the geologic history of venus: a stratigraphic vie geologic history of venus: a stratigraphic view...

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 103, NO. E4, PAGES 8531-8544, APRIL 25, 1998

The geologic history of Venus: A stratigraphic view

Alexander T. Basilevsky • Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow

James W. Head III

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

Abstract. On the basis of regional and global stratigraphic analyses, we outline the major events in the geologic history of Venus determined by photogeological study of surface features. Because the morphological signatures of terrain emplaced prior to the time of tessera formation are not preserved, the stratigraphic record presented comprises only the last 10-20% of the total history of Venus. The estimated range of the mean crater retention age of the surface (from -200 to 1600 million years) leads us to describe the timing and duration of different events in terms of fractions of the mean surface age T. The beginning of the observed history of Venus was characterized by intensive tectonic deformation of global or semi-global scale which formed the tessera terrain. Termination of the compressional stage is estimated to have occurred at about 1.4T while the tensional stage lasted for another 0.1-0.2T. After tessera formation, several stages of extensive volcanism occurred, burying vast areas of tessera and forming what are now observed as regional plains. The combined duration of the emplacement of these plains is estimated to be about 0.2- 0.3T, with an implied average global rate of volcanism of a few cubic kilometers per year. Regional plains-forming matedhals can be subdivided and are separated from each other, and from underlying and overlying units, by unconformities. These unconformities are formed, from oldest to youngest, by tessera-forming deformation, dense fracturing, broad ridging, and, finally, wrinkle ridging. These tectonic episodes are interpreted to be generally globally synchronous and to represent successive episodes characterized by the dominance of compression, then tension, then again compression, and, finally, tension. The last global-scale tectonic episode, extensive wrinkle ridging, happened at about time T, which was very close in time to the emplacement of the most areally abundant plains unit. This marked the transition to the present stage of the history of Venus, which is characterized by a predominance of regional rifting and related volcanism. This stage appears to have lasted from about time T to the present, making it the longest time duration among the stratigraphic units considered, although the resulting tectonic and volcanic features and deposits cover only 10-20% of the surface of Venus. These observations mean that the general intensity of tectonics and the flux of volcanism (a few tenths of a cubic kilometer per year) in this latest period were much lower than those in earlier times. In summary, the morphologically observable part of the history of Venus was characterized by two key characteristics that stand in contrast to the comparable period of Earth history (approximately the Phanerozoic) when global geodynamic processes were dominated by plate tectonics: (1) Venus shows no signature of plate tectonics; instead, its global tectonic environment passed from an initial dominance of compression, through tension, then again compression, and finally tension, with the density of deformational structures and the strain rate declining with time. (2) In the beginning of this period of time on Venus, plains-forming volcanism occurred at a rate comparable to volcanism at mid- ocean ridges but was emplaced in an entirely different style. The predominant component of volcanism on Earth during this time was the extrusive volcanism at mid-oceanic ridges. For the last few hundred million years, Venus has been dominated primarily by dhft-associated volcanism emplaced at a production rate comparable to or even lower than present intraplate volcanism production rates on Earth.

1. Introduction

Recent extensive studies of Venus based mostly on the results of the Magellan mission have provided significant

1Also at Department of Geological Sciences, Brown University, Providence, Rhode Island.

Copyright 1998 by the American Geophysical Union.

Paper number 98JE00487. 0148-0227/98/98JE-00487509.00

progress in understanding the geology of this planet (see summary in Bougher et al., [1997]). Among the planets, Venus is the closest to Earth in its position in the Solar System, size and mass and evidently in its bulk composition. Thus in some earlier studies, it was thought to be Earth-like in its major endogenic geological processes and geodynamics. Prior to Magellan, three end-member ideas were proposed for the dominant planetary heat loss mechanism: plate tectonics (as on Earth), hot spots (as on Io), and conduction (as on the Mars, Moon, and Mercury) [Solomon and Head, 1982]. Surprisingly, it was found that Venus was quite different from

8531

8532 BASILEVSKY AND HEAD: GEOLOGIC HISTORY OF VENUS

Earth. Instead of a bimodal distribution of ages (e.g., equivalent of continents and seafloor, or lunar highlands and maria) Magellan images showed a relatively young surface (a few hundred million years old)with only about 1000 impact craters over the entire planet. Even more surprisingly, the areal distribution of impact craters was found to be indistinguishable from a random one [Phillips et al., 1992; $chaber et al., 1992]. This led to the suggestion that Venus may have been globally resurfaced relatively recently by tectonic and volcanic processes. To maintain the apparent randomness of areal crater distribution and to account for the

lack of embayment of the vast majority of its craters, the resurfacing had to be rapid compared to the rate of accumulation of the impact crater population [Schaber et al., 1992; Strom et al., 1994].

The question of the rapidity of the resurfacing has very important geophysical implications and is still a subject of debate [e.g., Phillips et al., 1992; $chaber et al., 1992; Phillips, 1993; Strom et al., 1994, 1995; Herrick, 1994; Basilevsky and Head, 1995a,b,c, 1996a,b; Herrick et al., 1995; Phillips and Izenberg, 1995; Price, 1995a,b; Price and Suppe, 1994, 1995; Price et al., 1996; Basilevsky et al., 1997a; Collins et al., 1997; McKinnon et al., 1997]. It is evident that in the near future, progress toward a solution to this question will be related to estimates of impact crater density that are used in geochronology across the Solar System [Basaltic Volcanism Study Project (B VSP), 1981 ]. In the case of Venus, however, with its relatively small crater population, application of the crater density approach is pesently limited to the mean age of the surface [Phillips et al., 1992; Strom et al., 1994; McKinnon et al., 1997] and an estimate of the surface age of only a few widespread geologic units. The latter need to be areally extensive enough to have sufficient craters on the unit to keep, in turn, the stochastic variations of crater density reasonably low [Ivanov and Basilevsky, 1993; Namiki and Solomon, 1994; Price and $uppe, 1994; 1995; Price, 1995a,b; Price et al., 1996; Gilmore et al. , 1997].

While crater density is often used as a practical tool for site- to-site stratigraphic correlations in studies of other planetary bodies, in the case of Venus only the globally averaged age of the unit can be estimated because of the small number of

craters. The question of whether a specific area is part of some rock stratigraphic unit seen elsewhere cannot be judged solely on the basis of crater density. Some alternate criteria are needed in order to assemble different local areas into global stratigraphic units. As is the case on other planets and satellites, these criteria can be developed through photogeologic mapping and stratigraphic studies, which are a goal of numerous ongoing mapping projects for Venus [Tanaka, 1994; Tanaka et al., 1997; Basilevsky et al., 1997b]. Although these projects are ongoing, a model of regional and global stratigraphy of Venus has been proposed [Basilevsky and Head, 1995a,b,c; Basilevsky et al., 1997a] and successfully tested for its applicability over about 30% of the surface of Venus.

The first goal of this paper is to consider the published age estimates in the context of this global stratigraphic model in order to further test the model. The second goal is to use the stratigraphy as a tool to summarize a scenario for the geologic history of Venus that is consistent with crater density esimates and other crater-based evidence for the duration of various

geologic processes and events.

2. Model of Regional and Global Stratigraphy In earlier studies we analyzed 36 random 1000 x 1000 km

areas with the goal of defining local stratigraphic units, determining their sequences, and attempting to correlate these local sequences into a model of regional and global stratigraphy [Basilevsky and Head, 1994, 1995a,b,c]. We then tested this model by mapping several new areas [Basilevsky, 1995, 1997; Basilevsky and Head, 1995c] including individual 1:5,000,000 (l:5M) quadrangles [Basilevsky, 1996a; Head and Ivanov, 1996] and several larger regions [Basilevsky and Head, 1996a; Basilevsky et al., 1997b; Ivanov and Head, 1997a,b]. These studies allowed the assessment and testing of the applicability of the model over approximately 30% of the surface of Venus and to update it adding new stratigraphic units and subdividing some earlier identified units into subunits. The following is the model of regional and global stratigraphy of Venus which will be analyzed and used for the subsequent consideration of absolute age estimates. It considers the surface materials observed on the Magellan images as representing six rock-stratigraphic units which we ranked as groups (Figure 1).

2.1. Fortuna Group

The oldest unit, the Fortuna Group includes the material(s) of tessera terrain (Tt) whose blocks stand over the surrounding plains and cover about 8% of the surface of Venus [Ivanov and Head, 1996]. Tessera morphology is dominated by intersecting systems of ridges and grooves of tectonic origin (Figure 2). This tectonic deformation typically does not extend into the surrounding plains, and this makes the embayment of tessera by these plains very evident. The stratigraphic rank of tessera is not clear. Depending on whether its material was emplaced within a certain relatively short time period or is an assemblage of materials formed at essentially different times, the Fortuna Group may be equivalent in stratigraphic rank to the majority of the overlying groups or it may be a kind of analog of the Precambrian assemblage of the basement of some continental platforms of Earth. Two important things are clear, however: (1) all other stratigraphic units overlie or

0.1T

Geologic Time Time-Stratigraphic Unite units Rock-Stratigraphic Units and Structures

Aurelian Period Aurelian System Aurella Group Cdp

•, ==. Aria Group Ps, Pl o

_ ._

Gulneverlan Gulneverlan • Rusalka Group Pwr, Psh

Period System m FRB• >e Lavlnla Group Pfr,

* • Slgrun Group Pdf _

Fortunlan Period Fortunlan System Fortuna Group Tessera, Tt

Pre-Fortunlan Pre-Fortunlan Period System ? ?

T-'

1.47 ñ0.46T -

Figure 1. Venus stratigraphic units and global correlations (from Basilevsky and Head [1995a,b] with minor modifications): Psh (plains with shields) and FB (fracture belts) are included). T, the average age of the surface of Venus, is estimated to be about 300-500 million years [Phillips et al., 1992; Schaber et al., 1992; Strom et al., 1994] or possibly up to about 800 million years [Zahnle and McKinnon, 1996]; see discussion in text.

BAS1LEVSKY AND HEAD: GEOLOGIC HISTORY OF VENUS 8533

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Pwr

Figure 2. Region of tessera terrain (Tt) embayed by plains with wrinkle ridges (Pwr). (Location is 34.5øN, 355.5ø; portion of C1MIDR-30N351;1; width of image is 200 km). For these and all subsequent images and sketch maps, north is at the top. Symbols on maps are as follows: Solid, continuous lines are contacts; thick solid lines with black diamonds are crests of broad highs and ridges; paired lines with internally oriented hachures are graben; thin lines with black dots are faults; sinuous lines with open diamonds are wrinkle ridges; continuous line with hachures toward interior crater rim crest is double dashed lines are lava channels.

embay tessera (Figures 2, 4, and 6), and (2) although the absolute age of the emplacement of tessera materials may be different, the time of heavy deformation of those materials into tessera terrain cannot vary significantly. The latter is strongly suggested by the fact that several of the largest (> 1,000,000 km 2) tessera blocks have approximately the same density of impact craters superposed on them [lvanov and Basilevsky, 1993; Gilmore et al., 1997].

2.2. Guinevere Supergroup

The Guinevere Supergroup is an assemblage of several plains-forming material units. It consists of four groups

separated from one another and from the underlying Fortunian Group by unconformities caused by episodes of tectonic deformation.

2.2.1. Sigrun Group. This unit consists of material of densely fractured plains (Pdf) covering about 3% of the surface of Venus. Pdf is typically present as relatively small kipukas among the younger plains (Figure 3); Figure 4 shows Pdf where it embays tessera. Sigrun Group materials are deformed by densely-spaced swarms of faults. Within each Pdf outcrop the faults are usually subparallel to each other, although in neighboring outcrops the orientation of dominant tectonic trends may be either the same or different. If one ignores the

Figure 3. Embayment of densely fractured plains (Pdf) by shield plains (Psh) and plains with wrinkle ridges (Pwr). (Location is 24.0øN, 12.7ø; portion of C1MIDR-30N009;1; width of image is 150 km).

8534 BASILEVSKY AND HEAD: GEOL(X•IC HISTORY OF VENUS

Pwr

Figure 4. Tessera terrain (Tt) embayed by densely fractured plains (Pdf), which is in tum embayed by shield plains (Psh) and plains with wrinkle ridges (Pwr). (Location is 46.0øN, 1.0ø; portion of C1MIDR-45N011;1' width of image is 150 km).

faults, the terrain composed of this material appears to be primarely plains. We consider this as evidence that the Pdf plains were emplaced as floods of mafic lavas.

2.2.2. Lavinia Group. This unit is represented by the materials of fractured and ridged plains (Pfr) (Figure 1). The ridges are typically rather broad (5-10 km wide) and sometimes are clustered, forming ridge belts (RB) (Figure 5). Part of the fractures present in this material are a result of its deformation, while some of the fractures are inclusions of Pdf material too

small to be mapped separately. The total areal abundace of Pdf and RB is about 3% of the surface, observed mostly as elongated islands among the younger plains. Pfr material is embayed and overlain by the younger plains but also embays

tessera (Figure 6) and Pdf (Figure 7). In its undeformed state the Lavinia Group material appears very similar to the overlying plains, and this is considered evidence that it was primarily emplaced as mafic lava floods.

2.2.3. Rusalka Group. This unit consists of materials of shield plains (Psh) (Figures 3, 4, 9) and plains with wrinkle ridges (Pwr)(Figures 2-5 and 7-12). Together they occupy about 70-75% of the surface of Venus. Pwr are areally dominant among the Rusalka Group, comprising about 60-65% of Venus. They are usually moderately dark on Magellan images, but mottled and radar bright varieties are also observed. These varieties often show distinctive age relations among them (Figure 8) that permit one to separate the Pwr stratigraphic unit

..: )

//

Pwr

Pfr/RB

Figure 5. Fractured and ridged plains (Pfr) and ridge belts (RB) of the Lavinia Group embayed by plains with wrinkle ridges (Pwr). (Location is 53.0øN, 215.0ø; portion of C1MIDR-60N208;l' width of image is 150 km).

BASILEVSKY AND HEAD: GEOLOGIC HISTORY OF VENUS 8535

Figure 6. Tessera terrain (Tt) embayed by fractured and ridged plains (Pfr) and ridge belts (RB), which in turn is embayed by smooth plains (Ps). (Location is 46.0øN, 247.0ø; portion of C1MIDR-45N244;1; width of image is 130 km).

into subunits [e.g. Basilevsky and Head, 1996a; Ivanov and Head, 1996; Kryuchkov, 1996; Basilevsky, 1997]. A definitive characteristic of the Pwr is the presense of wrinkle ridges, typically about 1 km wide, which often form a network with various dominant trends. Usually a single wrinkle ridge network deforms all Pwr subunits, thus separating them all from the younger stratigraphic units (Figure 9). In some rare cases, two subsequently emplaced wrinkle ridge networks are observed that may be used for identification of subunits [Basilevsky et al., 1997b]. Younger subunits of Pwr often form extended flow like features. This observation and in-situ

geochemical measurements made by the Venera 9, 10 and Vega

1 and 2 landers provide reliable evidence that the Pwr formed by extended floods of mafic lavas [Basilevsky et al., 1992; Weitz and Basilevsky, 1993].

Shield plains are another component of the Rusalka Group. This unit is represented by materials of plains formed by clustered and coalescing gently sloping shields (Figures 3, 4, and 9) of apparent volcanic origin. It occupies about 10% of the surface of Venus. In our earlier work [Basilevsky and Head, 1995a,b,c] we did not separate Psh and Pwr, considering both of them as plains with wrinkle ridges. However, mapping of different areas by ourselves and others showed that plains of this type can be distinguished as a separate stratigraphic unit

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Figure 7. Densely fractured plains (Pdf) embayed by fractured and ridged plains (Pfr) and ridge belts (RB), which in turn are embayed by plains with wrinkle ridges (Pwr). (Location is 35.0øN, 156.0ø; portion of C1MIDR-30N153;l' width of image is 90 km).


Figure 8. Plains with wrinkle ridges (Pwr) displaying two different subunits; radar-dark plains with wrinkle ridges (Pwrl), and radar-bright plains with wrinkle ridges (Pwr2). (Location is 19.0øN, 149.5ø; portion of C1MIDR-15N146;l' width of image is 175 km).

[e.g. Solomon et al., 1992; Ivanov, 1993; Aubele, 1995, 1996; Basilevsky, 1996a; Head and lvanov, 1996]. Psh are often wrinkle-ridged and embayed by Pwr material (Figures 3, 4, and 9). The latter relationship makes them in many areas of Venus the stratigraphically lower member of the Rusalka Group. In some cases, however, the age relations between Pwr and Psh materials are ambiguous, and in relatively rare cases, small Psh-type shields even appear superposed on Pwr and on the wrinkle ridges themselves. On the basis of the gentle slopes of their flanks, Psh volcanic shields are believed to be made mostly of mafic lavas. In-situ geochemical measurements made by the Venera 8 lander in the area with abundant Psh shields showed the presense of either alkaline basalt or even

more differentiated material (andesite?) [Nikolaeva, 1990; Basilevsky et al., 1992; Basilevsky, 1997].

In some areas of Venus there are belts of fractures and

narrow graben that cut Lavinian fractured and ridged plains and ridge belts and are typically embayed by shield plains and plains with wrinkle ridges. These fracture belts (FB) (Figures 9 and 10) often contain a few faults and graben that cut Psh and Pwr too. So within the fracture belts, materials of Sigrun, Lavinia, and partly Rusalka groups may be present in a heavily deformed state (Figure 1).

2.2.4. Atla Group. This unit is mostly made of the materials of lobate (P1) and smooth (Ps) plains undeformed by wrinkle ridges (Figures 11-12). Material of these plains covers

Pwr

Z;r Psh

Figure 9. Shield plains (Psh) embaying tessera terrain (Tt) and fracture belts (FB), which are in turn embayed by plains with wrinkle ridges (Pwr). (Location is 40.7øN, 132.5ø; portion of C1MIDR-45N138;l' width of image is 150 km).


Pwr

Figure 10. Fracture belt (FB)cutting ridge belt (RB) and embayed by plains with wrinkle ridges (Pwr). (Location is 41.0øS, 345.0ø; portion of C1MIDR-45S350;l' width of image is 180 km).

about 10-15% of Venus, overlies and embays the Pwr and all older units, and is overlain by the stratigraphically younger Aurelia Group. Most Atla Group materials are associated with rift zones, occurring there in the form of large, gently sloping volcanic constructs and subhorizontal lava flow fields. A

subordinate part of Atla Group materials are associated with some coronae, mostly forming lava flow aprons around corona annulae [Magee and Head, 1993, 1995]. The morphology of Atla Group materials, as well as in-situ geochemical measurements made by the Venera 14 lander, suggests that they are mafic lavas [Basilevsky et al., 1992; Weitz and Basilevsky, 1993].

2.3. Aurelia Group

The Aurelia Group is represented by the materials of radar- dark parabolas associated with the youngest impact craters [Campbell et al., 1992; Strom, 1993] (Figure 13) as well as by the material of eolian radar-dark patches (Sp) and streaks (Ss) contemporaneous with radar-dark parabolas, but not necessarily associated with them.

Impact craters lacking associated parabolas are present in practically all broad regions of Venus. In principle, their materials can belong to any pre-Aurelian group. However, the vast majority of them are superposed on Pwr, including the

Figure 11. Lobate plains (P1) embaying plains with wrinkle ridges (Pwr). Note lava channel in Pwr.. (Location is 53.5øS, 356.0ø; portion of C1MIDR-60S347;l' width of image is 225 km).


Figure 12. Plains with shields (Psh) embayed by plains with wrinkle ridges (Pwr), which are in turn cut by smooth plains (Ps). Note that the smooth plains contact is controlled by the position of preexisting wrinkle ridges. (Location is 40.0øN, 326.5ø; portion of C1MIDR-45N329;2; width of image is 90 km).

wrinkle ridges themselves [Basilevsky, 1996b], so these post- Pwr craters obviously have an Atlian age.

This sequence of stratigraphic units described above has been observed by us in the process of photogeologic analysis and mapping covering about 30% of Venus [Basilevsky and Head, 1994, 1995a,b,c, 1996a,b, 1997; Basilevsky, 1995, 1996, 1997; Basilevsky et al., 1997a; Head and Ivanov, 1997; Ivanov and Head, 1997a,b]. In presentations at the annual NASA Geological Mapping of Venus Program meetings in Flagstaff, Arizona (1995), Reno, Nevada (1996), and Pasadena, California (1997), and in personal communications there, we found that many other workers had observed and described much the same sequence [e.g., Tanaka, 1994; Senske et al., 1994; Tanaka et al., 1997]. Of course, for some units other mappers use designations different than ours (e.g., our "densely fractured plains" (Pdf) is often called "lineated plains"

and our "plains with wrinkle ridges" (Pwr)is called "regional plains" [Tanaka et al., 1997]). However, almost all workers use the terms "tessera," "ridge belts," "shield plains," "lobate plains," "smooth plains," etc. Although some units are lumped together and not subdivided, and in some cases features (e.g., ridge belts) are considered part of a larger unit which in our mapping we see subdivided on the basis of stratigraphic relations, we detected no contradictions to our general stratigraphic sequence. In summary, there is a correspondence of the units of other mappers to our units, and this supports the general stratigraphic sequence.

3. Synchroneity of Stratigraphic Units

A problem which requires additional analysis is the question: To what extent are the same members of the

Figure 13. Impact crater Uvaisi with dark parabola (Cdp)overlying a wide range of other units and structures. (Location is 5.0øS, 188.0ø; portion of C2MIDR-00N183;l' width of image is 700 km)


I Pwr J j Pwr i

I I Pdf I

• Pwr • •, • Pwr

• • '• Pwr Figure 14. Diagram showing what would happen if the stratigraphic sequence typically observed on Venus (from tessera (Tt), to densely fractured plains (Pdf), to ridge belts (RB), and finally, plains with wrinkle ridges (Pwr)) were not formed quasi-synchronously over the planet, but at different times in different areas. The stratigraphic columns (top of the diagram) are the same, but the sequences are shifted in time so tessera-forming deformation of the central area i s contemporaneous with the episode of wrinkle ridging of Pwr in the left-hand area and to the episode of RB-forming broad ridging in the right-hand area (see the dashed line showing the contemporaneity). In these hypothetical cases, tessera of the central area coming in contact with Pwr of the left area would show no embayment by these plains, being formed at the expense of these plains, but maintaining embayment relationships with the Pwr in the central area (solid contact). At the boundary between the central and right-hand area, tessera of the central area, if it meets ridge belts of the right- hand area, would have no embayment by that RB material. Instead, tessera forming deformation of this area would transform part of that RB into tessera while far to the right, where RB of the right-hand area meets tessera of that area, the stratigraphic realtions are normal: RB material embays tessera. This diagram shows that if quasi-synchroneity of stratigraphic units is not the case, numerous violations of the stratigraphic sequence should be seen.

with respect to Baltis Vallis. On the basis of this analysis it was possible to demonstrate the near-synchronous nature of the key stratigraphic units within this very large region [Basilevsky and Head, 1996a].

In the second step we addressed the question: Can these units be shown to be laterally continuous and correlative over larger parts of the globe? In order to accomplish this, we enlarged the area of analysis using two approaches. We first chose an area centered at 30øN and mapped a continuous sequence of C1-MIDRS completely around the planet at a scale of l:3M [lvanov and Head, 1997a,b; 1998a,b]. Second, we mapped regions to the north of this area, the northern high latitudes above 30øN, at a scale of l:10M [Basilevsky et al., 1997b]. Thus we were able to test for both the vertical

sequence and the lateral continuity of stratigraphic units at different scales and over an area comprising about 30% of the surface of Venus.

Although this mapping involved five different workers, each with different backgrounds and approaches, the stratigraphic sequences were essentially the same across the 30% of the planet analyzed. Furthermore, the major stratigraphic units could be traced laterally and continuously throughout the entire region. This consistency within this very large region supports the broad quasi-synchroneity of the proposed stratigraphic units and their separation by regional to global deformational episodes. If nonsynchroneity were the case (Figure 14), and Rusalka-type wrinkle ridging in one region were contemporaneous with Fortunian-type tesserization in a second region and to Lavinian-type broad ridging in a third region, the inevitable consequence of this nonsynchroneity should be a violation of the proposed sequence of stratigraphic units. In this case we should see not only embayment of tessera by the Pfr and Pwr (which we do see on account of local unconformable relationships (Figures 2, 4, and 6)) but also tesserization of the Pfr and Pwr (which we do not see).

We conclude that these analyses support the proposed stratigraphic sequence for a significant (-30%) part of the surface of Venus. Further tests of the proposed sequence will come with the completion of synoptic (l:5M to l:10M) geologic mapping of the planet. However, the good agreement of the proposed model with numerous studies over a significant part of the planet encourages us to utilize it to assess the existing estimates of absolute ages of different geologic units in order to work toward a better understanding of the geologic history of Venus.

stratigraphic column we described globally synchronous? Did emplacement of the materials of the same units, and their deformation by the characteristic structures, occur more or less contemporaneously in different regions of Venus? Or (Figure 14) is the sequence of units just a record of a local sequence of events which typically occurred in different places on Venus at different times?

In • attempt to address this problem, we first studied photogeologically the large region surrounding the 6800-km- long lava channel Baltis Vallis, which comprises about 5% of the surface of Venus. This channel is considered to be formed

geologically instantaneously [Kargel et al., 1994], thus making it an ideal stratigraphic marker. We first considered the age relations among various stratigraphic units themselves in different parts of this region, and we then considered their age

4. Absolute Age Estimates

The duration of the morphologically distinguishable part of the geological history of Venus is estimated on the basis of impact crater densities and is the subject of continuing debate. The average age of the surface of Venus is estimated to be 288+311/-98 m.y. by Strom et al. [1994], 400 to 800 m.y. by Phillips et al. [1992], and even 800+800/-400 m.y. by Zahnle and McKinnon [1996]. The weakest part of these estimates, which is a major cause of the great diversity, is the model- dependent transition from the crater densities themselves to absolute ages. Thus it is worthwhile to discuss problems in Venus geologic history not in terms of millions of years, but in terms of fractions of the average age of the surface of Venus (T), returning to conventional time units only when it is necessary.


Because T is an average crater retention age, it should be close to the average age of the most widespread unit on the planet, which is the Rusalkian plains with wrinkle ridges and shield plains which are the areally dominant surface units on Venus [Basilevsky et al., 1997b; Iranov and Head, 1997a,b]. The age of other stratigraphic units on Venus also may be estimated from crater densities if the units occupy sufficiently large areas to contain enough craters to provide statistically significant results. Such crater counts were done for the Fortunian Group tessera [lvanov and Basilevsky, 1993; Herrick, 1994; Gilmore et al., 1997], for a broad assemblage of regional plains [Price, 1995a,b], and for Atlian lava flows and volcanic constructs [Price and Suppe, 1994; Namiki and Solomon, 1994]. The total areas of Sigrunian and Lavinian Groups outcrops are insufficient for reliable crater counts (see summary by Basilevsky et al. [1997a]).

4.1. Tessera

The average surface age of the observed blocks of tessera was estimated to be (1.47+0.46/-0.46)T by Iranov and Basilevsky [1993]. Later work by Gilmore et al. [1997], which concluded that the average surface age of tessera was about 1.4T, confirmed this earlier estimate. The difficult part of the tessera age estimate, besides the fact that the morphologically rough and radar bright surface of this terrain camouflages smaller craters from identification (see Basilevsky et al. [1997a] and Gilmore et al. [1997] for details), is the large error bars. The latter are due to the relatively small number of on- tessera craters, which in turn is a result of the relatively small percentage of the surface of Venus occupied by tessera (-8% according to Iranov and Head [1996]) and the relatively small number of impact craters on Venus as a whole [Schaber et al., 1992].

Because none of about 80 craters observed on tessera show

evidence of early tessera-forming compressional deformation, and only seven craters are overprinted by later tessera-forming tensional deformation, one may conclude that the tessera- forming deformation ceased within a relatively short time period: less than 1/80 of the tessera age (about 0.01T) for the compressional stage and within about 7/80 of the tessera age (about 0.1-0.2T) for the tensional stage [Gilmore et al., 1997]. This also has two implications: (1) that the termination of the compressional stage of tessera-forming deformation was the starting point of the accumulation of the on-tessera crater population, and (2) that during the subsequent tensional phase of tessera formation, craters were mostly mildly deformed and not completely wiped out. Thus the estimated age value of the tessera surface apparently dates the termination of the compressional stage but not the end of the tessera-forming process.

4.2. Regional Plains

As was described above, the average age of the surface of Venus (T) is apparently a reasonable estimate of the age of the Rusalkian plains. Using this observation, combined with estimates of the age of the tessera, one may approximately estimate the combined time duration of the emplacement of Sigrunian, Lavinian, and Rusalkian volcanic plains (Figure 1). As was discussed above, the average surface age of tessera, which apparently dates the termination of the compressional stage, is somewhere between 1.01T and 1.93T [lvanov and Basilevsky, 1993]. If the lower extreme (1.01T) were correct,

it would imply that the combined duration of the Sigrunian, Lavinian and Rusalkian periods should be about 0.01T, that is, approximately a few million years. We know, however, that between the end of the compressional stage and the emplacement of post-tessera plains, there occurred the 0.1- to 0.2T-long tensional stage of tessera formation [Gilmore et al., 1997]. This implies that this lower extreme of the Iranov and Basilevsky [1993] estimate is unlikely. This suggests that the lower extreme of the estimated age of the compresional stage cannot be smaller than about 1.1-1.2T. Unfortunately, this conclusion per se does not constrain the time of termination of the tessera-forming tensional stage, so it also leaves unconstrained the combined time duration of emplacement of Sigrunian, Lavinian, and Rusalkian plains.

However, we have the higher extreme of the compressional stage time estimate, and the mean estimated value. If the higher extreme (1.93T) is correct, this implies that the combined duration of the Sigrunian, Lavinian, and Rusalkian periods was about 0.9T- (0.1-0.2T) -0.7-0.8T, that is, approximately hundreds of millions of years. Furthermore, if we consider that the mean estimated value of the age of the compressional stage of tessera formation (-1.4T [lvanov and Basilevsky, 1993; Gilmore et al., 1997]) is the most probable one, then the combined duration of the Sigrunian, Lavinian, and Rusalkian periods was about 0.4T- (0.1-0.2T) - 0.2-0.3T, that is, approximately a few tens of millions of years to about one hundred million years.

The next piece of information on the duration of the emplacement of the post-tessera regional plains was recently presented by Collins et al. [1997]. These workers compiled three data sets: (1) the percentage of impact craters on Rusalkian plains which were embayed by these plains materials (-1%), (2) estimations of the Rusalkian plains- forming material thickness (63% of the plains are thinner than 500 m), and (3) knowledge of impact crater relief [Sharpton, 1994]. On the basis of this information, Collins at al. [1997] estimated the time duration of emplacement of the Rusalkian plains that was required to maintain the observed number of embayed impact craters. This estimate showed that this time duration should be between 0.01T and 0.06T. Combining these results with results based on the crater counts on tessera, this estimation favors the hypothesis that the total duration of the Sigrunian, Lavinian, and Rusalkian periods was probably about 0.2-0.3T.

The formation of the wrinkle ridges which deform the surface of the Rusalkian plains is considered to have occurred in the proposed stratigraphic sequence at the boundary between the Rusalka Group and Atla Group (Figure 1). Wrinkle ridging of the Pwr followed within a very short period of time after the emplacement of Pwr materials. This is demonstrated by the fact that among about 650 impact craters superposed on the Rusalkian plains, only 7 craters have been found to be deformed by wrinkle ridges [Schaber et al., 1995; Basilevsky, 1996b]. At first glance, this means that the time interval between the emplacement of the Pwr and their wrinkle ridging should be about 0.01T (7/650 = -0.01). But if we consider the possibility that some craters emplaced before the wrinkle ridging episode might not be ridged because their sizes were smaller then the wrinkle ridge network spacing, then the time interval between the emplacement of Pwr and their wrinkle ridging may be increased up to 0.13T [Basilevsky, 1996b].

Price [1995a,b] subdivided the Venus plains into four stratigrapic units based on synoptic l:30M geologic mapping


and determined the surface age of each of them through crater density measurements. Her mapping approach was different than ours and was based on the conclusion of Arvidson et al.

[1992] that the morphological prominence of plains-forming lava flows visible on the Magellan images is degraded with time as a result of surface weathering and infilling by eolian deposits. The plains units proposed by Price [1995a,b] are thus defined on the basis of decreasing distinctiveness of flow boundaries as a function of increasing age and include, from youngest to oldest, PI-1, highly lobate plains, such as many of our P1 plains, which occupy about 5% of the total plains; P1- 2, plains with distinct but less dramatic flow morphology, comprising about 25% of the total; P1-3, plains with subtle lobate appearance, which occupy about 30% of the total; and Ps, smooth plains with no discernible flow morphology, occupying about 35% of the total plains. Crater densities increase from PI-1 to Ps, yielding age estimates of 0.7+/- 0.25T for PI-1; 0.8+/-0.13T for P1-2; 1.09+/-0.15T for P1-3; and 1.23+/-0.15T for Ps.

To correlate the Price [1995a,b] units with the units of Basilevsky and Head [1995a,b,c], a special study was undertaken [Basilevsky et al., 1996]. For the area 40ø-80øN, 140ø-260øE, the results of the mapping of Price [1995a] were coregistered with the results of the mapping by Kryuchkov [1996], who used the stratigraphic sequence of Basilevsky and Head [1995a,b,c]. It was found that the plains units of Price [1995a,b] are mixtures of our units. In the sequence from P1-1 to Ps, the percentages of our younger plains units (Atlian and Upper Rusalkian) systematically decrease, while the percentage of our older units (Lower Rusalkian, Lavinian, and Sigrunian) increases. This comparison shows that the results of Price [1995a,b] are in general agreement with the discussion above of the absolute ages of the plains of Venus.

As was discussed above, wrinkle ridges were evidently emplaced within a relatively short period of time (0.01T to 0.13T long) after the time of emplacement the Rusalka Group materials [Basilevsky, 1996b]. Therefore, theoretically the lower boundary of the Atlian period should be somewhere between times 0.87T and 0.99T. However, keeping in mind the very approximate level of all these estimates, it is probably more reasonable to consider that the lower boundary of the Atlian period is T (Figure 1). The upper boundary of Atlian time (that is, the lower boundary of Aurelian time) was independently estimated from the percentage of craters with radar-dark parabolas (the latter is an essential component of the Aurelian Group) by Basilevsky [1993] and Strom [1993] to be about 0.1T prior to the present. The upper boundary of the Aurelian time is, of course, the present time. These estimates of the time limits of the Atlian period are in agreement with the craters counts of Namiki and Solomon [1994], Price and Suppe [1994], and Price [1995a,b] which show that the average age of the Atlian volcanics is about 1/2T (see Basilevsky et al. [1997a] for details).

These absolute age estimates of different components of the surface of Venus, which we are able to correlate with our

stratigraphic units, show that our younger stratigraphic units have absolute ages smaller than the global mean surface age, while our older units have an age larger than the mean value. It is necessary to emphasize that those age estimates were made for all areas of the occurrence of the unit, including those which have not yet been mapped by us. On the basis of the successful application of the proposed stratigraphic sequence to a very large part of the surface of Venus, we now turn to

outline a schematic scenario of the geologic history of Venus, not only in terms of the relative sequence of events, but also dating those events in terms of absolute time.

5. A Proposed Scenario for the Geologic History of Venus

The observed geological history of Venus (based on the photogeological analysis of Magellan images) records only the last 10-20% of the total history of Venus because the morphological signatures of pre-Fortunian time (Figure 1) are not preserved. In the subsequent discussion, we will continue to describe the times when different events occurred in terms of

fractions of the mean surface age of Venus (T), which is estimated to be 288+311/-98 m.y. [Strom et al., 1994]; 400 to 800 m.y. [Phillips et al., 1992]; and even 800+800/-400 m.y. [Zahnle and McKinnon, 1996]. Further determination of the true duration of T is contingent on progress in the understanding and correlation of the interplanetary flux of crater-forming objects.

The beginning of this part of the history of Venus (Figures 1 and 15)was characterized by intensive tectonic deformation of global or semi-global scale which formed the tessera terrain. Early stages of that deformation were evidently compressional and later changed into tensional [Bindschadler and Head, 1991; Ivanov and Head, 1996], but differences of opinion exist about the sequence in different places [Hansen and Willis, 1996] and whether the large presently preserved tessera blocks might represent downwelling [Bindschadler and Head, 1991; Bindschadler and Parmentier, 1990; Bindschadler et al., 1992a,b; Ivanov and Head, 1996] or upwelling [Herrick and Phillips, 1990; Ghent and Hansen, 1997]. Termination of the compressional stage is estimated to have occurred at about time 1.4T, while the tensional stage lasted for another 0.1-

TECTONICS VOLCANISM

:• .... ' '-'"'" '"':':'•:'•' •••••'".-_!i:..-. •' Z

Rift-assochated

Extension ............. •:"'""•••••-':"' :..-' "••-'..-.••____ •_-•11i;.----.-,,. -•-:.•:•. ß ::.:.:...:::.•::: ................................................................ ............................ '1' Corr•ression '!. •d-oceanic rid9e

LU .................. ............................. • ••""•'•••""••••:••:•:•i :::-•"'""'"'":':':•: :'-': ;:'"'-•'#-':•::•__ -':"):q• T 1.5 1.0 0.5 0 1.0 0.5 0

Figure 15. Summary of the geologic history of Venus compared with Earth. On Earth the plate tectonic system contains complementary amounts of compression and tension throughout this period. On Venus, tectonics are dominated by phases that oscillate between predominantly global tension and compression. On Earth during this period, the rate of volcanism appears to have been relatively constant; the majority of volcanism is at mid-ocean ridges and the minority is at intraplate sites. On Venus, rates of volcanism peaked early in this period, and the dominant style changed with time.


0.2T. A number of hypotheses have been proposed to account for the tessera-forming deformation (see summary by Ivanov and Head [1996]). Among these are gravitational instabilities causing mantle overturn [Parmentier and Hess, 1992; Head et al., 1994], an oscillatory convective regime of the mantle [Arkani-Hamed and Toksoz, 1984; Arkani-Hamed et al., 1993], episodic plate tectonics [Turcotte, 1993], a "catastrophic" convective episode caused by a phase transition in the mantle [Steinbach and Yuen, 1992; Weinstein, 1993; Herrick and Parmentier, 1994], or something else. This intensive tectonism was accompanied by volcanic activity [Ivanov and Head, 1996]. Thus emplacement of tessera-forming material and its deformation into tessera terrain are the major geologic events of Fortunian time.

After tessera formation, several stages of extensive volcanism occurred (Figures 1 and 15). These were of basaltic composition at the Venera landing sites (see discussion by Basilevsky et al. [1992] and Weitz and Basilevsky [1993]) and of probable basaltic composition elsewhere. These events buried vast areas of tessera and formed what we see now as the

regional plains of Sigrunian, Lavinian, and Rusalkian age. In order to bury tessera terrain with typical kilometer-scale vertical relief, a few kilometers' thickness of plains-forming lavas are required [e.g., Iranov and Head, 1996, Figure 17]. The combined duration of the emplacement of these plains is estimated to be about 0.2-0.3T. If T is somewhere between 300

and 800 million years, this implies that the average global rate of volcanism was about a few cubic kilometers per year, which is comparable to the present rate of terrestrial oceanic ridge extrusive volcanism [e.g., Head et al., 1992, Figure 16; Basilevsky and Head, 1996b; Head et al., 1996]. This, however, does not mean that the style and geotectonic environment of emplacement of plains-forming lavas on Venus and mid-oceanic ridge basalts on Earth are similar.

Plains-forming materials of the Sigrunian, Lavinian, and Rusalkian Groups are separated from each other, from the underlying Fortunian Group, and from the overlying Atlian and Aurelian Groups, by unconformities formed by tessera-forming deformation, Pdf-type dense fracturing, Pfr/RB-type broad ridging, and finally, wrinkle ridging (Figures 1 and 15). These tectonic episodes had to be generally synchronous in different areas of Venus because otherwise, multiple violations of the described stratigraphic sequence would be observed (Figure 14), and this is not the case. This also means that in the part of the geologic history of Venus presently exposed, there were episodes characterized by the dominance of compression, then tension, then again compression, and finally tension [Head and Basilevsky, 1998] (Figure 15).

It is not specifically known if there were distinct Sigrunian, then Lavinian, then Rusalkian volcanic pulses, separated by tectonic episodes, or if on the global scale, the emplacement of the Sigrunian, Lavinian, and Rusalkian plains-forming materials was more or less continuous. For example, if volcanism was relatively continuous but declining in rate and changing in style, the changes in tectonic style as a function of time could help to make the portions of the volcanically emplaced materials morphologically distinct: e.g., densely fractured, broadly ridged, or wrinkle ridged. These tectonically imprinted morphologic signatures together with primary formational signatures, such as the dominance of small volcanic shields (Psh)or lobate flow-like features (P1), were the reasons for the initial identification of the proposed stratigraphic units. Of course, thorough photogeological study of crosscutting and embayment relations of each of the

proposed units with other units was undertaken to construct the • stratigraphic sequence. The presence of tectonically determined uncomformities among the majority of the proposed units makes this procedure similar to normal stratigraphic analyses in terrestrial geology. Of course, this approach was possible not only because of the visible change in the tectonic deformation style with time, but also because of the significant decrease with time of the intensity of tectonic deformation, so that in most cases the later deformation does

not overwrite the earlier structural imprints. The last globally distributed tectonic episode, extensive

wrinkle-ridging, happened at about time T, which was very close in time to the emplacement of the Late Rusalkian plains. This marked the transition to the present stage of the history of Venus, which is characteristized by a predominance of regional rifting and localized rift-associated volcanism in the form of large shield volcanic constructions and lobate volcanic plains-forming units. The majority of this stage is represented by the Atlian period, which appears to have lasted from about T to 0.1T from the present. This makes this period the longest in duration among the stratigraphic units considered, although the resulting tectonic and volcanic features and deposits cover only 10-20% of the surface of Venus. This observation means that the general intensity of tectonics and the flux of volcanism in the Atlian period were much lower than that in pre-Atlian time. According to estimates by Basilevsky and Head [ 1996a], Head et al. [ 1996; Figure 1] and Basilevsky et al. [1997a], the rate of this latest volcanism was a few tenths of a cubic kilometer per year, which is comparable to the average present rate of intraplate volcanism on Earth [Crisp, 1984; Head et al., 1992].

The upper part of this recent stage of the history of Venus (Figures 1 and 15) is the Aurelian period, which started at about 0.1T and continues up until the present. Its defining characteristic, a preservation of crater-associated radar-dark parabolas, simply reflects a certain level of reworking of the surface by eolian processes [Greeley et al., 1992]. In the sense of tectonic and volcanic processes, it is apparently just a continuation of the Atlian period. Finally, documented cases of rifting and volcanism postdating the dark parabolas [Basilevsky, 1993] show that Venus may be endogenically active at the low level characteristic of the Atlian even today.

6. Conclusions

The considerations stated above demonstrate that the

morphologically observable part of the geologic history of Venus (Figures 1 and 15)was characterized by several key characteristics that stand in contrast to the comparable period of geologic history of Earth. Depending on the magnitude of the true value of the mean surface age of Venus (T), the corresponding part of the geologic history of Earth may comprise (1) the post-Paleozoic (•-245 m.y.), (2) the Phanerozoic (-570 m.y.), or (3)even extend into the late (-900 m.y.) or middle (-1600 m.y.) Proterozoic. In any of these cases, during that time period the global geodynamic processes of Earth were certainly dominated by plate tectonics, characterized by its general global balance of tension (at divergent plate boundaries) and compression (at convergent plate boundaries), and a variable but relatively narrow rate of plate movement. In the comparable time period, Venus shows no certain signature of plate tectonics, particularly in post- tessera time; instead, its global tectonic environment passed from an initial dominance of compression, through tension,


then again compression, and finally tension (Figure 15). In addition, the density of the deformational structures and probably the strain rate [Grimm, 1994] were definitely declining with time.

The predominant component of volcanism on Earth during this time period was extrusive volcanism at mid-oceanic ridges, characterized by a somewhat variable, but generally stable production rate on a global scale. Subduction zone volcanism and intraplate volcanism played a subordinate role. For some short periods of time the formation of large igneous provinces dominated intraplate volcanism and the global production rate [Larson, 1991; Coffin and Eldholm, 1994; Head and Coffin, 1997]. In the beginning of that same period of time, Venus was characterized by the emplacement of plains-forming volcanism at a rate comparable to terrestrial volcanism at mid-ocean ridges (but in a much different, non- plate tectonics style), while for the last few hundred million years, Venus was dominated primarily by rift-associated volcanism emplaced at a production rate comparable to or even lower than than that of present intraplate volcanism on Earth.

In summary, this geologic history of Venus underlines the sequence of important events that must be explained by models of the geodynamic evolution and thermal history of Venus. It also illustrates the distinctive difference between the recent

history of Venus and the Earth and will hopefully serve as a basis for the further investigation of the reasons for the differences between two planets that in other ways are so similar.

Acknowledgments. We gratefully acknowledge fruitful discussions with M. A. Ivanov, V. P. Kryuchkov, A. A. Pronin, Martha Gilmore, Geoff Collins, and George McGill, and financial support from NASA Planetary Geology and Geophysics Program grant NAGW-1873 to J. W. H. Detailed reviews by Robert Strom and others were also very helpful. Figures were prepared by Anne C. C6t6, and photographic work was done by Peter Neivert.

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A. T. Basilevsky, Vernadsky Institute of Geochemistry and Analytical Chemistry, ulitsa Kosygina, 19 117975 Moscow, Russia. (e-mail: abasilevsky @glasnet.ru)

J. W. Head, Department of Geological Sciences, Brown University, Box 1846, Providence, RI 02912. (e-mail: [email protected])

(Received August 18, 1997; revised February 3, 1998; accepted February 10, 1998.)

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