general morphology resembles certain fretted channels which may still be active today (see example of Cart [ 1981, pp. 154-155]; also, compare valley classes IV and VI, Figure 3). Carr considers the young fretted channels to result from some combination of valley-side mass wasting and downvalley debris flow, possibly assisted by interstitial ice. Such processes may also have been active in the ancient past, and large environmental changes are not required for their occurrence.

Other Evidence for Valley Mo;phogenesis

Figures 9a (Viking frame) and 9b (map of the frame) provide a close view of the morphology and stratigraphy of one reach of

valley originating at the breached modified-crater rim first the above-discussed valley. In the frame, a flat-floored modified transects a rounded, northwest oriented ridge. It is then crater separates two valley segments, and a relatively dark, thin,


A

Fig. 9. (a) Location map and (b) image of a small portion of the flat-floored branching valley system mapped in Figure 8. The image is cropped from Viking frame 427S03 and is approximately 32 km in width. See text for descriptions at letters A, B, and C. In Figure 9a, the heavy stippling is inferred dark igneous material, and the white arcs transecting the valley floors are scarps discussed in the text.

and resistant stratiform unit occupies the central portions of the crater's interior (above letter "A"). This may be either an exhumed lava sill or a lava flow. Stratigraphically below this unit is a higher albedo, slope-forming stratum that is much thicker and could represent impact ejecta, volcanic ash, or eolian-reworked materials, perhaps once associated with interstitial ice. The light stratum rests, in turn, on another thin, dark unit (below letter "B"

in Figure 9). The entire sequence is now bounded by a deep, trough-shaped depression (immediately adjacent to "B"), which is developed at the approximate position of the old crater rim. It is clear that subsidence has occurled in this area. It is possible that it could be involved, also, in the initial genesis of nearby valley segments along lhults or fractures.

Two other geomorphic features preserved along the present valleys may also relate to genetic processes: (1) Below the letter

"A" in Figure 9, and also to the left of the letter "C", are small, faintly lobate scarps situated across the fiat valley floor. The scarps may mark the distal termini of downvalley. freezing water, ice, and/or debris flows. Episodic valley growth and modification would then be implied, and also valley erosion by wall-to-wall fluid flow. This valley may actually be a relict channel. !,2) Valley-filling fluid flows are seemingly supported by the presence of a long narrow interfiuve downstream from the junction of two tributary valleys: imlnediately above "C" in Figure 9. Such interfiuves are typical of glaciated piedmont valley junctions on Earth and also some fretted channel junctions on Mars. These small-scale features are near the limit of frame resolution, and

alternative genetic models exist. They do indicate, however, that new images from Mars Observer will be useful in constraining valley origin models and associated climatic change inferences.

17,306 BRAKENRIDGE, AEOLiS QUADRANGLE, MARS

B

o ©

c

A

Fig. 9. (continued)

CONCLUSIONS

The valleys mapped in Aeolis exhibit strong preferred alignments that suggest the past operation of structural geologic controls over valley location. This supports the general conclusion that headward spring sapping is important in valley genesis [Sharp and Malin, 1975; ?ieri, 1980]. Also, evidence exists at a variety

of image scales for ancient volcanic activity near many Aeolis •talleys. Probable fissure eruption ridges, small volcanos, exhumed lava sills, and collapsed, (scabby) karst-like morphologies all occur within the fluvially dissected plateau sequence, and these features suggest the occurrence of extensive subsurface igneous activity. A possible valley genesis model is that, in response to widespread effusive volcanism in interstratified, ice-rich terrains, local subsidence occurred along fractures and faults and produced scarps that intersected local aquifers. Heated spring discharges issuing from these scarps may then have carved the valleys through a combination of (probably

episodic) downvalley water, ice, and debris flows, and headward erosion along stratal discontinuities and individual conduit faults and fractures.

Although Aeolis valleys might have been carved by seepage flows without the intervention of hot springs [Pieri, 1980], this alternative requires that past mean temperatures and pressures were much higher in order to allow spring conduits to remain open. In the absence of independent, non-ambiguous evidence for the required large amount of climatic change, the cold spring sapping hypothesis is more complex. Also, certain aspects of' Martian fluvial morphology are not explained by climate-induced valley cutting but are by the thermal spring model. These aspects are (1) the nearly complete restriction of valleys to the possibly ice-rich plateau sequence units, (2) the common spatial association of densely dissected plateau sequence materials with nearby volcanic plains, (3) the intermittent gaps along branching valley networks that suggest actual lack of continuity during valley formation, and (4) the maintenance of constant trunk valley


widths along hundreds of kilometer-long valley reaches, into which debouch numerous tributary valleys. It is unlikely that the branching valley networks ever functioned, as terrestrial networks do, to collect and transport water from headwaters to the mouth.

Theoretical models of early atmospheric evolution are sometimes cited incorrectly as independent evidence for the putative ancient dense and warm Mars atmosphere. All such models are uncertain, but several do allow Mars to evolve without

ever developing an Earth-like atmosphere. For example, an ancient warm and dense atmosphere is not predicted for Mars if the later stages of planetary accretion were slow: the planet's atmosphere would remain cold as H20, CO 2 and other volatiles condensed on the planet's surface [Matsui and Abe, 1987]. Although enriched D/H ratios suggest atmosphere depletion [Owen et al., 1988], the oxygen isotope data do not, and a variety of histories and controlling processes are possible [Jakosl•y, 1988]. Mars may indeed be volatile-rich [Cart, 1986, 1987] without ever having experienced a thick atmosphere.

Under modern climatic conditions, most newly outgassed and condensed volatiles would be deposited in high latitudes. For Noachian and early Hesperian time, however, Jakosk.w and Cart

[1987] conclude that the planet's higher, pre-Tharsis Montes, spin axis obliquity favored near-surface ice stability in equatorial and temperate regions such as Aeolis. Low-latitude ice may have become increasingly unstable as Tharsis Montes developed and obliquity decreased: direct sublimnation is the probable ice removal process. This long term change in the Mars surface environment may be an important factor in the history of fluvial valley genesis [Clow, 1987; Jakosky and Cwv; 1987]. Instead of early atmosphere removal, two other processes adequately explain the relict nature of the valleys: (1) the progressive depletion of ice-rich terrain during obliquity reduction, which reduced the opportunities for volcanism-ice interactions, and (2) the reduction of widespread effusive volcanism in the cratered terrains by the end of early Hesperian time [Tanaka, 1986].

Depending on local conditions (e.g., sill thicknesses, burial depths, bulk ice concentrations, host rock permeability), magma injections into ice-rich strata may cause "thermokarst" landforms [Costard and DolIfus, 1987] or "mega-lahars" [Squyres et al., 1987] instead of springs and valley-carving fluid flows. Such features can form very quickly, and catastrophic interactions between igneous processes and ice have been a continuing theme in geomorphological investigations of the Martian surface [Allen, 1979; Wilhelms, 1986; Squyres et al., 1987]. Now the less dramatic geomorphological effects of thermal springs require additional study. Other workers infer that igneous activity melted subsurface ice near the large outflow channels, and that the resulting meltwater, when catastrophically released, carved such channels [McCauley et al., 1972; Masursky et al., 1977; Wilhelms, 1986]. The combined evidence presented here raises the possibility that fundamentally similar processes, and not climate, were responsible for carving abundant branching valleys in Aeolis.

Acknowledgments. The following reviewers provided helpful comments at various stages of this research and reviewed earlier versions of this manuscript: V. Baker, N. Barlow, V. Gulick, C. Kochel, and K. Tanaka. JGR reviewers D. Pieri and A. Howard are thanked for their

detailed criticism and helpful comments on the original manuscript. I thank George Brakenridge for photographic enlargements of many Viking orbiter frames. The research was supported by NASA Mars Data Analysis Program grant NAGW-1082 and is a contribution of the NASA-sponsored "Mars: Evolution of Volcanism, Tectonism, and Volatiles" project.

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