Enhanced surface temperatures and a thinner lithosphere on Venus relative to Earth have been cited as leading to increased lithospheric buoyancy. This would limit or prevent subduction on Venus and favor the construction of thickened crust through underthrusting. In order to evaluate the conditions distinguishing between underthrusting and subduction, we have modeled the thermal and buoyancy consequences of the subduction end member. This study considers the fate of a slab from the time it starts to subduct, but bypasses the question of subduction initiation. Thermal changes in slabs subducting into a mantle having a range of initial geotherms are used to predict density changes and thus their overall buoyancy. Finite element modeling is then applied in a first approximation of the assessment of the relative rates of subduction as compared to the buoyant rise of the slab through a viscous mantle

Burt, J. D.

Head, James W., III

English

NASA Technical Reports Server

rable temperatures, the crystallizationof babingtonitc requires
more hydrous conditions,lower CO 2,and slightlyhigher 0 2lug aci-
tiesinthe fluidphase than ilvaite.Since similartemperatures,CO l
pressures,and oxygen fugacitiesinduced within skarn depositsexist
on Venus, ilvaiteand perhaps babingtonitccould have alsoformed
on the surface of thisplanet by the interactionof the venusian
atmosphere with extruded basalticrocks. One factor that might
mitigate againstthe formation ofthese calcicFel÷-Fe_ silicateson
Venus, however, are the high abundances of Mg and AI measured
during the Venera 13/14 [32] and Vega 2 [33} missions. The Mg 2÷
and A] _ cadons are not accepted into the crystal struc lures of ilvait_
and babingtonite.
Discussion: Although magnetite is generally regarded to be
the predominant ferric-bearing mineral on Venus, other mixed-
valence FeZ+-Fe_ minerals known to exist on the surface of Earth
could be stable in the venusian atmosphere. Thus, in addition to
laihunite (which is probably metastable) and ilv aite and babingtonite
(both of which may be found in rocks depleted of Mg and All, oxy-
amphiboles and oxy-micas may also be major constituents of the
venusian surface. The opacities and high electrical conductiv ities of
such mixed-valence FeZ+-Fc _' silicate minerals, the properties of
which resemble magnetite [34], may also contribute to high radar-
reflectivity regions in the highlands of Venus [35].
References: [1]VolkovV.P.ctal.(19g6)Adv.Phys.Geochem.,
6, 136-190. [2] Gooding J. L. (1986) in The Solar System Observa-
tions and Interpretations (M. G. Kivclson, ed.), 208-299, Prentice-
Hall. [3] Fegley B. Jr. et at. (1992) Proc. LPSC, Vol. 22, 3-19.
[4] Florensky C. P. et al. (1983) LPSCXIV, 203-204. [5] Pieters
C, M. et at. (1986) Science, 234, 1379-1383. [6] Barsu.kov V. L.
et at. (1982)Proc. LPSC 13th, inJGR, 87, A3-A9. [7] Nozcttc S. and
Lewis J. S. (1982) Science, 216, 181-183. [8] Barrels K. S. and
Burns R. G. (1986) Eos, 44, 1270. [9] Bartcls K. S. and Bums R. G.
(1989) Intl. Geol. Congr., 28, 9293. [10] Barrels K. S. and Burns
R. G. (1989)LPSCXX,44--45. [11] Straub D. W. etal. (1991)JGR,
96, 18819-18830. [ 12] Straub D. W. and Burns R. G. (1992) LPSC
XXIII, 1375-1376. [13] Zhang R. et al. (1981) Tscher. Mineral.
Petrog. Mitt., 28,167-187. [14] Schaefer M. W. (1983) Nature,
303, 325-327. [15] Schaefer M. W. (1985) Am. Mineral., 70,
729-736. [16} Kondoh S. ctal. (1985)Am. Mineral., 70,737-746.
[17] Banfield J. F. ctal. (1990) Corttrib. Mineral. Petrol., 106,
11 O-123. [18] lishi K. et al. (1989) NeuesJahrb. Mineral. Mh., 1t6,
245-254. l 19] lishi K. et al. (! 989) NeuesJahrb. Mineral. Mh., 1"18,
345-356. [20] Kitamura M. eta]. (1984)Am. Mineral., 69,154--160.
[21} Shen B. et at. (1986)Am. Mineral., 7l, 1455-1460. [22} Wang
S.-Y. (1980) Geochimica (Chinese), 3, 31--42. [23] Khodakovsky I.
L. et at. (1979) Icarus, 39, 352-363. [24] Barnes V. E. (1930) Am.
Mineral., 15, 393---417. [25] Phillips M. W. et al. (1988) Am.
Mineral., 73, 500-506. [26] Phillips M. W. et al. (1989) Am.
Mineral., 74,764-773. [27] Popp R. K. et al. (1990) Am. Mineral.,
75, 163--I69. [28] Burr D. M. (1971) Annu. Rept. Geophys. Lab.
Yearb., 70, 189-197, [291 Burr D. M. (1971) Soc. Mining Geol.
Japan, Spec. Issue 3, 375-380, [30] Gustafson H. J. (1967) J.
Petrol., 15,455-496. [31] Gole M. J. (1981) Can. Mineral., 19,
269---277. [32] Surkov Y. A. et at. (1984) Proc. LPSC 14th, in JGR,
89, B393-B402. [33] Surkov Y. A. et al. (1986) Proc. LPSC 16th,
in JGR, 91, E215-E218. [34] Burns R. G. (1991) in Mixed.Valence
Systems: Applications in Chemistry, Physics and Biology (K.
Prassides, ed.), 175-200, NATO ASI-C Ser., Math. Phys. Sci., 343,
Plenum. [35] Research supported by NASA grant number NAGW-
2049.
LPI Contribution No. 789 17
THERMAL BUOYANCY ON VENUS: PRELIMINARY RE-
SULTS OF FINITE ELEMENT MODELING, L D. Butt and
J.W. Head, Department of Geological Sciences, B sown University,
Providence RI 02912, USA.
S (:
Introduction: Enhanced surface temperatures and a thinner
lithosphere on Venus relative to Earth have been cited as leading to
increased lithospheric buoyancy. This would limit[l] or prevent [2]
subduetion on Venus and favor the construction of thickened crust
through underthrusting. Undenltrusting may contribute to the for-
marion of a number of features on Venus. For example, Freyja
Montes, a linear mountain belt in the northern hemisphere of Venus,
has been interpreted to be an orogenic belt [3] and a zone of
convergence and underthrusting of the north polar plains beneath
Ishtar Terra, with consequent crustal thickening [41. Such mountain
belts lie adjacent to regions of tessera and contain evidence of
volcanic activity. Tessera is also considered to consist of thickened
crust [5] and crustal underthrusting is one possible mode for its
formation [4]. Models for the formation of the mountain belts and
associated features must then explain compressional deformation
and crustal thickening, as well as melt production.
In order to evaluate the conditions distinguishing between
underthrusting and subduction, we have modeled the thermal and
buoyancy consequences of the subduction end member. This study
considers the fate of a slab from the time it starts to subduer, but
bypasses the question of subduction initiation. Thermal changes in
slabs subducting into a mantle having a range of initial gcotherms
are used to predict density changes and thus their overall buoyancy.
Finite element modeling is then applied in a first approximation of
the assessment of the relative rates of subduction as compared to the
buoyant rise of the slab through a viscous mande.
Subductlon Modeh In the model, slabs, having a thickness set
L
by 90% of the basalt solidus, subduer at a 45 ° angle into the mantle.
The initial geotherms match surface thermal gradients of 10°C/kin,
15°C/Icm, and 25°C/kin [6]. Slabs heat via conduction, crustal
radioactivity, phase changes, and adiabatic compression. Phase
changes involving the conversion of basalt to eclogite at depths of
60 to 160 km and then enstatite to forsterite plus stishovite between
260 and 360 km generate 0.13 × 10 -s ergs/cm 3 s and 0.36 x 10 -s ores/
cm 3 s respectively [7]. The slab radiogenic heat production is 2,63 x
10 -7 ergs/g s [8]. Adiabatic compression adds 0.5°C per kilometer
of depth. Convergence rates ranged from 5 mm/yr to 100 mrrdyr.
The thermal evolution of the slab is followed using a finite
difference technique [7,9]. The model region measures 800 km
horizontally by 400 km deep. Processing ends when the slab tips
reach a 300-km depth, implying time intervals of I 0 m. y. to 100 m.y.
Slab density changes derive from the thermal results through
calculation of the thermal expansion (% = 3 × IO5/"K [8]) and the
effects of pressure (b = I x lO-3/kbar [8]) on initial densities set for
zero pressure and temperature. The assumed initialdensity stnxcture
includes a 10-km or 25-krn basaltic crust (density = 3.0 g/cm_), a
corresponding 25-kin or 65-kin-thick depleted mant|e zone (den-
sity= 3.295 g/cm3), and an underlying undepleted mantle (den-
sity= 3.36 g/cm3). Density changes due to the phase transitions are
also included. Results take the form of density distributions within
the model region.
Finite element modeling is then employed to gauge the rate at
which a slab having the density structure derived above will move
through a viscous mantle. Buoyancy body forces are applied to a
slab having a viscosity of 102t Pa s surrounded by a mantle with a
https://ntrs.nasa.gov/search.jsp?R=19930005114 2020-03-17T08:47:00+00:00Z
| 8 international Colloquium on Venus
viscosity of 10 t9 Pas. Zero stress boundary conditions are applied
to all sides of the model region, while no motion perpendicular to
region boundaries are allowed except at the top.
Results" Figure 1 shows a typical result of the computations
modeling the density changes. Density contours (0. I g/cm 3 spacing)
clearly delineate the slab and its crustal layer. In this case (10°CJkrn
gcotherm, 25 km crust, subduction rate of 5 kndm.y.) the net slab
densities in the region above the basalt-cclogite phase transition arc
lower than their mantle surroundings (the phase change is set for the
density analysis at 110 km depth). Above the 110-kin depth densi-
ties in the crustal portion of the slab arc lower than in the mantle
outside the slab. This causes the net slab density to be less than that
in the surrounding mantle. Below the basalt-eclogit¢ phase change,
net slab densities exceed those in the neighboring mantle. Net slab
buoyancy remains positive until the slab has lengthened to about
2"/5kin.Therea.fter,slabsbecome negativelybuoyant.
haitial/'mite clement results indicate that the positively buoyant
slabs will rise through the mantle at a rate of 5 to 10 km/m.y. This
analysis considers only the instantaneous velocity of the slab and
does not incorporate the full results of the density modeling or the
dynamics of slab subductive descent.
Discussion: Qualitatively, subduction is likely to be enhanced
by negatively buoyant slabs c¢ hindered by slabs that are positively
buoyant, Positive net buoyancy is found above the basalt-eclogite
phase change, tending to oppose subduction. Negative net slab
buoy ancy for the full- length slab was found for all conditions, while
neutral buoyancy was achieved for slabs at alength o f about 275 kin.
Thus, the slab must penetrate deeply into the mant/e before negative
buoyancy can help drive subduction. The rate of the slab's buoyant
rise through the mantle is then important in determining whether the
slab may descend deeply enough to become negatively buoyant.
Preliminary results of _nite element modeling indicate the slab
may rise at rates between 5 and 10 km/m.y. Thus, subducting slabs
will tend to rise into an underthrnsting position if their subducf_on
rate is slow. However, it may be that moderate to high rates of
subduction will overwhelm the buoyant rise of a slab. This could
lead to slabs being forced through the basalt-eclogite phase Iransi-
zion and lo great enough depths to become negalively buoyant, tb us
possibly producing a self-sustaining subduction system.
These initial results must be considered in light of the presump-
tion of subduction made in undertaking the analysis. Some process
still must be found that would carry the slab downward despite its
initial positive buoyancy. Further work will model more closely the
dynamics of the subductive motion of the slab and the effects of the
slab density evolution on slab buoyancy, its rate of rise through the
mantle, and the continuance of subduction.
These results indicate that for all cases of assumed Venus
geotherm a lithospherlc slab whose subduction has been initiated
will hnstead be forced _o underthrust the overriding lithosphere if the
0 200 400 600 BOO
_5
34
3_
3_
1 3
i J
i 1
Fig. 1. Final density distribution for 10°C/kin geothenm, 25 km crust, and 5
km/m.y, subduction rate. Contours have a 0.1 g/cm3 spacing.
subduction rate is slow. This could then lead to crustal thickening,
melting, and volcanism, and possibly provide one model to explain
the association ofcompressional mountain belts and blocks of high-
standing tessera, with apparent flexural rises and f0re, deeps, and
with large volumes of volcanic deposits.
References: [1] Phillips R. J. and Malin M. C. (19g2)in Venus
(D. M. Hunten et al., exls.), 159--214, Univ. of Arizona, Tucson.
[21 Anderson D, L. (1981) GRL, 8, 309-311. [31 Crurapler L. $.
et at. (1986) Geology, 14, 1031-1034. [4] Head I. W. (1990)
Geology, 18, 99-102. [5] Vorder Bruegge R. W. and Head J. W.
(1989) GRL, 16, 699-702, [6] Hess P. C. and Head I. W. (1989)
Abstracts of the 28th International Geological Congress, 2-55.
[7] Minear J. W. and Toksoz M. N. (19"/0) JGR, 75, 139"/-1419.
[8] Turcoue D. L. and Schubert G. (1982) Geodynamics: Applica-
tions of Continuum Physics to Geological Problems, Wiley, New
York, 450 pp. [9] Gerald C. F. (1978) App[iedNumericalAnalysis,
2at edition, AddisonrWesley, Re_ing, Massachusetts.
EROSION VS. CONSTRUCTION': THE ORIGIN" OF VENU-
SIAN CHANNELS. D.B.J. Bussey and J. E. Guest, University
of London Observatory, University Co[/ege London, London NW7
2QS, UK.
Lava channels are a common feature in the volcanic regions of
the Moon, and have now been observed on Venus { 1]. There h as been
much debate about the origin of lunar channels: Are they the result
of erosiona/(either thermal or mechanical) or constractional pro-
cesses? It is necessary to determine the criteria to distinguish
between the different types of channels. The clearest evidence is that
the presence of levees indicates that the channel experienced a
constructiona/phase for a period.
Greeley [2] has proposed that Hadley Rille, on the Moon, was
formed as a leveed channel and lava tube system. Evidence for this
is its location along the crest of a ridge. In addition, Hadley Rille and
other lunar mare sinuous lilies are discontinuous, suggesting that
their origin was, in part, a lava tube th at has subsequently undergone
partial roof collapse. Care [3} and Head and Wilson [4] have argued
that these rilles were produced by lava erosion. For lunar highland
channels, which tend to be larger than their mare counterparts,
mechanical erosion of the megaregolith is a possible process.
Channels of several different types have been observed on the
surface of Venus l11. They are probably formed by more than one
process. They range in size from a few kilometers to over 6800 km
[ 1]. The relatively short ("tadpolelike") channels [5] (e. g., 24 S 347)
appear similar to lunar mare sinuous rilles in morphology. They are
so like certain constructional terrestrial channels (e.g., Kalaupapa,
Hawaii [6]) that it appears reasonable to say that they too are
conslructiona] channels or collapsed lava tube systems.
However, the long sinuous channels referred to by B alter et al. [ 1]
as "canali" pose a different problem in the understanding of their
formation. One example of a channel of this type in the southeast
region of Aphrodite Terra appears to show both erosional and
constructional characteristics. This channel is represented in Fig. 1.
It is approximately 700 km long with an average width of about 1
kin. It drops a distance of 700 m from beginning to end, which means
that the average slope is 0.06 °. Its source may have been a graben
situated at the northwest end of the channel. It appears to have
different origins along its length.
The lack of levees near the source suggests that the channel "
is erosional in this region. An inferred profile is shown as AA' in
Fig. 1.


Thermal buoyancy on Venus: Preliminary results of finite element modeling

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