Geologic history of the Mead impact basin, Venus
Robert R. Herrick
Virgil L. Sharpton
Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, Texas 77058
ABSTRACT
The geologic history of the Mead impact basin on Venus, a basin similar in size to
Chicxulub (Mexico), may be a guide of what to expect from future exploration of Chicxulub.
During the collapse phase of crater formation in the Mead basin, radar-bright impact melt
material was deposited as a topographically flat surface within a large central area, burying
the transient cavity rim and other underlying structures. The central area is not flat now
and has been modified by viscous relaxation and thermal cooling effects. Substantial parts
of the ejecta deposits have been covered by postimpact volcanic flows that are not obvious
without the topographic data. Previous global surveys of Venusian impact craters, using
only image data, may have underestimated the number of craters embayed by volcanism.
INTRODUCTION
Data collected from the Magellan mission
reveal Mead (Fig. 1) to be the largest exposed impact structure on Venus (269 km
diameter) and a multiring basin. Magellan
collected synthetic aperture radar (SAR)
data at ;100 m resolution, and carried an
altimeter that provided topography, centimetre-scale roughness, power reflection coefficient, and radar emissivity data at ;20
km horizontal resolution (Saunders et al.,
1992). SAR images appear bright if the
imaged surface is sloping toward the radar,
is blocky at radar wavelengths, or is inherently reflective, and for most purposes SAR
data can be treated as aerial black-andwhite photographs. In most cases the reflection coefficient data and emissivity data are
mathematical complements; i.e., reflective
materials have a low emissivity (Pettengill et
al., 1992; Ford and Pettengill, 1992). For the
;80% of the basin covered by SAR data at
two different look angles, stereo photogrammetric techniques were used to generate the
high-frequency component of topography
and improve the horizontal resolution of the
topographic data for Mead to 1–5 km (technique described in Leberl et al., 1992). Stereo data were not collected for three orbits
(;60 km) just west of the basin’s center.
Here the resolution is 10 –20 km and some
minor artifacts in the final topography indicate the high-low frequency transition. Although we provide evidence that Mead has
been affected by some postemplacement volcanism and tectonism, Mead is far more
pristine than any terrestrial basin and thus
can provide important information about
basin formation on a planet with Earthlike
surface gravity. In particular, the Chicxulub
structure in Mexico may be similar in size to
Mead (Sharpton et al., 1993), suggesting
that analysis of Mead can aid the interpretation of current and future geologic and
geophysical data to be collected at
Chicxulub.
Geology; January 1996; v. 24; no. 1; p. 11–14; 4 figures.
MEAD BASIN
Bounded by the inner-ring scarp, the central area of Mead is 194 km in diameter and
radar bright relative to the surrounding
plains (Fig. 2); the strong radar returns over
this region are due primarily to the high reflectivity (and low emissivity) of this surface
unit rather than a roughness effect (Weitz,
1993). This high reflectivity (highest in the
northeast part of the central area) suggests
that the material composing the central area
has either a high dielectric constant or a high
porosity (Weitz, 1993). The central area averages about 700 m below the surrounding
plains and 1100 m below the elevated rim.
The steep inner-ring scarp averages ;400 m
in height and is highly irregular in outline,
reminiscent of a scalloped crater rim in lunar craters. There is considerable topographic variation within the central area; the
center and periphery are depressed 150 –300
m compared with the intermediate region
between r 5 30 km and r 5 60 km. Several
concentric linear features, some of which
are distinct grabens, modify the intermediate region. The central depressed area exhibits primarily radial features, some of
which are distinct scarps that appear to be
formed by shortening. The periphery of the
central area, particularly in the southwest
and northeast, is slightly more radar bright
in appearance and is mottled by small, very
radar bright spots (Fig. 3).
The area between the inner and outer
scarp rings (hereafter referred to as the terrace zone) of Mead averages 40 km in width
and has radar properties indistinguishable
from the plains surrounding the basin.
Where ejecta cover the terrace zone it resembles ejecta-covered plains outside the
outer-ring scarp; plains regions within the
terrace zone are similar to those outside the
outer ring, and emissivity signatures are similar for the terrace zone and the surrounding
plains. However, we cannot definitely identify tectonic features from the surrounding
plains that appear to continue into this zone.
The terrace zone is generally bounded on its
outer perimeter by a single scarp to the rim
(average height ;700 m), but on the northnortheast is a set of descending terraces, interpreted to be step-down faults, of a few
hundred metres each. The eastern quadrant
of the terrace zone stands 150 –300 m higher
than the remainder of the zone; there is a
corresponding decrease in the height of the
outer-ring scarp. The western quadrant is
generally lower than the remainder of the
terrace zone. Here radar-bright ejecta are
confined to high-standing areas, whereas in
the other areas of the terrace zone there is
no obvious correlation between ejecta location and elevation.
The topographic rim of Mead is ;400 m
higher than the surrounding plains. Radarbright ejecta are located around most of the
rim crest, but beyond the rim the radarbright ejecta are confined to high-standing
areas in the surrounding plains. Northwest
of the crater there is a volcanic feature ;20
km in diameter, the crest of which is ;900 m
above the surrounding terrain. The topography combined with the radar imagery
show clearly that a flow unit ;75 by 50 km
extends eastward from the volcano and covers part of the ejecta blanket (Fig. 4).
Geologic History
The data suggest that the current appearance of Mead has been affected not only by
the formation of the basin, but also by postemplacement viscous relaxation and volcanism. The similarity of the terrace zone to the
surrounding plains leads us to interpret the
terrace zone to be a down-dropped plains
region outside the transient cavity. The
available data are insufficient to allow us to
distinguish whether the inner-ring scarp is
the boundary of the transient cavity; it may
be that this boundary is underneath the central area. The volume of the hole (beneath
the elevation of the surrounding plains)
roughly represents the volume of material
ejected beyond the outer-ring scarp. Simple
calculations (e.g., Melosh, 1989, p. 90) and
comparison with other large craters on Venus and the moon indicate that the ejecta
blanket is far less extensive than would be
expected for a pristine basin of Mead’s size.
Typical estimates of the volume of melt
produced for a Mead-sized crater are 30%–
50% of the crater volume, and about half of
this will not be excavated from the crater
(Grieve and Cintala, 1992; O’Keefe and Ahrens, 1977). The entire central area gener11
Figure 1. A: Radar image of Mead impact basin. Outer rim averages 269
km in diameter, and radar-bright central area averages 194 km in diameter. Note 20-km-diameter volcanic feature 50 km northwest of
outer rim. B: Radar image of Mead basin with topography overlaid as
color. Each color change represents 150 m change in elevation. C:
Profiles taken every 45°. Bottom profile is radial average. Vertical line
segments mark approximate location of inner- and outer-ring scarps.
A
Figure 2. Sketch map showing morphologic units and significant tectonic features. Morphologic units increase in stratigraphic age from
top to bottom in legend. Dotted-line box outlines area of image in
Figure 1, A and B.
B
12
GEOLOGY, January 1996
Figure 3. Full-resolution image of northeast part of Mead impact structure. Outer ring is series
of step-down faults in this region. Terrace zone is similar in radar character to surrounding
plains and has ballistic ejecta superposed on it. Inner-ring scarp shows evidence for mass
wasting. Particularly bright features appear to poke through perimeter of radar-bright central
area. Image area is 75 3 75 km.
ally appears to have been emplaced as a single unit. We interpret this unit to be the
cooled impact melt sheet. The reflectivity
data suggest a distinctive composition or porosity for this unit. The irregular outline of
the inner ring suggests that mass wasting has
significantly modified this scarp. We propose that this irregular outline is a byproduct of melt material sloshing around within
this scarp during the formation of Mead.
The melt washed up against, and perhaps
partially melted, the steep scarp face of the
inner ring, causing collapse along the inner
scarp face. There may be buried terraces just
beneath the periphery of the central area,
and the radar-bright spots are features (e.g.,
large clumps of ballistically emplaced ejecta)
not entirely buried by the melt sheet.
If the central area was emplaced as a single unit, it seems likely that its upper
boundary (the surface) was flat at the time
of emplacement. If so, then this unit has
clearly undergone postemplacement tectonic modification. The basin-centered nature of the elevation anomalies in the central area and the lack of apparent rim
deformation indicate that the source of postemplacement deformation of the central
area is the impact feature itself. We propose
GEOLOGY, January 1996
that there has been postemplacement impact-related deformation caused by basin
cooling and viscous relaxation of topography. The topography and faulting in the central area are consistent with broad updoming of the central area while the centerpoint
is held down. Stretching of the rising broad
topographic ring produced concentric grabens, while near the centerpoint contraction
resulted in radial thrust faults. Viscous relaxation of an uncompensated hole produces updoming of the floor, whereas thermal contraction from cooling of an impact
basin causes basin-centered subsidence (Solomon et al., 1982; Bratt et al., 1985). Thus,
qualitatively the topography of the central
area could be explained by doming of the
floor due to viscous relaxation counteracted
in the center by thermal subsidence. To
match the observations, after cooling of the
melt sheet a thermal anomaly of several
hundred degrees must have extended below
the basin center for at least 20 km in order
to produce enough subsequent thermal contraction of the rocks to counteract the updoming by relaxation; laterally the anomaly
must have been concentrated within a few
tens of kilometres from basin center. The
amount and spatial dimensions of thermal
subsidence depend on the subsurface temperature field immediately after the melt
sheet cools, and this field is unconstrained
(Bratt et al., 1985). Another likely reason
that the basin center has domed less than
intermediate distances is that the basin was
not simply an uncompensated hole after
emplacement, but instead there is substantial subsurface structure influencing viscous relaxation and thermal subsidence
(e.g., an uplifted mantle plug below basin
center or a buried crater ring of ;140 km
diameter).
Volcanic flows with sources exterior to
the crater have covered substantial parts of
the ejecta blanket both exterior to the crater
and within the terrace zone. A volcanic
source is easily identifiable for the small flow
northwest of the crater. The flow boundary
is identifiable only from its onlapping relation to the high-standing ejecta, and neither
the flow nor its boundary have a radar signature distinguishable from the surrounding
plains. Similarly, crater ejecta exist only in
high-standing areas for the regions exterior
to the rim and in the western part of the
inner ring zone, implying that these regions
have also been covered by postimpact volcanic flows that do not have a distinctive radar signature. Coverage by widespread
postimpact volcanism also explains the unusually limited extent of the ejecta blanket.
Although this volcanism cannot be traced
directly to a single source, we note that the
potential for volcanic resurfacing is demonstrated by several large, stratigraphically
young volcanoes located in Eastern Eistla
Regio a few hundred kilometres to the west
of Mead. The high-resolution topographic
data are vital for making a compelling case
that Mead has been volcanically embayed;
without the topographic data, the dark areas
can be attributed to patchiness or a change
in radar character across the ejecta blanket.
We suggest that global surveys of Venusian
impact craters based on image data alone
may have missed many examples of ‘‘subtle’’
embayment of impact craters.
IMPLICATIONS FOR THE CHICXULUB
STRUCTURE
Current evidence suggests that the
Chicxulub structure is roughly the same diameter as the Mead basin (Sharpton et al.,
1993). However, the Chicxulub impact was
into a shallow-water marine environment
with perhaps 1–2 km of sediment overlying
the basement crustal layer. Nevertheless, to
first order, we might expect the general dimensions (e.g., rim height, depth) and appearance to be similar for the two structures,
and we can make some inferences about
possible drilling and seismic results. At
Chicxulub it may be that one or more of the
13
caused some researchers to conclude that
the planet is geologically quiescent (e.g.,
Schaber et al., 1992). If more craters are embayed than previously recognized, the possibility exists that significant volcanism is occurring in a widely distributed manner.
Second, Mead has undergone viscous relaxation, but the relaxation is not a simple
updoming of the central area. Thus, whereas
the relaxation of Mead must be indicative of
the bulk mechanical properties of the Venusian lithosphere, modeling of Mead as a
simple hole in the ground with no subsurface structure would clearly yield erroneous
values for those properties. However, the
knowledge gained about basin subsurface
structure from the ongoing exploration of
Chicxulub could pave the way for realistic
models of Mead’s relaxation, just as Mead’s
surface appearance is influencing interpretations of the data for Chicxulub.
ACKNOWLEDGMENTS
Supported by a National Aeronautics and Space Administration (NASA) grant to the Lunar and Planetary Institute
and a NASA grant to Sharpton under the Venus Data Analysis Program. Software supplied by Vexcel Corporation was
used to do stereo analysis of topography. Jeff Plaut at the Jet
Propulsion Laboratory supplied matching stereo images of
Mead. We thank T. Watters for his review. Lunar and Planetary Institute Contribution 871.
Figure 4. Volcano and associated flows northwest of Mead basin. Although distinct flow
boundary does not exist, combined topography and image data show clearly that flows from
20-km-diameter high-standing volcanic feature have covered parts of ejecta blanket immediately east of volcano. Thin dashed lines are topography at 300 m contour interval. Image is 112.5
3 150 km.
inner rings are buried by melt or breccia
units, and at intermediate distances melt
rocks may overlie ballistically emplaced
ejecta. However, there may be places drilled
that are just within the outer topographic
ring, and there ejecta deposits will directly
overlie apparently undeformed basement
material. In addition, postemplacement viscous relaxation and cooling may make the
top of the melt sheet vary in depth across the
structure. Finally, the sedimentary nature of
the uppermost target material at Chicxulub
may cause these layers to behave as a lesscoherent rock mass than the igneous surface
rocks on Venus. Thus, Chicxulub’s outer
rings may not be defined by a single fault
scarp, but rather as a series of smaller stepdown faults, as occurs for part of the outer
rim at Mead.
14
DISCUSSION
Two observations presented here have
particularly far reaching implications for
studies of the geologic and tectonic history
of Venus. First, Mead is clearly embayed by
postimpact volcanism, but an unambiguous
interpretation would have been difficult
without the topographic data. The material
that has covered parts of Mead’s ejecta does
not have obvious flow margins in the Magellan data and has radar properties indistinguishable from the surrounding plains.
Potentially hundreds of craters (and their
ejecta deposits) classified by previous workers as unembayed (e.g., Schaber et al., 1992;
Phillips et al., 1992) have been surrounded
and partially covered by subtle embayment.
The random and apparently unembayed
nature of the majority of the craters has
Printed in U.S.A.
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Manuscript received April 28, 1995
Revised manuscript received October 2, 1995
Manuscript accepted October 10, 1995
GEOLOGY, January 1996