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Vaalbara and Tectonic Effects of a Mega Impact in the Early Archean 3470 Ma

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T. E. Zegers
T. E. Zegers
T. E. Zegers
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Adriana C Ocampo at NASA Headquarters
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Abstract

The oldest impact related layer recognized on Earth occur in greenstone sequences of the Kaapvaal (South Africa) and Pilbara (Australia) Craton, and have been dated at ca. 3470 Ma (Byerly et al., 2002). The simultaneous occurrence of impact layers now geographically widely separated have been taken to indicate that this was a worldwide phenomena, suggesting a very large impact: 10 to 100 times more massive than the Cretaceous-Tertiary event. However, the remarkable lithostratigraphic and chronostratigraphic similarities between the Pilbara and Kaapvaal Craton have been noted previously for the period between 3.5 and 2.7 Ga (Cheney et al., 1988). Paleomagnetic data from two ultramafic complexes in the Pilbara and Kaapvaal Craton showed that at 2.87 Ga the two cratons could have been part of one larger supercontinent called Vaalbara. New Paleomagnetic results from the older greenstone sequences (3.5 to 3.2 Ga) in the Pilbara and Kaapvaal Craton will be presented. The constructed apparent polar wander path for the two cratons shows remarkable similarities and overlap to a large extent. This suggests that the two cratons were joined for a considerable time during the Archean. Therefore, the coeval impact layers in the two cratons at 3.47 Ga do not necessarily suggest a worldwide phenomena on the present scale of separation of the two cratons. Although the impact 3470 Ma impact may have been more limited in size than previously thought, it is interesting to test if geological events described for the Pilbara and Kaapvaal Craton may represent the structural and magmatic results of an impact. The time series correlation between Lunar and Earth impact history and periods of high volcanic activity (Abbott and Isley, 2002) suggest that there is a causal relationship between crustal growth and meteorite impacts. The era between 3490 and 3400 Ma represents one of the best documented periods in which felsic continental crust was formed by intrusion and extrusion of TTG (tonalite, trondhjemite, granodiorite) melt. The stratigraphy consists almost entirely of mafic to ultramafic volcanic rocks and minor felsic (TTG) volcanic rocks. In this stratigraphy it may not be possible to distinguish impact melts from normal volcanic rocks. In both the Pilbara and Kaapvaal Craton extensional faults have been described, which were active at ca. 3470 Ma, during felsic volcanism and broadly coeval with the impact layers (Zegers et al., 1996, Nijman et al., 1998). These extensional structures have been interpreted as the result of caldera collapse
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VAALBARA AND TECTONIC EFFECTS OF A MEGA IMPACT IN THE
EARLY ARCHEAN 3470 Ma
T.E. Zegers and A. Ocampo
European Space Agency, ESTEC, SCI-SB, Keplerlaan 1, 2201 AZ Noordwijk,
tzegers@rssd.esa.int
Abstract
The oldest impact related layer recognized on Earth occur in greenstone sequences of the
Kaapvaal (South Africa) and Pilbara (Australia) Craton, and have been dated at ca. 3470
Ma (Byerly et al., 2002). The simultaneous occurrence of impact layers now
geographically widely separated have been taken to indicate that this was a worldwide
phenomena, suggesting a very large impact: 10 to 100 times more massive than the
Cretaceous-Tertiary event.
However, the remarkable lithostratigraphic and chronostratigraphic similarities between
the Pilbara and Kaapvaal Craton have been noted previously for the period between 3.5
and 2.7 Ga (Cheney et al., 1988). Paleomagnetic data from two ultramafic complexes in
the Pilbara and Kaapvaal Craton showed that at 2.87 Ga the two cratons could have been
part of one larger supercontinent called Vaalbara.
New Paleomagnetic results from the older greenstone sequences (3.5 to 3.2 Ga) in the
Pilbara and Kaapvaal Craton will be presented. The constructed apparent polar wander
path for the two cratons shows remarkable similarities and overlap to a large extent. This
suggests that the two cratons were joined for a considerable time during the Archean.
Therefore, the coeval impact layers in the two cratons at 3.47 Ga do not necessarily
suggest a worldwide phenomena on the present scale of separation of the two cratons.
Although the impact 3470 Ma impact may have been more limited in size than previously
thought, it is interesting to test if geological events described for the Pilbara and
Kaapvaal Craton may represent the structural and magmatic results of an impact. The
time series correlation between Lunar and Earth impact history and periods of high
volcanic activity (Abbott and Isley, 2002) suggest that there is a causal relationship
between crustal growth and meteorite impacts. The era between 3490 and 3400 Ma
represents one of the best documented periods in which felsic continental crust was
formed by intrusion and extrusion of TTG (tonalite, trondhjemite, granodiorite) melt.
The stratigraphy consists almost entirely of mafic to ultramafic volcanic rocks and minor
felsic (TTG) volcanic rocks. In this stratigraphy it may not be possible to distinguish
impact melts from normal volcanic rocks. In both the Pilbara and Kaapvaal Craton
extensional faults have been described, which were active at ca. 3470 Ma, during felsic
volcanism and broadly coeval with the impact layers (Zegers et al., 1996, Nijman et al.,
1998). These extensional structures have been interpreted as the result of caldera collapse
(Nijman et al., 1998, Van Kranendonk et al, 2002) or as the result of delamination of
lower crustal eclogite (Zegers and Van Keken, 2001). The question remains if there is
any evidence in the geological record that this magmatic event could have been triggered
Large Meteorite Impacts (2003) 4038.pdf
by meteorite impacts. We will examine the possibility that these extensional fault patterns
represent multi ring faults associated with a large impact.
The early Archean greenstone terrains can be regarded as the closest analogue to Martian
geology. Therefore the study of impact features in the Pilbara Craton may have
implications for understanding impact features on Mars.
References:
Abbott, D.H., and A.E. Isley, 2002, Extraterrestrial influences on mantle plume activity, Earth
and Planetary Science Letters, v. 205, p.53-62.
Byerly, G.R., D.R. .Lowe, J.L. Wooden, and X. Xie, 2002, An Archaean impact layer from the
Pilbara and Kaapvaal Cratons. Nature, v. 297, p.1325-1327.
Cheney, E.S., C. Roering, E. Stettler, 1988, Vaalbara (Extended abstract). Geol. Soc. S. Afr.
Geocongr. v.88, p.85-88.
Nijman, W., B.A. Willigers, and A. Krikke, 1998, Tensile and compressive growth structures:
relationships between sedimentation, deformation and granite intrusion in the Coppin Gap
Greenstone Belt, eastern Pilbara, Western Australia. Precambrian Research, v. 88, p.83-108.
Van Kranendonk M. J., A. H. Hickman, R. H. Smithies, D. R. Nelson, and G. Pike, 2002,
Geology and Tectonic Evolution of the Archean North Pilbara Terrain, Pilbara Craton, Western
Australia. Economic Geology Bulletin, V. 97, p. 695-732.
Zegers, T.E., S.H. White, M. de Keijzer, and P. Dirks, 1996, Extensional structures during
deposition of the 3460 Ma Warrawoona Group in the eastern Pilbara Craton, western Australia.
Precambrian Research, v.80, p.89-105
Zegers, T.E. and P.E. van Keken, 2001, Middle Archean continent formation by crustal
delamination. Geology, v.29, p.1083-1086.
Large Meteorite Impacts (2003) 4038.pdf
... Cheney (1996) correlated these two widely dispersed continents, by comparing 100-1000 m thick unconformity bound sequences, which show remarkable similarities in their depositional environment. Sequence stratigraphic correlation is supported by the paleomagnetic data (Zegers et al. 1998;Strik et al. 2003) and chronostratigraphic data (Zegers and Ocampo 2003), which placed Vaalbara to be the earth's earliest craton. Further, it has been noted that the oldest impact-related layers, dated as ca. ...
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Vaalbara (Extended abstract)
  • E S Cheney
  • C Roering
  • E Stettler
Cheney, E.S., C. Roering, E. Stettler, 1988, Vaalbara (Extended abstract). Geol. Soc. S. Afr
  • Jan 1988
  • 85-88
  • E S Cheney
  • C Roering
  • E Stettler
Cheney, E.S., C. Roering, E. Stettler, 1988, Vaalbara (Extended abstract). Geol. Soc. S. Afr. Geocongr. v.88, p.85-88.