Statistics – Computation
Scientific paper
May 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agusm.t22a..02d&link_type=abstract
American Geophysical Union, Spring Meeting 2002, abstract #T22A-02
Statistics
Computation
3210 Modeling, 5420 Impact Phenomena (Includes Cratering), 8450 Planetary Volcanism (5480), 9305 Africa, 1517 Magnetic Anomaly Modeling
Scientific paper
Ron Girdler et al. suggested a possible impact origin for the Bangui magnetic and gravity anomaly in Central Africa.1 Paul DeCarli and Ron debated the subject vigorously but politely during each of his visits to California for the Fall AGU Meeting. We honor Ron's memory by continuing the debate. The striking feature of the inferred crater is its size, 800 km diameter. The three largest identified impact craters known on Earth do not exceed 200 km diameter. Grieve has estimated that, based on the record of craters on the Moon, there should be more than a hundred 1000 km-diameter impact structures on the Earth. Allowing for impacts on subsequently subducted oceanic crust, one would expect to find evidence for as many as thirty 1000 km-diameter impact craters on continental crust. If the estimated number of large craters is not grossly in error, one may infer that there could be some natural process that obliterates the evidence for very large craters. We recognize that most of these large craters will be old, ca. 4000 Myr. However, the ca. 200 km diameter Vredefort and Sudbury structures are approximately 2000 Myr old and clearly recognizable as of impact origin. We suggest that very large craters, ca 800 km dia, will probably be inundated by impact-induced volcanism. Impact-induced terrestrial volcanism has been proposed by other workers, but the detailed mechanism has not been treated quantitatively. Consequently, the concept has received little acceptance. We examine the consequences of an impact that would produce an 800 km diameter crater. We note that the final crater diameter is achieved by gravitational modification of a bowl-shaped transient crater. Studies of large lunar and terrestrial craters imply that the transient crater diameter is about half the final diameter. We therefore attempt to infer a range of impact parameters that would produce a 400 km diameter transient crater. Simple semi-empirical scaling relations have been derived from a combination of cm-scale laboratory experiments and generic computational simulations. These scaling relations imply an impacting object having a diameter in the range of 50 to 100 km, assuming a chondritic impactor and an impact velocity of 20 km/s. To assess the possibility that volcanism might be triggered, one must estimate the depth of the transient crater. The aspect ratio, crater depth/crater diameter will depend on the nature of the impactor. A nickel-iron impact will produce a relatively deeper crater than a chondrite impact. Preliminary estimates indicate that the lower bound on the depth is about 80 km. This depth appears sufficient to trigger large-scale decompression melting. Results of computational modeling of the formation of very large impact craters will be presented.. If the Bangui anomaly is the fossil of a very large impact, why is there no evidence of large scale volcanism? We suggest that the answer is simple; the Bangui anomaly is not of impact origin. 1R.W. Girdler, P.T. Taylor, and J.J. Frawley, Tectonophysics 212 (1992) 45-58
Decarli Paul S.
Jones Adrian P.
Price Geoffrey D.
Price Neville J.
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