Regional volcanic rises form a major part of the highlands in the equatorial region of Venus. These broad domical uplands, 1000 to 3000 km across, contain centers of volcanism forming large edifices and are associated with extension and rifting. Two classes of rises are observed: (1) those that are dominated by tectonism, acting as major centers for converging rifts such as Beta Regio and Alta Regio, and are termed tectonic junctions; and (2) those forming uplands characterized primarily by large-scale volcanism forming edifices. Western Eistla Regio and Bell Regio, where zones of extension and rifting are less developed. Within this second class of features the edifices are typically found at the end of a single rift, or are associated with a linear belt of deformation. We examine the geologic characteristics of the tectonic junction at Alta Regio, concentrating on documenting the styles of volcanism and assessing mechanisms for the formation of regional topography

Head, James W., III

Senske, D. A.

English

NASA Technical Reports Server

LPI Contribution No. 789 10"]
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Fig. 1.
in cycle 3. Some 98% of the planet will be mapped at the end of
mission cycle 3.
Planned observations in the fom'th mission cycle from mid-
September 1992 through mid-May 1993 will emphasize high-
resolution gravity observations of the equatorial regions of Venus.
A second Orbit Trim Maneuver (OTM) at the beginning of this
mission cycle will lower periapsis to below 200 km to improve the
gravity resolution. Magellan, with its large antenna and X-band
radio system, will also improve upon the venusian gravity maps
obtained from the Pioneer-Venus spacecraft. These new gravity
observations when coupled with superb radar images will provide
valuable insights to the interior processes occtm_g on Venus.
Scientific reports for the project include the "45 Day Report,"
which was published as a single issue of Science in March 1991. A
"6-Month Report" will also be published as a special issue of the
Journal of Geophysical Research (JGR) Planets in the summer of
1992. A "Geophysics Report" on the 360 ° of gravity observation in
cycle 4 will be one or more scientific articles submitted for
publication in the summer or fall of 1993.
Magellan data products, the SAR images, altimetry data and
radiometry data, are available as analog photographs and digital
compact disks (CD-ROMs) at the National Space Science Data
Center (NSSDC) at the NASA Goddard Space Flight Center (GSFC)
inGreenbelt, Maryland. As of May 1,1992, over 500 radar mosaics,
as well as the altimetry and radiometry data for the Fast mission
cycle, are available. In addition, some 250 photographs have been
released and are available to the public. The altimetry and radiom-
etry data for cycle 1 produced by thc Massachusetts Institute of
Technology (MYI'), as well as the cartographic products produced
by the U.S. Geological Survey (USGS), arc being released to the
science community, q
N93-14374 q o
ATLA REGIO, VENUS: GEOLOGY AND ORIGIN OF A
MAJOR EQUATORIAL VOLCANIC RISE. D.A. Senske and
J. W. Head, Department of Geological Sciences, Box 1846, Brown
University, Providence RI 02912, USA.
Introduction: Regional volcanicrises form a major part of the
highlands in the equatorial region of Venus. These broad domical
uplands, 1000 to 3000 km across, contain centers of volcanism
forming large edifices, and are associated with extension and
rifting. Two classes of rises are observed: (1) those that are
dominated by tectonism, acting as major centers for converging rifts
such as Beta Regio and Ada Regio, and are termed tectonic junctions
[1]; and (2) those forming uplands characterized primarily by large-
scale volcanism forming edifices, Western Eisda Regio and Bell
Regio, where zones of extension and rifting are less developed.
Within this second class of features the edifices arc typically found
at the end of a single fill or are associated with a linear belt of
deformation [1,2]. In thispaper, we examine the geologic character-
istics of the tectonic junction at Atla Regio, concentrating on
documenting the styles of volcanism and assessing mechanisms for
the formation of regional topography.
Topographic and Geologic Characteristics of Aria Regio:
Afla Regio is a 1000-kin x 1000-kin highland centered near 4°N,
200 ° and is a broad rise reaching an elevation of 3.0 km (all
elevations are referenced to a planetary radius of 6051.0 kin) (Fig.
1). The relationship between chasmata (rifts) and volcanic features
forms a pattern similar to that observed at Beta Regio, distinguish-
hag Alia as a major tectonic junction [3,4,5]. In addition, Pioneer
Venus gravity data show this highland to have a substantial gravity
anomaly, centered at Ozza Mons, along with a corresponding large
apparent depth of isostafic compensation (>200 km) [6,7]. Interpre-
tations from these data suggest that like Beta Regio, Aria Regio is
most likely a site of mantle upwelling.
Magellan altimetry data provide the first detailed coverage of the
topography of Ada Regio (Fig. I). The regional rise has gentle
slopes (0.1 ° to 0.2°), reaching its highest point at Ozza Mons, a 7.5-
kin-high peak. In plan view the central part of the highland is
triangular shaped with its apex pointing to the north where it
intersects the north-south-trending rift valley Ganis Chasma. The
legs of the triangle correspond to Dali Ch asma (southwest/northeast
orientation) and Parga Chasma (southeast/northwest orientation).
The more distal parts of all three rifts curve, are aligned along a more
east-west orientation, and form a "pinwheel" pattern centered on
Ozza Mons. At a point just to the south of where Dali Chasma
intersects Ozza Mons a second volcano, Maat Mons, is located on
the western edge of the rift and rises to an elevation of 9.2 km. To
the northwest of central Aria is a second gentle topographic rise
(elevation of 2.0 kin) on which is located Sapas Mons, a 4.0-km-
high volcano that has a substantial gravity anomaly (+25 regal at a
spacecraft aldtude of 200 km) (g). The presence of broad domical
topography, the large gravity anomaly, and the presence of |arge-
scale volcanism suggests that Sapas is the site of second thermal
anomaly.
Regional Geology of Aria Reglo: In order to understand the
relationships between regional texztonism and volcanism, we exam-
ine the geology of the central part of the Atla, concentrating on Ozza
Mona and Maat Mons. Geologic mapping of this area (Fig. 2) shows
it to contain five general units, the most abundant of which are radar-
dark plains. Plains to the northwest of Ozza Mona contain pervasive
sinuous ridges (spacing of 10 to 25 kin) with a general orientation
ofN 30°E, parallel to the trend of Ganis Chasma, and are interpreted
to be compressional in origin [5 ]. These structures may have formed
by the relaxation of topography or may represent surface deforma-
tion linked to large-scale flow in the mantle [5]. To the north of Ozza
Mous the plains are disrupted by faulting and fracturing forming a
150- to 250-km-wide rift, Ganis Chasma. Features mapped as
edifices correspond to the volcanos Sapas Mona, Maat Mons, and
Ozza Montes. An additional volcanic center with a corresponding
large gravity anomaly (+35 mgal) is located on the southwest edge
of Ganis Chasma (15°N, 195°). This region is located along a chain
of gravity highs stretching the length of Ganis Chasma. Lava flows
from the volcanic center lie on the edge of the rift, being deposited
to the southwest, apparently down the rift flanks and do not appear
to contribute to any rift infilling. A number of isolated regions of
https://ntrs.nasa.gov/search.jsp?R=19930005186 2020-03-17T08:42:55+00:00Z
108 lnternational Colloquiumon Venus
180"
i i
5o0 km
190" 200 ° 210"
Fig. 1. Magellan topography of Aria Regio contoured at 1.0-km intervals
and referenced to a planetary radius of 6051.0 kin. The rifts Dall Chuma,
Glmis Chasma,Parga Chasma. and two unnamed rifu trending to the northea._t
are topographically distinct depressions reaching depths of up to 3.0 kin. The
two highest features, Maat Mons and Ozza Mons. reach elevations of 9.2 km
and 7.5 Itrn respectively. The area oudined by the solid line corresponds to the
region shown in Fig. 2.
190 ° 195 ° 200*
200 km
Legend
Edifices
I_Ozza Mons []Maat Mons []Sapas Mons
]Volcanic Centers Other Than Edifices
_ Rifts
[]Plains
Structures
Faults and Fractures
,_ Graben
_:_.:_ Sinuous Ridges
' _i' Impact Craters
• _ Domes
Direction of Flow
Fig. 2. Sketch map of the central part of Aria Regio. Ozza Mons is located at the point where Ganis Chasma, Dali Chasma, and Parga Chasma converge, while
Maat Mons is located astride the western flank of Dali C-'hasma.
LPI Contribution No. 789 109
highly deformed terrain are mapped as tesserae. Unlike Beta Regio,
tesserae does not appear to be a major unit Atla [9].
Ozza Mons forms the central part of Aria and is located at the
point where Ganis, Dali, and Parga Chasma intersect. The relation-
ship between Oanis Chasma and Ozza Mons is examined along with
the general stratigraphic relationships between assemblages of
volcanic units. The rift valley, Oanis Chasma, is unique in that it is
relatively devoid of inf'flling from volcanism and sedimentary
material. The exception to this trend is evident where it merges with
the volcano. At the point where the rift intersects Ozza it narrows
from a width of 225 krn to 125 krn and deformation is con_f'med to
a trough on the flank of the volcano. The primary area of disruption
is a zone of scarps associated withnormal faulting that is offset from
the main trend of the rift, changing orientation from a strike that is
just west of north to one that is just east of north. Additional scarps
with a relatively uniform spacing of 5 Ion are arrayed with a
northeast/southwest orientation on the eastern flank of the edifice
and are associated with an unnamed rift aligned to the northeast. The
Ganis rift terminates at the summit of Ozza, a radar-dark oval
plateau (100 km× 60 kin) that rises to an elevation of 1.5 km above
its surroundings and contains numerous pits and collapse structures
(diameters of 0.5 to 7.0 kin). To the north of the summit, just below
the plateau, lies a dome field (domes 10 km in diameter) covering
an area 80 km × 100 kin. Deposits along the eastern edge o fthe dome
field are superposed on faults and fractures from Ganis Chasma,
indicating that at this location volcanism postdates faulting and
contributes to partial infilling of the rift. Further to the north, where
the rift narrows, other lava flows both superpose and are cross cut
by faults, indicating that rifting and the deposition of volcanics
occurred concurrently.
Four major classes of volcanic deposits are identified on Ozza
Mons; the first two are made up of the radar-dark material on the
summit and the adjacent dome field. The others correspond to flow
units mapped on the distal parts of the volcano, bright flows and
mottled bright flows. Bright flows have a relatively uniform texture,
with no apparent source regions for the deposits observed. In many
places the unit is cut by faults and fractures, deformation associated
with rifting. In comparison, it is possible to identify numerous
apparent source regions for mottled bright flows. These originate
mainly from vents on the outer flanks of the volcano. Like the bright
flow unit it is possible to identify places where the mottled bright
flows are cross cut by faulting. The boundaries between the units
typically form an embayment relationship, with some of the mottled
bright flows being channeled down graben, suggesting that they are
stratigraphically the younger of the two units. In general the dome
field and the mottled bright flows appear to be most recent units.
Maat Mons: Located 600 km southwest of Ozza Mons and
lying on the northwest edge of Dali Chasma is Maat MoPs. In plan
view, this volcano is elliptically shaped (195 km × 120 km) with its
major axis oriented in a northeast/southwest direction. Its summit
region differs from that of Ozza Mons in that it contains a 25-km-
diameter caldera with additional smaller nested pits and collapse
structures (diameters between 1.0 and 5.0 kin), indicating multiple
episodes of magma emplacement and withdrawal. Unlike Ozza
Mons, this volcano is not disrupted by faulting, but instead, along
its southeast flank, appears to be filling the rift.
Volcanic deposits mapped on Maat range from bright to mottled
dark. The most abundant flows have a bright homogeneous texture,
are found on the more distal flanks of the volcano, and are inter-
preted to be associated with the large-scale effusion of lava. Mottled
bright flows have a range of textures on the scale of tens of
kilometers and are found mainly in the area where the rift is being
fdled in. The central part of Maat contains deposits with mottled
dark textures that are arrayed in a pattern radial to the summit. In
several locations the dark material has a diffuse boundary, suggest-
ing the presence of a mantling deposit. One example corresponds to
a cluster of domes (diameters ranging from 5 to 10 kin) located in
a 2.0 km deep depression 250 krn to the north of the summit (3.2"N,
194.9") and mapped as a dome field. The terrain associated with this
area appears etched and is covered by a dark diffuse deposit,
suggesting the possible presence of an air fall deposit associated
with pyroclastic activity. In general the boundaries between the
different flow units are gradational, making stratigraphic relation-
ships difficult to determine.
Summary and Conclusions: Aria Regio is a complex region
of converging rifts and volcanism. The largest (areally extensive)
volcanic center corresponds to Ozza Mons and has characteristics
similar to Theia Mons in Beta Regio [5]. Stratigraphic relationships
at Ozza suggest that both the deposition of volcanics and rifting have
occurred concurrently. Maat Mons is broadly similar to volcanos
identified in Western Eistla Regio (Sif Mons) and contains a well-
defined summit ealdera. On the basis of regional stratigraphic
relationships and the analysis of geophysical data, Aria is inter-
preted to be a site in which a mantle plume, centered near Ozza
Mons, has uplifted plains causing faulting and rifting and volcanic
construction at Ozza Mons and Maat Mons.
In addition to Ozza Mons, Sapas Mons, Ganis Chasma, and the
area mapped as a volcanic center (separate from the large volcanos),
all have substantial positive gravity anomalies. Sapas is interpreted
to be a second site of upweUing adjacent to Aria. On the basis of its
large gravity anomaly and a correstxmding large apparent depth of
compensation (>200 kin) [7] the zone of extension and rifting at
Ganis is intertn_eted to be linked to mantle dynamics. This array of
surface features separate from those at central Aria (Ozza and Maat)
may be associated with multiple upwellings, or possibly a single
large plume that has produced smaller instabilities [10].
References: [1] Senske D. A. (1990) Earth Moon Planets,
50/51, 305-327. [2] Senske D. A. et al. (1991a) Earth Moon
Planets, 55, 163-214. [3] Campbell D. B. et al. (1984) Science, 226,
167-170. [4] Stofan E. R. et al. (1989) GSA Bull., 101,143-156.
[5] Senske D. A. et al. (1992)JGR, 97. [6] Sjogren W. L. et al. (1983)
JGR, 88, 1119-1128. [7] Smrekar S. E. and Phillips R. J. (1991)
EPSL, 107, 582-597. [8 ] Sjogren et al. (1983) JGR, 88, 1119-1128.
[9] Senske D. A. et al. (1991b) GRL, 18, 1159-1162. [10] Griffiths
R. W. and Campbell I. H. (1991) JGR, 96, 18295-18310.
iN93-14375
THE GEOLOGIC MAPPING OF VENUS USING C-I FOR-
MAT: SHEETS 75N254, 60N263. I.V. Shalimov. Lomonosov
Moscow University, Moscow, Russia
The results of geologic mapping of Venus, produced on the base
of Magellan images, are presented. We submit two C-1 format
geologic maps with the appropriate legend.
The mapping territory has been taken from Venera 15 and 16
missions and geologic maps have been composed by Sukhanov et al.
[2] and Kotelnieov et aI. [1]. Magellan images allow us to divide
some types of the plains units to determine the lava flow direction
and to map with better accuracy.
C-I sheets 75N254 and 60N263 are shown in Figs. 1 and 2. The
correlation of the mapped units is shown in Fig. 3. We regret the
difficult perception without the color legend.
References: [1] Kotelnicov V.A. et al. (1989)Atlas of Venus,
GUGK, Moscow (in Russian). [2] Sukhanov A.L. et al. (1989) Atlas


Atla Regio, Venus: Geology and origin of a major equatorial volcanic rise

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