Systematics of large-scale tectonics and age progressions in Alpine and Circum-Pacific blueschist belts

Abstract

The early Alpine tectonics and relatively high-pressure metamorphic parageneses of the Helvetic, Pennine and Sesia—Lanzo realms are compared with analogous circumpacific terranes in western California, southwestern Japan, west-central Chile, and southern Alaska. Petrotectonic relationships appear to be compatible with a process involving syntectonic recrystallization and pervasive deformation in an imbricated subduction zone for each of these regions. Successive underthrusting of younger portions of the downgoing low-density material, followed by decoupling and rise towards the surface may account for the systematic decrease in both age and metamorphic intensity proceeding away from the old plate junction in these terranes. The environs of the present Aleutian trench serve as a modern analogue. Several other circumpacific blueschist belts (i.e., New Zealand, New Caledonia and eastern Papua) exhibit possible departures of timing, metamorphic polarity or direction of tectonic transport from that appropriate to the imbricated convergent plate-junction model presented in this paper, but the overall metamorphic sequences and tectonic settings are nevertheless strikingly similar. Many relatively high-pressure belts seem to be characterized by the occurrence of a major post-metamorphic transcurrent fault. This phenomenon suggests that blueschist belts, which must be exhumed rapidly in order to preserve the relatively high-pressure, low-temperature mineral assemblages, typically are obliterated through higher-temperature recrystallization unless a profound change from convergent to conservative plate motion occurs. Variation in convergence rate is yet another important factor, more rapid underflow promoting generation of blueschistic metamorphic parageneses, slower underflow favoring shallow decoupling and buoyant return towards the earth's surface of the successive underplatings. Other things being equal, a steeper angle of underflow would provide lower temperatures at a given depth for the subducted slab, thus promoting glaucophane schist-type metamorphism.