Title: Large impact craters and basins: Mechanics of syngenetic and postgenetic modification
Abstract: The impact crater is the ubiquitous landform of the solar system.
Theoretical mechanical analyses are applied to the modification stage
of crater formation, both syngenetic (immediate or short term) and
postgenetic (long term). The mechanical stability of an impact crater is analyzed via a
quasi-static, axisymmetric slip line theory of plasticity. The yield
model incorporated is Mohr-Coulomb and a simplified rectangular
profile is used for the transient cavity. The degree of stability
(or instability) is described as a function of internal friction angle,
depth/diameter ratio, and a dimensionless parameter ρgH/c (ρ = density,
g = acceleration of gravity, H = depth, and c = cohesion strength).
To match the observed slumping of large lunar craters the cohesion
strength of the lunar surface material must be low (less than 20 bars) and the
angle of internal friction must be less than 2°. It is not implausible
that these failure strength characteristics are realized by freshly
shocked rock. A theoretical description of impact crater collapse is
evolved which accounts for the development of wall scallops, slump
terraces, and flat floors. A preliminary set of scale model experiments
performed in a centrifuge corroborate the theory. The strength of
terrestrial planet surfaces under impact is seen to vary by as much as
a factor of two. Shortly after the excavation of a large impact crater the transient
cavity collapses, driven by gravity. It is shown that at least one
concentric fault scarp forms about the crater, if the strength of the
target material decreases sufficiently rapidly with increasing depth.
This is demonstrated by two classes of models: extrusion flow models
which assume a weak layer underlying a strong layer, and plastic flow
models which assume a continuous decrease of cohesion strength with
depth. Both classes predict that the ratio of the radius of the scarp
to the transient crater radius is between 1.2 and 2 for large craters. Large impact basins on Ganymede and Callisto are characterized by
one or more concentric rings or scarps. The number, spacing, and
morphology of the rings is a function of the thickness and strength of
the lithosphere, and crater diameter. When the lithosphere is thin and
weak, the collapse is regulated by flow induced in the asthenosphere.
The lithosphere fragments in a multiply concentric pattern (e.g.,
Valhalla, Asgard, Galilee Regio, and a newly discovered ring system on
Callisto). The thickness and viscosity of a planetary lithosphere
increases with time as the mantle cools. A thicker lithosphere leads
to the formation of one (or very few) irregular normal faults concentric
to the crater (e.g., Gilgamesh). A gravity wave or tsunami induced by
impact into a liquid mantle would result in both concentric and radial
extension features. Since these are not observed , this process cannot
be responsible for the generation of the rings around the basins on
Ganymede and Callisto. The appearance of Galilee Regio and portions
of Valhalla is best explained by ring graben, and though the Valhalla
system is older, the lithosphere was 1.5-2.0 times as thick at the time
of formation. The present lithosphere thickness is too great to permit
development of any rings. It has been proposed that a mascon may be in the form of an
annulus surrounding the Caloris basin on Mercury, associated with the
smooth plains. The effects (stresses, deformation, surface tectonic
style, gravity anomalies, etc.) of such a ring load on a floating
elastic lithosphere of variable thickness are investigated. The main
characteristics of the surface tectonic pattern are normal faulting
within the basin and thrust faulting beneath the ring load~ both in
agreement with observation Moreover, the dominant concentric trend
of the basin normal faults is consistent with the ring load hypothesis
provided the mercurian lithosphere was ≤125 km thick at the time of
faulting. Simple updoming within the basin would produce normal
faults of predominantly radial orientation.
Publication Year: 1981
Publication Date: 1981-10-01
Language: en
Type: dissertation
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Cited By Count: 5
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