Driving forces of orogeny, with emphasis on blue-schist facies of metamorphism (test-case III: The Japan arc)

Blue-schists are the product of an eo-alpine metamorphic event in former trenches (foredeeps), under conditions of high confining pressures and relatively low temperatures. The metamorphic sedimentary rock sequences are generally associated with basic to ultra-basic rocks of the ophiolitic suite (and their metamorphic equivalents), both appearing in belts of partly plastically deformed, partly highly imbricated Pennine-type nappes.The metamorphism and tectonization of the foredeeps (trenches) with their sedimentary and igneous contents occurred during the overriding of major Austroalpine nappes from the concave (inner) side of the orogenic arcs. The foreland is passively subducted during this major orogenic phase (the ‘Flysch phase’ of Alpine-type orogeny).In those cases where the foreland is a continental lithosphere (such as the foreland of the Alpine belt in the Mediterranean) the subduction is less deep than in those cases where the “foreland” is formed by an oceanic lithosphere (such as in the case of the Japan arc).The geological setting of blue-schists in the Alpine system of the western Mediterranean shows that the load pressures produced by the Austroalpine nappes were generally lower than the confining pressures required by experimental data. Tectonic overpressures, produced during phases of high strain-rates which temporarily sealed off the pore fluids, may have produced the supplementary amount of confining pressure. An additional advantage of this hypothesis is, that it accounts for the occurrence of glaucophane-type metamorphism without such unusually low thermal gradients as those prognosticated by petrological experiments.Circum-Pacific blue-schist belts might be the result of deep subduction. However, the problem of rapid exhumation is then the stumbling block for the geodynamic model of plate-tectonics (Model I). The field data of observation do not confirm the expectation of this model, that subsequently dozens of kilometers of isostatic uplift occurred, exposing erosion levels from such great depths.On the other hand, the model of mantle-diapirism (Model II) suggests, that even if the trench deposits were subducted to great depths (passive subduction) upthrusting movements radiating from ‘Stockwerke’ of gravitational instability would drag up the high-P, low-T metamorphism immediately after its formation, in one and the same major phase of orogeny (flysch phase).The geological case-history of the Japan arc is analysed and its diagnostic facts are compared with the predictions (prognoses) of Model II. This verification leads to a confirmation of the functional correctness of this model of mantle-diapirism.A geodynamic analysis of the tectonic evolution and the present structural and geophysical situation of the Japanese realm leads to the conclusion that the driving forces of orogeny and seismicity are produced by stress-fields of gravitational instabilities that may occur in various structural levels (“Stockwerke”). These Stockwerke range in depth from near-surface to the top of the lower mantle, that is over a range about one thousand kilometers. This is a more complicated, but yet more realistic interpretation of the driving forces of orogeny, than the oversimplified views of plate-tectonics.