HR: 0800h
AN: T11C-1278    [Abstracts]
TI: Plate Tectonics: From Plate Boundary Zone Tectonics To Extensive Intraplate Tectonics
AU: * Ishikawa, M
EM: ishikawa@ynu.ac.jp
AB: Plates makes up earth's surface, and tectonic activity is generally concentrated on plate boundary zones. In restrict meaning, plate tectonics of the earth is regarded as mixture of plate boundary zone tectonics and extensive intraplate tectonics. For example, the Asian continent never behaves as rigid plate that was deformed extensively when the Indian continent collided with it. I infer that extensive intraplate tectonics reflects rheological weakening of wet mantle. To demonstrate effect of H$_{2}$O component on plate strength, one-dimensional rheological profiles of 100 km depth were constructed by assuming 20km thick upper crust and 20km thick lower crust. Temperature-depth profiles were calculated based on one-dimensional steady-state static heat transfer at given surface heat flows. Power law creep and Byerlee_fs law were used to estimate strength in ductile regime and brittle regime respectively. Creep strength for upper crust, lower crust, dry mantle and wet mantle were calculated using creep parameters of granite, granulite, dry dunite and wet dunite. The minimum value between power law creep strength and Byerlee_fs law strength gives the strength of the lithosphere. Strength profile at surface heat flow of 55mW/m$^{2}$ (continental average is 56.5mW/m$^{2}$) and strain rate of 10$^{-15}$/s (intraplate deformation is about 10$^{-15}$/s - 10$^{-16}$/s in Asia) shows a significant difference in strength for using dry mantle and wet mantle. In case of dry mantle, the uppermost mantle is quite strong. However, if wet peridotite represent the upper mantle, there is very little strength in the uppermost mantle. The cumulative lithospheric strength, i.e. integral strength from surface to 100km depth, and the cumulative mantle strength, i.e. integral strength from 40km to 100km depth were calculated with changing strain rate. For example, to deform continental lithosphere at strain rate of 10$^{-15}$/s, wet mantle has a cumulative strength of about 2x10$^{12}$N/m whereas the cumulative strength of dry mantle is about 15x10$^{12}$N/m. The weakening of wet mantle, therefore, result in the dramatic reduction of cumulative strength of lithosphere from 17x10$^{12}$N/m (dry) to 3x10$^{12}$N/m (wet). Forces of ridge push appear to be in the range of 3.9x10$^{12}$N/m. Ridge push is probably the principal driving force for intraplate deformation after amalgamation of continent. Dry upper mantle carries much of the ridge push force. The magnitude of cumulative lithospheric strength is much higher than that of wet lithosphere and far exceeds the ridge push forces. In contrast, the cumulative strength of wet lithosphere is comparable to the ridge push force, suggesting ridge push forces are enough to cause intraplate deformation of wet continental plate. I infer that slab dehydration along numerous subduction zones during collision of microcontinent and island arcs at late Paleozoic/Mesozoic caused extensive wetting of uppermost mantle beneath Asia, resulting in extensive weakening of plate. Since Proterozoic/Phanerozoic boundary, collision events such as East African orogeny, Variscan orogeny, Alpine orogeny and Himalayan orogeny have involved relatively extensive intraplatel tectonics in Gondwana supercontinent, Pangea supercontinent, Europe continent and Asia continent respectively whereas Archean/Proterozoic continental collisions give rise to plate boundary zone tectonics. It probably reflects initiation of return flow of H$_{2}$O component into mantle at about 750 million years ago (Maruyama, 1999). I conclude that fragile continental plate have been established during late Proterozoic to Phanerozoic assembly of continent, and extensive intraplate tectonics started to operate on earth at Proterozoic/Phanerozoic boundary.
DE: 8100 TECTONOPHYSICS
DE: 8102 Continental contractional orogenic belts
DE: 8125 Evolution of the Earth
DE: 8149 Planetary tectonics (5475)
DE: 8159 Rheology--crust and lithosphere
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting