NASA Technical Reports Server (NTRS) 19930005144: Venus tectonic styles and crustal differentiation

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LPI ContributionNo. 789 55
solar radiation, the actual solar flux requh'ed to create "moist" or oI(v)=MI-_ (I)
"runaway" conditions would be higher than the values quoted
above. (Indeed, some authors [12] have argued that cloud feedback where ot(v ) is the rms magnitude of anormalized spherical har-
would prevent arunaway greenhouse from ever occurring.) Early in monic coefficient of degree 1.A concentrated flow, characterized
solar system history, solar luminosity was about 25% to 30% less by large segments moving together, has asteep slope, thence ahigh
than today, puu.ing the flux at Venus' orbit in the range of 1.34 So valun of n, while adistributed flow, with small segments,has asmall
to 1.43 So. Thus, it is possible that Venus had liquid water on its value of n. We cannot measure velocities directly on Venus. But in
surface for several hundred million years following its formation. aplanet dominated by astrong outer layer, inwhich the peak stresses
Paradoxically, this might have facilitated water loss by sequestering are at a rather shallow depth, the magnitudes of gravitational
atmospheric CO 2in carbonaterocksand by providing an effective potential V and poloidal velocity v, are coupled [3]
medium for surface oxidation.
Continued progress in understanding the history of water on M(_V)/M (vt) = 12JrG_q/g (2)
Venus requires information on the redox state of the aunosphere and
surface.The lossofan ocean ofwater (orsome fraction thereof) where xl is the effective viscosity of the lithosphere, the ratio of
shouldhave leftsubstantialamounts ofoxygen behindtoreactwith stress to strain rate over long durations. The value inferred from the
the crust. This oxygen would presumably be detectable if we had magnitudes M for Earth is4× 10_1Pa-s, probably most influenced
core samples of crustal material. Barring this, its presence or by subduction zones. Support for this model isthat the gravity and
absence might be inferred from accurate measurements of lower velocity spectra on Earth have the same slope n to two significant
atmospheric composition. Another spacecraft mission to Venus figures, 2.3 [3,4]. On Venus the spectral slope of gravity, n(_V), is
could help to resolve this issue and, atthe same time, shed light on appreciably lower over degrees that can be determined reli-
the question of whether clouds will tend to counteract global ably-about 1.4 [4], strongly suggesting a more regional, less
warming on Earth. global, velocity field than on Earth.
References: [1] Donahue T. M. and Hedges R. R. Jr. (1992) Abasic constraint on the velocity field that issomewhat indepen-
Icarus, in press. [2] Lewis J. S. (19"/2) Icarus, 16, 241-252. dent of stresses, and thence theology, is that, at the mantle depth
[3] Wetherill G. W. (1980) Annu. Rev. Astron. Astrophys., 18, where convection dominates---more than 150 _ere must be a
77-113. [4] Prima R. G. and Fegley B. Jr.(1989) In Origin and correlation of vertical velocity vr(coupled to thepoloidal velocity
Evolution of Planetary and Satellite Atmospheres (S. K. A_eya vs by continuity) and temperature variations AT that lead to an
et al., eds.), 78-136, Univ. of Arizona, Tucson. [5] Donshue T. M. integral accounting for most of the total heat delivery Q from greater
et at. (1982) Science, 216, 630-633. [6] McElroy M. B. etat. (1982) depths
Science, 215, 1614-1618. [7] C-rinspoon D. H. and Lewis J. S.
(1988) Icarus, 74, 21-35. [8] Kasting J. F.et at. (1984) Icarus, 57, Q = JpCveATdS (3)
335-355. [9] Kasting J.F.(1988)Icarus, 74,472-494. [10] Kasdng
J. F. and Pollack L B. (1983) Icarus, 53,479--508. [1 l] Kumar S. For a mean heat flow of 60 mW/m 2 and average temperature
et al. (1983) Icarus, 53, 369-389. [12] Ramanathan V. and Collins variationAT of100°(2,equation (3)givesanestimateof0.6cm/yr
W. (1991) Nature, 351, 27-3-2. for vr.In the Earth, plate tectonics lead to such concentrations of vr
N93-14332 and AT atshallower depths that itisdifficult to draw inferences from
observed heat flow relev ant toequation (3). However, the constraint
VENUS TECTONIC STYLES AND CRUSTAL DIFFEREN- exists, and its implication for the velocity spectrum of Venus should
TIATION. W. M. Kaula and A.Lenardic, University of California, be explored.
The altimetry and imagery of Venus also indicate a regional@
Los Angeles CA 90024, USA. p_
! of Venus tectonics, even though magnitudes of velocities cannot be
Two of themost important constraints arcknown f_m Pioneer inferred because of dependence on unknown viscosity. For ex-
Venus data: the lack of a system of spreading rises, indicating ample, Maxwell Montes iscomparable to the Andes in height and
distributed deformation rather than plate tectonics; and the high steepness (suboceanic). But the material subducted under the Andes
gravity:topography ratio, indicating the absence of an astheno- clearly comes from the southeast Pacific Rise, over 4000 km away
sphere. In addition, the high depth:diameter ratios of craters on (despite the interruption of the Nazca Rise), while only 500 km from
Venus [1]indicate that Venus probably has no more crust 111anEarth. the Maxwell front is ascarp, and beyond that a much more mixed,
The Problems of the character of tectonics and crustal formation and apparently unrelated, variety of features. Clearly, Maxwell ismore
recycling are closely coupled. Venus appears to lack arecycling local than the Andes. A significant difference of Venus tectonics
mechanism aseffective assubduction, but may also have alow rate from Earth is the absence of erosion, which removes more than
of crustal differentiation because of amantle convection pattern that 1km/100 m.y. from uplands.
ismore "distributed," less "concentrated," than Earth's. Distributed Hypotheses for why Venus does not have crustal formation in a
convection, coupled with the nonlinear dependence of volcanism on ridge system, but rather amore distributed magmafism correlated
heat flow. would lead to much less magmatism, despite only with amore regional tectonism, include (1) the lack of plate pull-
moderately less heat flow, compared to Earth. The plausible reason apart due to inadequate subduction; (2) the lack of plate pull-apart
for this difference inconvective style is the absence of water inthe due to drag on the lithosphere from higher viscosity: i.e., no
upper mantle of Venus [2]. asthenosphere; (3) the lesser concentration of flow from within the
The most objective measure of the nature of motion that we can mantle, also due to higher viscosity; (4) lower temperatures, due to
hope to infer isthe spherical harmonic spectrum of its surface, or less initial heating and more effective retention of lithophiles in the
near-surface, velocities. A compact expression of this spect_-um is crust; (5) higher me]ring temperatures, due to lack of water conlcnt,
aspectxal magnitude M and slope n and (6) lower mobility of magma relative to matrix, due to (a) low
56 International Colloquium on Venus
HzO content. Co)low-density overburden, or (c) lesser horizontal with diameters greater than 1(30kin. axe the sites of some of the most
length scale of flow. voluminous eruptions. Head et al. [1] have identified 158 of these
Relevant to these hypotheses is that differentiation on Venus structures. Their spatial distribution isneither random (see Fig. 1)
rexluires more than adiabatic upwelling and pressure-release melt- nor arranged ha linear chains as on the Earth; large volcanos on
hag. In models of this process on Earth [e.g.. 5,6] the availability of Venus are concentrated ha two large, near-equatorial clusters that
mantle material to flow into the region is taken as given; the concern are also the site of many other forms of volcanic activity [1].
isabout the rising of magrna within asolid matrix, inparticular, the The set of conditions that must be met on Venus that controls the
mechanism(s) concentrating the melt in anarrow vertical slab. The change from widespread, distributed volcanism to focused, shield-
viscosity of the melt, and hence its ability to separate, isaffected by building volcanism isnot well understood. Future studies of transi-
water content. Differentiation from a plume, without pull-apart, tional features will help to address this problem. It is likely,
under alayer of higher strength or lower density (as ismore likely however, that the formation and evolution of aneutral buoyancy
on Venus), requires asurplus of heat, and thus is likely to lead to a zone (NBZ) plays an important role inboth determining the style of
lower rate of magmatism for agiven heat flow. Also the inhibition the volcanism and the development of the volcanic feature once it
of upward flow by alow-density overlying layer may lead to less has begun toerupt. Head and Wilson [2] have suggested that the high
differentiation of crust. Application of models of aplume under a surface pressure on Venus may inhibit volatile exsolution, which
lithosphere [7] to Venus features such as Atla and Beta indicate that may influence the density distribution of the upper crust and hence
appreciably higher upper mantle viscosities may cause pressure control the nature and location of a NBZ. The extreme variations In
gradients to account for these great peaks in the geoid. pressure with elevation may result hasignificantly different charac-
We have applied fhaite element modeling to problems of the teristics of such a NBZ at different locations on the planeL Inorder
interaction of mantle convection and crust on Venus [8]. The main to test these ideas regarding the importance of NBZ development in
emphasis has been on the tectonic evolution of Ishtar Terra, as the the evolution of a large shield and to determine the style of
consequence of convergent mande flow. The early stage evolution volcanism, three large volcanos that occur at different basal eleva-
is primarily mechanical, with crust being piled up on the down- tions were examined and the distribution of large volcanos as a
stream side. Then the downflow migrates away from the center. In function ofaltitude was determined.
the later stages, after more than 100 m.y., thermal effects develop The evolution of Sapas Mons, a 600-km-diameter shield vol-
due to the insulating influence of the thickened crust. An important cano, was studied [3]. Six flow units were identified on the basis of
feature of this modeling isthe entrainment of some crustal material radar properties and spatial and temporal relations. The distinctive
in downflows. variation between units was attributed to the evolution of magma
An important general theme in both convergent and divergent ha a large chamber at depth. The presence of summit collapse
flows isthat of mixing vs. stratification. Models of multicomponent suructures and radial fractures, interpreted to be the surface expres-
solid-state flow obtain that lower-density crustal material can be sion of lateral dikes, supports this suggestion. Theory predicts that
entrained and recycled, provided that the ratio of low-density to voleanos located at the altitude of Sapas Mons should have large
high-density material is small enough (as in subducted slabs on magma chambers located at zones of neutral buoyancy at relatively
Earth). The same considerations should apply in upflows; asmall shallow depths beneath the subsa'ate [2]. Not ordy does the evidence
percent partial melt may be carded along with its matrix and never from the flow units suggest that such azone ispresent, but the size
escape to the surface. Models that assume melt automatically rising ofthe summit collapse and the near-surface nanL,'e of the radial dikes
to the crust and no entrainment or other mechanism of recycling indicates that the chamber is both large (on the order of 100 km in
lower-density material [e.g., 9] obtain oscillatory behavior, because diameter) and shallow.
it takes a long time for heat to build up enough to overcome a In comparison to Sapas Mons, two other volcanos at different
Mg-rich low-density residuum. However, these models develop elevations were examined. Theory predicts that the volcano at the
much thicker crust than consistent with estimates from crater
depth:diameter ratios [1].
References: [1] Sharpton V. L. and Edmunds M.S. (1991)Eos, 2_ i I 1 t I I I I I I I I _ I I I I
72,289. [2] Kaula W. M. (1992a) Proc. IUGG Syrup. Chem. Evol. • expected#
Planets, in press. [3] Kaula W. M. (1980) JGR, 85, 7031. [4] Kaula 20 [] actual #
W. M. (I 992h) Proc. IAGSymp. Gray. FieldDet. Space Air Meas.,
in press. [5] McKenzie D. P, (1985)EPSL, 74, 81. [6] Scott D. R.and
Stevenson D. J. (1989) JGR, 94. 2973. [7] Sleep N. H. (1990) JGR,
95, 6715. [8] Lenardic A. etal. (1991) GRL, 18, 2209. [9] Parmentier
E. M. and Hess P. C. (1992) LPSCXXII1, 1037
,N93-14333
LARGE SHELD VOLCANOS ON VENUS: THE EFFECT OF
NEUTRAL BUOYANCY ZONE DEVELOPMENT ON EVO- 0 iII l I I I I I I I I I I I I I I 1I 1IL
LUTION AND ALTITUDE DISTRIBUTION. S. Keddic and 85N 65N 45/'/ 25N 5N 15S 35S 555 75S
latitude
J. Head, Department of Geological Sciences, Brown University,
Providence RI 02912, USA.
Fig. 1. Location of large volcanos asafunction of latitude. Thedark-shaded
cohnnns indicate where volcanos would be located if they were randomly
The Magellan mission to Venus has emphasized the importance
distributed on the surface as afunction of the pew.enrage of area at agiven
of volcanism in shaping the surface of the planet. Volcanic plains
latitude. The striped columns show where the volcanos actually occur. Note
make up 80% of the terrain and hundreds of regions of localized the paucity of volcanos at higher latitudes and the concentration in the
eruptions have been identified. Large volcanos, defined as edifices equatorial region of theplaneL