Plate tectonics and island arcs

The plate-tectonic concepts that developed rapidly in the late l960s made possible the understanding of island arcs. Before that time, mobilistic concepts evolved slowly, hindered, particularly in the United States, by an obstructionist geoscience establishment.The volcanic belts of island arcs form about 100 km above subducting plates. Convergent-plate boundaries evolve complexly with time and, at any one time, vary greatly along their lengths. Seismicity defines positions, but not trajectories, of descending slabs, which sink more steeply than they dip and are overridden by advancing upper plates. Subduction occurs beneath only one side at a time of an internally rigid plate, and the common regime in an overriding plate, behind a surficial accretionary wedge, is extensional, except where a collision is underway. Back-arc-basin lithosphere is built behind, or by, migrating island arcs, which lengthen and increase their curvatures. A collision can involve two active arcs, in which case, intervening lithosphere sinks beneath both of them, or an active margin and a passive one. Either type of collision generally is followed by the breaking through of new subduction, beneath the composite mass of light crust, from a new trench on the outside of the aggregate; conversely, a new subduction system commonly is a by-product of collision. A strip of back-arc-basin crust is in many cases left attached to the aggregate, in front of the new trench, and becomes the basement for a fore-arc basin, the leading edge of which is raised as melange is stuffed under it.Sedimentation in trenches is predominantly longitudinal and can be from distant sources. Accretionary wedges are dynamic, being thickened at both toes and bottoms by tectonic accretion and thinned by gravitational forward flow; melange is largely a product of tectonic imbrication and flowage driven by these conflicting processes, not of submarine sliding. High-pressure metamorphic rocks form beneath overriding plates, not within wedges in front of them.Arc magmas incorporate much material from the lithosphere through which they rise and vary correspondingly with the evolving composition of that lithosphere. Arc crust is inflated into geanticlines by intrusive rocks and thermal expansion. Submarine island-arc volcanic rocks are widely spilitized, with Na enrichment and Ca depletion, by hydrothermal reaction with sea water. The lower crust of mature island arcs consists of granulite-facies rocks of mafic, intermediate, and felsic-intermediate compositions. The Mohorovicic discontinuity may be primarily a constructional boundary, representing the shallow limit off crystallization of voluminous rocks of ultramafic composition or plagioclase-free mineralogy.