Pre-Magellan analysis of the Venera 15/16 data indicated the existence of abundant small volcanic edifices, each less than or equal to 20 km diameter, interpreted to be predominantly shield volcanoes and occurring throughout the plains terrain, most common in equidimensional clusters. With the analysis of Magellan data, these clusters of greater than average concentration of small volcanic edifices have been called 'shield fields'. Although individual small shields can and do occur almost everywhere on the plains terrain of Venus, they most commonly occur in fields that are well-defined, predominantly equant, clusters of edifices. Major questions include why the edifices are concentrated in this way, how they relate to the source of the eruptive material, and what the possible relationship of shield fields to plains terrain is. There are three possible models for the origin of fields and small shields: (1) a field represents an 'island' of higher topography subsequently surrounded by later plains material; and (2) a field represents the area of magma reservoir

Aubele, J. C.

Crumpler, L. S.

English

NASA Technical Reports Server

LPI Contribution No. 789 7
over geologic time by aeolian activity. The widespread distribution
of thin ejecta deposits indicates that the rate of aeolian erosion is
low, perhaps only a fraction of a micrometer per year. We thus
conclude that most flow degradation in locations such as Sedna
Planitia is due to in situ weathering. In addition, elevation-depen-
dent weathering is inferred in western Ovda Regio, where plains
above 6054 km radius have enhanced reflection coefficients (>0.20)
as eompared to adjacent plains at lower elevations. Furthermore, the
presence of deposits with normal reflection coefficients blown in
from lower elevation plains indicates that the conversion to high
dielectric materials occurs at a slower rate than the rate of sediment
accumulation by winds. Combined vertical rates of surface modifi-
cation of meters over hundreds of millions of years are inferred from
the extent of surface modification for plains and the impact crater
abundance. This rate is orders of magnitude lower than the terres-
trim value and suggests that it will be possible to constrain relative
ages of surfaces on the basis of degree of preservation of volcanic
landforms and microwave signatures. /) t..! ¢1[q i -I _
.+:,+ e 4-t St 9 5
SHIELD FIELDS: CONCENTRATIONS OF SMALL VOL.
CANIC EDIFICES ON VENUS. J.C. Aubele and L. S.Crumpler,
Department of Geological Sciences, Box 1846, Brown University,
Providence RI 02912, USA.
Observations: Pre-Magellan analysis of the Venera 15/16
data indicated the existence of abundant small volcanic edifices,
each <20 km diameter, interpreted to be predominantly shield
volcanos [1,2] and occurring throughout the plains terrain, most
common in equidimensional clusters. With the analysis of Magellan
data, these clusters of greater than average concentration of small
volcanic edifices have been called "shield fields" [3,4]. A typical
shield field consists of volcanos numbering =102 and ranging in
density from 4 to 10 edi flees per 103 km 2 within an area that covers
>104 km 2. Most of these fields are roughly equant in outline, but a
small percentage are elongate or consist of diffuse concentrations of
edifices over larger areas. Typical field diameters mostly range
from 50 to 350 kin, with a mode from 100 to 150 km (Fig. 1). The
cumulative size distribution (Fig. 2) of shield fields more closely
follows the trend of coronaelarachnoids/novae (features assumed to
be dominantly intrusive) than features assumed to be dominantly
extrusive (such as large or intermediate-sized volcanos); this simi-
larity apparently reflects reservoir and source dimensions. The
volcanic edifices within an individual shield field are generally <10
km in diameter, and are predominantly radar-bright and shield-
shaped in profile with a single summit pit [5]. A small number of
fields are composed predominantly of a less common edifice type
such as radar-dark shields, edifices with radar-bright aureoles or
100 200 300 400 500 600
Di_r_elct [kanl
Fig. 1.
700 800 900
I , , = I L , , I t , r I I I , I , • , 1 + , ,
I000 I _ SFI\, l---++
"\
-....__--+..
200 400 600 800 1000
Horizontal Dtn_i_Lon (k.rnl
IZ00
Fig. 2.
halos, elongated small shields with bright radial flow patterns
("anemones"), or domical or conical proFde edifices [5]• The radar-
bright or radar-dark material locally surrounding shield field edi-
fices, which sometimes covers local structural lineaments, is
interpreted to represent associated volcanic material, probably thin
lava flow units, although minor amounts of ash or cinder may
produce a very thin local veneerin some areas [5]. If the visible flow
fields associated with some shield fields are of average size, then the
area of resurfacing associated with a shield field appears to be
comparable to that of the area of a single large volcano. Shield
formation did not apparendy occur planetwide as a single event, as
there appears to be a range of shield field ages in relation to the
surrounding regional plains units based on stratigraphic relation-
ships. A few vents within a shield field may be aligned along
dominant structural trends, and summit pits frequently occur along
dominant structural trends; however, the clustering characteristics
of edifices within a shield field appear to be most similar to that of
terrestrial cinder cone fields lacking in well-defined structural vent
control.
Distribution: At the conclusion of cycle 2 coverage, 556
shield fields (Fig. 3) have been identified in the catalog of volcanic
features [3,6] prepared for the Magellan Science Analysis Team,
Volcanism Working Group; shield fields are the most abundant
single category type of volcanic or magmatic features. Approxi-
mately 70% of shield fields occur on 50% of the surface of Venus.
Shield fields are somewhat more distributed over the surface than
are large single magmatic or volcanic features such as coronae or
large volcanos [3,6,7]; however, Magellan global analysis has
confirmed the previous observation made from the Venera dataset
[1 ] of at least one and possibly two dominant global concentrations.
The region of greatest concentration, which also shows h.igh concen-
trations of all other volcanic features [4,8,9], has been informally
named the Beta-Atla-Themis or "BAT" region, centered at longi-
tude 250 °. Density of shield fields within this region ranges from 2
to 7 fields per 10 _ km 2 and high density of shield fields appears to
def'me the margins of the BAT area. Magellan has also confirmed
the previous observation based on Venera data [1] that small
volcanos do not occur in large numbers in the areas dominated by
ridge belts or in the very lowest or very highest elevations on the
planet. Approximately 59% of shield fields occur in elevations
between mean planetary radius and 2 km above MPR, 36% occur in
regions below MPR in elevation, and only 5% occur in regions
greater than 2 km above MPR. When normalized for percentage of
surface area at these elevations, 76% of shield fields occur in regions
1 to 2 km above MPR. Fields are commonly spatially associated
https://ntrs.nasa.gov/search.jsp?R=19930005107 2020-03-17T08:49:47+00:00Z
8 lnleraational Colloquium on Venus
78.5
60
40
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0
-20
-40
-60
-78.5
I, ,1_1 )_[ iiI, ,I,,I t, I, ,I, ,.I,,I,_,,-!
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60 120 180 240 300 360
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with larger volcanic features. Shield fields frequently occur within
the inner rings of corona•; those asociated with large volcanos often
occur around the distal edges of, and occasionally are surrounded
by, the radial lava flows forming the volcano flanks, but they also
occur near the summit of a few large volcanos.
Implications: Although individual small shields can and do
occur almost everywhere on the plains terrain of Venus, they most
commonly occur in fields that axe well-defined, predominantly
equam, clusters of edifices. Major questions include why the
edifices are concentrated in this way, how they relate to the source
of the eruptive material, and what the possible relationship of shield
fields to plains terrain is. There are three possible models for the
origin of fields and small shields: (1) a field represents an "island"
of higher topography subsequently surrounded by later plains
matcrial; (2) a field r_presents the area of a region of anomalous
melting; or (3) a field represents the area of a magma reservoir.
Model 1 would imply that the flclds represent portions of a strati-
graphic "layer" of small edifices produced globally in an earlier
period of greater small shield productivity and that there has been
a change in eruption style with plains formation occurring predomi-
nantly after the production of the small edifices. If the shield fields
arc isolated "islands" surrounded by flooded plains, the equant
aspect of most fields could be explained; however, some fields show
associated flows superimposed on surrounding plains and the
manner in which shield fields appear to cover local structural
patterns suggests that they are associated with plains-forming
material themselves. In addition, local strafigraphic relationships
show that there is a range of shield field ages in relation to the
surrounding regional plains units and the associated larger volcanic
features, implying that shield formation did not occur planetwide as
a single event. Models 2 and 3 imply that the fields represent a_:as
of melting anomalies. Model 2 implies that the area of the field is
controlled by the extent of the region of melting. A variation of
Model 2 uses small reservoirs to explain local groups and align-
ments of edifices or differences in edifice type due to variations in
eruptive style or melt chemistry. Model 3 implies that the area of the
field is controlled by the areal extent ofa magmareservoir. The areal
shape and density of most shield fields could be explained by
postulating a shallow regional reservoir or trap located between the
melt source region and the surface and approximately equal in size
to the areal extent of the field. Given the strafigraphic evidence of
the range of shield field ages, models 2 and 3 are favored over model
1 for most cases. Whether the shape and size of a field reflects the
area of the melt anomaly or the area of a reservoir is difficult to
determine. The formation of a field of small volcanos, rather than
a single large volcano, must imply a difference in magma rates or
reservolr/source area characteristics. The reservoir or source area
characteristics of shield fields can apparently be related to the scale
ofthe feature,as has previouslybeen postulatedfor coronae [7].An
asso_iatexlquestion is the relationshipof shield fields to plains
terrain.This can bc expressed as four possibilities,some of which
are also related to the model of origin of the "fields" described
above. The possibilitiesare as follows: (1)The edificesmay be the
source of lava flows thatform or resurfacethe plains,which would
imply thatthe extrusivevolume from each edific_ isgreaterthan the
visiblevolume of the edifice and thatthe plains terrainiscreated
from a strafigraphicsequence of edifices and associated flows;
(2) the edifices and plains may bc formed simultaneously, which
would imply that the edifices are localized point sources within a
large extrusive mechanism that creates plains; (3) the edifices may
predate the plains, which would imply an early global edifice-
building stage and subsequent change in eruption style and heat fiow
to large-volume-flow field-type eruptions; or (4) the edifices may
postdate the plains, which would imply a change in eruption style to
late-stage localized small-volume extrusions or hot-spot-type
anomalies.
Detailed studies of several shield fields are continuing in an
attempt to answer these fundamental questions and to select appro-
priate models for understanding shield fields and their role in
volcanic resurfacing processes and crustal volume con_butions.
References: [1] Aubele and Slyuta (1990) Earth Moon Plan-
ets, 50/31,493-532. [2] Garvin and Williams (1990) GRL, 17.
1381-1384. [3] Head et al. (1992) JGR, special Magellan issue,
submitted. [4] Aubcle L C. et al. (1992)LPSCXXIII,47--48; Aub¢le
et al. (1992) Eos, 73, 178. [5] Guest et al. (1992) JGR, special
Magellan issue, submitted. [6] Crumpler et al. (1992) LPSCXXlil,
277-278; Crumpler et al. (1992) in preparation. [7] Stofan E. R. et
al. (1992)JGR, special Magellan issue, submitted. [8] Head et al.
(1992) LPSC XXIII, 515-516. {91 Crurnpler and Aubele (1992)
LPSC XXII[, 275-276.
!,:1 ©_ _:. _ik
'_" _' r'" = _' ..... <" :" :$'
THE GEOLOGY OF THE VENERA/VEGA LANDING SITES..
A. T. Basilevsky _ and C. M. Weilz _. !Vernadsky Institute of
Geochemistry and Analytical Chemist.,y, Russian Academy of
Sciences, Moscow 117975. Russia, 2.1et Propulsion Laboratory,
California Institute of Technology, Pasadena CA 91109, USA.
We have performed a photogeological analysis of the Venera/
Vega landing sites using Magellan radar images. These seven sites
axe the only places on Venus where geochemistry measurements
were taken. In this study, the updated coordinates of the landing sites
are used and the landing circle has a radius with an admissable error
of about 150 km [1].
Photogeologic Description of the Landing Sites: Venera 8
landed on the equatorial plainswithin a small localtopographic rlse
eastward of Navka Planitia.The coordinates of the landing site are
10.70°S, 335.25°E. Gamma-spectrometric analysis showed that the
surface material contains relatively high contents of K, U, and Th
[2,3]. A comparison with terrestrial KzO-U-Th analogs of this
material suggests that it may represent evolved subalkaline mag-
marie rock of intermediate silica content [4,5] or alkaline basalt
[6,7,8,9], particularly lamprophyres [10].


Shield fields: Concentrations of small volcanic edifices on Venus

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