Friday, December 16, 2011

The Evolution of Plate Tectonics

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A central problem in understanding the Earth system is the relationship


between mantle convection and near-surface structure, geophysics and


geochemistry. Unexplained anomalies of the plate tectonics (PT) model are the


postulated mechanism of generating the axial rift valley by upward movement of


Custom writing service can write essays on The Evolution of Plate Tectonics




marginal fault blocks, contrary to observations in the Icelandic rift, and the


mechanism by which a postulated broad upwelling plume can yield the required


narrow zone of axial volcanism. These discrepancies are the basis for a


critical reappraisal of other evidence. It is shown that the generation of


oceanic magnetic stripes, which led to the ingenious spreading hypothesis, is


a result of narrowing of a formerly expanded ocean ridge volcanic system, and


resultant sequential cooling of crust and upper mantle, and does not require


ocean floor spreading. An alternative ridge model postulates sub-continent


upwelling, sub-ridge convergent flow and generation of mid-ocean ridge basalt


(MORB) by heterogeneous volatile-promoted melting at sub-axial zones where


oceanic crest is recycled into the mantle. That model is favored by the


results of convection experiments and confirmed by a series of independent


indicators heat flow beneath ridge flanks, coriolis curvature of fracture


zones (FZ), downstream development of convective rolls and near-transform


tectonic rotations. Sub-axial downflow is confirmed by North Atlantic positive


geoid anomalies, by high P-wave velocities deep below the ridge axis, and by


the synclinal flexload structure and compressional stress regime of


near-axial crest. Oceanic island volcanism is attributed to the same process,


crustal recycling at local sites of downflow focused by deep residual masses


that are relatively cold (radioactivity depleted) and viscous (volatile


depleted). The recycling model is confirmed, for both MORB and OIB, and


decompression melting of mantle plumes rejected, on several grounds (1) local


recycling of crest and resultant mantle hydration are indicated by diapirs of


serpentinized mantle along fracture zones and ridge axes; () distinctive


isotopic signatures in oceanic basalt require local recycling of crest and


sediments to the magma source; () the principal oceanic island groups,


exemplified by the Hawaiian islands, lack the positive heat flow anomaly


expected over a plume and are underlain by zones of relatively high seismic


velocities. The proposed local recycling accounts for several previously


puzzling features of global geochemical systems, including the carbon budget,


the lead paradox and the isotopic array of heavy noble gases in oceanic


basalt. Deduced end-member components, defined by isotope ratios, can be


attributed to regionally variable mixtures of upper mantle with terrigenous


and pelagic sediments, subaerial and suboceanic basalt, and with depleted


lower mantle, proposed to have been available during a Pacific-centered


Mesozoic mantle surge. There is no need to appeal to the several currently


favored isolated reservoirs in the upper and lower mantle. Seismic data for


continental ranges, exemplified by the Alpine region, show that the plate


collision model is misleading in view of increasing evidence for upper mantle


deformation by viscous creep. Continental rift systems are re-examined on the


basis that the oceanic axial zone can be traced to the African rifts, via


Afar, and are therefore taken to be a result of a similar dynamic system. It


is shown that dominant upwelling plumes are focused beneath the thick keels of


ancient cratons and that rift systems, typically at craton margins, are


collapse structures in zones of convergence and downflow.


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