The surface characteristics and morphology of the equatorial region of Venus were first described by Masursky et al. who showed this part of the planet to be characterized by two topographic provinces, rolling plains and highlands, and more recently by Schaber who described and interpreted tectonic zones in the highlands. Using Pioneer Venus (PV) radar image data (15 deg S to 45 deg N), Senske and Head examined the distribution, characteristics, and deposits of individual volcanic features in the equatorial region, and in addition classified major equatorial physiographic and tectonic units on the basis of morphology, topographic signature, and radar properties derived from the PV data. Included in this classification are: plains (undivided), inter-highland tectonic zones, tectonically segmented linear highlands, upland rises, tectonic junctions, dark halo plains, and upland plateaus. In addition to the physiographic units, features interpreted as coronae and volcanic mountains have also been mapped. The latter four of the physiographic units along with features interpreted to be coronae

Head, James W.

Senske, D. A.

English

NASA Technical Reports Server

N9o-12447
43
GEOLOGY OF THE VENUS EQUATORIAL REGION FROM PIONEER VENUS RADAR
IMAGING. D. A. Senske and J. W. Head, Brown University, Providence, RI 02912.
_. The surface characteristics and morphology of the equatorial region of Venus were first described
by Masursky et al. (1) who showed this part of the planet to be characterized by two topographic provinces, rolling
plains and highlands, and more recently by Schaber (2) who described and interpreted tectonic zones in the highlands.
Using Pioneer Venus (PV) radar image data (15" S to 45' N), Senske and Head (3,4) examined the distribution,
characteristics, and deposits of individual volcanic features in the equatorial region, and in addition classified major
equatorial physiographic and tectonic units on the basis of morphology, topographic signature, and radar properties
derived from the PV data (5). Included in this classification are: plains (undivided), inter-highland tectonic zones,
tectonically segmented linear highlands, upland rises, tectonic junctions, dark halo plains, and upland plateaus. In
addition to the physiographic units, features interpreted as coronae and volcanic mountains have also been mapped. In
this paper, we briefly describe the latter four of the physiographic units along with features interpreted to be coronae.
Uoland rises are defined as broad, radar-dark, topographic highs containing individual peaks which are commonly
radar-bright and express typically 1.0- to 3.0-km of relief (4). Two regions designated as upland rises are Bell Regio
and Eisila Regio. Bell Regio is a radar-dark topographic rise on which are located four peaks, Api Mons, Nefertiti,
Tepev Mons, and an unnamed peak. On the basis of the presence of both radar-bright and radar-dark flows these
mountains are interpreted to be volcanoes (4,6). Gravity modeling by Janle et al. (7) suggests a deep apparent depth
of compensation (-200 km) for this region which they interpret along with geologic mapping to indicate support of
topography by dynamic processes in the mantle. The second upland rise, Eisila Regio, length of 8000 kin, forms the
westward extension of Aphrodite Terra. The western part of Eisila Regio is a broad radar-dark topographic rise which
exhibits properties similar to the surrounding plains. Located on the crest of the high topography are two volcanic
mountains, Sif Moils and Gula Mons. Flows associated with these volcanoes appear to be of limited extent, forming a
thin veneer, suggesting that this region was not built up entirely by volcanic construction. On the basis of the
interpretation of volcanic deposits representing a thin veneer, the presence of material on broad topographic rises
similar to that on the adjacent plains, and deep apparent compensation, formation of upland rises related to doming due
to thermal uplift is proposed.
Tectonic iunctions are radar-bright highlands located at the convergence of three or more inter-highland
tectonic zones, and are centers of volcanism possessing individual mountains typically located at the crest of a domal
topographic rise. Features mapped as tectonic junctions include Beta Regio, Atla Regio, Asteria Regio, northern
Phoebe Regio, the region of convergence of Ulfrun Regio and Hecate Chasma, and an elevated region southeast of Atla
Regio. The largest tectonic junctions, Atla Regio and Beta Regio are broad domal topographic rises on which are
located large volcanic edifices, Ozza Mons and Maat Mons in Aria Regio, and Theia Mons and Rhea Mons in Beta
Regio. On the basis of geologic mapping (8,9) and modeling of gravity data (10), these regions have been interpreted
to be associated with deep mantle thermal anomalies. The tectonic junctions located at Asteria Regio and northern
Phoebe Regio possess characteristics similar to Beta and Atla, and may have formed in similar manners, but are
smaller. The combination of inter*highland tectonic zones and tectonic junctions forms an interconnecting network
extending over half the circumference of the planet.
]_ark halo 171ains are defined as broad quasi-circular regions of very low radar backscatter located in low lying
areas and extending for hundreds of kilometers (4). A radar-bright circular feature or ring, which itself has a radar-dark
interior is often found in the dark material. In high resolution image data these circular features correspond to craters,
several of which are located on local topographic rises and are surrounded by lobate flow deposits suggesting a
volcanic origin (5). The most extensive regions of dark halo plains are located to the west of Eisila Regio and west of
Atla Regio. In the region west of Eisila Regio the radar-dark material appears to embay areas of higher topography
suggesting emplacement by lava flooding. On the basis of the embayment relations expressed by the radar-dark
material and the presence of deposits associated with the interior craters which are interpreted to be volcanic, it is
suggested that the dark halo plains are regions of smooth lava flows with the interior crater being a source region of
some of these deposits.
Upland plateaus are broad, bright to mottled-bright, plateaus covering areas of hundreds of square kilometers
typically bounded by steep scarps standing 1 to 2 kilometers above the surrounding plains (5). Few individual peaks
are present or they are entirely absent. Central troughs similar to those associated with Beta and Aphrodite are not
observed. Specific upland plateaus are located north of Asteria Regio, on the eastern flanks of Beta Regio, at Phoebe
Regio, adjacent to Ovda Regio and Thetis Regio, in the southern hemisphere at Alpha Regio, and at Tellus Regio.
Venera imaging of Tellus Regio shows it to possess a complex tectonic structure of intersecting valleys and ridges and
is mapped as tessera (11). A comparison of radar properties indicate that both upland plateaus and tessera are
characterized by high roughness, low values of uncorrected reflectivity, and that they contain a large percentage of
wavelength-scale (5-50 cm) diffuse scatterers (12). On the basis of topographic signature, radar properties, and
correlations with units mapped as tessera from Venera imaging, it is suggested that areas mapped as upland plateaus are
tectonic units similar to tessera (5).
Within the PV data £.9.r.Rllgg are characterized by locally elevated topography, narrow radar-bright discontinuous
rims, and radar-dark interiors (13). Similar characteristics are observed for cornnae mapped in Mnemosyne Regio from
both Arecibo and Venera radar images (14). On the basis of this characterization, two large circular features interpreted
as coronae are identified in the area imaged exclusively by PV. The first, Pavlova, is located in eastern Eisila Regio
(15' N, 40" E), is elliptical with dimensions of 525 km x 370 km, and is characterized by a 50-kin wide discontinuous
https://ntrs.nasa.gov/search.jsp?R=19900003131 2020-03-20T00:59:13+00:00Z
44 GEOLOGY OF THE VENUS EQUATORIAL REGION
Senske, D. A. and J. W. Head
rim which exhibits 200 to 600 m of relief. The discontinuous part of the rim corresponds with flanking topographic
depressions mapped as very dark that are similar to units mapped elsewhere as volcanic plains and suggests that lava
flows have breached the rim. The second (840 km diameter) corona is located to the south of western Eisila Regio
(2.0* N, 355.0°). Like Pavlova, this structure is characterized by a discontinuous rim which is radar-bright and elevated
to the south while the northern rim possesses no topographic relief and is mottled dark (5), Stratigraphic relations
between the rim and radar-dark interior deposits suggests lava flooding, forming plains, has occurred in the interior of
this structure. From this analysis, we find that the number of large coronae in the equatorial region to be much less
than in the northern high latitudes (14,15).
Discussion and Conclusions. On the basis of variations in structure and morphology, the physiographic
units are divided into three distinct longitudinal zones: upland rises (330" to 55°), tectonicall_ _se_mented linear
highlands-upland .lateaus (55 ° to 145°), and "ntnter:highland tectonic zones-tector_j_ iunctiop_s (145 ° to 315°). The zone
of upland rises contains the two highland structures of Bell Regio and Eisila Regio whose volcanic nature has been
previously established (4,6,7). Eisila Regio is characterized by three distinct regions, western Eisila _eith its volcanic
peaks Sif Mons and Gula Mons, central Eisila with the volcanic peak Sappho (16,17), and eastern Eisila Regio with its
corona structure, Pavlova. Detailed analysis of western Eisila Regio suggests that the volcanoes and their deposits
represent a thin veneer of material superimposed on a broad topographic high. In addition, ridges in the lowlands
along the eastern flanks of western Eisila Regio are interpreted to be normal faults (18) whose relation to high
topography is consistent with a model of uplift under extension (t9). This arrangement of structures and a_sociation
with volcanism in western Eisila suggests formation by doming associated with thermal uplift. A similar model has
been previously proposed for Bell Regio (6,7). The region of _l]2t_ _ggtag..al.cA_Li_l_ld__
_lateaus is made up of the highlands of western Aphrodite and Tellus Regio. Previous studies of the tectonically
segmented linear highland of western Aphrodite show it to be characterized by bilaterally symmetric topography and an
en echelon central trough offset in a right lateral sense along cross-strike structural and topographic discontinuities
(CSD's) (20,21). The structure of western Aphrodite is similar to that of terrestrial mid-ocean ridges, and has on this
basis been interpreted to be a site of possible spreading (20,21). The upland plateaus of Tellus Regio and the units
flanking northern Aphrodite form a second distinctive set of highland features in the equatorial region (13). The
location of an upland plateau adjacent to Thetis Regio, a region interpreted to be the site of crustal spreading (20,21),
suggests two possible models for formation of this upland plateau: 1) the plateau is a preexisting old crustal block;
doming and rifting analogous to that of old terrestrial continental crust, followed by spreading as suggested by Head
and Crumpler (20), have split the block and formed the intervening high topography associated with Thetis: 2) the
upland plateau originated by crustal spreading. In this second model, developed by Sotin et al. (22), an increase in
mantle temperature and associated production of thicker crust created the high topography in central Thetis Regio. The
region of inter-highland tectonic zones and tectonic junctigns has previously been described as zones of extension (2).
Unlike the upland rises and tectonically segmented linear highlands, which are predominantly linear features striking
east-west, the inter-highland tectonic zones have a variety of orientations forming an interconnecting radial pattern
between tectonic junctions (t3). The presence of a bilaterally symmetric topographic rise with a central trough offset
along CSD's suggests that the inter-highland tectonic zone of eastern Aphrodite is a site of rifting and possible crustal
spreading (23). This and other inter-highland tectonic zones converge at tectonic junctions. Two of these junctions,
Beta Regio and Atla Regio, have been interpreted to be associated with deep mantle thermal anomalies (5,9,10).
The highlands in the equatorial region of Venus form a near-global network of volcanic centers and interconnecting
tectonic zones, composed of several distinctive terrain types. The relationship between tectonic junctions and inter-
highland tectonic zones suggests that the junctions are nodal points of the network. The inter-highland tectonic zones
which extend to the north and do not connect with tectonic junctions die out in this direction. In some places in the
equatorial network crustal spreading may be occurring (inter-highland tectonic zones) (23) whereas at other places
(tectonic junctions) hot spots and thermal uplift activity is apparently occurring. These characteristics and correlations
suggest that both vertical thermal uplift and lateral movement are occurring in the Venus equatorial highlands. In
contrast to the equatorial region, the northern high latitudes are characterized by several broad zones of compression
forming highlands and orogenic belts (24,25).
References: (1) Masursky, H., et al., JGR, 85, 8230, 1980. (2) Schaber, G., GRL, 9, 499, 1982. (3) Senske,
D.A. and J. W. Head, LPSC XVIII, 908, 1987. (4) Senske, D. A. and J. W. Head, LPSC XIX, 1061, 1988, (5) Senske,
D. A., M. S. Thesis, in preparation, 1989. (6) Basilevsky, A. T. and P. Janle, Astron Vestnik, 21,109, 1987. (7)
Janle, P. et al., Earth Moon, and Planets, 39, 251, 1987. (8) Stofan, E. R., et aI.,GSA Bulletin, 143, 1989. (9)
Senske, D. A. and J. W. Head, LPSC XIX, 1063, t988. (10) Esposito, P. B., et al., Icarus, 51, 448, 1982. (11)
Barsukov, V. L. et al., JGR, D378. 1986. (12) Bindschadler, D. L. and J. W. Head, Earth, Moon, and Planets, 133,
1988. (13) Senske, D. A. and J. W. Head, LPSC XX, 986, 1989. (14) Stofan, E. R. and J: W. Head, submitted to
Icarus, 1989. (15) Nikoleava, O. V. et aL, Geokhimiya, 5, 579, 1986. (16) Burns, B. and D. B. Campbell, JGR, 90,
3037, 1985. (17) Stofan, E. R., M. S. Thesis, 1-64, 1985. (18) Senske, D. A. and L W. Head, IGC, in press, 1989.
(19) Withjack, M. D. and C. Scheiner, AAPG Bull., 66,302, 1982. (20) Head, J. W. and L. S. Crumpler, Science, 238,
1380, 1987. (21) Crumpler, L. S., and J. W. Head, JGR, 93, 301, 1988. (22) Sotin, C. et aI., submitted to EPSL,
1988. (23) Crumpler, L. S. and J. W. Head, LPSC XIX, 223, 1988. (24) Crumpler, L. S. et al., Geology, 14, 1031,
1986. (25) Vorder Bruegge, R. M. S. Thesis, 1987.
45
VENUS GRAVITY: MEASUREMENTS, REDUCTIONS AND RESULTS; W.L. Sjogren,
Jet Propulsion Laboratory
This presentation will include a description of what gravity data are
and how gravity data measurements are obtained. There will be a brief
discussion of the error sources that corrupt the raw data and how Magellan
data will be superior to the previous Pioneer Venus data set.
A summary of present data coverage and what will be obtained by
Magellan will be shown. The various reduction techniques using spherical
harmonics, line-of-sight profiling and direct mass estimates for local
feature modeling will be described The published results from these
various approaches will be summarized and inferences suggested. There'll
be a list of things we hope to resolve in the MGN extended mission with
new high resolution data. For the consciencious participant at this
conference, he would be in good shape to ask intelligent questions if he
reads the Venus V-gram #14, May 1988.


Geology of the Venus equatorial region from Pioneer Venus radar imaging

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