Though there is no plate tectonics per se on Venus, recent Magellan radar images and topographic profiles of the planet suggest the occurrence of the plate tectonic processes of lithospheric subduction and back-arc spreading. The perimeters of several large coronae (e.g., Latona, Artemis, and Eithinoha) resemble Earth subduction zones in both their planform and topographic profile. The planform of arcuate structures in Eastern Aphrodite were compared with subduction zones of the East Indies. The venusian structures have radii of curvature that are similar to those of terrestrial subduction zones. Moreover, the topography of the venusian ridge/trench structures is highly asymmetric with a ridge on the concave side and a trough on the convex side; Earth subduction zones generally display the same asymmetry

Sandwell, David T.

Schubert, Gerald

English

NASA Technical Reports Server

LP! Contribution No. 789 97
where I = 3NuK/a 2, "y= agV/Cp, t is time, to is total ascent time,
Nu = 0.8Pc tt2, Pe= Va/K, Pe is the Peclet number, V is the velocity
of magmatie ascent, a is the body radius, K(=I x IOaS m2s -1) is the
thermal diffusivity, ct (=6 x 10 -s deg -t) is the coefficient of thermal
expansion, g is the gravitational acceleration, and Cp(=1.25 x 10 -4
ergs Kg-t*K -t) is specific heat capacity. T is the mean magma
temperature, TOis the magma temperature in the source region, and
n is a constant that def'mes the shape of the planetary thermal
gradient (equation (1)). Equation (2) reduces to the expression for
the thermal gradient (equation (1)) for an infinitely slow ascent
(dimensionless ascent time Jt 0 = **), and to the adiabatic curve
T/T0= e-_t for an infinitely fast ascent (Jto = 0). A typical plot of the
resulting cooling curves for terrestrial conditions, contoured in It 0
values, is illustrated in Fig. 1. In order for the magma to reach the
surface unsolidified, the cooling curve must not cross the solidus
before it reaches the surface. The allowable Jt0 values obtained from
the cooling curve plots for Venus and Earth can be directly com-
pared to obtain relative minimum magma ascent velocities, source
depths, and body sizes. The results are shown in Fig. 2.
The cooling model for the buoyant crack ascent has previously
been described briefly in [2]. It is the problem of a magma at an
initial temperature T O placed in contact with the wall rock of
temperature Tm. This problem was initially solved by [7], and their
solution for the average temperature T as a function of time is
T- Tm 8 {-(2m - 1)2_ 2 Ktl4a 2
m=l (2m - 1)2 (3)
where the notation is the same as in equation (2), and the right side
is constant for any single dimensionless ascent time (Kt/a2). This
result is for a constant wall rock temperature, but can be adapted to
a variable wall rock (thermal gradient) temperature by approximat-
ing an incremental magma ascent in a simple numerical scheme
where the initial magma temperature T Oat any location m is, instead
of the source depth temperature, the final magma temperature at the
previous location m-I [2]. If heat is conducted ahead of the magma
body, the boundary temperature of the magma and wall rock will not
be constant (l'conutc t = Tin) at any given location, but will be the
average of the two initial temperatures of the magma and wall rock
12
OB
06
O2
0
O_
• i
Z_o n=2 _J2o n_3
"""--._........
-_Vrnmn,2
............v_n:3 ................
1 | l i i
OB I 12 1_ a6
_[helmai G_ad_enl I_TIdZ)
Fig. 2. Plot of the results for basaltic pluton ascent on Venus. The results are
nondimensionalized by the corresponding terrestrial values, and the source
depth values are the direct result of the assumed planetary thermal structure
(equation (1)) . For example, for a Venus thermal gradient the same as the
terrestrial thermal gradient and n = 2, the corresponding minimum source
depth is about 60% of the terrestrial value the minimum ascent velocity is
about 30% of Earth's and the minimum body size is about 70% of Earth's.
Thermal structure parameters: n = 2; dT/dZ = 1.0 (dT/dZ of Earth); Z0 = 1.0
fZ0 of Earth).
at any location for the majority of the cooling time [8]. For this case,
the contact temperature at the ruth position is the average of the f'mal
temperature of the magma at the m-1 position (TIn_l) and the initial
wall rock temperature at the ruth position (Tmo). The preliminary
results from this model indicate that the effect of the planetary
thermal structure is of the same order of the effect seen in the pluton
model.
In general,for both ascentmechanism models presented here,
the influence of the planetary thermal structure parameters for
Venus in the probable order of decreasing importance is surface
temperature, surface temperature gradient, thermal gradient curva-
ture with depth, and planetary mantle temperature. The higher
surface temperature of Venus, for otherwise similar planetary
thermal structures, allows considerably smaller minimum possible
crack sizes and/or magma body sizes, and slower ascent velocities
than would be possible on Earth for a reasonable range of Venus
source depths and surface thermal gradients. This surface tempera-
ture effect is greater for more primitive magma compositions, and
may be greater for magmas of higher crystallinity. A higher venu-
sian surface thermal gradient has the same effect of the higher
surface temperature on magma transport, but to a much lesser
degree. Similarly, for higher values of thermal gradient curvature
with depth (higher n in equation (1)), the minimum possible ascent
velocity and body/crack size also decreases slightly. If the mantle
temperature for Venus is elevated by a hundred degrees or so over
that of Earth [9], it should result in a modest increase of melt
production and magma transport to the surface compared toEarth.
The effect of the range of Venus surface temperatures with elevation
(390°--470°(2 or 660*--740 °K [ 10]) is under investigation, and is also
anticipated to have a significant effect on magma transport, possibly
greater than that of the higher mantle temperature.
References: [l]HeadJ.W.etal.(199I)Science,252,276-288.
[2] Marsh B. D. (1978) Phil. Trans. R. Soc. Lond., A288, 611-625.
[3] Marsh B. D. and Kantha L H. (1978) EPSL, 39, 435--443.
[4] Sakimoto S. E. H. et at. (1992) LPSC XXIli, 1203--1204.
[5] Marsh B. D. (1982) Am J. Sci., 282,808-855. [6] Sclater J. G.
et at. _1980) Rev. Geophys., 18, 269-312. [7] Williamson E. D. and
Adams L H. (1919) Phys. Rev., 14, 99-114, [8] Jaeger J. C. (1964)
Rev. Geophys., 2,443--466. [9] Sotin C. et at. (1989) EPSL, 95,321.
[ l 0] Seiff A. (1983) In Venus, 215-279, Univ. of Arizona, Tucson.
[111 Lambert I. B. andp_y'VllTte P._J. (I972) 1. Geol.,80, 693--708.
N93-14367
EVIDENCE FOR RETROG RADE LITHOSPHERIC SUBDUC-
TION ON VENUS. David T. Sandweil I and Gerald Schubert 2,
lScripps Institution of Oceanography, La Jolla CA 92093, USA,
2Deparmaent of Earth and Space Sciences, University of California,
Los Angeles CA 90024, USA.
Though there is no plate tectonics per se on Venus [1], recent
Magellan radar images [2] and topographic profiles [3] of the planet
suggest the occurrence of the plate tectonic processes of li thospheric
subduction [4] and back-arc spreading [5]. The perimeters of several
largecoronae (e.g., Latona, Artemis, and Eithinoha) resemble Earth
subducfion zones in both their planform and topographic profile.
McKenzie et al. [4] have compared the planform of arcuate struc-
tures in Eastern Aphrodite with subducfion zones of the East Indies.
The venusian structures have radii of curvature that are similar to
those of terrestrial subduction zones. Moreover, the topography of
the venusian ridge/trench structures is highly asymmetric with a
ridge on the concave side and a trough on the convex side; Earth
subduction zones generally display this same asymmetry.
https://ntrs.nasa.gov/search.jsp?R=19930005179 2020-03-17T08:44:33+00:00Z
98 International Colloquiura on Venus
Trench/Outer Rise Topography: Latona Corona (eastern
Aphrodite, Venus) provides a striking resemblance to the South
Sandwich Trench (South Atlantic, Earth) as shown in Fig. I. The
Sandwich subduction zone is a deep aremtte trench having a radius
of curvature of about 330 kin. A topographic profile across the
Sandwich trench(A-A')displaysthecharacteristictrench/outerise
signaturethatiscausedby downward flexureofthelithosphereprior
to subduetion[6].We have modeled thislithosphericflexureto
determinetheelasticplatethickness,thecurvatureof theplateand
thebending moment thatisneeded tosupportthetopographyof the
outerrise;the best-fitmodel has a 40-kin-thickplate.Like most
othertrencheson theEarth,theextreme curvatureof topographyon
the outer trench wall is sufficient to permanendy deform the
lithosphere prior to subduetion [7]. This inelastic behavior is
marked by trench-parallel normal faults extending from the trench
axis to the outer rise.
Latona Corona displaysessentiallythe same topographicand
flcxuralcharacteristicssccnattheSandwich arc(Figs.2 and 3).The
deep areuatetrenchalong the southernmargin of Latona has the
-60"
32e" 340"
- _-,_T • -la" El
g
-:,_-,_ _ _
-JO"
1711"
t
f
200 #00 600
Du_ranc¢ (kin)
Fig. 1. (Top image) Bathymetry of South Sandwich Trench (Earth) as a tall
volcanic ridge, a deep ocean trench, and the characteristic trench/outer rise
signature associated with lithospheric flexure prior to subduction (A-A').
(Bottom image) Topography of Latona Corona (East Aphrodite, Venus) at the
same horizontal and vertical scale as the Sandwich Trench (top). The southern
arc displays a prominent ridge-trench-outer rise signature (B-B'). The south-
ernmost ridge is not massive enough to maintain the bending moment of the
trench/outer rise flexure. Additional bending moment can be supplied by a
subducted slab or by the second interior ridge.
same radius of curvature as the Sandwich trench (340 kin) and
inboard of the trench axis lie two tall arcuate ridges. A topographic
profile across the Latona trench (Fig. 3) displays the char_teristic
uench/outer rise signature associated with lithospherie flexure
althoughitsoverallamplitudeissomewhat lessthanattheSandwich
trench. As in the case of the Sandwich trench, we applied the flexure
model to the trench/outer rise topography at Latona and estimated
an elastic plate thickness of 30 kin, which is similar to the elastic
thickness at the Sandwich trench. Like trenches on the Earth, the
outer trench wall of the Latona trench has a high curvature,
suggesting that the plate is flexed beyond its elastic limit, perhaps
in preparation for subduetion. SAR images display prominent
circumferential fractures on the outer trench wall in agreement with
the plate flexure model [5].
Since vonusian trenches display the major characteristics of
Earth trenches and since the Earth's lithosphere is known to be
subducting, it is possible that the vonusian lithosphere is also
subducting. We have tested this hypothesis by determining whether
the overriding coronal ridge is massive enough to supply the
measured bending moments; if the ridge cannot supply the required
moment then one must invoke a negatively buoyant subducted slab
to make up the deficit. At Artemis, about two-thirds of the coronal
ridge is needed to balance the trench/outer rise bending moment; all
of this topography lies within 100 kin of the trench axis. At Latona,
however, the outermost coronal ridge (southernmost ridge in Fig. 2)
is insufficient to balance the trench/outer rise moment; the second
interior ridge must be included but in this ease most of this
topographic moment lies more than 150 km from the trench axis (i.e.
more than one-half of the flexural wavelength). We performed the
same calculation at the Sandwich Trench and found that only about
one-halfofthevolcanicareisneeded tobalancethetronch/outerise
moment. However. on theEarth.thedownward forceexertedby the
topography of the volcanicarc islargelybalanced by the upward
buoyancy ofitsthickcrustalrootsand thetopographicmoment does
not support the trench]outerrisemomcnL The same isostatic
compensation might occur on Venus. Thus, while the Venus mo-
ment balance calculation does not demonstrate the need for a
negatively buoyant subducted slab, it cannot disprove the subdue-
tion hypothesis either.
Retrograde Subduction Around Coronae: These observa-
tions and calculations can be explained by a model of lithospheric
foundering, trench rollback, and back-arc extension that was devel-
oped to explain the geometries and kinematics of many subduction
zones on Earth including the Sandwich Trench. The corona is
essentially a hole in the lithosphere whose edges sag downward
beneath its outer rim. Sinking and rollback (or retrograde migration)
of the exterior lithosphere is accompanied by inflow of mantle
material beneath the corona and extension of the corona interior
(back-arc spreading). The interior extension occurs in a more
distributed and disorganized way than the ridgelike spreading in
terrestrial back-arc basins. The downward bending of the exterior
lithosphere beneath the rim of the corona produces the trench or
moat around the corona; the outer rise is an elasdc flexural response
to the downwarping.
The occurrence of retrograde subduction on Venus is consistent
with our understanding from fluid dynamics [8] that convection in
a largely internally heated mantle should be dominated by
downwelling instabilities from the cold upper thermal boundary
layer (lithosphere). Thus, we propose that the same physical process
that drives plate tectonics on Earth is also important in the tectonics
of Venus even though the lithosphere subduction forces on Venus do
LP[ Contribution No. 789 99
not apparently result in a completely analogous platelike tectonic
configuration.
On Earth, the mantle is m_nly cooled by the cold subdueting
lithosphere [8]. Lithospherie subduetion on Venus should play a
similar role in cooling the mantle. We can evaluate the potential
importance of subduetion on Venus to planetary heat transfer by
estimating the total length of subduetion zones on Venus [9] and
comparing it with the total length of terrestrial subduefion zones
(37,000 km [10]). Retrograde subduetion on Venus may be occur-
ring not only at the marginal trenches of the large coronae Artemis,
Latona, and Eithinoha, but also at _reuate trenches such as Daft and
Diana ehasmatain Eastern Aphrodite Terra [4] and elsewhere on the
planet (e.g., Hecate Ch_trta, Heads Rupes, Nightingale Coron_
and Parga Chssma). The total length of these areuate trenches is
about 15,000 km [ 11 ]. The estimate would be greater if features such
as Quetzalpetlatl Corona and the margins of plateauhighlands such
as Western Ishtar Terra mad Thetis Regiones [12] were included. Of
course, we do not know if any of these features are presently active.
Assuming that they _ all active and have terreslriai convergence
rates, this trench-length estimate indicates that mantle cooling by
lithospherie subduction is apotentially important process on Venus.
References: [llSolornonS.C.etal.(1991)Science,252,297;
Solomon S. C. et al. (1992)JGR, in press. [2] Saunders R. S. et al.
(1992)JGR, in press. [3] FordP, G. and Pettengill G. H. (1992)JGR,
in press. [4] MeKenzie D. P. et al. (1992) JGR, in press. [5] Sandwell
D. T. and Schubert G. (1992) JGR, in press; Sandwell D. T. mad
Schubert G. (1992) Science, submitted. [6] Caldwell J. G. and
Turcotte D. L. (1979) JGR, 84, 7572. [7] Turcotte D. L. et al. (1978)
Tectonophysics, 47, 193; MeNutt M. K. and Menard H. W, (1982)
Geophys. J. R. Aaron. Soc., 71, 363. [8] Schubert G. (1992) Annu.
Rev. Fluid Mech., 24, 359. [9] Stevenson D. J., personal communi-
cation. [I0] Reymer A. and Schubert G. (1984) Tectonics, 3, 63.
ill] The following features were identified in a gridded Venus
topography map [5] as possible subduetion zones based on their
ridge-trench-outer rise topography and areuate planform: Artemis
Corona, Latona Corona, Eithinoha Corona, Nightingale Corona,
Dali Chasma, Diana Chasma, Hecate Chasms, Parga Chasma,
Hestia Rupes, and a number of unnamed arcuate features. There are
many other more subtle trcnehlike expressions that would almost
double the estimated trench length of 15,000 kin. [ 12] Bindschadler
D. L. et al. (1992) JGR, itrpre_. - - .._
N93-14368 '
I
OVERVIEVt/OF VENUS GEOLOGY: PRELIMINARY DE-
SCRIPTION OF TERRAIN UNITS FOR VENUS GLOBAL
GEOLOGIC MAPPING. R. StephenSaunders, Ellen R. Stofan,
Jeffrey J. Plant, and Gregory A. Michaels, Jet Propulsion Labora-
tory, California Ins dtute of Technology, Pasadena CA 91109, USA.
Venus terrain units can be categorized on the basis of morphol-
ogy, reflecdvity, backscatter, roughness, and emissivity. Morphol-
ogy can be inferred from Magellan left-looking nominal incidence
angle image mosaics, fight-looking coverage, and more limited left-
looking stereo. The typical resolution is about 300 m down to about
120 m near periapsis in the cycle one nominal coverage. The scale
of geologic mapping governs definition of mappable terrain units.
Initial global mapping is being compiled at a scale of 1:50 million.
At this scale, the smallest individual features that can be mapped is
about 125 km. The categories of terrain types are plains, complex
ridge terrain, features with morphology suggesting volcanic or
volcano-tectonic origin, features interpreted to be tectonic in origin,
crater units, and surficial units such as splotches mad streaks. The
following are brief descriptions of terrain units that axe being
mapped globally at the 1:50 million scale.
Plains:
Smooth plairm---Planar surfaces with low radar backscatter and
smooth texture.
Smooth plains with wrinkle ridges---Plains with widely spaced (10
km or more) linear to sinuous ridges.
Mottled plalns--Plalm with many apparently overlapping lobate
outline subunits creating a light and dark mottled appearance.
Gridded plans and lineated pl_----One or more crmsing sets of
parallel radar bright lineaments cross the plains.
Complex ridge terrain (CRT):
CRTl_Single trend of generally parallel but irregular ridges and
troughs of various wavelengths from a few kilometers to tens of
kilometers; individual ridges may exceed 50 km in length.
CRT2---CRT1 disrupted by sets of crosscutting lineaments; Mor-
phology varies from long, sinuous ridge sets to blocky.
CRT3--Two prominent sets of ridges and troughs that are approxi-
mately orthogonal; relatively short ridges (25 kin) and longer
disrupting linear zones (50 km).
CRT4---Chaodc arrangement of ridges and troughs intersecting at
a variety of angles and scales; may have lensate or hummocky
appearance.
Morphology suggesting volcanic and volcano-tectonic origin:
Corona---Annular arrangement of ridges and t_ughs; interior var-
ies.
Arachnoid----Circular to quasielreular features with radial systems
of lineaments.
Shield--Low h/w often with radial flow-like patterns; diameter 50
to several hundred kilometers.
Domo---Small shield structure with steep sides and broad convex or
concave summit; generally simple circular in plan.
Dome field---Cluster of approximately one hundred or more 3- to
10-krn dome forms.
Scalloped dome--Dome with radial ridges separating scoop or
theater-shaped grooves. Many have surrounding landslide de-
bris.
Flow field---Complex of individual lobate flow-like features typi-
cally with parallel alignments and common source.
Channel--Sinuous trough or sinuous linear contrasting pattern on
plains.
Features Interpreted to be tectonic in origin:
Linear troughs and bright lineaments resembling graben or frac-
tures---Generally steep-sided straight troughs and bright linea-
ments interpreted to be graben faults and fractures.
Lineament zone.--Bands of linear features and broad troughs and
basins.
Ridges---Positive linear features similar to lunar wrinkle ridges.
Ridge belts--Bands of ridges having a generally common trend.
Crater units--Various crater related features including ejecta, fl0w
material and pababolic halos.
Splotclv--Dark and bright quasicircular features.
Windstreak--Bright and dark streaks generally terminating at one
end at a topographic feature such as a dome or ridge.


Evidence for retrograde lithospheric subduction on Venus

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server