Authors:
McKinnon, William Beall
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 fric (...)
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.
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