The Salme ridge belt can be interpreted as being the leading edge of a venusian crustal unit that moved against the highland foreland unit. It is indicative of a compressional zone, with a thrust front facing west. The Salme ridge belt with adjoining structures is an evident indication of lateral stresses and adjoining crustal movements on Venus. It supports the idea of southeast compression against and over the foreland planitia, which has bent under the load and/or lateral stress, resulting in trough and bulge formation in front of the ridge belt. The origin of the driving force for the movements remains masked. Laima Tessera is located in the direction from which the thrust is thought to apply but there are no appropriate candidates for a rift zone although a thrust from the southeast would be in good agreement with structures of Laima Tessera. The temperature gradient suggests that the lithosphere is approximately 12 km thick, while its elastic layer is approximately 3 km thick based either on the load-induced flexure model or on the compressional buckling model

Kauhanen, K.

Raitala, J.

English

NASA Technical Reports Server

LPI Contribution No. 789 93
direction bias, and comparison of the reflectivity and rms slope
parameters will also be presented at the colloquium.
The association of anomalous scattering behavior and impact
parabola features suggests that impact-derived materials are in-
volved. These probably have unique properties among Venus sur-
face materials with respect to their sizedistribution (a large component
of fines?) and possibly with respect to composition or mineralogy.
The presence of streaks suggests that wind processes are involved.
Under a surface change scenario, the variations in SAR backscatter
are suggested to result from redistribution of a layer of loose
material, possibly by wind, during the interval between data acqui-
sitions. However, the scale of the observed differences requires
large volumes of material to be redistributed over a short period of
time. Furthermore, the common occurrence of impact parabola
features on the planet indicates that the age of the population of
parabolas is probably on the order of tens to hundreds of Ma. It
seems unlikely, if these surfaces were vulnerable to such rapid
change, that they would retain their distinctive signatures over such
a time span.
The viewing geometry hypothesis requires surfaces that contain
geometrical elements that favor or diminish back, scatter strength,
depending on the side from which the surface is observed. The very
nearly identical incidence angles (though east- and west-looking),
require that the surfaces have a strong asymmetrical component in
the east-west direction. The dominant asymmetry in the parabolic
features is in the east-west direction (parabolas "open" to the west),
so it may be inferred that structures at a smaller scale have an east-
west component of asymmetry. Aeolian bedforms (ripples, dunes)
are the prime candidates for such structures in the parabola areas.
Terrestrial transverse dunes typically have stoss slopes of 5°-10 °.
and slip face slopes of 300-35 °. The absence ofthe"speckly" returns
commonly observed in SAR images of dunes implies that the
bedforms responsible for the extensive bright patches contain faces
at scales of tens of centimeters to no more than a few tens of meters.
The visibility of aeolian bedforms in SAR imagery is known to be
highly sensitive to look azimuth, relative to the dominant strike
direction of slopes [4]. In the high southern latitudes discussed here,
the look azimuth between cycles differed by -160 ° , which may
further affect the visibility of features. Several difficulties remain
with the bedform hypothesis. The large enhancement in backscatter
on steep faces should have a corresponding smaller enhancement on
the shallow faces. This effect is not observed. "me reversal in
"sense" of the anomalies is also somewhat inconsistent with bed forms
developed within a wind regime dominated by one persistent
direction of flow, necessary forsuch widespread, consistent slope
distributions.
References: [1] Arvidson R. E. et al. (1991) Science, 252,
270-275. [2] Campbell D. B. et al. (1992) JGR, special Magellan
issue, in press. [3] Pettengill G. H. et al. (1991) Science, 252,
260--265. [4] Blom R. G- f198"g) [d[. J.'Rem. Seas.. 9, 945-965.
N93-14364 !
CYTHEREAN CRUSTAL BENDING AT SALME DORSA.
J. Raitala and K. Kauhanen, Department of Astronomy, University
of Oulu, Finland.
The horseshoe-like,narrow (I00 km wide and 600 km long)
SalineDorsa consistsof arcuateridgesand grooves opening south-
easton the planitia to the south of Ishtar Terra. Magellan radar data
was studied in order to find tectonic style and lithospheric thickness
of the area. The S alme ridge belt indicates folding and thrust faulting
of surface layers due to compression against the tessera foreland
zone. The western edge of Saline Dorsa is scarplike. Most ridges
follow the main course of the arc and the overall ridge orientation
is north-south while ridges subparallel the arcuate edge. The ridge
trend at the northeastern end is northeast-southwest, while the
southern part of the horseshoe has more northwestern ridges. The
ridge belt has elements of normal compression against the zone. It
has widened due to the new ridge formation. As this process repeats
itself, the crust also becomes thicker making the topographically
high ridge belt act as a load.
The scarp along the western edge of Saline Dorsa indicates that
the movement has been to the west or northwest. West of Saline
Dorsa the crust has bent due to the load and/or thrust of Saline Dorsa,
resulting in a trough outside the scarp. The compressional ridge belt
has acted either as a nappe overthrust or as a mere surface load due
t I O0 km I
C
;4 Xb 0
Fig. 1. Radar image of the crustal bending at central Saline Dorsa (a). The load and compression/tension stress system is displayed (b) and modeled
with vertical exaggeration (c). The trough depression is next to a surface load and/or compres sional massif. Grabens ate located on the anticlinal bulge.
https://ntrs.nasa.gov/search.jsp?R=19930005176 2020-03-17T08:44:37+00:00Z
t94 International Colloquium on Venus
to crustal thickening under stress from the east or southeast. Saline
Dorsa cuts through the ridges of older Signm Fossae. Saline Dorsa
has quite a recent border scarp and is obviously relatively young.
Interlocking nearby ridge belts indicate repeated compressions.
The trough and bulge west of Salme Dorsa is caused by crustal
bending due m the ridge belt load or nappe thrust to the west.
Tensional grabens along the crest of the bulge indicate crustal
extension. The grab_s parallel the ridge belt and trough as gentle
arcs that open in the direction of Saline Dorsa on the bulge crest.
Elastic deformation of the layered crust and adjoining fracturing of
the uppermost brittle surface might be reasonable assumptions. The
set of narrow grabens on top of the bulge are due to excess tensile
stress in the uppermost brittle layer of the lithosphere. The lack of
corresponding troughs, bending, or grabens on the eastern side of
the ridge belt may indicate that both the load and the thrust from the
east have to be taken into accounL The volcanic area inside the
Saline Dorsa horseshoe have also weakened the crust on that side.
The elastic layer is confined by the temperature above which the
upper mantle has negligible strength [1]. The elastic pan of the
lithosphere is defined by isotherms 450°C and 650°(2 and this elastic
lithosphere is considerably thinner than the low-attenuation seismic
lithosphere. The temperature gradients of 15°C/kin [2] and 20°C/
km [3] and the surface temperature of 470°C suggest that the lower
boundary of the elastic lithosphere is about 12 km or 9 km deep.
respectively.
The thickness of the elastic layer is estimated using a flexural
approach and a two-dimensional model of a semi-infinite broken
elastic lithosphere under a linear load [4] with the only acting force,
V0, applied vertically to its end, where the bending moment is zero.
The only measurable quantity, x b, is the distance between the force
and the bulge. The equation for the deflection of a plate includes
loading of the lithosphere by vertical forces, hydrostatic restoring
force, and the position of the bulge. Assuming a basaltic composi-
tion for the crust, re =3000 kgm -3, E = 0.6 x 10 It Pa and x) = 0.25
and using rm = 3300 kgm -3, g = 8.6 ms -2 and xb = 50 km we find
h _ 2.9 km for the elastic thickness of the lithosphere.
Variations in the magnitudes of the vertical or horizontal loads
or the bending moment will alter the displacement but will not alter
the position of the top of the bulge. Changes in the model, applica-
tion of a horizontal force or a bending moment, will have more
dramatic effects on the distance of the bulge and the lithospheric
thickness. If a bending moment is assumed, the thickness of the
lithosphere increases due to the increasing effect on plate curvature.
The free edge boundary of our model is justified by volcanic activity
that has weakened the lithosphere. Both a three-dimensional model
and a continuous elastic plate model will reduce the elastic thick-
ness. Allowing a bending moment to act upon the plate end, the
elastic thickness of the lithosphere increases by a factor of 2.
The elastically thick lithosphere can support high compressional
stresses or fail by faulting rather than buckling. Compressional
stress critical to deformation can be estimated [4] to be approxi-
mately 0.4 GPa, taking the previous values. The wavelength of the
buckling at the critical stress is about 94 km, and is reduced as the
stress increases. This value corresponds well to the distance be-
tween Saline Dorsa and the bulge, and thus horizontal forces cannot
be totally neglected. Recent research has revealed that buckling of
the lithosphere can occur at stress levels much less than the elastic
strength of the lithosphere [5]. Horizontal forces may have contrib-
uted to the buckling but it is difficult to find out which one of the
forces has been active for a thickness of the elastic lithosphere of
about 3 kin. If both forces are allowed [6], the thickness of this layer
of the lithosphere is slightly increased to 3.1 km.
Summary: The Saline ridge belt can be interpreted as being
the leading edge of a venusian crustal unit that moved against the
highland foreland unit. It is indicative of a compressional zone, with
a thrust front facing west. The Saline Dorsaridge belt with adjoining
structures is an evident indication of lateral stresses and adjoining
crustal movements on Venus. It supports the idea of southeast
compression against and over the foreland planitia, which has bent
under the load and/or lateral stress, resulting in trough and bulge
formation in front of the ridge belt. The origin of the driving force
for the movements remains masked. Laima Tessera is located in the
direction from which the thrust is thought to apply [7] but there are
no appropriate candidates for a rift zone although a thrust from the
southeast would be in good agreement with structures of Laima
Tessera. The temperature gradient [2] suggests that the lithosphere
is approximately 12 km thick, while its elastic layer is approxi-
mately 3 km thick based either on the load-induced flexure model
or on the compressional buckling model.
References: [1] Kirby S. H. (1983) Rev. Geophys. Space
Phys., 21, 1528-1538. [2] Smrekar S. E. and Solomon S. C. (1992)
Workshop on Mountain Belts on Venus and Earth, 35-37.
[3] Solomon S. C. and Head J. W. (1989)LPSCXX, 1032-1033.
[4] Turcotte D. L. and Schubert G. (1982) Geodynamics: Applica-
tions of Continuum Physics to Geological Problems, Wiley,
New York, 450 pp. [5] Stephenson R. A. and Cloetingh S. A. P. L.
(1991 ) Tectonophysics, 188, 27-37. [6] McAdoo D. A. and Sandweil
D. T. (1985) JGR, 90, 8563---8569. [7] Raitala J. and Tormanen T.
(1990) Earth Moon Planets, 49, 57--83.
N93-14365 ,
!
: : ( /
COMPUTER SIMULATIONS OF COMET- AND ASTER-
OIDLIKE BODIES PASSING THROUGH THE VENUSIAN
ATMOSPHERE--PRELIMINARY RESULTS ON ATMO-
SPHERIC AND GROUND SHOCK EFFECTS. D. Roddy _.
D. Hatfield _, P. Hassig 2, M. Rosenblatt 2, L. Soderblom z, and E. De
Jong 3, zU.S. Geological Survey, Flagstaff AZ, USA, 2California
Research & Technology, Chatsworth CA, USA, 3Jet Propulsion
Laboratory, Pasadena CA, USA.
We have completed computer simulations that model shock
effects in the venusian atmosphere caused during the passage of two
cometlike bodies 1130m and 1000 m in diameter and an asteroidlike
body 10 km in diameter. Our objective is to examine hypervelocity-
generated shock effects in the venusian atmosphere for bodies of
different types and sizes in order to understand (1) their deceleration
and depth of penetration through the atmosphere and (2) the onset
of possible ground-surface shock effects such as splotches, craters,
and ejecta form ations. The three bodies were chosen to include both
a range of general conditions applicable to Venus as well as three
specific cases of current interest.
These calculations use a new multiphase computer code
(DICE-MAZ) designed by California Research & Technology for
shock-dynamics simulations in complex environments. The code
has been tested and calibrated in large-scale explosion, cratering,
and ejecta research. It treats a wide range of different multiphase
conditions, including material types (vapor, melt, solid), particle-
size distributions, and shock-induced dynamic changes in veloci-
ties, pressures, temperatures (internal energies), densities, and other
related parameters, all of which were recorded in our calculations.
DICE-MAZ is especially useful in our Venus study because of the
advance capability in muhiphase adaptive zoning and because of the
color coding associated with displaying the complex variations in


Cytherean crustal bending at Salme Dorsa

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