EVIDENCE FOR A PLATE TECTONICS DEBATE

Abstract

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 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 crust 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 crust. 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 crust and resultant mantle hydration are indicated by diapirs of serpentinized mantle along fracture zones and ridge axes; (2) distinctive isotopic signatures in oceanic basalt require local recycling of crust and sediments to the magma source; (3) 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.