Plate tectonic emplacement of upper mantle peridotites along continental edges

Abstract

Recently developed ideas of global tectonics haye provided a new framework within which to consider the origin of alpine-type peridotites. In plate theory, compressional zones associated with island arcs are considered to represent plate boundaries where oceanic lithosphere is subducted. The subduction zones are characterized by lithospheric underthrusting, andesitic volcanoes, and deep seismic activity that generally dips under the continental edge (the Benioff zone). The presence of large oceanic-mantle crustal slabs thrust over or into continental edges contemporaneously with blueschist metamorphism in New Caledonia and New Guinea establishes an important variant of plate tectonics in the zones of compression. The ‘obduction’ zones are characterized by a complete lack of volcanic activity and by high-pressure metamorphism. During formation, they can be represented by shallow seismic zones dipping oceanward. The common association of peridotites and blueschists in these orogenic belts may result from the initial stage of compressional impact (or orogeny) between an oceanic and a continental lithospheric plate. Disturbed zones combined with a lack of high-temperature contacts at boundaries between cold mantle-peridotite slabs and trench sediments provide geologic evidence of emplacement by obduction (tectonic overriding). Internal subsolidus plastic deformation of these peridotites can be attributed to deep-seated strain within the upper mantle during spreading. Serpentinites represent alteration developed during tectonic emplacement into wet sediments of the continental plate, which produces a less dense and plastic envelope that facilitates further tectonic movement in these compressional zones. Recognition of these peridotite-serpentinite-blueschist belts within exhumed subduction or obduction zones will allow delineation of ancient compressional impacts between moving lithospheric plates.