Other
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.v24b..01a&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #V24B-01
Other
1065 Major And Trace Element Geochemistry, 8103 Continental Cratons, 8125 Evolution Of The Earth (0325)
Scientific paper
Tectonic processes on the early Earth were controlled largely by heat transfer from the deep Earth to the surface. Thermal modeling shows that convective heat transfer driven by heating from below results in hot material rising in columns, which is thought to be the dominant mode of Hadean heat transfer. In contrast, Phanerozoic-style plate tectonics probably requires liquid water to cool lithospheric plates through hydrothermal convection, followed by the sinking of these cool, dense plates gravitationally. We present a speculative model for the formation of greenstone belts at the sites of plume-like up-wellings to form oceanic plateaux, and their preservation at zones of convergence. This model is based on a thermal convection model (Zhong et al 2000, JGR) in which hot plumes rise from the core-mantle boundary and melt to form a proto-oceanic crust that is thicker than modern oceanic crust, and resembles oceanic plateau crust. Sinking of this proto-oceanic crust at zones of convergence is essentially vertical and symmetrical along convection cell walls. Geometric constraints require that the thick plateau must spread laterally as it moves radially away from the rising plume, in order to preserve crustal volume as the surface area increases. Convergence at triple junctions where three cells meet will focus this radial flow back into a single descending "anti-plume." Converging proto-oceanic crust may tend to be preserved here, especially if the position of this triple-junction is unstable. This will form deltoidal blocks of greenstone above the triple junction that are subject to compressive stress during convergence. Proto-oceanic crust that does sink will partially melt to form TTG suite magmas that rise and intrude the overlying greenstone belt. The residue of this melting will be eclogite similar to that formed by subduction processes today. It is expected that this process would be disrupted by basin-forming impacts that culminate around the end of the Hadean (circa 3.85 Ga) as seen on the Moon. The transition to modern-style plate tectonics is marked by (1) the formation of oceanic crust at mid-ocean ridge spreading centers, (2) the formation of island arc volcanic and plutonic complexes, and (3) the formation of accretionary mélange complexes during the subduction of oceanic crust. Rock assemblages characteristic of oceanic crust and island arc suites may also be formed during Hadean-style convection, making the recognition of accretionary mélange complexes the most reliable indicator of Phanerozoic-style tectonics. Accretionary mélange complexes result from the juxtaposition of oceanic crust and pelagic sediments in a matrix of arc-derived constituents, and may be recognized even after high-grade metamorphic events. These lithotectonic elements are now intimately intermixed but could not have formed in a single tectonic setting. The Farmington Canyon Complex (Utah) may represent an example of an Archean accretionary complex, with both oceanic components (gabbro, basalt and komatiite lavas, chert) and arc components (represented by the mélange matrix, now felsic gneiss with a greywacke composition). Recognition of accretionary mélange complexes may be a reliable indicator of Phanerozoic-style plate tectonics, and suggest that the initiation of recognizable plate tectonic processes occurred by 3.0 Ga, which coincides with the oldest Phanerozoic-style ophiolite assemblages. This transition may have started as early as 3.8 Ga, which coincides with the end of the terminal lunar bombardment.
Andreasen K.
Shervais John W.
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