PLEASE NOTE:
*
CCNet-ESSAY, 14 May 1999
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POST-LATE HEAVY BOMBARDMENT ASTEROID/COMET IMPACT RATES &
OCEANIC
MEGA-IMPACTS: IMPLICATIONS FOR EPISODIC CRUSTAL EVOLUTION AND
PLATE
TECTONICS
Andrew Glikson <geospectral@spirit.com.au>
Research School of Earth Science,
Australian National University,
Canberra, A.C.T. 0200
Recent discoveries of comet clusters, large buried multi-ring
impact
structures, and impact fallout deposits from impact basins on the
scale
of >500 km-diameter, are providing new perspective on the
terrestrial
impact history and its consequences for crustal evolution. Lunar
mare
crater counts, the terrestrial impact flux, and astronomical
observations of asteroids and comets define impact rates within
the
same order of magnitude (4-6*10^-15.km^-2.yr^-1 - for craters
>= 20
km-diameter) within the inner solar system since the end of the
Late
Heavy Bombardment (LHB) ~3.8 Ga ago [1]. When coupled with the
observed
crater size vs cumulative crater size-frequency power law
relationship
(N ~ Dc^-1.8; N = cumulative number of craters of diameter >
Dc), these
rates imply formation on Earth of > 450 craters of Dc >=
100 km, > 50
craters of Dc >= 300 km, and > 20 craters of Dc >= 500
km since 3.8 Ga
[2]. Extraterrestrial geochemical signatures, including 53Cr/52Cr
isotopes, Ni-chromites-bearing quench-condensation spherules, and
PGE
anomalies showing volatile species (Pd and Au) depletion
patterns,
allow identification of fallout deposits of Archaean impact
basins
several hundred km-large [3-4]. Geochemical and isotopic
constraints
require that more than 80 percent of the projectiles impacted on
time-integrated oceanic crust since the LHB. The modelled
consequences
of injection of shock energies on the scale of >10^8 megatons
TNT-equivalent by oceanic impact of projectiles of Dp > 10
km-diameter
suggest development of propagating fractures, rift networks,
thermal
perturbations, and ensuing magmatic activity. Impact clusters are
observed in the late Devonian, late Triassic, Jurassic-Cretaceous
boundary, Cretaceous-Tertiary boundary, and late Eocene. The
Mesozoic-Cainozoic impact flux requires ~ 30 craters >=100
km-diameter
and ~ 3 craters >= 300 km-diameter, including 12 oceanic
impacts. A
minimum observed rate is defined by the 6 documented continental
craters of >=80 km-diameter. Isotopic ages of crater clusters
and flood
basalts increasingly allow tests of possible genetic
relationships
between these clusters, episodic continental flood volcanism,
rifting
and continental breakup [5,6].
[1] Shoemaker, E.M., Shoemaker, C.S.. The Proterozoic
impact record of
Australia. AGSO J. Aust. Geol. Geophys. 16 (1996) 379-398;
[2]
Glikson, A.Y., 1999. Oceanic mega-impacts and crustal
evolution.
Geology, 27:387-341; [3] Byerly, G. R., Lowe, D. R.,
1994. Spinels
from Archaean impact spherules: Geochim. et Cosmochim.
Acta,
58:3469-3486 ; [4] Shukolayukov, A., Kyte, F.T., Lugmair, G.W.
and
Lowe, D.R., 1998. The oldest impact deposits on Earth - first
confirmation of an extraterrestrial component (abstract),
Cambridge
meeting on Impacts and the Early Earth; [5] Alt, A.D., Sears,
J.W.,
Hyndman, D.W., 1988. Terrestrial maria: The origins of large
basalt
plateaus, hotspot tracks and spreading ridges. J. Geol., 96:647–662;
[6] Rampino, M.R., Caldeira, K., 1993. Major episodes of geologic
change: correlation, time structure and possible causes. Earth
Planet. Sci. Lett., 114, 215–227.
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