The impact crater is the ubiquitous landform of the solar system.Theoretical mechanical analyses are applied to the modification stageof crater formation, both syngenetic (immediate or short term) andpostgenetic (long term).
The mechanical stability of an impact crater is analyzed via aquasi-static, axisymmetric slip line theory of plasticity. The yieldmodel incorporated is Mohr-Coulomb and a simplified rectangularprofile 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 cohesionstrength of the lunar surface material must be low (less than 20 bars) and theangle of internal friction must be less than 2°. It is not implausiblethat these failure strength characteristics are realized by freshlyshocked rock. A theoretical description of impact crater collapse isevolved which accounts for the development of wall scallops, slumpterraces, and flat floors. A preliminary set of scale model experimentsperformed in a centrifuge corroborate the theory. The strength ofterrestrial planet surfaces under impact is seen to vary by as much asa factor of two.
Shortly after the excavation of a large impact crater the transientcavity collapses, driven by gravity. It is shown that at least oneconcentric fault scarp forms about the crater, if the strength of thetarget material decreases sufficiently rapidly with increasing depth.This is demonstrated by two classes of models: extrusion flow modelswhich assume a weak layer underlying a strong layer, and plastic flowmodels which assume a continuous decrease of cohesion strength withdepth. Both classes predict that the ratio of the radius of the scarpto the transient crater radius is between 1.2 and 2 for large craters.
Large impact basins on Ganymede and Callisto are characterized byone or more concentric rings or scarps. The number, spacing, andmorphology of the rings is a function of the thickness and strength ofthe lithosphere, and crater diameter. When the lithosphere is thin andweak, 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 onCallisto). The thickness and viscosity of a planetary lithosphereincreases with time as the mantle cools. A thicker lithosphere leadsto the formation of one (or very few) irregular normal faults concentricto the crater (e.g., Gilgamesh). A gravity wave or tsunami induced byimpact into a liquid mantle would result in both concentric and radialextension features. Since these are not observed , this process cannotbe responsible for the generation of the rings around the basins onGanymede and Callisto. The appearance of Galilee Regio and portionsof Valhalla is best explained by ring graben, and though the Valhallasystem is older, the lithosphere was 1.5-2.0 times as thick at the timeof formation. The present lithosphere thickness is too great to permitdevelopment of any rings.
It has been proposed that a mascon may be in the form of anannulus surrounding the Caloris basin on Mercury, associated with thesmooth plains. The effects (stresses, deformation, surface tectonicstyle, gravity anomalies, etc.) of such a ring load on a floatingelastic lithosphere of variable thickness are investigated. The maincharacteristics of the surface tectonic pattern are normal faultingwithin the basin and thrust faulting beneath the ring load~ both inagreement with observation Moreover, the dominant concentric trendof the basin normal faults is consistent with the ring load hypothesisprovided the mercurian lithosphere was ≤125 km thick at the time offaulting. Simple updoming within the basin would produce normalfaults of predominantly radial orientation.
