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Home , Beta Regio, Chasma, Devana Chasma, Rift, Rift zone, Volcanism on Venus

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E R STOFAN ) > Department of Geological Sciences, Brown University, Providence, Rhode 02912 J. W. HEAD t D. B. CAMPBELL NAIC Arecibo Observatory, Arecibo, Puerto Rico 00612 S. H. ZISK Massachusetts Institute of Technology/NEROC Haystack Observatory, Westford, Massachusetts 01886 A. F. BOGOMOLOV Moscow Power Institute, Moscow, USSR O. N. RZHIGA Institute of Radiotechnics and Electronics, Moscow, USSR A. T. BASILEVSKY Vernadsky Institute, USSR Academy of Sciences, Moscow, USSR N. ARMAND Institute of Radiotechnics and Electronics, Moscow, USSR

ABSTRACT , suggest that their origins (Sjogren and others, 1983). The anomaly sug- may be linked. gests a compensation depth of 330 km for Beta Beta Regio is a region of rifting and volcan- or dynamic support for the topography (Espo- ism on Venus. The of Beta, a major INTRODUCTION sito and others, 1982). topographic rise and , is herein char- This preliminary information on the charac- acterized using Pioneer Venus, Arecibo, and The nature and distribution of tectonic struc- teristics and distribution of chasmata on Venus 15/16 data. High-resolution (1-2 km) tures on terrestrial is closely linked to the and the nature of Beta Regio raises several signif- Arecibo and Venera radar images reveal de- major mechanisms of lithospheric heat transfer icant questions concerning their origin and evo- tails of faulting and , and Pioneer (Solomon and Head, 1982). On Venus, some of lution. Is the topography associated with the rift Venus altimetry illustrates the density and lo- the most topographically distinctive and areally zones (both the broad topographic rises and the cation of faults in relation to topography. extensive tectonic features are the deep linear narrow flanks of the ) due to uplift or vol- Faults are distributed throughout Beta but valleys known as "chasmata," which are inter- canic construction? Are these rift-like structures are concentrated in Devana Chasma, where preted by most workers to be the result of litho- of active or passive origin (Sengor and Burke, they are spaced 5-20 km apart. The pattern spheric extension and rifting (Pettengill and 1978)? The association of Devana Chasma with of faulting and distribution and sequence of others, 1980; Masursky and others, 1980; the broad topographic rise of Beta would seem volcanic activity in Beta can be used to help McGill and others, 1981; Schaber, 1982; to argue for localized uplift and faulting (McGill understand how the rift has evolved, includ- Campbell and others, 1984). Schaber (1982) and others, 1981) and an active origin, whereas ing the origin of the high topography of Beta has shown that systems of chasmata define tec- the major global-scale extensional zones defined and the origin and nature of Devana Chasma. tonic zones as much as 20,000 km in linear by Schaber (1982) suggest large-scale horizontal On the basis of geologic mapping relations extent. extension and the possibility of passive rifting. If and map patterns, Beta appears to have One of the most distinctive occurrences of the elastic of Venus is thin (in the formed as a result of doming in response to a chasmata is in Beta Regio, a 2,300 x 2,000 km 1-10 km range), as suggested by thermal- mantle anomaly. At the same time, highland region rising >5 km above mean gradient estimates, by laboratory data for the Mons, a major shield , was formed. planetary radius and cut by a north--trend- behavior of materials, and by the characteristics Formation of Devana Chasma followed, with ing linear trough (Devana Chasma) in excess of of deformation in (Solomon and extensive faulting in the rift trough. Geometry 1 km deep (Figs, la and lb). Recently obtained Head, 1984a), then why are the chasmata so of the trough and patterns suggests that high-resolution radar images of central Beta wide and deep? Is lithospheric stretching an im- some degree of lithospheric stretching has oc- Regio (Fig. 2) show details of the tectonic struc- portant process in the formation of rifts on curred. Later volcanism produced a second ture and associated volcanic deposits (Campbell Venus (Solomon and Head, 1984b; Zuber and major , , which is and others, 1984). Two scales of tectonic fea- Parmentier, 1986)? What is the explanation for superimposed on the western bounding fault tures are seen: (1) major faults, which generally the two scales of faulting observed in Beta Regio of the rift zone. Both uplift and extension define the edges of the 300-km-wide rift zone, (Campbell and others, 1984)? Are the two have been involved in forming Devana and (2) abundant minor faults, spaced 10-20 scales related to different scales of extension or Chasma and Beta Regio and may be impor- km apart and concentrated in the central part of differential-strength layering in the lithosphere tant in the formation of other equatorial high- the rift zone. The faults lie in and on the flanks (Zuber and Parmentier, 1986; Zuber, 1987)? land regions with systems of chasmata on of the central trough, Devana Chasma, which The purpose of this study is to examine the Venus, such as Aphrodite Terra. The bifurca- has an average width of 160 km. Beta Regio is nature of Beta Regio as an example of a major tion of Devana Chasma in the vicinity of also one of the most prominent features in grav- topographic rise and rift zone on Venus, utilizing Theia Mons, and the extension of the rift sys- ity data of Venus, with a strong positive anom- a variety of data sets. We examine the questions tem south to Phoebe Regio and west toward aly highly correlated with the topography of the origin of the high topography and the

Geological Society of America Bulletin, v. 101, p. 143-156, 16 figs., January 1989.

143

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Figure 1. (a) Pioneer Venus topography of Beta and Phoebe Regiones, illustrating the location of Devana Chasma, Theia Mons, and Rhea Mons. The map is in a Mercator projection, with the scale shown for the equator. The contour interval is 500 m. (h) Map based on radar-brightness (surface roughness) patterns from Arecibo and Pioneer Venus imaging data, showing Beta Regio, Devana Chasma, and Phoebe Regio. Areas indicated in the stippled pattern are the relatively brightest areas, or areas with the highest surface roughness, which tend to correlate with the topographically highest areas shown in Figure la and the areas characterized by chasmata.

active versus passive nature of the rift. We also maps of central Beta Regio (Fig. 5) (Campbell surface of Venus can be described by radar- examine the significance of volcanism associated and others, 1984), as well as a general unit map scattering models such as that expressed in Hag- with the rift to determine the sequence of tec- (Fig. 6). Arecibo data of northern Beta Regio fors law: tonic and volcanic activity in Beta Regio. were also mapped (Figs. 7 and 8) and compared C to radar images of the same region obtained by <*fi) = (P y) (œs40 + Csin20)15, METHODS AND RESULTS the and 16 orbiters. (3) The Venera radar images (resolution 1-2 km), with an inci- where a is the radar cross section per unit area at Three sets of data were used in examining the dence angle of 10° and a look direction to the angle of incidence 0, p is the Fresnel reflection Beta region. (1) Pioneer Venus roughness and west (Fig. 9), were used to map brightness pat- coefficient at normal incidence angle, and C is reflectivity data (resolution about 100 km) were terns associated with surface slopes (Figs. 10 the Hagfors parameter (Hagfors, 1970) out to used to characterize the general surface proper- and 11). Rose diagrams of lineaments mapped 40° and following cos20 beyond 40° (Fig. 14). ties of the region, whereas Pioneer Venus altime- from the Arecibo and Venera 15/16 data were At incidence angles less than 20° (for example, try profiles across the rift allowed examination produced (Figs. 12 and 13) and compared to Venera 15/16), the curve slopes steeply, so that of detailed relations between radar brightness theoretical fault patterns. The Venera 15/16 surface-slope changes control the amount of patterns and topography (Figs. 3 and 4). (2) mission mapped Venus from orbit during backscatter. Between 20° and 60° (for example, High-resolution (approximately 2 km) images of 1983-1984, obtaining images and altimetry Arecibo data of Beta Regio), the slope of the the Beta region obtained at the Arecibo radar north of 30°N latitude (Kotelnikov and others, scattering-law curve is relatively flat, so that facility in Puerto Rico in 1983 were also studied 1985). The two radar image sets, Venera 15/16 changes in small-scale (wavelength-size; 12.6 (Fig. 2). The incidence angle of the Arecibo and Arecibo, provide different information cm) surface roughness dominate the returned radar varies approximately with the latitude, about the venusian surface, although they are of signal, with changes in the dielectric constant (a from about 30°-45°, with the look direction of similar resolution. They are complementary be- function of porosity and composition) also con- the radar oriented approximately perpendicular cause of differences in their viewing geometries tributing to the radar return. At incidence angles to the curved image edge seen in Figure 2. Sur- (incidence angles and look directions). greater than 60°, surface slopes again begin to face roughness variations were used to produce The way in which radar interacts with the dominate the returned signal (Campbell and

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still detected. The look directions of the two data 37*N sets are sufficiently different to provide at least some detection at all angles, while mitigating suppression effects.

RESULTS

General Setting

Beta Regio is a major highland region on Venus, rising at Theia Mons to a maximum ele- vation of >5 km above the mean planetary ra- dius. The over-all topography of Beta is that of an elliptical , 2,300 x 2,000 km, with its long axis oriented about north-south (Fig. la). The dome is cut by a north-south-trending trough, Devana Chasma, which continues south of Beta through Phoebe Regio (Fig. lb). Beta has regional slope characteristics similar to those of Aphrodite Terra, with steep slopes at low elevations corresponding to chasmata walls, as well as steeply sloping outer margins (Sharpton and Head, 1985). The topography of the Beta dome is approximately symmetrical about its long axis. In Arecibo images of central Beta (Fig. 2), the dome is characterized by a broad zone of roughness (80-300 km wide) with north- south-trending linear features. The linear fea- tures occur over both the central trough, which originates to the south of Beta in Phoebe Regio, and the high topography of the dome (Figs. 3 and 4). The trough and faults are not visible in the vicinity of Theia Mons, but reappear to the north of Theia offset to the east (Campbell and others, 1984) (Figs. 2 and 6). Devana Chasma varies in width and depth through Beta, reaching its greatest depth (2.5 km) in the vicinity of Rhea Mons and is shallowest at Theia Mons. North of Rhea, the trough widens and loses its topographic expression in the plains.

Figure 2. Arecibo radar image (1-2 km resolution) of central Beta Regio. Relatively rough Surface Properties surface areas are radar-bright, whereas radar-dark areas are smooth. The radar-bright lin- eaments are interpreted to be faults, concentrated along the rift zone. Rhea and Theia Mons, Pioneer Venus data have been used to charac- topographic highs seen in Figure la, appear here as radar-bright features with a central terize the surface of Beta Regio (Head and oth- radar-dark region. The lineaments appear to be overlain by bright deposits associated with ers, 1985). Root mean square (rms) slope data Theia Mons (bottom), whereas bright deposits associated with Rhea Mons (top) are dissected indicate that the region is moderate or transi- by lineaments. The image is in a Mercator projection, with the scale referenced to the equator, tional in roughness (2.5°-5°), as is most of the surface of Venus (Head and others, 1985). Areas of increased roughness, as much as 10° rms Burns, 1980). Therefore, Venera 15/16 images imately N60°W, whereas the Venera 15/16 slope, occur on the flanks of Rhea and Theia predominantly portray surface slope variations, look direction is about east-west (Fig. 13). Lin- Mons. Basilevsky and others (1982) found only whereas Arecibo images illustrate changes in eaments parallel to the look direction of a radar a weak correlation between altitude and degree centimeter- to meter-scale surface roughness. system will be suppressed (McDonald, 1980; of surface roughness at Beta Regio, with no ap- This difference allows lineaments that result Ford, 1980). Ideally, perpendicular look direc- parent correlation between roughness and ra- both from surface roughness and from slope tions would provide the most complete detec- gional slope. The Beta region is generally changes to be studied in the Beta region. tion of linear features. The Venera and Arecibo moderate in reflectivity (0.1-0.2), indicating a The look directions of Venera 15/16 and data sets each show a suppression of linear fea- rock-dominated surface (Head and others, Arecibo also differ in the Beta images. The Are- tures parallel to their look direction (Figs. 12 1985). lander images from the eastern cibo data of Beta has a look direction of approx- and 13), but lineaments at these orientations are flanks of the Beta dome show a rocky surface

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patterns. The lineaments bounding the rift ap- pear wider in the Arecibo image, but all the lineaments are about 1-2 km across, at the limits of resolution. The greater brightness of linea- ments bounding the rift may result from increased surface roughness attributable to age, exposure of higher reflectivity (younger?) mate- rials, or multiple fault offsets producing corner reflectors. Campbell and others (1984) defined two scales of lineaments: (1) major bounding lineaments, 160 km apart on average and defin- ing the edge of the flanks of the rift, and (2) minor lineaments, spaced 10-20 km apart and located in the central rift zone. A rose diagram of lineament orientations shows a pre- dominant N15°E trend, paralleling the crest of the dome, which in places also parallels the orientation of the trough of Devana Chasma (Fig. 12).

Origin of Lineaments

Detailed characteristics of the lineaments to the north were obtained from the Venera 15/16 radar images. In this data set, the linear features also appear both radar bright and dark (Figs. 9 and 1 la). The lineaments correspond to scarps, some with apparent slump blocks (at A, Fig. 11a). Narrow (<5 km) graben can also be ob- served as the lineaments splay out into the plains to the north (at A, Fig. lib). A rose diagram combining lineament orientations from both the Arecibo and Venera 15/16 data reveals three dominant orientations: north-south, N15°E, and 280° N20°W (Fig. 13). A group of narrow (<5 km 400 wide) radar-bright lines are seen to the north km of the rift zone, separated from it by a 100- Figure 3. Arecibo image of central Beta Regio with superimposed Pioneer Venus altimetry to 200-km-wide patch of smooth plains (at A, profiles. The profile altitude is in kilometers above a reference level of 6,051.0 km. The profile Fig. 10). This smaller set, spaced 5 km apart, represents the topography along the base line of each profile. Bright lineaments occur both trends north-south and does not appear to be within the trough, Devana Chasma, and on the high topography of the Beta dome. The directly related to the rift. topographic highs associated with Theia and Rhea Mons correlate with radar-bright (relatively Combining the Arecibo and Venera 15/16 rough) regions. data sets allows the nature and distribution of lineaments in Beta Regio to be determined in with soil localized between rock fragments (Flo- ample, 45.5°N, 282°; 45°N, 283°) and west (for more detail because of the different look direc- rensky and others, 1977; Garvin and others, example, 36.5°N, 274°) of the rift. Some of the tions and incidence angles of the two radar sys- 1984). Garvin and others (1984) interpreted the impact craters have central peaks, but no ejecta tems. All major lineaments can be identified in morphology of the surface to be similar to that patterns or secondary craters have been dis- both data sets by general shape and location, but of terrestrial basaltic flows. Areas of high cerned in this region. some minor lineaments are unique to each data reflectivity (>0.2), with a significant component set; for example, the abundant narrow (<5 km) of high dielectric materials, are located at Theia Lineament Characteristics radar-bright lines north of the rift in the Venera and Rhea Mons. Reflectivity decreases in the 15/16 data (Fig. 10, at A) do not appear in the region surrounding Beta, indicating soils or a The general characteristics of lineaments in Arecibo data. The low incidence angle and look diffusely scattering surface (Bindschadler, 1986). central Beta Regio can be seen in Figures 2 and direction of the Venera data indicate the pres- The combined roughness and reflectivity proper- 5. The linear features are both radar dark and ence of scarps and their facing direction, where- ties of the majority of Beta are similar to those of bright and are located in a broad zone of radar as higher-incidence-angle Arecibo brightness Aphrodite Terra (Head and others, 1985). brightness in the Arecibo images that varies patterns indicate a high degree of surface rough- Few impact craters are seen in the vicinity of from 80-300 km across. The lineaments are ness associated with the scarps. Individual lin- the Beta Regio rift in either Venera 15/16 or generally spaced 10-20 km apart, and 5 km eaments may have a different appearance in Arecibo radar images, but a few impact craters, apart in the vicinity of Rhea Mons. They are 25 each radar data set. For example, radar bright in with diameters of less than 60 km, are recog- to several hundred kilometers in length, parallel Venera and no change in Arecibo indicates nized several hundred kilometers north (for ex- to subparallel, and some occur in en echelon smooth, shallow east-facing slopes (at A, Fig.

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TOPOGRAPHIC PROFILES - BETA REGIO nature, and associated roughness, we interpret the lineaments in Beta Regio as faults. The majority are interpreted as normal faults pro- duced by extension because of the close associa- tion of the faults with the Beta rift zone. Distinct Figure 4. Pioneer scarps with high degrees of roughness can be Venus altimetry profiles detected, with scarps generally facing toward the from Figure 3 correlated central . For example, the western with Arecibo radar-bright- bounding fault of the rift (at A, Fig. 8) can be ness patterns. Profile alti- seen to have a relatively steep scarp facing east tude is in kilometers into the central depression (see arrow, profile g, above 6,051.0 km. The Figs. 3 and 4), with a high degree of associated stippled pattern indicates roughness. As the fault loses its topographic ex- the location of bright de- pression to the north, the associated roughness posits associated with becomes more diffuse. Scarps with shallower Theia or Rhea Mons. The slopes and less associated roughness become dark lines illustrate the more abundant as the rift faults splay to the location of radar-bright north. The deformation in Beta Regio occurs on lineaments in the Arecibo three scales: (1) a zone of faulting and associated image and do not indicate roughness 80-300 km wide, (2) characteristic exact location or dip of a trough width of 160 km, and (3) fault spacings fault. The arrow indicates of 10-20 km in the trough and over the dome in the position of the west- central Beta Regio, and 5 km near Rhea Mons. ern bounding fault of the rift. Distribution of Faults

To the south of Beta (Figs, la and lb), the observed faults are concentrated in a zone of lineaments less than 150 km wide, distributed ' ,ault km over the relatively low topography of the rift. At L3 radar bright area T Arecibo image edge Theia Mons, the faults disappear beneath the volcano, which lies on top of the western bound- ing fault of the rift zone (Campbell and others, 286° 1984) (profile a, Figs. 3 and 4). To the north of °N Theia, in central Beta Regio, the faults appear to Figure 5. Structural map of radar- be concentrated on the western rift flank or bright and -dark lineaments in central more on the crest of the dome (profiles b and c, Beta Regio from the Arecibo image Figs. 3 and 4). The western bounding fault tends seen in Figure 2, after Campbell and to lie along the highest points on the flanks of others (1984). The lineaments are the trough, whereas the eastern bounding fault spaced 10-20 km apart, 5-10 km apart appears in some areas to lie along the inner wall near Rhea Mons. The map is in a Mer- of the trough. The trough itself varies in width cator projection, with the scale corre- from 50 to 200 km and depth from 2.5 km to sponding to 30° latitude. <500 m. The trough takes several steps 20-30 km to the west along strike between Theia and Rhea Mons. The flank heights in this region are generally less than 1 km relative to the surround- ing terrain. South of Rhea Mons, the trough loses its distinct profile, and the faults are con- centrated in a crestal low on the dome (profile d, 15), whereas lineaments that appear radar bright Figs. 3 and 4). At Rhea Mons, the flank heights in Arecibo and dark in Venera indicate rough, increase and the trough narrows and deepens. west-facing slopes (at B, Fig. 15), and so on. The Here, the faults are concentrated in the trough combination of different incidence angles pro- and spaced only 5 km apart (profile e, Figs. 3 vides both slope and roughness information, and 4). North of Rhea Mons, the rift broadens whereas the differing look directions increase the and becomes shallower as the topography de- detection of lineaments, as seen in Figure 13. creases. The fault patterns splay out, distributed The two look directions are not perpendicular over the flanks and trough of the rift (profiles f-i, but are at a sufficiently different angle to have Figs. 3 and 4). The trough everywhere appears different enhancement patterns, thus providing a to have faults on its floor and interior walls, but more complete set of lineaments to characterize the faulting extends over a much broader zone the rift. corresponding to the rift zone. On the basis of linearity, continuity, scarp-like

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because the east-west linear features are at low angles to the look direction of both radar data sets, it seems likely that they represent features earlier than the rift faults which appear to cut them. In Venera 15/16 images, the majority of faults appear to terminate at the ridges, whereas a few faults appear to be offset by the ridges (at B, Fig. 1 lb), suggesting that some of the ridges formed contemporaneously with rift faulting but most ridges predated rift faulting. The western bounding fault of the rift cuts a northeast- trending group of faults to the north of the rift (at A, Fig. 8; B, Fig. 11a), which are spaced approximately 5 km apart. This relationship can be seen in both Arecibo and Venera 15/16 data and appears to be a relative age relation. These two sets of lineaments (at A and B, Fig. 8) are within the Beta highland and may be associated with rifting and formation of the dome. An east- northeast-trending set of lineaments to the northwest of the rift (at C, Fig. 8) appears to be embayed by plains-forming material in Venera 15/16 data and is apparently cut by the rift faults. On the basis of general morphology, we interpret these lineaments to be compressional ridges that surround the northern end of the Beta dome and that may be related to initial relaxa- tion or elastic flexure.

Fault Patterns

The fault patterns of the rift can be used to help constrain the processes that formed the high topography of Beta. Withjack and Scheiner (1982) experimentally and analytically studied fault patterns associated with circular and ellip- tical domes. Homogeneous clay models were subjected to doming, with and without simul- taneously applied regional horizontal strain. Without regionally applied strain, normal faults 0 400 formed on the crest and flanks of the dome, with 1 • km the normal faults on the flanks trending radially to the dome. Under regional extension, the Figure 6. Unit map of central Beta Regio (see Figs. 2 and 5). Major volcanic units associated dome developed normal faults on its crest per- with Theia and Rhea Mons are shown, as well as the zone of roughness and lineaments that pendicular to the applied extension direction, characterizes the Beta rift zone. Arrow indicates an area where flows from Rhea Mons appear whereas on the flanks, normal faults trend to overlie lineaments associated with the rift zone. The map is in a Mercator projection, with obliquely to the applied extensional direction. the scale relative to 30° latitude. Analytical models predict fault patterns similar to those produced by doming the clay models. The fault patterns associated with the Beta rift Relative Ages but the enhanced surface roughness could be (Figs. 12 and 13) were compared to the experi- attributed either to age or to the alignment of the mental and analytical results of Withjack and Crosscutting and superposition relations are brighter feature more perpendicular to the look Scheiner (1982). In the central part of the rift commonly used to determine relative age rela- direction of the radar system. Therefore, cross- (Fig. 12), where the faults have a predominant tions of geologic features, but in the Arecibo cutting relations of surface roughness patterns N15°E trend, the fault pattern most resembles data, crosscutting and superposed radar-bright- have to be interpreted with caution. The major that of doming under limited external regional ness (that is, surface roughness) patterns may not fault trends of the rift appear to cut across a set extension (applied extension rate and uplift rate necessarily reflect relative age relations. A of east-west-trending linear ridges (at B, Fig. 8) approximately equal) (Withjack and Scheiner, lineament with enhanced radar brightness may at the northern end of the Beta highland. Some 1982). The faults trend nearly parallel to the appear to be superposed on another lineament, evidence of offset of the rift faults is seen, and long axis of Beta Regio on the crest of the cen-

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/101/1/143/3380394/i0016-7606-101-1-143.pdf by guest on 30 September 2021 Figure 7. -based Are- cibo image (1-2 km resolu- tion) of northern Beta Regio. This area was also imaged by the Venera 15 and 16 space- craft (see Fig. 9). The radar- bright feature at the center bottom of the image is Rhea Mons. The bright-ringed cir- cular feature at 42°N latitude, 272° longitude is the corona Rauni. The image is in a Mercator projection, with the scale corresponding to 30° latitude. A map based on the surface roughness pat- terns in this image is shown in Figure 8.

400 km

Figure 8. Lineament map of surface roughness variations, based on Arecibo image of northern Beta Regio (Fig. 7). The letter A indicates the western bounding fault of the rift, which is one of the brightest (roughest) lineaments in the image. The western bounding fault appears to overlie a set of northeast-trending lineaments. At B, the rift faults appear to crosscut a set of north west-trending lineaments. A set of regional east-west-trending lineaments that appear to predate the rift are indicated at C. The map is in a Mercator projection, with the scale relative to 30° latitude.

KEY ^bright lineaments f .^ dark lineaments

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/101/1/143/3380394/i0016-7606-101-1-143.pdf by guest on 30 September 2021 Figure 9. Venera 15/16 radar image of northern Beta Regio (quadrangle 19). The region is approximately the same as shown in Figures 8 and 9. The image was digitally mosaicked and has a resolution of 1-2 km. The 10° incidence angle of the Venera radar results in variations in brightness that reflect changes in surface slope.

275° 280° 285° 290°

fci > - - ^ Figure 10. Structural map of surface slope vari- ations, based on the Ven- i era 15/16 image of north- ern Beta Regio seen in / ' V^.' .7, Figure 9. At A, a group of

- © narrow radar-bright lin- eaments are mapped that r: - 'w. are separated from the rift lineaments by a smooth plains region. These lin- eaments do not appear to be related to faulting as- sociated with the rift. The coLr-t-^'t 1X^V-S'i ''?,/ 285 — — x^'iA>"' • letter B indicates the posi- W • '••>.• 280" / I /o, , tion of crosscutting lin- eaments arranged in a diagonal pattern, which •71 SMOOTH PLAINS 7^/^'VENERA 9 may be strike-slip in ori- k gin. Boxes 1, 2, and 3 in- •77] BETA UPLIFT, SMOOTH UNIT 290' dicate the locations of FT?] BETA UPLIFT, HUMMOCKY UNIT Figures 11a, lib, and 11c,

|FIIFI% CORONA respectively.

GROOVES & SCARPS, PROMINENT X^T RIDGES

GROOVES & SCARPS, SUBDUED • : VOLCANIC CONSTRUCTS & DOMES

^^ FAULTS 0 IMPACT CRATERS

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Figure 11. (a) Venera 15/16 radar image of northern Devana Chasma. Faults associated with the rift splay outward as the topography of Beta Regio decreases to the north. At A, a possible slumped block associated with a west-facing scarp can be seen. The letter B illustrates a group of northeast-trending lineaments that appear to be cut by the rift faults, (b) Venera 15/16 radar image of a region to the northeast of Devana Chasma. Faults associated with the rift are visible in the lower left corner. A narrow (< 10 km wide) graben can be seen at A. The letter B indicates a group of northeast-trending faults that appear to be offset by west- northwest-trending ridges. A small dome, possibly of volcanic origin, is seen at C. (c) Venera 15/16 radar image of the central portion of the Beta dome, illustrating hummocky terrain (arrow) composed of intersecting trends of lineaments.

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ty, and low regional slopes. The plains in the northern quarter of Venus have been inter- preted by and others (1986) to be vol- canic in origin, on the basis of general morphology and compositional measurements from the Venera landing sites. No clear sources for the volcanism or flow features can be identi- fied in Arecibo or Venera images of the plains in the Beta region. In the Venera 15/16 image (Fig. 9), two major terrain units can be identi- fied within the northern part of the Beta upland: (1) hummocky terrain and (2) smooth terrain (Fig. 10) (Basilevsky, 1988). The hummocky terrain is characterized by subdued small ridges Figure 12. Rose diagram of lineament orientations in central Beta Regio south of Rhea and hummocks in subparallel, crosscutting, and Mons. The lineaments were digitized from Figure 2, then plotted in arbitrary 5° bins to circular patterns (Fig. 1 lc). The terrain is mostly illustrate the general trends of lineaments. The region south of Rhea Mons is covered only in localized in the summit region of the dome and the Arecibo data. The predominant lineament orientation in this region is approximately in some areas resembles regions of finely pat- N15°E. The look direction of the radar is approximately perpendicular to the largest peak in terned parquet (for example, Lakheside ), the rose diagram. indicating that the terrain is the result of earlier tectonic deformation of the surface of the dome. The areas of hummocky terrain correspond to tral high topography and in the trough. At the The structural evidence in Beta Regio thus relatively high Pioneer Venus roughness (Head northern end of the Beta rift (Fig. 13), the com- suggests that uplift or doming has been a pre- and others, 1985). The hummocky terrain is bined fault patterns (using both data sets) most dominant process. Lithospheric stretching has crosscut by the major rift faults, indicating that resemble fault patterns produced by doming also affected the rift, especially in the central the dome underwent an earlier stage of deforma- without regional strain. Although many faults region, and probably also to the south, where tion prior to rift formation. Smooth terrain is are oriented approximately parallel to the axis the trough and zone of faulting continue into localized on the lower slopes of the dome and of the dome, a greater number of faults trend Phoebe Regio in a linear pattern (Fig. 1) with- merges into the smooth plains of adjacent obliquely and nearly perpendicular to the long out exhibiting the splaying of faults seen at the . Domes are frequently found axis of the dome. Some faults are at orientations northern termination of the rift. on this unit, which includes the area of the predicted by analytical models (Withjack and Venera 9 landing site. At the northern end of the Scheiner, 1982) to be occupied by strike-slip Associated Volcanic smooth terrain is a system of crosscutting faults, but no evidence of strike-slip motion is and Tectonic Features lineaments arranged in a diagonal pattern (at B, seen along these faults. A small zone of potential Fig. 10), which may be strike-slip faults formed thrusting is also predicted, but no evidence of Various features of apparent volcanic origin as a result of east-west extension. thrusting has been recognized. The Beta fault can be identified both within Beta Regio and in patterns were also compared to fault patterns the surrounding plains, with some features produced by one or more holes in a plate clearly linked to the rift. The evidence for vol- (Timoshenko and Goodier, 1970) and those canism includes volcanic plains, domes, and Figure 13. Rose dia- produced by a crack in a plate (Jaeger and shield-like structures, with distinct flows mapped gram of lineament orien- Cook, 1976). These theoretical fault patterns do in the vicinity of the shield volcanoes. tations in northern Beta not produce the characteristic splaying of faults The plains surrounding Beta Regio are char- Regio (north of Rhea seen at Beta Regio. acterized by moderate roughness and reflectivi- Mons), based on Arecibo and Venera 15/16 data. The lineaments were dig- o i-mmumiilniu itized from Figures 3 and 9, and the rose diagrams were constructed as in Figure 12. Both the Are- cibo and Venera 15/16 data sets show a look- direction enhancement of lineaments approximately perpendicular to look di- rection. Combining the patterns from each data set gives a more accurate set of lineament trends that are associated with the Beta rift.

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Figure 14. Scattering-law regional slopes from the central topographic .O high (Figs. 2 and 6). Campbell and others T3 curve for Venus, based on the ' -10 Arecibo (1984) interpreted these characteristics as sup- C Hagfors relationship. Incidence O Venera angles of Arecibo data (30°-45°) porting the hypothesis of a volcanic origin for o ffl -20 are in the flat part of the Theia. Theia is superposed on the rift (Fig. 6 and w profile a, Figs. 3 and 4, illustrate that the trough <0 scattering-law curve, where in changes in surface roughness and zone of faulting are visible to the north and o •30- will dominate the returned sig- south of the structure), indicating that the con- nal. Venera 15/16 data, at struction of Theia postdated formation of the 10°-15° incidence angles, are in rift. <0 CE the steep portion of the curve The other shield-like structure in Beta Regio -50 where changes in surface slope is Rhea Mons, at 33°N, 283°. Rhea is a 4.7-km- 0 10 20 30 40 50 60 70 80 90 dominate the returned signal. high broad dome interrupted in the center by e° Angle of incidence The vertical axis indicates the Devana Chasma. In the Arecibo image, Rhea relative power returned for a appears as an irregular radar-bright patch, 230 x cross section I', in decibels (db). 270 km, with the radar-bright areas overlying Abundant domes, 10-15 km in diameter, are the abnormally high rift rims (profile e, Figs. 3 located north and northeast of the rift zone (at and 4; Fig. 6). The radar-bright area is dissected C, Fig. lib), and Barsukov and others (1986), position similar to tholeiitic (Surkov and by a radar-bright stripe (80 km across) overlying noting the presence of summit craters on some others, 1976; and others, 1976; the trough, which contains radar-bright faults of the domes, interpreted them to be of volcanic Florensky and others, 1977) in a region of (spaced 5 km apart) parallel to the trough walls. origin. The high surface pressure and lower sub- moderate reflectivity and roughness in Pioneer Flow-like radar-bright and -dark features trend- surface pressure gradient on Venus suppress ex- Venus data (Head and others, 1985). ing down regional slopes are common on the solution of volatiles from a and thus eastern outer flank of the rift; some of these inhibit large-scale explosive eruptions (Garvin Shield Volcanoes flows overlie faults on the dome (see arrow, and others, 1982), but the domes may form by Fig. 6). Rhea Mons is interpreted to be a large smaller strombolian-type eruptions (Head and Two shield volcanoes with diameters >250 volcanic shield on the basis of its dome-like Wilson, 1986) or may be analogous to some km have been identified, Theia and Rhea Mons, topography, flow-like features, association with terrestrial oceanic edifices (Slyuta and others, both located south of the Venera 15/16 the rift, and similarity (both topographically and 1988; Aubele and others, 1988). No flow fea- coverage. Theia Mons is at 23°N latitude, 281° morphologically) to Theia Mons. tures are observed around the domes, but they longitude, where it forms a radar-bright circular Rhea and Theia are similar in size and mor- may be undetectable at the resolution of the im- area 350 km across with an irregularly shaped phology, as well as general radar properties. ages. The Venera 9 spacecraft landed to the radar-dark central region over its highest point Both have high rms slope values on the flanks northeast of the rift in a region that contains (Fig. 2). Theia is surrounded by lobate flow-like (Head and others, 1985), possibly corresponding many domes. Venera 9 detected a surface corn- radar-bright features that trend radially down to rough lava flows, and a dark patch at the highest point on the structure, possibly indicat- ing smooth volcanic deposits and perhaps a cen- radar look direction tral . Flows surrounding both volcanoes trend down regional slopes of the dome, indicat- ing that shield-forming volcanism postdated for- Venera 15/16 mation of the high topography. Flows from both features also overlie faults on the dome (Fig. 6), indicating that faulting was earlier than at least sseSCv. Arecibo some eruption. Both volcanoes have reflectivity EMM 3gjj values >0.2 near their summits, indicating the presence of high dielectric materials (Head and | | Bright return others, 1985). Theia and Rhea have different relations to the rift. Theia is clearly superposed | | Medium return on the rift, whereas Rhea is interrupted by it. [ I Dark return Devana Chasma is very deep (>2 km) in the vicinity of Rhea Mons, and distinct faults can be •<& Surface roughness mapped in the trough, but this is not the case at Figure 15. Effects of differing incidence angles on detection of surface slope and roughness Theia. From this relationship, we can conclude variations. The Arecibo system has high incidence angles (>30°) and is more sensitive to that flooding and construction of Rhea largely variations in surface roughness, whereas the Venera system (incidence angle of about 10°) is predated the formation of Devana Chasma. In more sensitive to changes in surface slope. The letters A and B represent two hypothetical addition, the presence of the hummocky terrain, situations to illustrate the differences in the radar systems. At A, a smooth, shallow slope facing with evidence of deformation in the vicinity of the radar beam will appear bright in the Venera image but will not be detected in the Arecibo Rhea Mons, indicates that construction of Rhea image. In contrast, a relatively rough region, as seen at B, will be detected by Arecibo but not may have largely predated this early stage of by Venera 15/16. tectonic deformation of the Beta dome.

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In Arecibo radar images of the Beta region, What Are the Implications of Fault Spacings? volcanoes indicate that volcanism has contrib- volcanic flow features are seen only around uted at least 1 km of topography. The strong Theia and Rhea Mons. No flows are visible in Scales of deformation may be indicative of association of updoming and volcanic activity in the plains or in the vicinity of the small domes. the strength and thickness of layers in a plane- Beta is interpreted as evidence for a mantle The flows around the shield structures are both tary lithosphere. The scales of deformation asso- heterogeneity beneath Beta Regio. radar dark and bright, indicating varying degrees ciated with the Beta rift have been used in an of surface roughness, and are as much as attempt to characterize the structure of the Is the Beta Rift Active or Passive? hundreds of kilometers long. Flows are consist- Venus lithosphere. Zuber (1987) modeled the ently associated both with large and lithosphere of Venus as two strong layers sepa- Active rifts originate above a mantle hetero- shield volcanoes on Venus, and with smaller rated by a weak layer, deformed by the growth geneity, whereas passive rifts result from litho- (<100 km diameter) shields elsewhere on the of small-amplitude perturbations. An upper spheric stretching (Sengor and Burke, 1978). (Stofan and others, 1987). crust of 2-8 km is underlain by a weaker lower The early-stage uplift and volcanism and broad Other structures in the Beta region linked to crust, with total crustal thickness not exceeding flank width of the Beta rift are perhaps indica- tectonic and volcanic processes are coronae, de- 30 km, underlain by a strong upper mantle layer tive of an active origin for the rift (Keen, 1985). fined by Barsukov and others (1986) as struc- of perhaps only a few kilometers in thickness The geometry of the rift and the fault patterns, tures 150-600 km in diameter surrounded by (Zuber, 1987). The model was found capable of however, indicate that some degree of litho- concentric ridges and grooves. The corona explaining the regular spacings of faults and the spheric stretching has occurred. McGill and oth- Rauni can be seen in both Arecibo and Venera characteristic rift width in Beta Regio. The 10- ers (1981) calculated that bending and an images at 42°N, 272° (Figs. 7 and 9). The struc- to 20-km fault spacing indicates a layer thick- increase in arc length due to uplift can account ture has a diameter of 270 km and is surrounded ness of about 2-8 km, which is comparable to for only a few kilometers of extension at Beta, by concentric ridges spaced approximately 5 km the 1- to 10-km elastic layer thickness predicted indicating that tens of kilometers of crustal ex- apart. The central region of the corona is slightly by Solomon and Head (1984a). The presence of may have taken place across Devana raised (<500 m) above the surrounding terrain, two wavelengths of deformation in Beta Regio Chasma. If the rift is a product of active proc- as are other coronae in the northern hemisphere indicates that the upper crust is either weaker or esses, uplift of the rift flanks should follow the (Stofan and Head, 1986). The corona in north- not significantly stronger than the upper mantle. formation of a central depression, which would ern Beta Regio appears to be overlain by vol- We interpret the closer spacing of the faults in form because of gravity-sliding stresses after canic flows and is also cut by a system of the vicinity of Rhea Mons as indicating a thinner domal uplift (Zuber and Parmentier, 1986). grooves, which follow the same orientation of lithosphere at the time of deformation, perhaps Present evidence suggests that the formation of the rift faults, indicating that they may be related linked to volcanism. the central depression postdated the formation to rift deformation. The corona appears to pre- of at least Rhea Mons, but stratigraphic evidence date formation of some of the plains volcanism What Is the Origin of the Beta Regio for an early-stage central depression is not de- as well as the latest rift deformation. Topographic Rise and Rift Flanks? tectable in radar images. Volcanism has been an important process in the Beta region. Both the dome of Beta and the The origin of the high topography of the What Is the Role of Volcanism in surrounding plains appear to be composed of broad topographic rise of Beta and the narrower Beta Regio? volcanic plains material. Several types of vol- flanks of the rift along its strike has been ad- canism have been identified, from 10- to 15-km- dressed by using a variety of geologic evidence. Volcanism has also been a major geologic wide domes to shield structures with diameters If the topography resulted only from uplift, fault- process in the Beta region. Typical Venusian of >200 km (Rhea and Theia Mons). The shield ing would reflect stresses due to doming, and the styles of volcanism, volcanic plains and domes, structures have a complex relationship with the heights of the rift flanks would not be expected are found in the lowlands surrounding Beta. The rift, with Rhea Mons predating and Theia Mons to be higher in the vicinity of volcanic features. volcanism associated with the rift is dominated postdating its formation. In Beta Regio, shield- On the other hand, if topography resulted solely by two large central volcanoes, Theia and Rhea like volcanic structures are closely associated in from volcanic construction, the rift and faults Mons. The shield-like structures have smooth a spatial sense with extensional . would have formed as a result of lithospheric areas at their highest point and rougher flanks stretching due to gravity-sliding stresses. The ob- and are both characterized by high-dielectric CONCLUSIONS served fault patterns associated with the rift and materials. If high-dielectric materials are asso- dome most resemble theoretical patterns asso- ciated with relatively recent volcanic activity Beta Regio has had a complex geologic his- ciated with doming under regional tension (Head and others, 1985), the Beta highland may tory, with multiple episodes of tectonic and vol- (Withjack and Scheiner, 1982). The concentra- be relatively young. The location of Theia and canic activity. Analysis of the Beta rift zone tion of voclanism at major constructs Rhea and Rhea Mons is controlled or appears to control indicates the benefits of using multiple radar Theia Mons, built coincident with or after the structural features. Rhea is dissected by the data sets. The different look directions of the dome had formed, indicates that uplift has trough of the rift, whereas Theia is superimposed two radar systems result in more complete struc- dominated over volcanic construction to pro- on the western bounding fault. In the Beta re- tural maps, which makes comparisons to theo- duce the high topography of the dome. At the gion, shield volcanoes are closely associated retical fault patterns easier and has enabled us to major volcanic shields, increased rift-flank with the rift zone, although shields do occur in address the following questions. heights associated with the topography of the other tectonic environments on Venus (Stofan,

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1985). The concentration of volcanism along mation of Rhea, with continued rift-related lowed by extensive volcanism (Baker and Woh- the axis of the rift is similar to the situation along faulting in the trough (Fig. 16, stage II). The lenberg, 1971). Trough formation occurred terrestrial rifts, where rifts tend to propagate and closer spacing of faults at Rhea Mons may be later, with continued faulting within the trough. up between regions of hot-spot-related due simply to crustal thinning or to the existence The similarities in topography (McGill and oth- volcanism (Dewey and Burke, 1974). At Bell of layers of differential strength in the litho- ers, 1981) and evolution to the Regio, also an extensional environment, the sphère at the time of deformation. The concentra- indicate that the tectonic style and evolution of major volcanic construct is located apart from tion of faults in the trough at Rhea may indicate the Beta rift is more similar to continental rather the zone of faulting (Janle and others, 1987), that faulting which was originally associated than oceanic rifts. The northern segment of De- unlike the nodal arrangement of shield volca- with a broad zone of extension has become pro- vana Chasma (from Theia Mons to the north) noes at Beta Regio. gressively localized in the style of behavior pre- does not appear to be characterized by an active dicted by finite-amplitude necking models for divergent plate boundary. What Is the Sequence of Events in the origin of the rift (Parmentier and others, The crater-retention age of the surface of the Beta Regio? 1987). Lastly, further volcanic activity resulted northern quarter of Venus has been estimated at in the formation of Theia Mons, after the end of 0.5-1.0 b.y. by Ivanov and others (1986). The Crosscutting and superpositional relations be- the episode of major faulting (Fig. 16, stage III). area of Beta is not large enough to make the tween volcanic features and faults in Beta Regio Some of the events may have overlapped in paucity of craters in the vicinity of the dome and allow a sequence of events to be determined time. The volcanic activity associated with the rift statistically significant. The lack of craters (Fig. 16). Flows around Rhea Mons trend down rift zone has moved to the south over time, sim- combined with the distinct fault scarps and lack regional slopes of the dome and overlie major ilar to lateral migration of tectonic and volcanic of soils (Head and others, 1985), however, indi- faults. We interpret this to indicate that domal activity at terrestrial rifts (lilies, 1975). The se- cates a relatively young age for the surface of uplift was accompanied by faulting, lithospheric quence of events at the terrestrial East African Beta. It is possible, however, that the dome itself extension, volcanic flooding, and formation of rift is similar to the proposed sequence at Beta may be older, with impact craters on the dome Regio. In Kenya, faulting was accompanied by Rhea Mons (Fig. 16, stage I). The formation of covered by a thin veneer of volcanic deposits. doming and stretching of the lithosphere, fol- the trough, Devana Chasma, postdated the for- The sequence of events outlined above may have occurred over an extensive period of time.

Rhea Mons. In summary, current data suggest that Beta Regio formed as a result of doming attributable -Domal Uplift to a mantle heterogeneity which also produced a major shield volcano, Rhea Mons. The doming -Formation of Rhea Mons was accompanied and followed by rifting and faulting to form Devana Chasma, indicating a migration in volcanic, and probably tectonic, ac- tivity to the south over time. Later volcanism occurred at Theia Mons. Chasmata similar to Devana have been recognized in Aphrodite Terra and are also likely to be sites of major crustal uplift and extension. Isolated topographic -Faulting faults highs identified along those chasmata may be -Rifting shield structures comparable to Theia and Rhea trough Mons. Recent regional studies have led to the hypothesis that chasmata in the Aphrodite re- gion may be the sites of crustal divergence and spreading (Head and Crumpler, 1987) and that crustal divergence may be widespread in the equatorial highlands. We find no specific evi- dence for crustal spreading in northern Beta III. -Continued Faulting Regio but, as shown in Figure 1, Devana Theia Mons Chasma bifurcates at Theia Mons and appears -Formation of Theia Mons to be connected to a chasmata system extending to the west in Aphrodite and toward the south in Phoebe Regio, perhaps indicating that the Beta rift may be linked to other styles of tectonic activity in the equatorial highlands (Head and Crumpler, 1987). The area of rifting north of Figure 16. Sequence of events in Beta Regio. I. Formation of Rhea Mons after initial uplift, this bifurcation splays out into the northern low- probably accompanied by some faulting. II. Formation of Devana Chasma and associated lands (Fig. 1) and does not appear to be linked faults. III. Rifting and faulting was then followed by the formation of Theia Mons.

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Basilevsky, A. T., 1988, Northern Beta: Photogeologic analysis of Venera rifting and the origin of Beta Regio: Geophysical Research Letters, v. 8, to the more regionally extensive extensional de- 15/16 images and maps [abs.]: Lunar and Planetary Science 19, p. 737-740. formation in Phoebe Regio and westward to- p. 41-42. Parmentier, E. M., Stofan, E. R., and Head, J. W., 1987, A finite amplitude Basilevsky, A. T., Bobina, N. N., Shashkina, V. P., Shkuratov, Yu. G., model for the formation and evolution of rift zones: Application to the ward Aphrodite Terra. The pattern associated Kornienko, Yu. Y., Usikov, A. Ya., and Stankevich, D. G., 1982, On Beta Regio rift [abs.]: Lunar and Planetary Science 18, p. 764-765. with Beta/Phoebe (convergence of three arms of geological processes on Venus: Analysis of the relationship between Pettengill, G. H., Eliason, E., Ford, P. G., Loriot, G. B„ Masursky, H„ and altitude and degree of surface roughness: Moon and Planets, v. 27, McGill, G. E., 1980, Pioneer Venus radar results: Altimetry and surface rift zones) is very similar to that seen in terres- p. 63-89. properties: Journal of Geophysical Research, v. 85, p. 8261-8270. Bindschadler, D. L., J 986, Characterization of Venera 15/16 geologic units Schaber, G. G., 1982, Venus: Limited extension and volcanism along zones of trial triple junctions. from Pioneer Venus reflectivity and roughness data [M.S. thesis J: Provi- lithospheric weakness: Geophysical Research Letters, v. 5, p. 419-421. dence, Rhode Island, Brown University, 94 p. Sengor, A.M.C., and Burke, K., 1978, Relative timing of rifting and volcanism Campbell, D. B., and Burns, B. A., 1980, Earth-based radar imagery of Venus: on Earth and its tectonic implications: Geophysical Research Letters, ACKNOWLEDGMENTS Journal of Geophysical Research, v. 85, p. 8271-8281. v. 5, p. 419-421. Campbell, D. B., Head, J. W., Harmon, J. K., and Hine, A. A., 1984, Venus: Sharpion, V. L., and Head, J. W., 1985, Analysis of regional slope characteris- Volcanism and rift formation in Beta Regio: Science, v. 226, tics on Venus and Earth: Journal of Geophysical Research, v. 90, p. 167-170. p. 3733-3740. We would like to thank Maria Zuber and Dewey, J. F., and Burke, K., 1974, Hot spots and continental break-up: Impli- Sjogren, W. L., Bills, B. G., Birkeland, P. W„ Esposito, P. B., Konopliv, A. R„ Marc Parmentier and our colleagues at the Ver- cations for collisional : Geology, v. 2, p. 57-60. Mottinger, N. A., Ritke, S. J., and Phillips, R. J., 1983, Venus gravity Esposito, P. B., Sjogren, W. L„ Mottinger, N. A., Bills, B. G., and Abbott, E., anomalies and their correlations with topography: Journal of Geophysi- nadsky Institute for helpful discussions and sug- 1982, Venus gravity: Analysis of Beta Regio: Icarus, v. 51, p. 448-459. cal Research, v. 88, p. 1119-1128. Florensky, C. P., Ronca, L. B., Basilevsky, A. T., Burba, G. A., Nikolaeva, Slyuta, E. N., Nikolaeva, O. N., and Kreslavsky, M. A., 1988, Distribution of gestions. We would like to thank the USSR O. V., Pronin, A. A., Trakhtman, A. M., Volkov, V. P., and Zasetsky, small domes on Venus: Venera 15/16 radar data [abs.]: Lunar and Academy of Sciences for providing the Venera V. V., 1977, The surface of Venus as revealed by Soviet Venera 9 and Planetary Science 19, p. 1097-1098. JO: Geological Society of America Bulletin, v. 88, p. 1539-1545. Solomon, S. C., and Head, J. W., 1982, Mechanisms of lithospheric heat 15/16 data. We also gratefully acknowledge re- Ford, J. P., 1980, Seasat orbital imagery for geologic mapping: Tennessee- transport on Venus: Implications for tectonic style and volcanism: Jour- Kentucky-Virginia: American Association of Petroleum Geologists Bul- nal of Geophysical Research, v. 87, p. 9236-9246. views by Kevin Burke and George McGill, as letin, v. 64, p. 2064-2094. 1984a, Venus banded terrain: Tectonic models for band deformation Garvin, J. B., Head, J. 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The research was also emissions and radar Radio Science, v. 5, p. 189-227. sity, p. 2-41. supported by National Aeronautics and Space Head, J. W., and Crumpler, L. S., 1987, Evidence for divergent plate boundary Stofan, E. R., and Head, J. W., 1986, Pioneer Venus characteristics of ovoids characteristics and crustal spreading: Aphrodite Terra: Science, v. 238, on Venus: Preliminary results [abs.]: Lunar and Planetary Science 17, Administration Grants NAGW-713 and NGR p. 1380-1385. Supplement, p. 1033-1034. Head, J. W., and Wilson, L., 1986, Volcanic processes and landforms on Stofan, E. R., Head, J. W., and Campbell, D. B„ 1987, Geology of the southern 40002088, Jet Propulsion Laboratory Contract Venus: Theory, predictions and observation: Journal of Geophysical Isbtar Terra/Guinevere and Sedna Planitae region, Venus: Earth, Moon 957088, and the William F. Marlar Foundation. Research, v. 91, p. 9407-9446. and Planets, v. 38, p. 183-207. Head, J. W„ Peterfreund, A. R., Garvin, J. 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