Statistics – Computation
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.p22b0406o&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #P22B-0406
Statistics
Computation
5104 Fracture And Flow, 5420 Impact Phenomena (Includes Cratering), 5460 Physical Properties Of Materials
Scientific paper
New hydrocode impact calculations of the formation of the Chicxulub structure describe the detailed formation of the overturned flap marking the rim of the transient crater, the broad central uplift with an annular depression of the Moho-outside the central region, the outward collapse of the central peak, and the formation of annular faults and rings outside the radius of the initial transient crater. The structure formed in about 5 minutes. We use the Mohr-Coulomb-Anderson-Holmquist (MCAH) brittle damage model for rock fracture employing parameters for pristine and fractured rock measured in well understood laboratory experiments. We do not evoke other weakening mechanisms (e.g., acoustic fluidization). The projectile and the Earth's crust (thickness = 33 km) was modeled with the equation of state of granite and the underlying mantle (depth > 33 km) with the equation of state of dunite. We varied the 20 km/s velocity impactor radius from 5.0 to 7.5 km. For the target rocks, we assume a MCAH rheology with an undamaged internal friction (μ u ) of 0.5 to 1.5, and shock and deformation-induced damaged internal friction (μ d), of 0.1 to 0.5. The limiting von Mises strength was 2.4 GPa. The size of the damage zone is a function of integrated strain to failure. The rock damage distribution which is calculated during the impact evolution is approximately hemispherical and has a maximum radius of approximately twice that of the 40 to 55 km radius of the transient crater cavity. We found that the damage distribution determines: 1) the transient cavity dimensions ( e.g. depth of penetration), 2) Moho undulations, 3) ejecta lofting angles, 4) the occurrence of a central peak and the detailed dimensions, 5) the number and radii of terrace/slump faults, and 6) the radii and amplitude of final surface undulations (rings) extending outward of the circular faults. Within an integrated computation, we calculate the projectile penetration through 200 km of atmosphere, the formation of a 50 km deep transient cavity, and the collapse of the transient cavity to form the final crater morphology. Upon impact, the projectile lines the transient cavity and produces an associated melt layer. During the transient cavity collapse, the melt flows near the centerline and forms a thin layer on top of the peak ring. The peak ring forms as a result of the collision of the down ward flowing transient central peak with the nearly vertically launched cavity flow. The radius of the overturned stratigraphy is a measure of the transient cavity size and thus the energy of impact. The terraced zone faulting is initiated during the over folding of the ejecta curtain and proceeds during the slumping of material in front of the ejecta curtain. The slumping is part of the transient cavity collapse flow field. An asymmetric ring fault is formed that terminates the faulting in the terrace zone and also extends downward to the Moho. This ring is often designated as the crater rim. We calculate this diameter to be 150 km. We find ~20 km of central uplift of material above the Moho, and small positive and negative undulations of the Moho near the centerline. Finally a ~200 km diameter exterior topographic high ring is formed which is the result of the secondary impact of ejecta deposited upon the region of damaged surface material.
Ahrens Thomas J.
Okeefe J.
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