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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