Geology of a rift zone on Venus: Beta Regio and Devana Chasma E R STOFAN ) > Department of Geological Sciences, Brown University, Providence, Rhode Island 02912 J. W. HEAD t D. B. CAMPBELL NAIC Arecibo Observatory, Arecibo, Puerto Rico 00612 S. H. ZISK Massachusetts Institute of Technology/NEROC Haystack Observatory, Westford, Massachusetts 01886 A. F. BOGOMOLOV Moscow Power Institute, Moscow, USSR O. N. RZHIGA Institute of Radiotechnics and Electronics, Moscow, USSR A. T. BASILEVSKY Vernadsky Institute, USSR Academy of Sciences, Moscow, USSR N. ARMAND Institute of Radiotechnics and Electronics, Moscow, USSR ABSTRACT Aphrodite Terra, suggest that their origins (Sjogren and others, 1983). The anomaly sug- may be linked. gests a compensation depth of 330 km for Beta Beta Regio is a region of rifting and volcan- or dynamic support for the topography (Espo- ism on Venus. The nature of Beta, a major INTRODUCTION sito and others, 1982). topographic rise and rift zone, is herein char- This preliminary information on the charac- acterized using Pioneer Venus, Arecibo, and The nature and distribution of tectonic struc- teristics and distribution of chasmata on Venus Venera 15/16 data. High-resolution (1-2 km) tures on terrestrial planets is closely linked to the and the nature of Beta Regio raises several signif- Arecibo and Venera radar images reveal de- major mechanisms of lithospheric heat transfer icant questions concerning their origin and evo- tails of faulting and volcanism, and Pioneer (Solomon and Head, 1982). On Venus, some of lution. Is the topography associated with the rift Venus altimetry illustrates the density and lo- the most topographically distinctive and areally zones (both the broad topographic rises and the cation of faults in relation to topography. extensive tectonic features are the deep linear narrow flanks of the rifts) due to uplift or vol- Faults are distributed throughout Beta but valleys known as "chasmata," which are inter- canic construction? Are these rift-like structures are concentrated in Devana Chasma, where preted by most workers to be the result of litho- of active or passive origin (Sengor and Burke, they are spaced 5-20 km apart. The pattern spheric extension and rifting (Pettengill and 1978)? The association of Devana Chasma with of faulting and distribution and sequence of others, 1980; Masursky and others, 1980; the broad topographic rise of Beta would seem volcanic activity in Beta can be used to help McGill and others, 1981; Schaber, 1982; to argue for localized uplift and faulting (McGill understand how the rift has evolved, includ- Campbell and others, 1984). Schaber (1982) and others, 1981) and an active origin, whereas ing the origin of the high topography of Beta has shown that systems of chasmata define tec- the major global-scale extensional zones defined and the origin and nature of Devana Chasma. tonic zones as much as 20,000 km in linear by Schaber (1982) suggest large-scale horizontal On the basis of geologic mapping relations extent. extension and the possibility of passive rifting. If and map patterns, Beta appears to have One of the most distinctive occurrences of the elastic lithosphere of Venus is thin (in the formed as a result of doming in response to a chasmata is in Beta Regio, a 2,300 x 2,000 km 1-10 km range), as suggested by thermal- mantle anomaly. At the same time, Rhea highland region rising >5 km above mean gradient estimates, by laboratory data for the Mons, a major shield volcano, was formed. planetary radius and cut by a north-south-trend- behavior of materials, and by the characteristics Formation of Devana Chasma followed, with ing linear trough (Devana Chasma) in excess of of deformation in Ishtar Terra (Solomon and extensive faulting in the rift trough. Geometry 1 km deep (Figs, la and lb). Recently obtained Head, 1984a), then why are the chasmata so of the trough and fault patterns suggests that high-resolution radar images of central Beta wide and deep? Is lithospheric stretching an im- some degree of lithospheric stretching has oc- Regio (Fig. 2) show details of the tectonic struc- portant process in the formation of rifts on curred. Later volcanism produced a second ture and associated volcanic deposits (Campbell Venus (Solomon and Head, 1984b; Zuber and major shield volcano, Theia Mons, which is and others, 1984). Two scales of tectonic fea- Parmentier, 1986)? What is the explanation for superimposed on the western bounding fault tures are seen: (1) major faults, which generally the two scales of faulting observed in Beta Regio of the rift zone. Both uplift and extension define the edges of the 300-km-wide rift zone, (Campbell and others, 1984)? Are the two have been involved in forming Devana and (2) abundant minor faults, spaced 10-20 scales related to different scales of extension or Chasma and Beta Regio and may be impor- km apart and concentrated in the central part of differential-strength layering in the lithosphere tant in the formation of other equatorial high- the rift zone. The faults lie in and on the flanks (Zuber and Parmentier, 1986; Zuber, 1987)? land regions with systems of chasmata on of the central trough, Devana Chasma, which The purpose of this study is to examine the Venus, such as Aphrodite Terra. The bifurca- has an average width of 160 km. Beta Regio is nature of Beta Regio as an example of a major tion of Devana Chasma in the vicinity of also one of the most prominent features in grav- topographic rise and rift zone on Venus, utilizing Theia Mons, and the extension of the rift sys- ity data of Venus, with a strong positive anom- a variety of data sets. We examine the questions tem south to Phoebe Regio and west toward aly highly correlated with the topography of the origin of the high topography and the Geological Society of America Bulletin, v. 101, p. 143-156, 16 figs., January 1989. 143 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/101/1/143/3380394/i0016-7606-101-1-143.pdf by guest on 30 September 2021 144 STOFAN AND OTHERS Figure 1. (a) Pioneer Venus topography of Beta and Phoebe Regiones, illustrating the location of Devana Chasma, Theia Mons, and Rhea Mons. The map is in a Mercator projection, with the scale shown for the equator. The contour interval is 500 m. (h) Map based on radar-brightness (surface roughness) patterns from Arecibo and Pioneer Venus imaging data, showing Beta Regio, Devana Chasma, and Phoebe Regio. Areas indicated in the stippled pattern are the relatively brightest areas, or areas with the highest surface roughness, which tend to correlate with the topographically highest areas shown in Figure la and the areas characterized by chasmata. active versus passive nature of the rift. We also maps of central Beta Regio (Fig. 5) (Campbell surface of Venus can be described by radar- examine the significance of volcanism associated and others, 1984), as well as a general unit map scattering models such as that expressed in Hag- with the rift to determine the sequence of tec- (Fig. 6). Arecibo data of northern Beta Regio fors law: tonic and volcanic activity in Beta Regio. were also mapped (Figs. 7 and 8) and compared C to radar images of the same region obtained by <*fi) = (P y) (œs40 + Csin20)15, METHODS AND RESULTS the Venera 15 and 16 orbiters. (3) The Venera radar images (resolution 1-2 km), with an inci- where a is the radar cross section per unit area at Three sets of data were used in examining the dence angle of 10° and a look direction to the angle of incidence 0, p is the Fresnel reflection Beta region. (1) Pioneer Venus roughness and west (Fig. 9), were used to map brightness pat- coefficient at normal incidence angle, and C is reflectivity data (resolution about 100 km) were terns associated with surface slopes (Figs. 10 the Hagfors parameter (Hagfors, 1970) out to used to characterize the general surface proper- and 11). Rose diagrams of lineaments mapped 40° and following cos20 beyond 40° (Fig. 14). ties of the region, whereas Pioneer Venus altime- from the Arecibo and Venera 15/16 data were At incidence angles less than 20° (for example, try profiles across the rift allowed examination produced (Figs. 12 and 13) and compared to Venera 15/16), the curve slopes steeply, so that of detailed relations between radar brightness theoretical fault patterns. The Venera 15/16 surface-slope changes control the amount of patterns and topography (Figs. 3 and 4). (2) mission mapped Venus from orbit during backscatter. Between 20° and 60° (for example, High-resolution (approximately 2 km) images of 1983-1984, obtaining images and altimetry Arecibo data of Beta Regio), the slope of the the Beta region obtained at the Arecibo radar north of 30°N latitude (Kotelnikov and others, scattering-law curve is relatively flat, so that facility in Puerto Rico in 1983 were also studied 1985). The two radar image sets, Venera 15/16 changes in small-scale (wavelength-size; 12.6 (Fig. 2). The incidence angle of the Arecibo and Arecibo, provide different information cm) surface roughness dominate the returned radar varies approximately with the latitude, about the venusian surface, although they are of signal, with changes in the dielectric constant (a from about 30°-45°, with the look direction of similar resolution. They are complementary be- function of porosity and composition) also con- the radar oriented approximately perpendicular cause of differences in their viewing geometries tributing to the radar return. At incidence angles to the curved image edge seen in Figure 2. Sur- (incidence angles and look directions). greater than 60°, surface slopes again begin to face roughness variations were used to produce The way in which radar interacts with the dominate the returned signal (Campbell and Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/101/1/143/3380394/i0016-7606-101-1-143.pdf by guest on 30 September 2021 GEOLOGY OF A RIFT ZONE ON VENUS 145 still detected.