Yap Trench Research Articles

Horizontally-moving subducted slab may generate enigmatic features of the Palau and Yap Trench-Arcs

The Philippine-Sea plate in the northwestern Pacific Ocean is moving horizontally toward N55°W at rates of 40 to 90mm/yr (greater southward) relative to underlying asthenosphere. The Palau and Yap Trenches situated at its southeastern margin are moving together with the Philippine-Sea plate. Resistance of viscous asthenosphere against vertically subducted rigid slabs is estimated based upon a simple viscous flow model. It has been shown that appreciably large uplifting pressure is generated in the frontal zone of the vertical slab, while tectonic erosion in the trench bottom proceeds both under the influence of the viscous flow. This model may reasonably explain enigmatic features of the Palau and Yap Trench-Arc systems revealed by detailed topographic mapping and submersible investigation such as (1) large water depths (exceeding 8000m) in spite of their extremely small plate convergence rates, (2) exposure of serpentinized peridotite, gabbro and basalt despite a large amount of slope failure deposits containing a huge reefal limestone block, (3) abnormally close proximity of islands to the trench axis, (4) deficiency of barrier reefs on their trench-facing side in spite of the easterly to northeasterly prevailing wind.

Introduction to the special volume on the Izu-Bonin - Mariana arc system

Introduction to the special volume on the Izu-Bonin - Mariana arc system

Seismic observations at the Yap Islands and the northern Yap Trench

We conducted two seismic surveys in the Yap region to investigate tectonic activity in this area. One was conducted in the northern Yap Trench for ten days using ocean bottom seismometers, the other was conducted on the Yap Islands for eight months using a small array. From these observations we detected many earthquakes beneath the Yap region and found a characteristic pattern of seismicity. Many earthquakes occurred in the inner trench slope, no seismicity was observed in the axial region of the trench, and a few earthquakes occurred in the outer trench slope. This pattern is similar to the typical pattern in active subduction zones. This result indicates that the Yap Trench is still actively subducting. We did not detect earthquakes deeper than 40 km during the observations.

Cache Creek terrane entrapment: Oroclinal paradox within the Canadian Cordillera

Exotic and far‐traveled oceanic crustal rocks of the Cache Creek terrane (CC) are bordered by less exotic Quesnel (QN) and Stikine (ST) arc terranes to the east, north, and west. All of these terranes are enveloped by an arcuate belt of displaced continental margin rocks; the Kootenay (KO), Nisling (NS), and parts of the Yukon‐Tanana (YTT) terranes, that have indirect ties to ancestral North America (NA). Initial 87Sr/86Sr isopleths conform to this arcuate pattern. Such a pattern of concentric belts presents a geological conundrum: How did the QN, ST, and CC come to be virtually enveloped by terranes with ties to NA? Past and current models that explain assembly of the Canadian Cordillera are deficient in their treatment of this problem. We propose that Early Mesozoic QN and ST were joined through their northern ends as two adjacent arc festoons that faced south toward the Cache Creek ocean (Panthalassa?). Oceanic plateau remnants within the CC today were transported from the Tethyan realm and collided with these arcs during subduction of the Cache Creek ocean. Counterclockwise oroclinal rotation of ST and NS terranes in the Late Triassic to Early Jurassic caused enclosure of the CC. Rotation continued until these terranes collided with QN in the Middle Jurassic. Paleomagnetic declination data provide support for this model in the form of large average anticlockwise rotations for Permian to Early Jurassic sites in ST but moderate clockwise rotations for sites in QN. Specific modern analogues for the Cordilleran orocline include the Yap trench, where the Caroline rise is colliding end‐on with the Mariana Arc and the Banda Arc, located on the southeastern “tail” of the Asian plate, which is being deformed into a tight loop by interactions with the Australian and Pacific plates.

Deformed soft sediments at the junction between the Mariana and Yap Trenches

Highly deformed unconsolidated hemipelagic sediments were found below the sea bed surface at anomalously shallow depths near the junction between the Mariana and Yap arc-trench systems. The order of deformation is (1) sigmoidal veins and spaced foliations, (2) layer-parallel shear planes, (3) reverse faults and kink bands and (4) more veins. All the deformation structures are considered to have been developed progressively during shear parallel to bedding, accompanied by volume decrease due to pore reduction and fluid expulsion during consolidation, while the sediments are not yet lithified.

Obduction processes in ancient, modern and future ophiolites

Summary Geologic and tectonic processes of ophiolite obduction deduced from structural mapping, timing of deformation and metamorphism and palinspastic reconstruction of ancient ophiolite complexes compare closely with modern processes operative in the western Pacific. Many ongoing arguments about the origin of ophiolite complexes (oceanic, island arc or marginal basin) and their emplacement (subduction-related or not) are largely semantic and depend on exactly what individuals regard as island arcs, marginal basins or subduction zones. A review of tectonic processes currently active in the western Pacific region shows a wide range in size, shape and duration of arc-basin complexes and some of these show uniformitarian models similar to those proposed for emplacement of ancient ophiolites. The Tethyan ophiolites of the Semail (Oman Mountains) and Spontang (Western Himalaya of Ladakh-Zanskar) and the Ordovician western Newfoundland ophiolites are compared with recent examples of the Mariana basin, Yap trench, Banda arc, Luzon arc and Papua New Guinea. It is concluded that they are all connected in some way with marginal basins and island arcs in varying stages of development and maturity. Emplacement of these ophiolites is related to continental underthrusting of a trapped marginal basin and accretionary prism comparable to present-day examples of the N Australian margin underthrusting Timor and the Banda Sea and the SE China margin underthrusting the Luzon arc.

Metamorphic rocks of the Yap arc-trench system

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.