Metamorphic rocks of the Yap arc-trench system

Abstract

The Yap trench-arc is a link between the Mariana and Philippine arcs; the latter are both loci of acive volcanism and seismicity but the Yap arc is formed of metamorphic rocks and has had few historic earthquakes. It does not appear to be an active subduction zone. The 8–9 km deep Yap trench has a steep west well, it has little or no sediment ponds on the trench slope. There is no modern volcanism and the few volcanic rocks present on Yap may be large fragments in a tectonic breccia. The main rock type on Yap is greenschist with textural evidence for intense penetrative deformation. The chemistry of these rocks, especially the Mg, Cr, Ni and Co abundances, suggest that the protolith was an ultramafic rock. Rocks from the trench wall east of Yap include amphibolite facies metabasite and metasediments, e.g. (1) Ca-Al-rich hornblende + labradorite + epidote schist; (2) calcite + diopside marble; (3) grossularite-andradite + diopside + vesuvianite calc-gneiss; and (4) diopside + calcite + epidote calc-gneiss. These units (1–4) are overlain by Yap ultramafic schists; a major tectonic break characterized by a polymictic breccia including clasts representative of Yap schists, metabasites, metasediment, pyroxenite and feldspathic gabbro separates the trench lithologies from the ultramafic schist. Other sites sampled in the trench wall have rocks typical of seamounts. It is proposed that a Tertiary(?) subduction zone was terminated when “un-subductible” seamounts blocked the trench. This led to overthrusting, from the west, of seafloor crust and upper mantle. The volcanic arc ceased to exist, some of its fragments are seen in the tectonic breccia. Termination of subduction may be caused by the blocking of subduction zones by large seamount-atoll chains or thickened oceanic crust such as the Caroline Ridge. In some cases this may lead to obduction of oceanic lithosphere.