Tharsis province of Mars: Geologic sequence, geometry, and a deformation mechanism

ICARUS 3 8 , 4 5 6 - - 4 7 2

(1979)

Tharsis Province of Mars: Geologic Sequence, Geometry, and a Deformation Mechanism D O N A L D U. W I S E , M A T T H E W P. G O L O M B E K , AND G E O R G E E. M c G I L L Department of Geo~gy and Geography, University of Massachusetts, Amherst, Massachusetts 01003 Received October 25, 1978 The early history of Mars included two large-scale events of great significance: (1) the lowering and resurfacing of one-third of the crust, followed closely by (2) evolution of the Tharsis bulge. Tharsis development apparently involved two stages: (1) an initial rapid topographic rise accompanied by the development of a vast radial fault system, and (2) an extremely long-lived volcanic stage apparently continuing to the geologic present. A deformational model is proposed whereby a first-order mantle convection cell caused early subcrustal erosion and foundering of the low third of the planet. Underplating and deep intrusion by the eroded materials beneath Tharsis caused isostatic doming. Minor radial gravity motions of surficial layers off the dome produced the radial fault system. The hot underplate eventually affected the surface to cause the very long-lived volcanic second stage. Deep crustal anisotropy associated with the locally NE4rending boundary between the highland two-thirds and the lowland one-third caused the NE elongation of many features of Tharsis. INTRODUCTION T h e Tharsis Province of Mars, with its great volcanoes, high elevations, and v a s t fracture systems (Carr, 1974) is a key element in a n y discussion of the crustal evolution of the planet. T h e unifying surface structure of the p r o v i n c e - - t h e "teetonic g l u e " - - i s a radial s y s t e m of fractures a n d associated grabens which extends over a b o u t 2 5 % of the area of Mars. M o r e or less centrally located within the radial fault area are the aprons of giant volcanoes occupying a b o u t 5 % of the M a r t i a n surface area. Over the past few years our group has been examining various aspects of Tharsisrelated problems: we h a v e been m a p p i n g geologic quadrangles at either end of Tharsis (Wise, 1979; McGill, 1979); developing crater statistical techniques for

M a r t i a n events ( N e u k u m and Wise, 1976; McGill and Wise, 1972; McGill, 1977); and examining fault or stress models of the M a r t i a n crust (Wise 1974, 1975; Wise et al., 1978). On the basis of these Tharsisrelated studies, the general literature, and discussions with m a n y other workers in the field, this s t u d y a t t e m p t s to use the techniques of structural geology and crater dating to answer, at least in part, some basic Tharsis questions: (1) W h a t relationships exist a m o n g fault and stress orientations, m a j o r geologic m a p units, a n d m a j o r topographic features? (2) W h e n in the general evolution of the M a r t i a n crust did Tharsis first a p p e a r ? Were there a n y precursor events? (3) T o w h a t extent were Tharsis events synchronous t h r o u g h o u t the province?

456 0019-1035/79/060456-17502.00/0 Copyright O 1979 b y Academic Press, Inc. All rights of reproduction in a n y form reserved.

THARSIS TECTONICS

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458

WISE, GOLOMBEK, AND McGILL

H o w does t h e c e n t e r of t h e r a d i a l f a u l t i n g c o r r e l a t e in l o c a t i o n w i t h t h e c e n t e r s of volcanic activity and topographic uplift? (6) W h e n do t h e N E e l o n g a t i o n s of t h e v o l c a n i c c h a i n a n d f a u l t e x t e n s i o n s first appear? What other features show similar o r i e n t a t i o n a n d h e n c e a r e c a n d i d a t e s for genetic association with these structures? (7) W h a t l i m i t s do t h e s e c o n s t r a i n t s of time and structural geology place on deeper g e o p h y s i c a l m o d e l s for T h a r s i s e v o l u t i o n ? I n a t t e m p t i n g to a n s w e r s o m e of t h e s e questions, this paper outlines the sequence of m a j o r T h a r s i s f e a t u r e s a n d e v e n t s , t h e i r timing, their geometric characteristics, and presents a tentative tectonic model based on these constraints. Oldest Counts en Lightly Crotered Hemisphere 1,000,000

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Time I n h e r e n t in a n y t e c t o n i c d i s c u s s i o n a r e t h e e l e m e n t s of t i m e a n d s t r u c t u r a l e v o l u tion. R e l a t i v e age d a t i n g b y s u p e r p o s i t i o n of s t r u c t u r e s a n d m a p u n i t s is t h e t r a d i t i o n a l a n d o b v i o u s m e t h o d we h a v e u s e d to e s t a b l i s h m a n y geologic s e q u e n c e s in local areas. C o r r e l a t i o n of t h e s e s e q u e n c e s a m o n g n o n c o n t i g u o u s a r e a s of M a r s , in t h e a b s e n c e of r e t u r n e d r o c k s a m p l e s , depends on crater statistics. T h r o u g h o u t t h e T h a r s i s P r o v i n c e (Fig. 1), c r a t e r c o u n t s were p e r f o r m e d o n a r e a s w h i c h b y v i s u a l i n s p e c t i o n s e e m to h a v e a homogeneous crater population. Homog e n e o u s in t h i s c o n t e x t does n o t necess a r i l y i m p l y a single a g e of s u r f a c e f o r m a -

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FIG. 2. Relationship of faulting to crater number ages for map units or areas in four different regions of Tharsis. Numbers 1 to 4 at the top of each column indicate the region (listed in the box) from which the map units come. The surfaces represented by each dot have been characterized by their age relationship to faulting in their local area as indicated at the top of the columns. The elusterings of the data classes for the four areas indicate that, at this level of resolution, Tharsis faulting events were essentially synchronous throughout the province. The figure shows that the bulk of the faulting took place near crater number 10 000; for many areas it was complete by crater number 6000, but in other areas it continued for a long time.

THARSIS TECTONICS

tion, but rather that within the area of counting randomly selected subareas would yield similar crater counts. Cumulative size-frequency plots were prepared for these areas for all visible and apparently nonvolcanic craters. The Mars standard curve of N e u k u m and Wise (1976) was then used as an approximation to a crater production curve in order to project the crater density of the most reliable size range of the data plot to a 1-km crater density. This 1-km "crater density num-" ber" is used as a relative time standard throughout this paper and always refers to the cumulative number of craters greater than 1 km diameter/106 km 2 as derived by this method. Crater frequency plots for geologically simple, single-age surfaces approximate the 1,000,000

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459

standard curve quite closely in the range of their statistically reliable crater sizes. For areas which have a single blanketing or resurfacing event, the larger craters survive and produce a curve approximating the standard one in that size range. Using this size range, the data can be projected to a 1-km crater number for the older surface. Similarly, the smaller, younger craters produce a second section of the curve which can be projected to a 1-km crater number appropriate to the age of resurfacing. The existence of a double-age surface can be recognized by two sections of the curve, each approximating the standard curve but separated by a distinct bump (Neukum and Wise, 1976). For such areas, two ages were read for the surfaces. For areas with multiple resurfacing the

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WISE, G O L O M B E K , A N D M c G I L L

numerous bumps change the average slope of the curve and do not permit reliable ages to be obtained.

have crater numbers of about 6000 or less. The "pre-most-faults" column indicates that most surfaces immediately predating intense faulting in their areas have crater numbers in the 12 000 to 20 000 range. By contrast the surfaces which predate only some of the faulting in their areas have crater numbers of about 10 000 or less. More details on the crater count ages of Tharsis-related events, structures, and map units are given in Fig. 3. These data suggest that Tharsis tectonism begins about crater number 15 000 and reaches a peak in the 5000 to 10 000 period. Events subsequent to crater number 5000 are largely associated with the growth of the giant volcanoes, their aprons, and the local intense to diffuse faulting associated with them.

EVOLUTION OF THE THARSIS PROVINCE Growth versus Time

In Fig. 2, crater numbers for various geologic surfaces are plotted. The surfaces have been characterized by their age relationships to faulting in the same areas and have been separated into four regions of Tharsis: (1) north, (2) southeast, (3) south, and (4) southwest. The clusterings of data for the four areas suggest that, at this level of resolution, Tharsis events were synchronous. The right-hand column illustrates that surfaces postdating all Tharsis faulting in their immediate areas

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Fro. 4. Timing of some major Tharsis events in relation to the presently proposed range of absolute age versus crater n u m b e r curves for Mars. Cratering versus age curves of various authors are now converging within a factor of 2 of the lunar curve (Informal Conference, Planetary Geology Principal Investigators Meeting, Tucson, June 1978). This range (shaded) is shown in relation to Tharsis crater numbers. The steep slope of the curve above crater n u m b e r 5000 makes the timing of the main Tharsis events essentially model independent in the range 3.4 to 4.1 b y before present. The later volcanic stage occurs below the knee of the curve so t h a t its timing is highly dependent on the model selected.

THARSIS TECTONICS T h e precise relationship of M a r t i a n crater density to absolute age is not generally agreed upon. At present most authors' versions of curves for Martian crater density versus age tend to converge toward the lunar curve within a factor of ± 2 (Informal Conference at P l a n e t a r y Geology Principal Investigators Meeting, Tucson, June 1978). This range of currently proposed crater age curves is illustrated in Fig. 4. As illustrated in Figs. 1 to 3, the bulk of Tharsis tectonism takes place prior to crater number 5000. This is above the knee of the lunar curve, where there is general agreement among all the time scales. On all these scales, the 5000 to 15 000 crater numbers for the period of intense faulting suggest that Tharsis was born and went through a rapid tectonic evolution over a period of a few hundred million years somewhere in the time span of 3.3 to 4.1 b y before present. Because of the flatter slope of the curves of Fig. 4 below the knee, absolute ages of younger events are highly model dependent. F o r t u n a t e l y for our present discussion, crater numbers below 5000 relate largely to the later stages of Tharsis history involving geomorphic processes and growth of the giant shield volcanoes. When N e u k u m and Wise (1976) suggested t h a t the great volcanoes of Mars had been long dead, the youngest crater numbers then available for the volcanoes were about 500. For such a number there is an uncertainty of 4-10 ~ years, depending on the model time scale chosen. Subsequently, Carr et al. (1977) have found numbers on Olympus Mons of 20 to 50, indicating y o u t h on any time scale. These numbers are so low that, independent of the time scale, Tharsis must be considered at least a warm corpse if not a fire-breathing geologic entity. T h e tectonic evolution of Tharsis apparently has been very asymmetric in time, starting early in the planet's history

461

with a geologically brief period of major widespread faulting and general uplift. Subsequent events involved breakup into more local topographic units, m a n y geomorphic modifications, local basin filling, local faulting, and the construction of giant shield volcanoes. Because these events have been spread over the remaining 3 to 4 by of geologic time they represent a long, slow winding down of Tharsis tectonism. Whether this occurred as a rapid early slowdown which is now almost complete or whether it reflects a continuous low level of tectonic activity which continues at present are questions with answers t h a t depend on the cratering time scale selected. The key time statement about Tharsis tectonics stands, independent of the choice among any of the current cratering time scales : Tharsis had a vigorous and rapid development to maturity early in Martian history. Its old age continues to the geologic recent, a life span which is almost unbelievable, even by geologic standards.

Early Crustal Foundering T h e geologic history of Tharsis has been outlined in the paragraphs and figures above, but a number of aspects need amplification. Mars is commonly cited as having a heavily cratered southern hemisphere and a lightly cratered northern hemisphere (Mutch et al., 1976). In other words, Mars has two first-order asymmetries: the T h a r sis Province, and the heavily cratered vs lightly cratered "hemispheres." In reality the lightly cratered "hemisphere" comprises 34% of the area of the planet and might better be called the "lightly cratered third." For well-preserved areas showing the heaviest concentrations of large craters in the heavily cratered highland two-thirds of Mars, crater numbers range from 100 000 to 400 000. The other one-third of Mars averages about 3 km lower t h a n this

462

WISE, GOLOMBEK, AND McGILL

ancient highland and is much more lightly cratered, indicating that it has gone through a major resurfacing subsequent to the intense early bombardment recorded in the heavily cratered highlands. In an attempt to place some time limits on this resurfacing period, we made crater plots for three of the most densely cratered homogeneous areas we could find within the lightly cratered one-third of the planet. The locations and the crater numbers derived for these areas are: (a) 45N, 140W -- 100 000; (b) 72N, 295W = 75 000; and (c) 71N, 280W = 50 000. Area (a) is close to the boundary of the region and may represent only partially destroyed ancient crust rather than a new surface with a new cratering age developed upon it. The data suggest that the first-order process which created this major crustal dichotomy waned about crater numbers 50 000 to 75 000. Pre-Tharsis (?) Events

Broadly speaking there are several ages of blanketing deposits associated or possibly associated with the Tharsis Province. They are most readily separated by their crater numbers and superposition relations with major episodes of Tharsis faulting. A widespread series of surfaces and geologic units predating almost all significant faulting was developed in the time span of crater numbers 12 000 to 20 000. Most prominent of these is Lunae Planum (15N, 65W) with its wrinkle ridges suggesting analogy with lunar lava plains (Carr, 1973). Surfaces of similar age form much of the fractured plains of the lowlands north of Alba Patera (41N, 110W) and also comprise the old surface which was later disrupted to form Noctis Labrinthus (4S, 100W). The present distribution of these surfaces is not symmetric to Tharsis Province, being best developed on the eastern and northern parts of the province. These Lunae-Planum-related surfaces are

possible but by no means certain precursors of the Tharsis events. They permit the statement that Tharsis may have begun at about crater number 20 000 with outpourings of basaltic (?) lava plains. The presence of small areas of these older units near some of the central portions of Tharsis argues that the topographic high of Tharsis is largely the result of broad crustal uplift rather than construction of a vast volcanic and sedimentary pile. The presence of degraded large craters protruding through younger deposits in the Thaumasia quadrangle (McGill, 1976) and in the Tempe Plateau (Wise, 1978) supports this contention. Northern Tharsis Events

The first certain evidence for the existence of Tharsis as a distinct province is the development of the radial fault system. The oldest members of this system are developed on surfaces with crater numbers of 13000 or less in the Tempe Plateau area (35N, 80W). Older systems of faults in that area have variable trends possibly associated with the Mareotis Fossae (85W, 43N) and the nearby highland boundary. These major Tempe faults are buried by cratered plains deposits of crater numbers near 9000 and by upland, volatile-rich deposits of similar age (Wise, 1979). A second period of locally intense faulting disrupted the northern end of Tharsis with the spectacular fault system splaying around Alba Patera. The second system is developed on surfaces with crater ages 1000 to 2700 and also reactivates some members of the Tempe Plateau fault system. The Alba fault system, interpreted as deflecting around a solidified pluton, progressively changes strike from NE to N. A number of units with crater numbers 10 000 :t: 3000 are interspersed with the Tharsis radial fault system but are not readily separable as a distinct group on the Cart and Scott map (Fig. 1). How-

THARSIS TECTONICS ever, another group of old cratered plains units is separable and seems to lie unconformably across most of the radial Tharsis fault system. Crater numbers on these units range as high as 9000 in the north but for the most part have values in the 3000 to 5000 range. These units, outlined in Fig. 1, from a partial ring around central Tharsis. Commonly they are mildly faulted and locally intensely faulted to form a local younger "basement" complex to Tharsis. The initiation of the older of the great volcanic shields such as Alba Patera and Uranius Patera coincides approximately with these units at crater numbers around 5000. Central Tharsis is covered by the young volcanic aprons and central cones of the giant shield volcanoes of Tharsis Montes. Olympus Mons and its complexly lobate apron is a similar, even younger feature. Crater numbers for these blanketing aprons are in the range of a few hundred to 2000, whereas the numbers for the cones range from a few tens to a few hundreds. This essentially unfaulted blanket rests unconformably across the mildly faulted older plains deposits.

Southern Tharsis Events The Thaumasia area, at the southern end of the province, has four distinct fault systems (Wise et al., 1978): (1) an E - W system of scarps and lineaments on terrains yielding crater numbers of 70 000 or larger, (2), (3) N E and N to S fault systems which appear fresher than the E to W structures and which include structures with definite graben-like characteristics, and (4) the major system of grabens approximately radial to Syria Planum. This young, fanning or radial set cuts all the older fractures of the Thaumasia lobe and extends far into the ancient cratered highlands south of the lobe. It generally is covered with plains units having crater numbers between 3000 and

463

5000, but locally these units are cut by individual members of the radial graben set (McGill, 1976). In the Memnonia region (ON, 150W) the radial fault system is well developed on a surface with crater number 10 000 but is buried by a unit of crater number 5000 (Fig. 1). Ages of the major fault motions in southern Tharsis may also be obtained by consideration of the Syria-Soils Planum Block. This block is defined on the north by Valles Marineris, the west by the Claritas Fossae zone, and on the south by the large topographic lobe of Thaumasia. The block has been interpreted as a tectonic plate with great lateral motions by a number of authors (All~gre et al., 1974; SengSr and Jones, 1975; Courtillot et al., 1975; Masson, 1977; 1978). Under this model, Valles Marineris is interpreted as the gap left by the southward motion of the block. Claritas Fossae would be a right lateral fault zone and Thaumasia would represent a frontal lobe pushed up by the southward motion. We find a variation of this general geometry attractive, but the magnitude of proposed displacement quite improbable. The Thaumasia lobe has one large crater which might be interpreted as deformed by southward motion of the lobe and has a radially faulted foreland south of the lobe. Claritas Fossae and related areas of intense faulting yield consistent crater numbers of 7500 to 8500 with unconformably overlying or little-faulted nearby units yielding numbers of 4500. We interpret Claritas Fossae ridge as an anticlinorium at the edge of the block. Lateral motions, if any, on the block must be quite small. We have been unable to find any evidence of strike-slip motion on the Claritas Fossae faults, although modest amounts of distributed motion within en-echelon grabens of the zone are quite possible. Valles Marineris also does not require great lateral motion. If the average dimensions of the original canyon were approximately 3 km deep by

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WISE, GOLOMBEK, AND McGILL

100 k m wide and an absolutely rigid lithosphere were 100 k m thick, a lateral movement of only 3 km would be needed to account for the missing volume b y vertical displacement. We find the overall geometry of the S y r i a - T h a u m a s i a Block with its related structures to be an attractive b u t unproven candidate for very minor S to SE motion. The best dates we can now place on the main motion of the block are in the range of crater numbers 7500 to 8500. The most likely driving mechanism is simple gravity flow and slide of a surficial lithosphere off a newly risen Tharsis bulge. A similar

mechanism with even smaller lateral displacement is proposed' below for the origin of the main Tharsis radial fault system. THARSIS GEOMETRY AND STRESS A n u m b e r of the gross features of Tharsis Province m a y be treated with simple geometric techniques of structural geology in order to avoid the distortions of the Mercator projection on which most of these data are usually plotted. I n Fig. 5 the centers of the four great volcanoes of Tharsis Montes have been plotted on an equal area net along with the great circle best fitting the points. The four fall within

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FIG. 5. Alignment of the Tharsis Montes line of volcanoes. Locations of major volcanoes have been plotted on the upper hemisphere of a stereographic projection with 0 ° longitude at the right and 180° longitude at the left. The four volcanoes lie within a hag-degree of a great circle oriented N38E. Tempe Fossae lies on the NE projection of the same great circle, whereas Memnonia Fossae is parallel to the same circle but lies 1° east of its SW projection (dotted line). The great circle drawn through Olympus Mons and Alba Patera is not parallel to the first great circle. Abbreviations: OM, Olympus Mons; UP, Uranius Patera, Tharsis Montes from the North; AM, Ascraeus Mons; PM, Pavonis Mons; AM, Arsia Mons.

465

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Fie. 6. Intersections of the projected faults of the Tharsis radial system contoured for population density. Tharsis Montes volcanoes are shown b y the black dots. Only the locations and orientations of well-developed parallel fault swarms are included. Great circles have been drawn at the proper location and proper orientation on the equal area projection with the intersections contoured as described b y Billings (1972). The equidimensional character of the contoured area shows t h a t the faults are a good approximation to a radial orientation.

a half-degree of the great circle oriented N38E. T h e fact t h a t T e m p e Fossae lies on the same great circle suggests t h a t all these volcanic features m a y be related to weaknesses associated with an older fracture which formerly extended far to the SW of the present T e m p e Fossae. M e m nonia Fossae, near the SW projection of the same line, lies a p p r o x i m a t e l y 1 ° east of the same great circle. On M e r c a t o r maps, a n o t h e r line t h r o u g h Alba P a t e r a a n d O l y m p u s M o n s appears subparallel to the Tharsis M o n t e s line, but, plotted on

the stereographic net of Fig. 5, these lines can be seen to differ in strike b y 14 ° . T h e extent to which the radial fault system of Tharsis is radial a b o u t a point is considered in Fig. 6. T h e distal ends of the most strongly developed fault swarms of the radial s y s t e m are plotted on the u p p e r hemisphere of a n equal area net. T h e orientation of the s w a r m at each point is represented b y a great circle of proper orientation. T h e 190 intersections of these great circles were located a n d their concentration factor determined and

466

WISE, GOLOMBEK, AND McGILL least principal stress axis for compression positive (Anderson, 1951). The truly radial nature of the faults with their center on the top of the Tharsis topographic bulge indicates that at the time of their formation the near-surface sigma 3 stress trajectories were a series of concentric circles around the present Tharsis topographic center. A system of locally developed lunar-like wrinkle ridges sweep as a great arc along the NE, E, and SE flanks of Tharsis. These are best developed on Lunae Planum, Felis Dorsae, and Solis Dorsae. They are distinct in orientation from other wrinkle ridges of the region, as, for example, the young set developed on Chryse Planitia.

contoured by a standard structural geology running average, moving circle technique (Billings, 1972, p. 104). If the fault orientations were not radial about a point the contours of Fig. 6 would show marked elongation in some direction. The fact that the contours are highly concentrated into an essentially circular "bulls eye" indicates that the fault system is, to a good approximation, truly radial about a point. The center of this bull's eye, at 14S, 101W, lies in Syria Planum, close to the center of the present high point of the Tharsis bulge but about 1000 km SE of the Tharsis Montes line of volcanoes. These faults with their related grabens must be perpendicular to sigma 3, the N

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467

THARSIS T E C T O N I C S

The system probably has a range of ages but is approximately coincident in time with the radial fault system (Fig. 3). Insofar as these ridges are commonly considered compressional features, surface lines normal to the strike of the ridge swarm should represent a sigma 1 (maximum compression) stress trajectory. The extent to which these possible stress trajectories are radial to Tharsis is examined in Fig. 7 in the same manner that the faults were considered in Fig. 6. The elongation or girdle formed by the contour

pattern of Fig. 7 may indicate that the wrinkle system is not concentric about a single point. Instead, it suggests that the strikes of the wrinkles form a broadly elliptical map pattern centered at about I°N and 122°W or somewhat west of the Tharsis Montes line of volcanoes with the long axis of that ellipse subparallel to the volcanic line. The problem of the degree of circularity of various Tharsis features is examined in Fig. 8. Here the data are plotted on a stereographic net which has the charac-

o 0 0 0

/J

OM

I

\ o\

CENTEROF ~Fd)RADIAL

\ o

\00

0

\

"~ oC/ I

\

" 0 .._o"6-I--0

/

/

4000 kin.

/-.,.t.~...~ RADIU/

.~. 0

%0"f ELEVATION CONTOURS IN KM. FROM 1976 USGS TOPO MAP OF MARS 0

DISTAL ENDS OF THARSIS SEMI-RADIAL FAULT SYSTEM

Fro. 8. Circular and noncireular distribution of some Tharsis features. The data are plotted on the upper hemisphere of a stereographic projection which has the property of maintaining circles on a sphere as circles on the plot. 0 ° longitude is on the right and 180 ° longitude is on the left. Circles 2500 km and 4000 km in radius have been drawn about the center of radial fracturing from Fig. 6. The elevation contours show a good approximation to circles about the same point. The distribution of the distal ends of the radial fault system is quite elongated in a N E to SW direction.

468

WISE, GOLOMBEK, AND McGILL

teristic that circles on the surface of a sphere will also appear as circles on the net. The 0- and 5-km elevation contours from the topographic map of Mars (U.S.G.S., 1976) are also plotted in Fig. 8 along with the center of the radial fault system. As shown, these two contours approximate rather closely the trace of true circles on the Martian surface. These circles have radii of 2000 and 3500 km and are centered on the intersection center of the radial fault system. In contrast, the degree of development of the radial fault system is far from circular. Locations of the distal ends of the most strongly developed fault swarms of this system are indicated by the dots on Fig. 8. The deviation of these points from a circular distribution is obvious. Their development is much more extensive NE and SE of the Tharsis central region, more or less parallel with the line of Tharsis Monte volcanoes. The implication is that even though the stress may have been radial, anisotropies of some type caused the fracture development to be greatly elongated along NE trends. DEFORMATIONAL MODEL The preceding discussions place a number of limits on Tharsis deformation models: (1) Formation early in the planet's history but just subsequent to the destruction of the northern one-third of its ancient crust. (2) Rapid development to maturity followed by an extremely long old age. (3) Formation largely by major topographic uplift and bulging. The map pattern of the topographic bulge is essentially circular. The bulge apparently formed early and, although it has gone through some significant breakup into isolated blocks and subsidence under volcanic loads, as yet it shows no signs of wholesale collapse. (4) Early development of a vast radial

fault and graben system which has its center at the present center of topographic bulging. (5) Building of the great shield volcanoes as a second stage, essentially postdating the era of bulging and great faulting. This stage has dragged on through most of the last 3 by. (6) The larger volcanic edifices are located NW of the center of Tharsis doming and faulting. (7) Although the fault orientations and topography are radially or circularly symmetric, many of the older elements of Tharsis show a distinct NE elongation. Most obvious is the N38E trend of the Tharsis Montes line of volcanoes, but the intensity and lateral extent of development of the radial fault system, the distribution of older cratered plains units, and the Lunae Planum system of wrinkle ridges also show a similar tendency toward NE elongation. The most fundamental and ancient feature detectable as subparallel to this trend is the border zone between the highland two-thirds and lowland onethird of Mars. This diffuse border zone on the NW edge of the Tempe Plateau passes SW beneath the aprons of Alba Patera and Olympus Mons. As discussed by Phillips (1978 ; including oral presentation) simple vertical doming of a region involving a single-layer mantle should stretch the crust, creating not only radial faults but also concentric grabens. In addition, the uplift should extend at least to the limits of the fault system. The Tharsis uplift may have been formerly as extensive as the fault systems, and old concentric grabens may be buried under the central Tharsis volcanoes, but in the absence of other confirming evidence it seems safer to conclude that Tharsis does not correspond to this simple, single-layer model. Instead, we wish to propose a multiple-layer model with partial detachment and gravity spreading of the shal-

THARSIS TECTONICS

lower brittle layers as the topographic rise develops. The Syria-Thaumasia Block motion discussed above is the best example of this type of process. The requirements of lateral spreading necessary to produce significant graben formation are still so modest in terrestrial analogues (e.g., Stromquist, 1976, plate 2) that the actual lateral displacement necessary to form the radial system is very small. Golombek (1978; 1979) has shown that

ISOSTATIC

469

the lunar graben system could be produced with a stretching factor of .001% or an increase of the radius of the Moon by only 18 m. If the same magnitude of stretching were required to produce the radial Tharsis graben system in its distal regions, then, at a distance of 3500 km from the center of the radial faults (Fig. 8), the total lateral displacement need have been only 220 m. If the lowland one-third of Mars were

(A)

,~ 4 . 0 B.Y. = Convective Subcrustal Erosion, Isostatic Foundering with Lava Plain Filling.

(B)

,~5.5-4.0

(c)

' v 5 . 5 B.Y. to Present = Slow Transfer of Heat from Deep In'trusive Zone to Sufoce to Create L o n g - L i v e d Volcanic Province. Thinner, Weaker Crust near Lowlands Skews Volcanic Center i~ That Direction.

RISE

B.Y.=Deep Crustal Intrusion of Previously Eroded Subcrustal Material. Consequent Isostotic Rise and Gravity Creep off Top of Dome.

FIG. 9. Stages in the proposed underplating model for the evolution of Tharsis. (A) At about 4 by before present a first-order convection cell causes subcrustal erosion. This produces isostatic foundering with consequent lowering and lava plain burial of the northern one-third of Mars. (B) In the range 3.5 to 4 by convection and crustal destruction ceases and the previously eroded material is intruded and underplated beneath Tharsis. This lighter root produces a geologically rapid but permanent rise of the Tharsis bulge with relatively little associated volcanism. Minor gravity creep and sliding of surficial brittle layer off the bulge produces the radial fault system. (C) 3.5 by to present. Shallow asthenophere cools and becomes solidified with time bringing gravity sliding and radial faulting stages to an end even though topographic bulge continues to exist. Slow conduction and buildup of radioactive heat initiate the stage of extremely long-lived volcanic activity. Thinner crust toward the lowland region skews the center of volcanic activity away from the center of topographic uplift toward the lowlands to bury the boundary region under a thick volcanic pile.

470

WISE, GOLOMBEK, AND McGILL

a primary feature it should have a crater density comparable to the highlands. The much lower crater density coupled with the lower elevations requires not only that it be resurfaced but also that millions of cubic kilometers of missing rock be accounted for. The relatively thin sedimentary blanket over the craters of the ancient crust provides an almost insurmountable volumetric constraint against processes involving simple surficial erosion of several kilometers from one-third of the planet and the dumping of the debris on the other two-thirds. Instead some deeper tectonic process seems called for. A very attractive mechanism involves crustal thinning by subcrustal erosion with consequent isostatic foundering of one-third of the planet's crust. The surface would be lowered and upwelling lavas would cover most of the older intensely cratered surface. A mechanism for localizing the subcrustal erosion almost entirely to one-third of the planet's surface is the operation of a first-order convection cell. Mantle convection has been invoked as a cause of the Tharsis uplift and fracturing by Hartmann (1973) and Carr (1974) but to our knowledge has not been suggested as the major resurfacing mechanism for the low one-third of the planet. In our model, scouring and erosional thinning would presumably take place above the hot upward directed portion of the cell (Fig. 9A). Whatever the surficial destructional process, it involved little faulting or distortion of the surviving older craters. The few probable faults we detect that may possibly relate to this time span are some subdued E to W lineaments of the Thaumasia Region and the ancient Mareotis Fossae (84W, 43N) near the highland/ lowland boundary of the NW Tempe Plateau. The ancient scarp bounding the heavily and lightly cratered regions has continued to retreat and collapse, providing abundant younger debris cover for parts of the vast northern plains.

The wholesale crustal destruction caused by subcrustal erosion ceased by crater numbers 50000 to 100000. A time gap of a few hundred million years elapsed before Tharsis was born in the span of crater number~ 10 000 to 20 000 or 3.5 to 4 by before present on all the current time scales. This sequential development of two first-order asymmetric features of the planet suggests a common underlying mantle process which shifted in locus and nature of operation. Possible reasons for this shift are: the earlier first-order mantle convective system may have been disrupted by the growth of a core; an asymmetry about the planet's axis of rotation caused by subcrustal erosion may have produced a shifting of the crust with respect to the mantle; the convective process itself may have become unstable and shifted to a new location; or the thermal gradients necessary to drive the convection decreased to the point that the cell ceased operation. Perhaps the most attractive of these ad hoc hypotheses is that major convection ceased and the lighter material ingested into the cell by the subcrustal erosion of the lightly cratered third of the planet began to rise again. By unspecified mechanisms these lighter materials were underplated beneath and injected into the Tharsis crust (Fig. 9B). The addition of these lighter roots produced a geologically rapid and permanent rise of the Tharsis bulge. This rise at about crater number 15 000 would provide the gravity potential for radial sliding and faulting of the surficial layers. The net transfer of material into the Tharsis region would account for the mass excess indicated by gravity data for the region (Phillips and Saunders, 1975). The Tharsis deep injection and underplating mechanism provides a simultaneous solution for two major tectonic problems: where to put the missing material from the lower third of the planet's surface,

THARSIS TECTONICS and how to provide a rapid but permanent uplift of Tharsis. As discussed above, the bulk of Tharsis volcanism postdates the period of intense radial faulting. With the proposed mechanism of isostatic rise and near surface gravity sliding, there is little reason for having major volcanism intimately associated with the sliding and radial faulting. Instead, the hot underplate, possibly including concentrations of radioactive materials, could require a very long time for its thermal effects to be felt at the surface. The effect could be a later but very long-lived volcanic phase following the initial rapid uplift (Fig. 9C). Cooling, settling, spreading of the bulge, and shifting of volcanic materials at depth would result in continuing minor faulting and block motions through this long volcanic stage. Through tectonic heredity, many of the younger faults of the volcanic stage would make use of the weakness planes of earlier fault systems. Any deep crustal anisotropy could influence the direction of subcrustal migration of the volcanic materials and the locus of their outpouring. In particular, a tapering crustal thickness under the highland edge and deeper structures associated with the boundary of the foundered (?) third of the planet, could provide more ready access of volcanics to the surface. The result would be a shifting of the center of the volcanic province NW away from the center of Tharsis uplift and an elongation of the province northeastward parallel to the highland-lowland boundary zone. Some of the details of the NE elongation of the distal ends of the radial fault system can also be explained in terms of creation of the Tharsis bulge on the highland side of the highland-lowland deep crustal boundary. The near-surface gravity gradients would be radial to the dome so that any extensive faults would form a radial system. However, stress concentration might be expected along the crustal

471

boundary zone. The result would be enhanced development of the radial fault system along the NE to SW trend to produce the elongation of the province illustrated in Fig. 8. Another ad hoc corollary to the model involves the existence of an early d~collement (detachment) surface under only the highland part of the bulge. Possibilities for this surface include : volatile-related mantle phases and/or asthenosphere, remnants of early crustal stratigraphy, or melting at modest depths of ice trapped in the cratered ancient crust. Graben widths suggest that brittle deformation was restricted to the upper few kilometers, indicating a shallow detachment plane inappropriate to the mantle lithology model. The process which caused foundering of the northern one-third of the planet also could have eliminated the early stratigraphic units and/or the volatile horizons, thereby destroying the detachment surface. In the remaining intact older highland crust, radial creep of the brittle surface layer permitted the radial fault system to be active as long as the detatchment surface existed. By crater number 5000, Tharsis thermal effects, possibly involving volatile escape from the crust, eliminated this surface to halt major growth of the radial fault system even though the elevation differences necessary to drive the system continue to exist to the present day. We recognize that this model is highly speculative and requires mantle processes on Mars for which no evidence exists at present. We have not attempted to go into any of these geophysical aspects. However, based on the constraints imposed by geometry, structural geology, and timing of events we feel that a Tharsis model along the above lines is worthy of further consideration. ACKNOWLEDGMENTS This paper is the result of work done over the past few years under several research grants

472

WISE, GOLOMBEK, AND McGILL

awarded by the NASA Planetary Geology Office. It was also supported in part by the Viking Guest Investigator Program. Some of the crater work was done in conjunction with Gerhart Neukum while the senior author was a visiting scientist at the Max Planek Institute, Heidelberg. It has benefited by discussion with many colleagues and friends. Extended discussions with members of the Tharsis Working Group under Roger Phillips have helped crystallize many of the ideas. REFERENCES ALI~GRE, C. J., COURTILLOT, V. E., AND MATTAUER, M. (1974). Evidence for lateral movements of the Martian crust. Trans. Amer. Geophys. Union EOS 55, 341. (Abstract.) ANDERSON, E. M. (1951). The Dynamics of Faulting and Dyke Formation with Applications to Britain, 2nd ed. Oliver & Boyd, London. BILLINGS, M. P. (1972). Structural Geology, 3rd ed. Prentice-Hall, Englewood Cliffs, N.J. CARR, M. H. (1973). Volcanism on Mars. J. Geophys. Res. 78, 4049-4062. CARR, M. H. (1974). Tectonism and volcanism of the Tharsis region of Mars. J. Geophys. Res. 79, 3943-3949. CARR, M. H., GREELEY, R., BhASIUS, K. R., GUEST, J. E., AND MURRAY, J. B. (1977). Some Martian volcanic features as viewed from the Viking Orbiters. J. Geophys. Res. 82, 3985-4105. CARR, M. H., AND SCOTT, D. H. (1978). Geologic Map of Mars, preliminary version. U.S.G.S. COURTILLOT, V. C. ALI~GRE, C. J., AND MATTAUER, M. (1975). On the existence of lateral relative motions on Mars. Earth Planet. Sei. Left. 25, 279-285. GOLOMBEK, M. P. (1978). Geometry of lunar grabens: Implications for shallow crustal structure. Reports of Planetary Geology Programs, 1977-1978 NASA Tech. Mere. 79729, 103-105. (Exp. abstract.) GOLOMBEK, M. P. (1979). Structural analysis of lunar grabens and the shallow crustal structure of the Moon. J. Geophys. Res., in press. HARTMANN, W. K. (1973). Martian surface and crust: Review and synthesis. Icarus 19, 550-575. MASSON, P. (1977). Structural pattern analysis of the Noctis Labyrinthus-Valles Marieneris regions of Mars. Icarus 30, 49-62. MASSON, P. (1978). Structural evolution of the Claritas-Fossae area of Mars. Reports of Planetary Geology Program, 1977-1978, NASA Tech. Mem. 79729, 94-96. (Exp: abstract.) McGILL, G. E. (1976). Geologic history of Thau-

masia quadrangle, Mars. Reports of Accomplishments of Planetary Programs, 1975-1976, NASA Tech. Mem. TMX-3364, 224--228. (Exp. abstract.) McGILL, G. E. (1977). Craters as "fossils": The remote dating of planetary surface materials. Geol. Soc. Amer. Bull. 88, 1102-1110. McGILL, G. E. (1979). Geologic map of Thaumasia quadrangle, Mars. U.S.G.S. Misc. Inv. Map, in press. McGILL, G. E., AND WISE, D. U. (1972). Regional variations in degradation and density of Martian craters. J. Geophys. Res. 77, 2433-2441. MUTCH, T. A., ARRIDSON, R. E., HEAD, J. W., JONES, K. L., AND SAUNDERS, R. S. (1976). The Geology of Mars. Princeton Univ. Press, Princeton, N.J. NEUKUM, G., AND WISE, D. U. (1976). Mars: A standard crater curve and possible new time scale. Science 194, 1381-1387. PHILLIPS, R. J. (1978). Topical problems in Martian geophysics. Reports of Planetary Geology Program, 1977-1978, NASA Tech. Mem. 79729, 69-70. (Exp. abstract.) PHILLIPS, R. J., AND SAUNDERS, R. S. (1975). The isostatic state of Martian topography. J. Geophys. Res. 80, 2893-2898. SENG~R, A. M. C., AND JONES, E. C. (1975). A new interpretation of Martian tectonics with special reference to the Tharsis region. Geol. Soc. Amer. Abst. with Prog. 7, 1264. (Abstract.) STROMQUIS% A. W. (1976). Geometry and growth of grabens, Lower Red Lake Canyon area, Canyonlands National Park, Utah. Contr. No. 28. Dept. of Geology and Geography, Univ. of Massachusetts, Amherst, Mass. U.S. Geological Survey (1976). Topographic Map of Mars. Map 1-961. WISE, D. U. (1974). Martian fault pattern and time sequence in relation to volcanism, northern Tharsis ridge area. Trans. Amer. Geophys. Union EOS 55, 341. (Abstract.) WISE, D. U. (1975). Faulting and stress trajectories near Alba Volcano, northern Tharsis ridge of Mars. Proc. Internation. Colloquium of Planetary Geology. Geol. Romana 15, 430-433. (Exp. abstract.) WISE, D. U. (1979). Geologic map of Arcadia quadrangle, Mars. U.S.G.S. Misc. Inv. map. In press. WISE, D. U., GOLOMBEK, M. P., AND McGILL, G. E. (1978). Tharsis province of Mars: Deformational history and fault sequence. Reports of Planetary Geology Program, 1977-1978, NASA Tech. Mem. 79729, 203-204. (Exp. abstract.)