JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 95, NO. B5, PAGES 7119-7132, MAY 10, 1990

Venus Trough-and-Ridge Tessera: Analog to Earth Oceanic Crust Formed at Spreading Centers?

JAMES W. HEAD

Department of Geological Sciences, Brown University, Providence, Rhode Island

One type of tessera terrain on Venus, the trough-and-ridge tessera, exhibits a distinctive morphology composed of throughgoing generally parallel linear valleys and shorter orthogonal valleys and ridges. The similarity of this pattern to oceanic crustal topography and morphology is examined. Oceanic crust on Earth is characterized by a distinctive orthogonal pattern, with transforms and fracture zones in one direction and linear abyssal hills developed parallel to the rise crest in the other. Similarities between the components of the tessera terrain and terrestrial seafloor fabric and structure are found in terms of linearity, parallelism, length, width, spacing, orthogonality, and general morphology. A difference is that trough and ridge tessera tend to occur as plateau-like regions at intermediate elevations (average -•2 kin) above the surrounding plains, while terrestrial seafloor ranges in elevation from that observed at rise crests and oceanic plateaus such as Iceland, down to depths of several kilometers on the abyssal floors, where the orthogonal pattern is often masked by sedimentation. On the basis of the similarities it is proposed that trough-and-ridge tessera may have originated through processes analogous to those responsible for the ocean floor fabric on Earth, forming at a rise crest similar to divergent boundaries on the Earth's seafloor, and evolving to its present morphology and configuration through processes of crustal spreading. The plateau-like nature of the trough-and-ridge tessera distribution could be the result of localized crustal thickening in the spreading process, producing elevated Iceland-like plateaus whose texture is preserved from subsequent lowland volcanic flooding. Some regions of more complex tessera patterns may result from deformation of the basic trough-and-ridge pattern of tessera. This hypothesis can be tested with global high-resolution imaging and altimetry data and gravity data from the Magellan mission.

1. INTRODUCTION Kozak and Schaber, 1986; Markov et al., 1989; Smrekar and

Phillips, 1988] or gravitational relaxation [Bindschadler and Tessera ("tile" in Greek; also informally called "parquet") is Parmentier, 1987]; (4 seafloor spreading where structural

an upland terrain type occurring in areas known to be patterns are related to rise-crest processes [Bindschadler and distinctive on the basis of Pioneer Venus altimetry, roughness, Head, 1988c]. Recent work by Head and Crumpler [1987] and reflectivity data [Head et al., 1985; Bindschadler and Head, suggests that divergence and crustal spreading is occurring on 1989a,b]. This terrain was shown by Venera 15/16 radar Venus in Western Aphrodite Terra, to the south of the region images to have a distinctive morphology and texture dominated covered by Venera 15/16 data, although the global extent and by closely spaced intersecting grooves and ridges that produce significance of this process is uncertain [Kaula and Phillips, a variety of patterns (hence the term tessera) including 1981]. Analysis of terrestrial spreading centers extrapolated to orthogonal, diagonal, chevron, and chaotic [Barsukov et al., Venus conditions [Sotin et al., 1988; 1989a,b] shows that the 1986; Basilevsky et al., 1986; Sukhanov, 1986; Bindschadler gravity and topography data for western Aphrodite are and Head, 1988b,c]. The spacing between ridges in tessera consistent with a spreading hypothesis. There is also evidence terrain varies but generally is in the range of 5-20 km. Ridge that some of the equatorial regions thought to represent crustal heights are generally not more than several hundred meters spreading have the same radar characteristics as the tessera in [Basilevsky et al., 1986]. Tessera occur in both small isolated the north [Bindschadler and Head, 1988a, d; Senske and Head, patches and in large regional configurations that comprise 1989]. The purpose of this paper is to investigate further the about 15% of the area north of 30øN. The morphology of general similarity of morphology between the trough-and-ridge tessera has been interpreted to be related to deformation which has acted with primarily horizontal components over broad areas (the areal deformation of Basilevsky et al. [1986], but the origin of the deformation is still controversial.

A range of models for tessera origin has been outlined and summarized by Bindscharier and Head [1988c] and includes (1 horizontal compression and crustal thickening due to asthenospheric currents [Basilevsky, 1986; Pronin, 1986]; (2 vertical uplift due to shallow mantle processes [Phillips, 1986;

tessera and the fabric of the ocean floor and to investigate one of the hypotheses for tessera origin, that is, that the tessera texture might be related to a crustal fabric produced at spreading centers.

2. CHARACEERISTICS OF TESSERA ON VENUS AND

Ttm T•OUGH-aND RnX]E TE• iN LmM• Tv. ss•.RA

Pioneer Venus data show that tessera terrain lies at higher Bindschadler and Parmentier, 1987]; (3 gravity-driven (generally 2 +1 kin) elevations than the surrounding plains and deformation manifested as gravity sliding [Sukhanov, 1986; that it is characterized by relatively higher values of surface

roughness at the centimeter-scale and the meter to decmeter scale [Head et al., 1985; Bindscharier and Head, 1989b] than most other surface units. Gravity data for one of the major

Copyright 1990 by the American Geophysical Union. regions of tessera (Tellus) led Sjogren et al. [1983] to suggest Paper number 89JB03068. that Tellus is compensated at relatively shallow depths. The 0148-0227/90/89JB-03068505.00 range of surface morphology observed in the tessera led

7119

7120 HEAD: VE2qUS TESSERA-EARTH OCEANIC CRUST ANALOG?

60

280 320 0 40 120 160 200

-30

-60

24O 280 r 320 •) ' ' 40 80 120 160 200 240

Fig. la. Location and setting maps. Global map of Venus showing the location of Laima Tessera and its relation to Ishtar Terra to the north and to Aphrodite Terra to the south. Areas indicated in black lie below the 0-km datum. Contour interval is 1 km. Box shows location of Figure lb.

-30

-60

Bindschadler and Head [1988b; also Models for the origin and morphologic variability along strike. The individual features evolution of tessera terrain, Venus, submitted to Journal of themselves (Figures 2b-2c) have three modes of expression: (1) Geophysical Research, 1989] to propose a three-part trough-shaped in cross section, with inward dipping walls and a classification scheme, outlined in order of increasing flat floor which usually appears to be covered by smooth plains complexity of structural patterns: (1) subparallel ridged terrain units of probable volcanic origin [Basilevsky et al., 1986; (subparallel linear ridges with narrow zones of disruption Sukhanov et al., 1987] (Figures 2b-2c, marked T); (2) grooved- oriented at a variety of angles to the ridges; type area Western shaped in cross section, where the inward dipping walls Fortuna Tessera); (2) trough and ridge terrain (large troughs in converge without the development of a flat floor (Figures 2b- one direction, smaller ridges and valleys in the other, generally 2c, marked G); and, (3) lineaments, where a linear feature is orthogonal direction; type area Eastern Laima Tessera); and (3) observed, but topography is not as distinct as in the case of the disrupted terrain (complex and chaotic orientation of short to troughs and grooves (Figures 2b-2c, marked L). Troughs range intermediate length ridges and troughs; type area central Tellus Regio). In this analysis, the general characteristics of the type example of the trough-and-ridge terrain (Eastern Laima Tessera) (Figure 1) are assessed. eøø,

Laima Tessera covers over 2 x 10 6 km 2 and lies south of Fortuna Tessera and Eastern Ishtar Terra (Figure 1). It is bounded on the east by a generally north-south trending ridge belt (Kamari Dorsa), and along the rest of its borders it has a ' transitional boundary where the topography descends toward the surrounding plains and appears to be embayed by them, s0 o. particularly to the south and west (Figure lb). The major morphologic pattern associated with Laima Tessera is a structural fabric composed of two elements (Figures l b-lc). One of these elements consists of WNW to NW trending long and throughgoing lineaments and faults [Basilevsky et al., 1986; Sukhanov et al., 1987] and large troughs up to 30 km in width. The second is a NNE to NE trending set of much shorter 40ø- 30 o• linear elements of alternating ridges and valleys spaced 6 to 12 km apart and oriented locally orthogonally to the larger WNW

30 ø 50 ø 70 ø

/ / I I • \ \ 60 ø

50 ø

0 o

•.../ [ 400 4• o I 50 ø 6•3 ø

trending structures. Fig. lb. Sketch map showing the location of Laima Tessera and The troughs/lineaments (Figures 1 and 2a-2c) are nomenclature of adjacent features and structures. Dorsa are linear belts

characterized by their linearity, significant length, and of deformation. Planitia are plains interpreted to be of volcanic origin.

HEAD: VENUS TESSERA-EARTH •C CRUST ANALOG? 7121

ß .. _ i ß I I I t

Fig. lc. Structure of the tessera in central and northem Laima Tessera (centered on 48 ø, 53øN) [from Basilevsky et al. 1986]. 1, lavas; 2, ridges of linear belts; 3, lineaments in tessera; 4, circular blocks; 5, volcanoes; 6, impact craters; 7, main faults. Vertical white stripes are data gaps which were fdled in later in the mission. Ausra Dorsa, upper left; Kamari Dorsa, right.

in width from about 8-30 km with most in the range of 10-12 Detailed examination of the areas between major troughs often km. In some cases the trough walls become irregular and reveals the presence of much more subtle linear features convex outward, and a bead-like appearance results (Figures 1 oriented parallel and subparallel to the troughs and producing and 2c, marked B). These bead-like or oval structures range in minor disruptions in the orthogonal fabric (Figures 1 and 2b, width from 15 to 45 km and in length from 30 to 50 km and marked D). appear to be more common in the southern part of Laima The second structural element occurs in the terrain between Tessera than in the northern. Often, the flat-floored trough the troughs/lineaments and is characterized by parallel ridges narrows and the walls merge into a single linear valley, and valleys oriented generally perpendicular to the generally less than 8 km in width. A cross section across the troughs/lineaments, giving the impression of a corrugated strike of the troughs (Figure 2d) shows that the troughs average appearance. The flanks of the ridges and valleys are often sharp less than 500 m deep along this profile but that the most and linear and are interpreted to be fault bounded, in contrast to prominent trough (Baba-jaga Chasma)approaches 1 km depth the hills and swales typical of the mountain belts of Ishtar where it crosses this profile. The range and frequency Terra which are interpreted to be anticlines and synclines distribution of trough/lineament lengths are related in part to [Crumpier et al., 1986]. Therefore the hills and valleys of the their preservation, since many can be seen to be embayed by corrugated terrain appear to be more similar to horsts and plains units surrounding Laima Tessera (Figure 1). Several graben than to anticlines and synclines (Figure 2). troughs completely cross the width of Laima Tessera, and one In many cases, particularly where the edge of the corrugated (Baba-jaga Chasma) extends for a distance of about 1400 km. terrain is defined by a trough, individual linear elements do not Most of these features are less than 750 km in length, and there appear to cross from one domain across trough/lineaments into is a distinct difference in the length and continuity north and the adjacent corrugated segment. In other cases, particularly south of Baba-jaga Chasma (Figure 1). where the edge of a portion of the corrugated terrain is defined

The major troughs generally parallel each other (Figure 1) by a lineament, many linear segments can be traced across the with the distance between any two being similar along their structure, whereas others terminate against it. Few corrugated strike. The spacing between successive major elements carry further than across one trough/lineament zone trough/lineaments in a traverse normal to the strike of the (see map patterns in Figure 1). Many elements terminate features, however, is variable. ANNE oriented transect across against the more subtle lineaments between major eastern Laima Tessera shows that the spacing ranges from 20 to troughs/lineaments (see particularly central Laima Tessera, 100 km with an average spacing for 13 troughs of about 50 kin. Figure 1).

7122 HEAD: VENUS TESSERA-EARTH OCEANIC CRUST ANAIXX;?

.... '•-•::"i,• ........... • ....... ":;"•' ::?... • ?•"•'.• •-:•::---:::•'• • '•:• %[: ::.:•:•:..•: "' • •'..-.-.•i:::

.....• .•:• % - 60 • ......

'•":-•'t .... •, .. .....

.:.

..•.

55 ø

50 ø 55 o 60 o

45 ø

Fig. ld. Venera 15/16 image of eastern Laima Tessera showing the texture of the trough-and-ridge tessera and the location of the enlargements, sketch maps, and topographic profile shown in Figure 2. Kamari Dorsa, right side of image.

The lengths of the valleys and ridges in the corrugated interpreted to be of volcanic origin are noted adjacent to the terrain are thus generally comparable to the separation tessera and and are sometimes associated with smooth plains in distances between the WNW oriented troughs/fractures. These the tessera in general [Slyuta et al., 1988; Aubele, 1989]. The distances are commonly in the range 20 to 100 km and are abundance of domes within the tessera itself, however, cannot much shorter than the WNW oriented troughs/fractures. The be easily determined because of the extremely rough separation distances of the hills range fxom about 6 to 12 km, topography comprising the corrugated terrain (Figure 2). averaging about 8-10 kin, in general agreement with the In summary, the trough and ridge type of tessera in Laima average 8 km crest-to-crest distance found for Laima Tessera by Tessera is characterized by a distinctive pattern of Ivanov [1988]. throughgoing troughs/lineaments, and orthogonally oriented

Although Laima Tessera itself is surrounded by plains ridges and valleys comprising the corrugated terrain (Figure 2). interpreted to be of volcanic origin [Basilevsky et al., 1986], We now proceed to examine the basic characteristics of the the tessera terrain itself does not appear to display evidence of terrestrial seafloor in order to compare and contrast the abundant volcanic centers except for the smooth flat floors of characteristics. the trough/fracture zones, and the beaded areas or ovals within these zones (Figure 1). A few elongate dome-like features, 3. NATURE OF THESE•R FORMED having dimensions of 40 km x 70 kin, are oriented parallel to AT SPREADING CENTERS the fabric of the corrugated terrain and are of possible volcanic The terrestrial seafloor possesses three major landforms in origin (Figure 2a). Smaller dome-like hills (<20 km diameter) areas of oceanic ridges: (1) the linear rise crest at the spreading

HEAD: VENUS TESSERA-EARTH OCEANIC CR•T )dqALOG? 7123

--•':": :-.. ::.-'-•:.." ":•'•:':,....•:.':•. :'--:.': - '•:'"':'• "-" •--. ..... .....:• .•:.- ',i ':: .;.,-."?,.. :' %.. --½'•:-:. . "':..: "-.. ::" .,.'. ':":--:-::•'/-;..:i. ?' •:-::' .. ':.::.:: :---:':"". ::'." ... . ..... :•:.' '/.:...' .' ..... ::--' .-.::-/-::'w.?-{:.• '-'-;::"7:,:'•::.:"::'::.::'?'.{ :.' :..•

... •, ::'j:•-•-:•t •' :-- '-:?..i -. ........ '. ii"•::•3::•'•-:•:'•":. '•" '..':.--.---" "'.' ::':-?::'-'"',.... '.•-'•? :'•:•.:::• ..•;.•:•..•:.:•' ',: v ¾.-'*:::- ¾'""7:-•:•:. "::':"•:'•':"; .... .......... ...:-'-:. ......... :'i.;i:;:::•:-fi::•' :.¾q ' ':•--'";:---'..."•:-'--::.. :"-. ::•-:' ß -,' ..'""'-' ............. [ ........ * ¾'::• ........... --•--•:' .......... -.•';.-:-'::-•::-:" ½ ...•:" .}.•..-.' .-'"-'5 ::"-•: ':•;'"*, ':•. :'.' .............. ß --

.- • .,'":: '-.:"•'*:*•. ".: ) .":::::'"'::i:.: .':".•.•' ' ?"." i;•.::.:•:*:•::' :-?:;;;.'" :.:?.•.•:.•.::': ::¾'::..*t •:'"'! :: ""¾•..- '"' :....'-;.-..•...": ': :::,5•'• .... •:' ...... . ........... .....• ---.-'- . .... ; ........... ..;• ;,•-:.-. ....... . ....... :... :.,.,.•... ............ .....• ..: ........ .:.. ...... .:... ........... ................... ..... ;' •---:• It-': ........... ':':':'-.** .... ' ......... ' ........ '"--:' "' "•: '"'" ........... ' .................................. '.?'½:i i'":'--::.:• 1[ ..." :' ' *:71-: ...... :. -:. ":-- .- .'t' '%.....---:: '.. :?'.-: •..:.-: ..' •: -::•.'?....•:.:%)•:• '..:•i::. :: ::•." •" '-::- ;'-•i:'i'.;-'•. :-:-•.-..• '• ;'..•:":::'•..;.:'!•:..--' ....... ".!:':.:.7: ':--.-.::'•: '::

• i:i' ::'i:•: -• :.::•:..:--t:'•:•;..:..?'.½.., :?.•i:}, ."•:•c::::?ii:iii, i::: ........... '-:::..;.:.•.. :.•. ?:::'-') '"•:•.-:..!:' •":--• :" ":• ............. ========================= :i:':', ½•/*** ';'-:':. ..::. ......... ::':•-..i?,---..:•:::• ...... ' .......... ' .... :':;:'?':"" ':•:'½;•"";'•;'" ..... ..:) ." ,• ..5.: .... •' .' ".

....... '::"•:•:'•"•:• ..... ....•::....:.q.

:.

..... .......

........ ::•....:•:.

Fig. 2a. Venera 15/16 images and sketch maps of the troughs/fractures and the corrugated terrain in Laima Tessera. T, o o

troughs; G, grooves; L, lineaments. Southeastern Laima Tessera, centered on 51 , 48 N; width of image is 500 km. The generally orthogonal nature of the fabric is clearly visible, with troughs (paired dark lines parallel to the long axis of photo with hatchures indicating depressions) and parallel fractures (dark lines) orthogonal to the shorter, more closely spaced linear hills of the corrugated terrain. Note also the circular depression of probable impact origin and the two linear hills.

7124 tLEAD: VENUS TESSERA-EARTH OCEANIC CRUST ANAIX)G?

0 100 II I , , \

km Fig. 2b. East central Laima Tessera, centered on 55 ø, 52.5øN; height of image is 180 kin. Note the trough-like nature of the major trough/fracture in the southeastern part of the image (Mots Chasma), how it widens in the central part and then changes to a narrow fracture in the northwest. Most elements of the orthogonal corrugated terrain terminate against the trough/fracture, while some extend across it.

o lOO I i •

km Fig. 2c. Central Laima Tessera, centered on 48 ø, 53.5øN; height of image is 110 kin. Note the variation in width of the trough (Baba-jaga Chasma), the development of small oval structures filled with volcanic plains (center), and its evolution into a fracture on the northwest side of the image. Corrugated terrain is oriented generally normal to the trough/fracture. A mountain 40 x 50 km, of possible volcanic origin, is located in the southeast.

LAIMA TESSERA

I ̂ aaba-Jaga C. Venera16 2/16 I Mots C. IMezas-Mate C. I I Kozhla-Ava C. / / /Kaygus C. I

mountains or abyssal hills). These abyssal hills produce a closely spaced pattern of linear ridges and valleys extending away from the rise crest, and oriented generally normal to the transforms/fracture zones [Macdonald, 1986].

Following the terminology of Fox and Gallo [1986], Sandwell and Schubert [1982] and Kastens [1987], transforms are defined as fault zones (along which strike slip motion is

Fig. 2d. Venera 15/16 altimetry profde across Laima Tessera showing occurring) connecting offset rise-crest segments and oriented the topography associated with named troughs and chasma (labeled generally orthogonal to the rise crest (Figure 3). Fracture above profile) and unnamed troughs or trough extensions (labeled TR below profde). Location of profile shown in Figure 1. zones are the aseismic along-strike extension of transform

faults (Figure 3). For this analysis, the term fracture zone (FZ) is used to refer to both fracture zones and transform faults,

center, (2) orthogonally oriented patterns of transforms and unless indicated otherwise. fracture zones [Lonsdale, 1977; Macdonald, 1986; Fox and Fracture zones are pervasive in the ocean basins (Figure 3) Gallo, 1986; Kastens, 1987] (Figure 3), and (3) linear ridges and may range from 20 to 60 km in width, consisting of ridges and troughs that are formed parallel to the rise crest (rifted and troughs parallel to the general strike of the zone. A typical

HEAD: VENUS TESSERA-EARTH OCEANIC CRUST ANALOG? 7125

Morphology•

Structure _ / // -- •/ / ,/ '--- ...... ,--*-'*-• .,.,.F 1t )>:_//:.

f, / ...,,, • Tr,..__• '- -O? ",.,,....,•/•,,/ 1• Fracture Zone

Transform Domain (20-eO

Transv©rs©

• ',,.•.• FZ Valley

Fig. 3a. Earth's ocean floor. Morphology, structure, and nomenclature of fracture zone/transforms, and abyssal hills (modified from Fox and Gallo [1986]). At the top, the rift valleys are separated by about 100 km along the transform fault and the general topographic characteristics of the abyssal hills, the transform, and a small part of its aseismic extensions are shown. The structure and nomenclature are shown below. NVZ, neovolcanic zone; TFZ, transform fault zone; TTZ, transform tectonized zone; PTDZ, principal transform displacement zone. See Fox and Gallo [1986] for details.

fracture zone contains a central trough 5-20 km in width that and volcanism is often localized within fracture zones [Lowrie has regional inward dipping convex-downward slopes down to et al., 1986]. Fracture zones are observed to change their the trough floor. Subsidiary basins, troughs, and ridges are morphology along strike. A sonograph mosaic of the Charlie- present along the valley floor. Floor depth is a function of Gibbs transform and fxacture zone in the North Atlantic (Figure transform offset and may be as much as several thousand meters 3b) shows its distinctly linear appearance over short distances [Fox and Gallo, 1986] due to differences in age and subsidence and its complexity along strike and along its flanks [Searle, level across the fracture zone. Ocean floor topography 1979]. typically changes across the fracture zone due to the In map view, fracture zones can be linear, arcuate, or juxtaposition of crust and lithosphere of different ages. slightly sinuous, depending on a number of factors including Transverse ridges often parallel the edge of the fracture zone changes in spreading rate, location and variability of poles of valley (Figure 3a). The actual movement at any given time rotation, and interplate and intraplate deformation subsequent occurs primarily along a narrow transform fault zone (TFZ), but to crustal formation [Cande et al., 1988]. On Earth, fracture the movement can migrate and such migration tends to produce zones (FZ) reach thousands of kilometers in length and are a broader transform tectonized zone CITZ) (Figure 3a). Talus and apparently limited only by the width of the ocean basin. sedimentation often obscure the details of the floor structure, The spacing between major FZs is relatively constant

7126 •: VENUS TESSERA-•TH •C CRUST ANALOCr?

t -' .... ...... -.. .. --.

Fig. 3b. Section of sonograph mosaic and tectonic bte•retation from •e sou•em tr•sfo• fault (str•bg essentia•y east-west M •e •nff• pa• of •e •age] of •e Charlie-Gibbs Fracture •ne • •e No• Afl•tic Oc•n. •s•fficafion is fr• no• to sou• [from Searle, 1979].

between any pair of adjacent FZs, but the spacing between FZs is variable along the length of an oceanic rise. Fracture zone spacing increases with spreading rate [Fox and Gallo, 1984; Sandwell, 1986; Abbott, 1986]. Average spacing in the North Atlantic is about 55 km [Macdonald, 1986; Vogt, 1986]. In regions of complex plate interactions, there are fracture zones which converge toward each other, such as the Mendocino and Blanco FZs bounding the Gorda plate [Stoddard, 1987].

A distinctive linear fabric of abyssal hills occurs parallel to the rise crest and normal to the fracture zones (Figures 3a and 3b). Along the Mid-Atlantic Ridge (MAR), normal faults that dip toward the rift valley develop within 1-5 km of the spreading axis. This faulting creates the inner walls of the rift valley. Adjacent to this in some areas, in a zone 10-20 km wide, occur the relatively flat, faulted terraces, outwardly bounded by the outer wall, the rift mountains, and the abyssal hills [Macdonald, 1986]. The major scarps are generally continuous along the 40-60 km length between fracture zones [Macdonald, 1986]. The nature of the transformation of the rift valley outer wall into the abyssal hills is a matter of controversy but may involve tilting of the rift valley walls back to a horizontal position [Verosub and Moores, 1981], or overprinting by outward dipping normal faults [Macdonald and Atwater, 1978], or a combination of both. The presence and wide occurrence of abyssal hills distally from oceanic rises was noted early in the exploration of the ocean floor, and they were used in the definition of morphotectonic provinces [Heezen et

al., 1959] which cover up to 80% of the sea floor [Menard and Mammerickx, 1967]. Abyssal hills are elongate, oriented generally parallel to regional magnetic anomalies, and normal to fracture zones and possess relief of 40-1000 m and widths of 2-35 km [Krause and Menard, 1965]. Sonographs of the Mid- Atlantic Ridge show the linear nature and relatively regular spacing of abyssal hills (Figure 3c [Laughton, 1981]). R. A. Pockalny et al. (A morphological comparison of abyssal hill topography using high-resolution, multibeam bathymetry data, submitted to Journal of Geophysical Research, 1989) reviewed three models for the formation of abyssal hills: (1) the horst and graben model [Needham and Francheteau, 1974], where abyssal hills were formed by inward and outward dipping normal faults; (2) the listric fault model [Harrison and Stieltjes, 1977], where listtic faulting occurs to offset the effects of the inward facing faults of the rift valley walls, rotating crustal blocks and producing hills shaped triangularly in cross section, which result from the rotated half grabens; (3) the episodic magmatism model [Kappel and Ryan, 1986; Pockalny et al., 1987; Barone and Ryan, 1988], where abyssal hill topography is generated by the interplay between episodic magmatism and extensional tectonism.

Recent work (R. A. Pockalny et al., submitted manuscript, 1989; P. J. Fox, personal communication, 1989) has shown that the spacing of abyssal hills is inversely related to spreading rate. Slow spreading ridges (<30 mm/yr) have relatively high abyssal hills (about 200 m) and an average

HEAD: VENUS TESSERA-EARTH OCEANIC CRUST ANALO6'? 7127

30øW

39øN

29øW

..

.... ';';•::;;: ;5;

30øW

o 5o lOO I ,, I I

km

Fig. 3c. Two parallel two-sided sonographs on the crest of the Mid-Atlantic Ridge near the Azores sin, wing the linear tectonic fabric of the abyssal hills. The sonographs have been digitized, corrected for slant range errors, and merged by the Jet Propulsion Laboratory. Width of image is approximately 100 km [from Laughton, 1981].

29øW

39øN

spacing of 10 km. Abyssal hills derived from slow to 4.2 km). Overall, the variation in the length of abyssal-hill intermediate spreading ridges (-40 mm/yr) have an average ridges along strike is 10 km to 40-50 km. spacing of 8.2 km. Abyssal hills associated with intermediate- In some cases, the basic orthogonal pattern can be modified to-fast ridges (-100 mm/yr) show an average spacing of 4.4 by several processes. Acton et al. [1988] described a complex km, and those from fast spreading ridges (-160 mm[yr) are less curved seafloor fabric in the Galapagos rise which they elevated (50-60 m) and less widely spaced (average spacing of interpreted to be caused by rift propagation and changes in

7128 HEAD: VENUS TESSERA-EARTH •C CRUST ANALO•

spreading rate. Menard [1984] showed that serrate ridges and transforms could develop during asymmetrical spreading. Stoddard [1987] described sinuous and complex patterns of magnetic lineations and abyssal hills on the Gorda plate located between the convergent Blanco and Mendocino fracture zones, and he analyzed models related to mechanisms of rotation of rigid subplates and nonrigid deformation to explain the origin of these patterns on the plate moving toward the apex of the two fracture zones.

In summary, oceanic crust is commonly characterized by a distinctive orthogonal pattern (fracture zones and abyssal hills) whose orthogonal elements differ in their characteristics in the two directions (Figure 3). We now proceed to compare the general fabric of the ocean floor to the fabric of the trough- and-ridge tessera on Venus.

4. COMPareSONS

The basic dimensions, spacing, and geometric characteristics and relationships between the terrains examined on Venus and Earth are summarized in Table 1, and this provides a basis for comparison of similarities and differences between the two terrains. There are numerous similarities

between the components of the trough-and-ridge tessera terrain type and basic terrestrial seafloor fabric and structure.

TABLE 1. Comparison of Dimensions of Venus Troughs and Ridges and Earth Fracture Zones and Abyssal Hills

Dimensions. km

Venus troughs Earth fracture zones

Len• Widtl• Sp•ing

up to 1400 10-12 -50 up to 4000 5-20 30-70

Dimensions. km

Length_ Heidi Sp•$igg

Venus ridges 20-100 ? 6- ! 2 Earth abyssal hills 10-50 0.05-0.2 4-10

4.1. Orthogonal orientation of different components. Oceanic abyssal hills and fracture zones bear relationships to each other similar to those of elements in the corrugated terrain and troughs in the tessera, i.e., a much greater length of troughs and fracture zones, and orientation of the shorter corrugated terrain elements and abyssal hills orthogonal to troughs and fracture zones.

4.2. Fracture zones and troughs/fractures. Oceanic fracture zones and tessera troughs/fractures are similar in their general linearity, in their often arcuate tendency, in their length (measured in hundreds of kilometers with the largest extending over 1000 km across the width of Laima Tessera), and in the width of the central trough (Figures 1-3 and Table 1). On Earth, the "transform domain" is deftned as the width of the portion of the seafloor oriented perpendicular to the strike of the FZ that is affected by the presence of the FZ itself (i.e., thinned oceanic crust) (see Figure 3a). With existing Venus imaging data, we

can see surficial expressions of troughs/ridges but not the more subtle effects of the fracture zone thermal structure. Additional

high-resolution altimetry data are required to determine the actual width of the zones or domains in which the troughs occur, but preliminary analysis of Venera 15/16 altimetry data (Figure 2d) suggests that they may be wider than the trough observed in the images. The two features (FZs and troughs/fractures) are comparable in their general parallelism and spacing, although detailed analyses of tessera terrain are required to establish the full range and variability of trough/fracture spacing on Venus. The two features are also comparable in their general morphology (Figures 1-3), with both having similar shapes and along-strike variations (changes in strike, basins).

4.3. Abyssal hills and corrugated terrain. Oceanic abyssal hills are similar to the ridges and valleys of the corrugated terrain in their distinct parallellism and their consistent occurrence and relatively constant widths over large areas (Figures 2, 3a, and 3c). They are also similar in terms of the fault-like boundaries of the valleys and ridges, in their spacing (Table 1), and in their relation to the troughs/fracture zones (predominantly terminating against them). More detailed analyses of the spacing of the elements of the corrugated terrain are needed to establish variability within and between domains and between different areas of tessera and to compare these to terrestrial data which show relations of spacing to spreading rates (R. A. Pockalny et al., submitted manuscript, 1989). Additional data (radarclinometry) are also needed to establish the heights and slopes of the elements of the corrugated terrain on Venus.

Significant differences exist between the environment and characteristics of the Earth's seafloor and the general environment of Venus and the trough-and-ridge tessera as seen in Laima Tessera.

4.4. Differences in environment between Venus and Earth. Venus is characterized by much higher surface temperatures than the Earth (450øC) and the lack of oceans. These factors have potentially important influences on seafloor spreading processes in terms of crustal formation processes, crustal modification processes, and hydrothermal cooling. Assessment of terrestrial spreading centers under Venus environmental conditions [Sotin et al., 1988, 1989a,b] indicates that the major differences between Venus and the Earth would be the influence of the enhanced surface

temperature on upper mantle temperatures on Venus and the resulting increase in crustal thickness and elevation of isostatically compensated topography. Crust on Venus produced at average spreading centers would be about 15 km thick, in contrast to the average crustal thickness on Earth of about 5 km. Along-strike variations in upper mantle temperatures at Venus spreading centers (e.g., Icelandic-like hot spots) could produce enhanced crustal thickness and increased isostatically compensated topography (e.g., Icelandic-like plateaus). Application of these concepts to Western Aphrodite Terra (Figure la), proposed as a site of crustal spreading on Venus [Head and Crwnpler, 1987], showed that the topography and gravity data for Ovda Regio in Western Aphrodite were consistent with the hypothesis of crustal spreading [Sotin et al., 1988, 1989a,b]. Specifically, much of the crust on the flanks of Western Aphrodite may be about 15 km average thickness and could be produced by crustal spreading. Ovda Regio, a central oval-like plateau along the Aphrodite rise, appears to be an area of enhanced crustal

}IF.M): VENUS TESSERA-EARTH •C CRUST ANALOG? 7129

thickness (about 30 km) which was initiated by an increase in upper mantle temperature of about 100øC and which has been spreading at about 1 cm/yr for the last 200 m.y.

In general, the environmental variations between the two planets modifies but does not seem to preclude crustal spreading processes. Left undetermined by existing studies is the influence that the different environmental conditions will

have on the nature, morphology, and spacing of abyssal hills and fracture zones formed in the Venus environment. These

factors are of the utmost importance because it is known that the thermal structure and theology of the oceanic crest and the interplay of volcanism and tectonism have an important but not yet fully understood influence on the characteristics of fracture zones and abyssal hills on Earth. Some of the variations in otherwise generally similar features (Table 1) may be due to these factors, although it cannot be ruled out that differences in the environment preclude the formation of these features in a Venus crustal spreading regime. Thus particular attention must be paid to the continually improving models for the formation and evolution of fracture zones and abyssal hills on Earth and the application of these models to the Venus environment. Continued investigation of the nature of spreading processes under Venus conditions will also be of significance in terms of understanding the Earth's Archean, a time when the thermal structure of the Earth is thought to be similar to that of present-day Venus.

4.5. Plateau-like nature of the trough-and-ridge tessera. Tessera terrain in general lies at elevations of 2 km (+1 km standard deviation [Bindschadler and Head, 1989b]) above the mean planetary radius, while most plains units (which do not show the distinctive patterns of the trough-and-ridge tessera) lie at or within 1 km of mean planetary radius. Two types of topographic boundaries are observed at the edge of the trough- and-ridge tessera terrain: (1) abrupt, where the edge is marked by a distinctive topographic change over relatively short distances and by a zone of deformation, as in Kamari Dotsum at the eastern edge of Laima Tessera (Figure 1), and (2) transitional, where the trough-and-ridge tessera terrain slowly decreases in elevation and is embayed by the volcanic deposits forming the lowland plains, as in southern and western Laima Tessera (Figure 1). There are several possible explanations for these observations, many of which can be tested with existing data and data from the upcoming Magellan mission: (1) the trough-and-ridge tessera are high because they represent the thermally most youthful part of a crustal spreading system. This would require that spreading centers be located within zones of trough-and-ridge tessera and that they be currently active. (2) The trough-and-ridge tessera are high because of enhanced crustal thickness due to increased crustal thickness

production along strike at the spreading center (the Iceland hot spot effect). This would require evidence that this process is operating at spreading centers and would predict that the terrain is in isostatic equilibrium. (3) The trough-and-ridge tessera are high because of enhanced crustal thickness related to deformational processes operating subsequent to its formation in the spreading center environment. This would seem to require significant surface deformation reflecting the deeper processes of crustal thickening. (4) The trough-and-ridge tessera are completely unrelated to processes associated with crustal spreading and the morphological and geometrical similarities are unrelated in tern of processes of formation.

Explanations 1-3 must also account for the apparent lack of the trough-and-ridge pattern in the lowlands, while no such

explanation is required for 4. The apparent lack of trough-and- ridge pattern in the lowlands could be due to volcanic flooding of the pattern, as suggested by the embayment relationships along the southern and western margins of Laima Tessera (and preferential preservation of tessera terrain in areas of younger and/or thicker crest). In this case, lava thicknesses in the range of several hundred meters would be required to cover the texture, and one would expect to see transitional areas and patchy areas of exposed trough-and-ridge tessera. Sukhanov [1986] cited the patterns of irregular polygons in volcanic plains units that are adjacent to some tessera as evidence for regions of buried tessera. On the basis of the distribution of these patterns, he proposed that the actual abundance of tessera might be a factor of two more than what is presently exposed, perhaps representing some global process of "tesserization." Comprehensive analysis of existing and Magellan data will be required to evaluate fully this possibility. Alternatively, if the trough-and-ridge pattern is produced preferentially in regions of thicker crest, perhaps there is some factor in the production of normal thickness crust at Venus spreading centers (different thermal regime or different ratio of extrusion to intrusion) that might result in the lack of production or preservation of the orthogonal texture (i.e., thicker crust is better preserved than thinner crust, or only thick crest exhibits the orthogonal texture). Additional analyses and data are clearly required to test these ideas and to establish the relationship of the trough-and- ridge tessera to the surrounding plains.

Several additional properties of tessera may help to resolve the origin of the trough-and-ridge tessera. Tesserae in general are characterized by distinctive and anomalously high surface roughness at a range of scales from centimeters to decameters compared to plains units in general [Bindschadler and Head, 1989b]. Localized volcanism and pervasive faulting producing rift mountains/abyssal hills are responsible for the abnormally rough topography at the centimeter to decameter scale seen on the seafloor [Gallo et al., 1984, Figures 8-9; Searle, 1984], which is very similar to the type of pervasive roughness seen in the tessera terrain [Bindschadler and Head, 1988a, 1989b]. High-resolution data obtained by the Magellan mission will permit the correlation of areas of enhanced surface roughness with specific geologic features and will allow further evaluation of the similarities and differences between the features on

Venus and Earth.

Line-of-sight (LOS) gravity data exist for all or part of several tessera regions (Tellus Regio, Laima Tessera, and Alpha Regio) [Sjogren et al., 1983]. Maps of LOS gravity anomalies show that these three regions are characterized by very small anomalies despite their topographic elevation, in contrast to the very large anomalies associated with other areas of high topography, such as Beta Regio. The small LOS gravity anomalies, together with the high topography characteristic of the tessera, may indicate compensation due to crustal thickness variations or shallow mantle processes. More widespread coverage and higher resolution gravity data will be of extreme importance in distinguishing between models for the formation and evolution of tessera in general, and trough-and-ridge tessera in particular.

5. DISCUSSION AND CONCLUSIONS

On the basis of the similarities in morphology, geometry, and spacing, it is concluded that processes analogous to those responsible for the ocean floor fabric on Earth are good

7130 HEAD: VENUS TESSERA-EARTH •C CRUST ANALOG?

candidates for origin and production of the trough-and-ridge very similar to those of the tessera (such as Laima and Tellus) tessera terrain type on Venus. In this model, elongate troughs in the regions covered by Venera 15/16. High-resolution are analogous to fracture zones, and linear elements of the Magellan data will permit a comparison between Laima Tessera corrugated terrain to abyssal hills; the trough-and-ridge tessera originated at a rise crest analogous to divergent boundaries and spreading centers on the Earth's seafloor and evolved to its present morphology and configuration through similar processes of crustal spreading.

If trough-and-ridge tessera fabric is produced by crustal spreading, where is the spreading center located in Laima Tessera? On the basis of the asymmetry of the fabric, candidate spreading centers would be expected to be found parallel to the corrugated terrain and normal to the trough/fractures, oriented in an approximately NNE-SSW direction (Figure lc). If the crust and lithosphere is relatively old, topographic variations due to thermal evolution may no longer be detectable. At active spreading centers on Venus topographic rises should be preserved and detectable for relatively slow spreading rates [Kaula and Phillips, 1981; Phillips and Malin, 1983]. Alternatively, regional slopes might provide information on the direction to higher, younger crust and lithosphere. Preliminary analysis of topograpic contour maps (500-m contour interval) has revealed no distinctive local rises, but more detailed analysis is underway using individual altimetry profiles. Although there does not appear to be a systematic trend in regional topography in an E-W direction, the extent of volcanic flooding of Laima Tessera appears to be greater in the west (Figure lc), suggesting that the eastern portion of Laima may be slightly higher, and thus a candidate for younger crust and lithosphere. The eastern boundary of Laima Tessera is characterized by a complex NNW striking ridge belt and a topographic drop of several kilometers, suggesting the presence of a tectonic boundary (Figure lc). Detailed mapping from Venera 15/16 altimetry and images of Laima Tessera is presenfiy underway to document topographic trends and to locate possible spreading centers.

The trough-and-ridge tessera terrain in Laima Tessera lies between Ishtar Terra in the north (a region of compressional deformation and crustal thickening [Crumpier et al., 1986; Vorder Bruegge and Head, 1989]), and equatorial regions of extensional deformation [Schaber, 1982] in Aphrodite Terra to the south (Figure la). Patterns of topography, morphology, and structure comparable to terrestrial spreading centers have been observed in the Western Aphrodite Terra region several thousands of kilometers to the south of Laima Tessera

[Crumpier et al., 1987; Head and Crumpier, 1987; Crur•ler and Head, 1988] (Figure la). Although high-resolution images of the Venera 15/16 mission are not available at these low

latitudes, evidence exists from Pioneer Venus and Arecibo data for the presence of a spreading center. Observations from Western Aphrodite Terra show (1) a segmented rise crest offset right-laterally along fracture zone-like cross-strike discontinuities (similar to the troughs in Laima Tessera); (2) bilaterally symmetrical topography normal to the rise crest; and (3) a thermal boundary layer topography suggesting spreading rates of about a centimeter a year and Iceland-like plateaus. Also revealed in the moderate resolution images and topography data are patterns of orthogonal elements parallel to the rise crest and in the general direction of the cross-strike discontinuities (Crumpier and Head, Crustal spreading on Venus: Evidence from topography, morphology, symmetry and map patterns, submitted to Tectonophysics, 1989). In addition, the radar properties of the Western Aphrodite Iceland- like plateaus along the rise crest (Ovda and Thefts regiones) are

and the terrain and structure in Aphrodite Terra and the nature and relationship of the intervening region.

The more complex tessera patterns observed by Venera 15/16 (subparallel ridged terrain and disrupted terrain [Bindschadler and Head, 1988b] are not discussed in this paper but could be due to deformation of the basic trough-and-ridge tessera pattern (analogous to more complex patterns seen on the terrestrial seafloor [Acton et al., 1988; Stoddard, 1987]), or to complex tectonic patterns from other sources of deformation (Bindschadler and Head, Models for the origin and evolution of tessera terrain, Venus, submitted to Journal of Geophysical Research, 1989). Recognition of the key basic patterns of the trough-and-ridge tessera (long troughs/fractures and short orthogonal corrugated terrain) could provide a "marker" structural type and help in deciphering the origin of the more complex tessera occurrences.

In conclusion, the similarities in morphology, geometry, and spacing of elements of the trough-and-ridge tessera to the terrestrial ocean floor fabric suggest that trough-and-ridge tessera may have formed by similar processes of divergence and crustal spreading on Venus. This hypothesis can be further tested by detailed analyses of the nature of Laima Tessera and other tessera in the northern hemisphere, application of models for the formation of fracture zones and abyssal hills on Earth to the conditions of the Venus environment, and the

analysis of global high-resolution imaging, altimetry, and gravity data from the Magellan mission.

Acknowledgments. The financial support of National Aeronautics and Space Administration grants NGR-40-002-116 and NAGW-713, from the Planetary Geology and Geophysics Program of the Solar System Exploration Division, Office of Space Science and Applications, are gratefully acknowledged. Mary Ellen Murphy, Angel Hilliard, and Peter Neivert assisted greatly in the preparation of the manuscript. Particular thanks are extended to Duane Bindschadler, Ellen Stofan, and Marc Parmentier for helpful discussions relating to concepts in this paper and to John Mustard, Jack Hermance, Scott Murchie, David Senske, Larry Crumpier, Richard Vorder Bruegge, Kari Roberts, Sharon Frank, John Grant, Susan Keddie, Jack Kliever, Phil Horzempa, and Annette deCharon for reviews of earlier drafts of this paper. Helpful comments and reviews of the final manuscript were provided by Paul Morgan, Robert Pockalny, Roger Seafie, and Paul Stoddard. Thanks are extended to the USSR Academy of Sciences for permission to use the data and to colleagues at the Vernadsky Institute for helpful scientific discussions.

Abbott, D., A statistical correlation between ridge crest offsets and spreading rate, Geophys. Res. Lett., 13, 157-160, 1986.

Acton, G., S. Stein, and J. F. Engeln, Formation of curved seafloor fabric by changes in rift propagation velocity and spreading rate: Application to the 95.5øW Galapagos propagator, J. Geophys. Res., 93, 11,845-11,861, 1988.

Aubele, J., Characteristics and geologic/terrain associations of small dome-like hills on Venus, Lunar Planet. Sci. XX, 28-29, 1989.

Barone, A.M., and W. B. F. Ryan, Along-axis variations within the plate boundary zone of the southern segment of the Endeavour Ridge, J. Geophys. Res., 93, 7856-7868, 1988.

Barsukov, V. L. and A. T. Basilevsky, The geology of Venus (in Russian), Priroda, 6, 24-35, 1986.

Barsukov, V. L., et al., The geology and geomorphology of the Venus surface as revealed by the radar images obtained by Veneras 15 and 16, Proc. Lunar Planet. Sci. Conf., Part 2, J. Geophys. Res., 91, suppl., D378-D398, 1986.

Basilevsky, A. T., Stmcture of central and eastern areas of Ishtar Terra and some problems of venusian tectonics (in Russian), Geotektonika, 20, 282-288, 1986.

HEAD: VENUS TESSERA-E•TH •C CRUST ANALOG? 71B 1

Basilevsky, A. T. and J. W. Head, The geology of Venus, Annu. Rev. Laughton, A. S., and R. C. Searle, Tectonic processes on slow- Earth Planet. Sci., 16, 295-317, 1988. spreading ridges, in Implications of Deep Drilling Results in the

Basilevsky, A. T., A. A. Pronin, L. B. Ronca, V. P. Kryuchkov, and A. Atlantic Ocean: Ocean Crust, Maurice Ewing Ser., vol. 2, edited by L. Sukhanov, Styles of tectonic deformations on Venus: Analysis M. Talwani et al., pp. 15-32, AGU, Washington, D.C., 1979. of Venera 15 and 16 data, Proc. Lunar Planet. Sci. Conf., Part 2, J. Laughton, A. S., The first decade of Gloria, J. Geophys. Res., 86, Geophys. Res., 91, D399-D411, 1986. 11,511-11,534, 1981.

Bindschadler, D. B., and J. W. Head, Diffuse scattering of radar on the Lonsdale, P., Regional shape and tectonics of the equatorial East surface of Venus: Origin and implications for the distribution of Pacific Rise, Mar. Geophys. Res., 3, 295-315, 1977. soils, Earth Moon Planets, 42, 133-149, 1988a. Lowrie, A., C. Smoot, and R. Batiza, Are oceanic fracture zones locked

Bindschadler, D. B., and J. W. Head, Definition and characterization of and strong or weak?: New evidence for volcanic activity and subtypes of Venus tesserae, Lunar Planet. Sci. XlX, 76-77, 1988b. weakness, Geology, 14, 242-245, 1986.

Bindschadler, D. B., and J. W. Head, Models for the origin of tessera Macdonald, K. C., The crest of the Mid-Atlantic Ridge: Models for terrain on Venus, Lunar Planet. Sci. XFX, 78-79, 1988c. crustal generation processes and tectonics, in The Geology of North

Bindschadler, D. B., and J. W. Head, Distribution of tesserae on Venus: America, vol. M, The Western North Atlantic Region, edited by P. Prediction using Pioneer-Venus and Venera data, Lunar Planet. Sci. R. Vogt, and B. E. Tucholke, pp. 51-68, Geological Society of X/X, 80-81, 1988d. America, Boulder, Colo., 1986.

Bindschadler, D. B., and J. W. Head, Models of Venus tectonics: Macdonald, K. C., and T. M. Atwater, Evolution of rifted ocean ridges, Evaluation and application to tessera terrain, Lunar Planet. Sci. XX, Earth Planet. Sci. Lett., 39, 319-327, 1978. 76-77, 1989a.

Bindschadler, D. B., and J. W. Head, Characterization of Venera 15/16 geologic units from Pioneer Venus reflectivity and roughness data, Icarus, 77, 3-20, 1989b.

B indschadler, D. B., and E. M. Parmentier, Tectonic features due to gravitational relaxation of topography, Lunar Planet. Sci. XVIII, 75-76, 1987.

Cande, S.C., J. L. LaBrecque, and W. B. Haxby, Plate kinematics of the South Atlantic: Chron C34 to present, J. Geophys. Res., 93, 13,479-13,492, 1988.

Crumpier, L. S. and J. W. Head, Bilateral topographic symmetry patterns across Aphrodite Terra, Venus, J. Geophys. Res., 93, 301-

Markov, M. S., L. F. Dobrzhinetskaya, and Ya. B. Smirnov, Tectonics of the Venus and the early Precambrian, Earth Moon Planets, 45, 2, 101-113, 1989.

Menard, H. W., Evolution of ridges by asymmetrical spreading, Geology, 12, 177-180, 1984.

Menard, H. W., and J. Mammerickx, Abyssal hills, magnetic anomalies and the East Pacific Rise, Earth Planet. Sci. Lett., 2,465- 472, 1967.

Needham, H. D., and J. Francheteau, Some characteristics of the rift valley in the Atlantic Ocean near 3604831, Earth Planet. Sci. Lett., 22, 29-43, 1974.

Phillips, R. J., A mechanism for tectonic deformation on Venus, 312, 1988. Geophys. Res. Lett., 13, 1141-1144, 1986.

Cramplet, L. S., J. W. Head, and D. B. Campbell, Orogenic belts on Phillips, R. J., and M. C. Malin, The interior of Venus and tectonic Venus, Geology, 14, 1031-1034, 1986. implications, in Venus, edited by D. M. Hunten et al., pp. 159-

Crumpier, L. S., J. W. Head, and J. K. Harmon, Regional cross-strike discontinuities in western Aphrodite Terra, Venus, Geophys. Res. Left., 14, 607-610, 1987.

Fox, P. J., and D. G. Gallo, A tectonic model for ridge-transform-ridge boundaries: Implications for the structure of the oceanic lithosphere, Tectonophysics, 104, 205-242, 1984.

Fox, P. J., and D. G. Gallo, The geology of North Atlantic transform plate boundaries and their aseismic extensions, in The Geology of North America, vol. M, The Western North Atlantic Region, edited by P. R. Vogt, and B. E. Tucholke, pp. 157-172, Geological Society of America, Boulder, Colo., 1986.

214, University of Arizona Press, Tucson, 1983. Pockalny, R. A., R. S. Detrick, and P. J. Fox, A comparison of off-axis

Sea Beam data collected at slow, intermediate, and fast spreading centers, EOS Trans. AGU, 68, 1491, 1987.

Pronin, A. A., The structure of Lakshmi Planum, an indication of horizontal asthenospheric flows on Venus (in Russian), Geotektonika, 20, 271-281, 1986.

Sandwell, D., Thermal stress and the spacing of transform faults, J. Geophys. Res., 91, 6405-6417, 1986.

Sandwell, D., and G. Schubert, Lithospheric flexure at fracture zones, J. Gallo, D. G., W. S. F. Kidd, P. J. Fox, J. A. Karson, K. Macdonald, K. Geophys. Res., 87, 4657-4667, 1982.

Krane, P. Choukroune, M. Seguret, R. Moody, and K. Kastens, Schaber, G. G., Venus: Limited extension and volcanism along zones Tectonics at the intersection of the East Pacific Rise with Tamayo of lithospheric weakness, Geophys. Res. Lett., 9, 499-502, 1982. transform fault, Mar. Geophys. Res., 6, 159-185, 1984. Searle, R. C., Side-scan sonar studies of North Atlantic fracture zones,

Harrison, C. G. A., and L. Stieltjes, Faulting within the median valley, J. Geol. Soc. London, 136, 283-292, 1979. present state of plate tectonics, Tectonophysics, 38, 137-144, Searle, R., GLORIA survey of the East Pacific Rise near 3.5øS: Tectonic 1977. and volcanic characteristics of a fast-spreading mid-ocean rise,

Head, J. W., and L. S. Crumpier, Evidence for divergent plate boundary Tectonophysics, 101, 319-344, 1984. characteristics and crustal spreading on Venus, Science, 238, 1380- Senske, D. A., and J. W. Head, Synthesis of Venus equatorial geology: 1385, 1987.

Head, J. W., A. R. Peterfreund, J. B. Garvin, and S. H. Zisk, Surface characteristics of Venus derived from Pioneer-Venus altimetry, roughness, and reflectivity measurements, J. Geophys. Res., 90, 6873-6885, 1985.

Heezen, B.C., M. Tharp, and M. Ewing, The floors of the oceans, 1, The North Atlantic Ocean, Spec. Pap., Geol. Soc. Am., 65, 122 pp., 1959.

Iranov, M. A., The results of morphometric study of the tessera terrain of Venus from Venera 15/16 data, Lunar Planet. Sci. XVII, 537-538, 1988.

Kappel, E. S., and W. B. F. Ryan, Volcanic episodicity and a non-

Variations in styles of tectonism and volcanism and comparison with the northern high latitudes, Lunar Planet. Sci. XX, 984-985, 1989.

Sjogren, W. L., B. G. Bills, P. W. Birkeland, N. A. Nottinger, S. J. Ritke, and R. J. Phillips, Venus gravity anomalies and their correlations with topography, J. Geophys. Res., 88, 1119-1128, 1983.

Slyuta, E. N., O. V. Nikolaeva, and M. A. Kreslavsky, Distribution of small domes on Venus: Venera 15/16 radar data (in Russian), Astron. Vest., 22, 287-297, 1988.

Smrekar, S. and R. J. Phillips, Gravity-driven deformation of the crust on Venus, Geophys. Res. Lett., 15, 693-696, 1988.

steady-state rift valley along Northeast Pacific spreading centers: Sotin, C., J. W. Head, and E. M. Parmentier, Terrestrial spreading Evidence from Sea MARC I, J. Geophys. Res., 91, 13,925-13,940, centers under Venus conditions: Effects on thermal structure and 1986. crustal thickness, Lunar Planet. Sci. XIX, 1109-1110, 1988.

Kastens, K. A., A compendium of causes and effects of processes at Sotin, C., D. A. Senske, J. W. Head, and E. M. Parmentier, Terrestrial transform faults and fracture zones, Rev. Geophys., 25, 1554-1562, spreading centers under Venus conditions: Evaluation of a crustal 1987. spreading model to Western Aphrodite Terra, Earth Planet. Sci.

Kaula, W. K., and R. J. Phillips, Quantitative tests for plate tectonics Lett., 95, 321-333, 1989a. on Venus, Geophys. Res. Lett., 8, 1187-1190, 1981. Sotin, C., D. A. Senske, J. W. Head, and E. M. Parmentier, Analysis of

Kozak, R. C., and G. G. Schaber, Gravity-spreading origin of the topography and line of sight (LOS) accelerations over Western venusian tesserae, Lunar Planet. Sci. XVII, 444-445, 1986. Aphrodite: Evaluation of a spreading center model, Lunar Planet.

Kranse, D.C., and H. W. Menard, Depth distribution and bathymetric Sci. XX, 1034-1035, 1989b. classification of some sea-floor profiles, Mar. Geol., 3, 169-193, Stoddard, P. R., A kinematic model for the evolution of the Gorda plate, 1965. J. Geophys. Res.,92, 11,524-11,532, 1987.

7132 HEAD: VENUS TESSERA-EAR'1I-I OCEANIC CRUST ANALOG?

Sukhanov, A. L., Parquet: Areas of plastic deformation (in Russian), Vorder Bmegge, R. and J. W. Head, Fortuna Tessera, Venus: Evidence of Geotektonika, 4, 60-76, 1986. horizontal convergence and crustal thickening, Geophys. Res.

Sukhanov, A. L., et al., Geological-morphological description of the Lett., 16, 699-702, 1989. Laima Tessera, Tellus Regio and Leda Planitia (photomap of the Venusian surface, sheet B-13) (in Russian), Astron. Vest., 21, 195- 206, 1987.

Verosub, K. L., and E. M. Moores, Tectonic rotations in extensional J.W. Head, Department of Geological Sciences, Brown University, regimes and their palcomagnetic consequences for oceanic basalts, Providence, RI 02912. J. Geophys. Res., 86, 6335-6350, 1981.

Vogt, P. R., The present plate boundary configuration, in The Geology of North America, vol. M, The Western North Atlantic Region, (Received June 14, 1989; edited by P. R. Vogt, and B. E. Tucholke, pp. 189-204, Geological revised August 14, 1989; Society of America, Boulder, Colo., 1986. accepted August 14, 1989)