Physics
Scientific paper
May 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agusm.v11a..03g&link_type=abstract
American Geophysical Union, Spring Meeting 2004, abstract #V11A-03
Physics
5420 Impact Phenomena (Includes Cratering), 5455 Origin And Evolution, 5499 General Or Miscellaneous
Scientific paper
Previous models of the effects of larger scale impacts on early (~ 4.6 - 3.8 Ga) terrestrial crustal evolution have relied on analogies with the moon. There are, however, some important differences. Due to the Earth's larger gravitational cross-section, it will have been subjected to more impacts of unit mass and these impacts will have a higher relative energy density than the moon. Most importantly, the relatively higher planetary gravity of the Earth will have resulted in more severe effects due to the differential scaling of crater dimensions relative to the volume of the target recording shock metamorphic effects with event size. As a consequence, impact melt volumes will exceed the volumes of target material excavated and displace by the cratering flow-field at transient cavity diameters > 400 km on Earth, compared to > 3000 km on the moon. At these sizes, the terrestrial equivalents of lunar impact basins will have formed in strengthless melt pools and would not have the "traditional" morphology of lunar multi-ring basins. The Earth's earliest crust was likely basaltic. Basaltic bodies in the terrestrial environment differentiate, as a function of thickness. Thus, these multi-tens of kilometer deep melt pools would have differentiated, in much the same manner as the impact melt sheet (Sudbury Igneous Complex) at the 1.85 Ga ~ 250 km in diameter Sudbury Structure. The largest envisaged impact event(s) have the potential to result in the production of ~ 0.5 x 109 km3 of felsic differentiates. Averaged over the ~ 800 Ma of "heavy bombardment", this is ~ 30% the present annual rate of growth of continental crust. The character of the earliest continental crust on Earth is speculative, but the partial melting of hydrated oceanic crust beneath some unspecified buoyant crust is a common theme for the production of the earliest tonalitic and trondhjemitic rocks. The felsic differentiates of the large impact melt pools are candidates for this earliest buoyant crust and may have played a role in the formation and evolution of protocontinents.
Cintala Mark J.
Grieve Richard A.
Therriault Ann M.
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