Large-scale mapping of tectonic structures on Venus shows that there is an organized global distribution to deformation. The structures we emphasize are linear compressive mountain belts, extensional rafted zones, and the small-scale but widely distributed wrinkle ridges. Ninety percent of the area of the planet's compressive mountain belts are concentrated in the northern hemisphere whereas the southern hemisphere is dominated by extension and small-scale compression. We propose that this striking concentration of fold belts in the northern hemisphere, along with the globe-encircling equatorial rift system, represents a global organization to deformation on Venus

Bilotti, Frank

Connors, Chris

Suppe, John

English

NASA Technical Reports Server

10 International Colloquium on Venus
measurements indicate low contents of K, U, and Th, typical for
thole]]tic basalt, [18,4].
Vega 2 landed at the transitional zone between Rusalka Plarfitia
and the eastern edge of Aphrodite Terra rise. The landing circle is
centered at 7.14°S, 177.67°E, about 1500 kin south of the Vega 1
landing site. Most of the landing site consists of a densely fractured
radar-bright plain with radar-dark plain to the northeast. The
southwestern portion of the site has ridges and fractures associated
with a 300-kin-diameter eoronalike feature. No television panora-
mas were made at the site but both gamma ray spectroscopy and
X-ray fluorescence spectroscopy measurements were taken. The
gamma ray measurements of K, U, and Th and the X-ray fluores-
cence spectroscopy measurements of bulk chemistry indicate a
composition dose to thole]]tie basalt [ 18,4].
Discussion: We have shown that within all the Venera/Vega
sites the dominant type of terrain is plains. These plains include (1)
mottled (Venera 8, 10, and Vega 1), (2) homogeneously dark
(Venera 13), (3) dominated by prominent lava flows (Venera 14),
and (4) fractured (Venera 9 and Vega 2). The plains are associated
with coronalike features (Venera 8(?), 13, 14, and Vega 2), fracture
belts and swarms (Venera 8, 13), and tesserae (Venera 8(?), 9, 10).
This diversity reflects the global diversity of the venus]an plains.
For the two sites (Venera 8 and 13) where nontholeiitic compo-
sitions of the surface material were determined, the steep-sided
domes resembling those described by Head et al. [11] and Pavri et
al. [12] have been found. For the five sites where geochemical
signatures of tholeiitelike basalts were identified (Venera 9, 10, 14,
and Vega 1.2), these steep-sided domes have not been observed
inside the landing circle. We believe that this correlation favors a
nontholeiitic origin for these steep-sided domes.
The television panoramas from the Venera 9, 10, 13, and 14
landers show a microlandscape with characteristics that correlate
well with the Magellan observations. At these four sites, the
panoramas all showed bedded outcrops. Before the Magellan mis-
sion, these bedded rocks were considered as either basaltic tufts or
lavas [14,15]. Magellan imagery does not resolve the lava vs. tuff
issue, although the presence of prominent lava flows at the Venera
14 site favors a lava interpretation of the bedded rocks. The very low
bulk density of these rocks estimated from Venera 13 and 14
overload measurements (! .5 g/cm 3) favors the tuff interpretation.
Therefore, the origin of these bedded rocks still remains an enigma.
Conclusion: Joint analysis of Magellan and Venera/Vega data
on the Venera/Vega landing sites has shown that the panoramas and
geochemistry measured by the landers agree well with the morphol-
ogy seen in Magellan imagery of the site. This observ at]on suggests
that it is possible to extrapolate our geochemical and morphologic
results to the remaining Magellan imagery of Venus.
References: [1] Akim E. L. and Stepanyantz V. A. (1992)
Astronornichesky Vestnik (in Russian), in press. [2] Vinogradov et
al. (197 3 ) lcarus,2 0,25 3-259. [3 ] Surkov et al. (197 6 ) Kosmicheskeye
Issledovaniz, 14 (5), 704-709 (in Russian). [4] Surkov et al. (1986)
Proc. LPSC 17th, in JGR, 91, E215-E218. [5] Nikolaeva O. V.
(1990) Earth Moon Planets, 50/51,329-342. [6] Surkov Yu. A. and
Fedoseev G. A. (1978) in Advances in Space Research in Soviet
Union, 313-351, Nauka, Moscow (in Russian). [7] Taylor S. R. and
McLerman S. M. (1985) The Continental Crust: Its Composition
and Evohaion, Blackwell, Oxford, 312 pp. [8] Taylor S. R. (1989)
Tectonophysics, 161,147-156. [9] Taylor S. R. (1991)Nature,350,
376-377. [10] Basilevsky et al. (1992) JGR, in press. [1 I] Head et
al. (1992) JGR, in press. [12] Parr] et al. (1992) JGR, in press.
[ 13] Basilevsky A. T. and Weitz C. M. (1992) LPSC XXIII, 67-68.
[14] Florensky et al. (1977) Bull. GSA, 88, 1537-1545.
[15] Basilevsky et al. (1985) Bull. GSA, 96, 137-144. [16] Surkov
et al. (1984) Proc. LPSC 14th, in JGR, 89, B393--B402. [ 17] Ivanov
M. A. (1992) LPSC XXIII, 579--580 [18] Barsukov et al. (1986)
Geok.himia, 3,275-288 (in Russian).
.p. ,2 c_i
GLOBAL DEFORMATION ON THE SURFACE OF VENUS.
Frank Bilotti, Chris Cormors, and John Suppe, Department of
Geological and Geophysical Sciences, Princeton University.
Princeton NJ 08544, USA.
Large-scale mapping of tectonic structures on Venus shows that
there is art organized global distribution to deformation. The struc-
tures we emphasize are linear compressive mountain belts, exten-
sional rifted zones, and the small-scale but widely distributed
wrinkle ridges. Ninety percent of the area of the planet's compres-
sive mountain belts are concentrated in the northern hemisphere
whereas the southern hemisphere is dominated by extension and
small-scale compression [1,2]. We propose that this striking con-
centration of fold belts in the northern hemisphere, along with the
globe-encircling equatorial rift system, represents a global organi-
zation to deformation on Venus. A great circle that connects the
northernmost branches of a globe-encircling rift system roughly
separates the tectonic hemispheres [ 1 ] (Fig. 1). South of this tectonic
equator [1] there are a few well-formed fold belts at the edges of
small crustal blocks within the global rift system, but the globally
significant deformation is the rift system and the wrinkle ridges.
Compressional structures on Venus can be divided into two
major styles: linear fold belts and wrinkle ridges. Figure 2 shows
that the major difference between these two styles is the distribution
and localization of strain. Wrinkle ridges represent deformation that
is distributed over thousands of kilometers whereas fold belts show
concentrated deformation along narrow bands. Venus is the only
terrestrial planet other than Earth that has linear compressive
mountain belts [3]. Venus]an fold belts, similar to those at plate
margins of the Earth, are the dominant compressional structures in
the northern tectonic hemisphere. They are typically about 100 km
, wide and thousands of kilometers long. The relatively sm all amount
of erosion on Venus allows us to image fault-related folds forming
over regionally extensive deeollement horizons within the belts [4].
This supports the notion that these mountain belts are analogous to
the thin-skinned fold-and-thrust belts of Earth. Wrinkle ridges are
the dominant style of compressional deformation in the southern
lit,-., t
@: \
Fig. I. Generalized tectonicmapofVenus after 131.Rift zones are indicated
by the heavy black lines. Ninety percent of the area of foldbelts lies in the
noahern hemisphere while the southern teclonic hemisphere is dominated by
extension and small-scale compression in the form of wrinkle ridges.
https://ntrs.nasa.gov/search.jsp?R=19930005109 2020-03-17T08:49:36+00:00Z
12 International Colloquium on Venus
Furthermore, there is a high correlation between wrinkle ridge
trends and long-wavelength topography. Within these trends we
observe quite regular interridge spacing of 15-30 kin. Preliminary
studies of wrinkle ridges reveal that individual structures may
accommodate up to 1.5 km of shortening, Therefore the distributed
shorteningin the southern plainsthatisaccorranodated by wrinkle
ridges isabout I-5%. A higher-resolutioninvestigationof wrinkle
ridges isnecessary torefine theseresultsmad gain a betterunder-
standing of the stresses involved in wrinkle ridge formation on
Venus. Thus the low-intensity wrinkle ridge deformation of the
plains has consistent patterns that approach a global scale.
References: [l]SuppeLandConnorsC.(1991)Workshopon
Mountain Belts on Venus and Earth, [2] Bilotd F. and Suppe J.
(1992) LPSC XXIII, 102. [3] Solomon S. C. and Head J. W. (1991)
Science. 252,252-260. [4] Suppe L and Cormors C. (1992) JGR. 97,
ha press. [5] Solomon S. C. et at. (1992) JGR, 97, in press.
[6] Squyres S. W. et el. (1992) ]GR. 97, in press. [7] Frank S. H. and
Head/. W. (1990) Earth Moon Planets, 50/51,421-470. [8] Suppe
J. and Cormors C. (1992) LPSC XXIII, 1389-1390, [9] Morgan
W. J. and Morgan L P. (1991) AGU Progr. w/Abslr., 72,284.
...... • , _:s'_ ,:.
COLDSPOTS OR HOTSPOTS? THE ORIGIN OF PLATEAU-
SHAPED HIGHLANDS ON VENUS. D.L. Bindschadlcr, De-
partment of Earth and Space Sciences, University of California, Los
Angeles CA 90024. USA.
A compelling question for the terrestrial planets is the origin of
the highland regions on Venus. Data on the topography, gravity
signature, and surface morphology returned by the Pioneer Venus,
Venera 15/16, and Magellan spacecraft represent a basis for divid-
ing these highlands into two distinct groups: volcanic rises and
plateau-shaped highlands [I ]. Volcanic rises are generally thought
to be due to mantle upwellings in the form of large mantle plumes
[2] and are thus similar in origin to terrestrial hotspots. There is less
agreement as to the origin of plateau-shaped highlands (PSH)
[1,3,4], Coldspot mande downwelling can lead to the formation of
a highland region under Venus conditions [3], and previous to
Magellan some PSH (particularly W. Ishtar Terra and erda and
Thetis Regiones) were suggested to be compressionally deformed
regions of thickened crust created by mantle downwelling [5,6,7].
A hotspot model proposes that suchregions are formed by magmatism
and tectonism related to the near-surface ascent of either the diapir-
shaped large mantle plume [8] or a solitary disturbance propagating
up a plume conduit [9]. The intent of this absa-act is to (1) briefly
review the characteristics of both volcanicrises and plateau-shaped
highlands on Venus and the models for their formation and (2)
consider tests that may help to make clear which model best explains
the plateau-shaped highlands.
Characteristics of Venus Highlands: Highlands on Venus
can be divided into volcanic rises and plateau-shaped highlands
(PSH) on the basis of their topography, long-wavelength gravity-
topography relationships, and geologic featazres (Table 1). Volcanic
risesare characterized by roughly circulartopographic riseswith
domical cross sections,relativelylarge gcoid-topography ratios
(GTRs), and the presence of largevolcanic constructs,widespread
flows,and extensional tectortism(commonly manifested asradially
trending rifts).They includeBeta,Atla,Bell,lrndr.and Western and
Central Eistla Regiones. Plateau-shaped highlands (PSH) are so
named fortheirshape incrosssectionsand are characterized inplan
view by a varietyof shapes, by steepmargins and rugged interiors.
by lower GTRs than volcanic rises,and by surfaces dominated by
the heavily deformed complex ridged terrain(or tessera),They
generally appear to share a common sequence of tectonism: short-
ening (commonly margin-parallel) followed by relatively small
amounts (<10%?) of extension [1 ]. These regions include erda,
Thetis, Alpha, Tellus, and Phoebe gegiones, Fortuna and Laima
Tesserae, and W. Ishtar Terra [ I ]. The inclusion of W. lshtar Terra
is arguable given its large apparent depth of compensation (ADC)
[10] and GTR values as well as the presence of extensive volcanic
plains and two large volcanic constructs (see Table I). For the
purposes of this work, W. lshtar Terra is considered a PSH on the
basis of its topography and the overwhelmingly compressional
tectonics manifested in its orogenic belts.
Models for Plateau-shaped Highlands: Observations that
must be explained by a successful model for PSH include topogra-
phy, long-wavelength gravity anomalies, and surface morphology.
The latter includes the abundant deformational features that domi-
nate their surfaces. A successful model should therefore explain the
style(s) of deformation (i.e., compression, extension, etc.) and the
reladve ages and geometries of these various structural features,
information that can be derived from Magellan images because of
TABLE I. Characteristics of Venus highlands.
Characteristic Volcanic Rises Plateau-shaped Highlands
Topographic cross section
Topographic planform
Gravity anomaly
GTR/ADC values
Volcanicfeatures
Extensional tectonics
Compressions[ tectonics
Domica[
Circular to elongate
Positive, centrally located
-20--35 m/kin
~150--300 km
Shield volcanos,
ext_sive plains, and flows
Rifts, numerous graben,
and fractures
Largely absent
Steep-sided, rugged interior
Circular to polygonal
Positive or negalive, offset from center
-10.-15 m/kin
-30-90 km
Minor plains, flows,and domes
Small (<10 kin), late-stage graben
Broad (10-20 kin) ridges,
marginal bells


Global deformation on the surface of Venus

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