Plate Subduction Zone Research Articles

深海底から地球深部を探る 南海トラフにおけるメタンガスの湧出現象

Plate boundaries on the deep seafloor, such as mid-oceanic ridges and plate subduction zones, can be regarded as openings or windows through which much amount of chemical components comes up from or goes down to the earth interior chiefly by seawater convection. Recent observations with research vessels, submersibles, and drilling vessels have brought about many new findings on geochemical characteristics of hydrothermal fluid venting at mid-oceanic ridges and of cold seepage at subduction zones, both of which bear significant parts of chemical flux between the ocean and the lithosphere. This review article focuses on the behaviour of methane gas (CH4) associated with pore fluid emanation (cold seepage) from sediment of the Nankai Trough accretionary prism, which is tectonically compressed by the subduction of the Philippine Sea plate under the Japanese Islands. The origin of CH4 gas is shown to have been investigated in terms of methane/ethane ratios and carbon isotope ratios of CH4. It is also mentioned that CH4 plays an important role in producing hydrogen sulfide in surface sediment to support enormous communities of giant clams (Calyptogena) which have endosymbiotic sulfur oxidizing (chemosynthetic) bacteria in their gills.

A Course for Training Preservice Elementary Teachers in the Content and Processes of Earth Science

Wright State University's preservice teacher-training program has developed a new course for elementary-education majors that is designed to foster both the content and science-process skills they will need to successfully prepare scientifically literate children in their elementary-school classrooms. Each topic covered in the course is complemented by hands-on group activities designed to give the students the opportunity to apply the concept. The nature of scientific inquiry is modelled by an activity using a box containing wrapped chocolates. Students are led to form operational definitions of working hypotheses and theories by answering open-ended questions about the contents. Students are asked to predict the path of lava from a hypothetical volcano given a topographical map. Real-time earthquake data from the National Earthquake Information Service are incorporated into the course. Using these data, students are able to trace plate boundaries and subduction zones. All hands-on activities in the cour...

A kinematic model for evolution of island arc-trench systems

SUMMARY Plate subduction zones are typically characterized by the patterns of surface topography and gravity anomalies consisting of island-arc high, trench low, and outer-rise gentle high. These patterns are stable on the time scale of 106-107 yr. At some subduction zones, regardless of its age, steady uplift of marine terraces formed by eustatic sea-level changes during the last lo5 yr can be also observed. The stable patterns of topography and gravity anomalies and the steady uplift of marine terraces seem to contradict each other. We constructed a kinematic model which could explain the evolution process of island arc-trench systems and demonstrated that this puzzle could be solved by considering the effect of accretion at plate boundaries. In our model the lithosphere-asthenosphere system is represented by a stratified visco-elastic half-space under gravity, which consists of a high-viscosity surface layer and a low-viscosity substratum. The rheological properties of both layers are assumed to be a Maxwell fluid in shear and an elastic solid in bulk. Interaction between oceanic and continental plates is represented by the steady increase of discontinuity in tangential displacement across the plate boundary. The other essential factors considered in our model are accretion of oceanic sediments at plate boundaries, erosion on land, and sedimentation on inner trench wall. We computed the evolution process of island arc-trench systems by using the kinematic model and obtained the following results: the island arc-trench systems grow at nearly constant rates in the early stage of plate subduction. Therefore, at young subduction zones, we can generally expect the steady uplift of marine terraces. The stable patterns of topography and gravity anomalies are formed within several million years after the initiation of plate subduction. When the accretion process continuously proceeds at plate boundaries, these stable patterns gradually migrate seaward as a whole. In such a case, we can expect the steady uplift of marine terraces even at old subduction zones. We demonstrate that our model can consistently explain the observed surface topography, gravity anomalies, and uplift rates of marine terraces at subduction zones.

Earthquake focal mechanisms, moment tensors, and the consistency of seismic activity near plate boundaries

Catalogs of moment tensors for more than 8000 earthquakes provide a more objective and complete description of the earthquake source than do focal mechanisms derived from first motions, and therefore moment‐tensors provide a valuable resource for tectonic analysis. We here present background information about the properties of moment tensors and examples of moment tensor analysis. We also introduce a new statistic, the seismic consistency Cs, which measures the similarity of earthquakes within a group. Cs is 1.0 if earthquakes are all alike and 0.0 if they cancel one another. Triangle diagrams provide a practical method for defining the fraction of normal, strike‐slip, and thrust fault components for an earthquake and are a new graphical method for displaying source properties of groups of earthquakes. We apply these methods to the Harvard centroid moment tensor catalog to study the characteristics of shallow earthquakes (<50 km depth) within 200 km of typical ridge‐transform and subduction zone plate boundaries. In this way, we have reached four major conclusions. First, even along relatively simple plate boundaries, there is considerable variation in the type and orientation of earthquake mechanisms. Second, along individual plate boundaries, groups of thrust, normal, or strike‐slip earthquakes generally have Cs equal to 0.8 or higher. Thus for many types of tectonic analyses it is unnecessary to add moment tensors to study seismic deformation; rather, it is sufficient to add scalar earthquake moments of the individual events. Third, moment tensors for some individual earthquakes are quite different from those produced by slip along a planar fault. However, summing up moment tensors shows that the deformation pattern produced by groups of earthquakes is generally more like fault slip than the pattern of typical earthquakes within the group. Fourth, by dividing the sum of moments by the velocity of plate motion and the length of the boundary, we calculate Rmom, the efficiency of seismic moment produced along individual plate boundaries. For the 12.75 years of data available, normal fault earthquakes along spreading ridges produce moment less efficiently than strike‐slip earthquakes along transforms. These in turn produce moment less efficiently than thrust earthquakes along subduction zones. For ridge‐transform earthquakes, boundaries with fast plate velocities produce seismic moment less efficiently than do boundaries with slow velocities.

The origin of alkaline magmas in an intraplate setting near a subduction zone: the Ngatutura Basalts, North Island, New Zealand

Abstract The Ngatutura Basalts are one of a series of Pliocene-Quaternary alkalic basalt volcanic fields in North Island, New Zealand. They are situated in an intraplate tectonic setting behind the currently active Taupo Volcanic Zone, and 300 km above the subducting slab. The volcanic field consists of 16 small-volume monogenetic volcanic centres composed mainly of eroded scoria cones and lava flows, that occupy an extensional tectonic environment characterized by NE-striking block faulting. In some cases the faults have controlled the localization of volcanic vents. The lavas have restricted compositions, ranging from hawaiites to nepheline hawaiites, and are characterized by enriched LILE, LREE, and HFS elements, with particularly high Nb and Ta, low Ba/Nb, and high Zr/Y and Ce N /Yb N ratios. Nepheline hawaiites are slightly more differentiated than hawaiites and have higher Ce N /Yb N ratios. Petrogenetic modelling suggests that the range in composition was mainly controlled by fractional crystallization of olivine, clinopyroxene, and minor plagioclase and titanomagnetite, which is consistent with the modal phenocryst abundances. Fractionation is explained by side-wall crystallization and flowage differentiation during rapid ascent, rather than gravitative settling in a magma chamber. Ngatutura magmas were probably derived from an enriched garnet lherzolite source within the low-velocity mantle. The process of source enrichment is speculative but our preferred model calls on metasomatizing fluids in the low-velocity zone. There is no geochemical evidence for any influence of the subducted slab on their composition, even though they overlie the Pacific plate subduction zone. This implies that the extent of subduction-related contamination in the mantle wedge is not pervasive, but is confined to a limited region overlying the subducted slab. Also, the “deep mantle plume” responsible for alkalic magmatism must have originated above the slab, because it seems unlikely that such a plume could have occurred at a deeper level and penetrate the slab without some evidence. This therefore limits the depth of origin of these “deep mantle plumes” to less than 300 km.

Subduction-induced pore fluid venting and the formation of authigenic carbonates along the cascadia continental margin: Implications for the global Ca-cycle

Pore fluid venting associated with subduction-induced sediment deformation causes precipitation of calcium carbonate as prominent carbonate chimneys or cement in the accreted sediments across the active continental margin off Oregon and Washington. A depletion of interstitial Ca 2+ with a maximum decrease of 50% relative to seawater Ca 2+ over only 1.5m depth and reduction in porosity in the deformed sediments suggest that interstitial Ca 2+ is removed to form calcium carbonate cement. In contrast, the pore waters of the undeformed abyssal plain sediments show no depletion in dissolved Ca 2+. They are either enriched to a maximum of 5% or show no change in dissolved Ca 2+. Here the background level of CaCO 3 content in the sediment is only 0.1 to 1%. Calcium carbonate precipitation in the deformed sediments probably occurs as the result of upward migration and oxidation of biogenic methane and of the increase in carbonate saturation due to release of excess pore pressure during fluid venting. Upward advection of fluids at rates of 1–28 cm y −1 is predicted from diffusion-advection-reaction models applied to the downcore concentration profiles of dissolved Ca 2+ and NH 4 + in the tectonically-deformed sediments. The range of predicted flow rates is related to the type of calcium carbonate lithification; i.e. slow rates generate cement and fast rates generate chimneys. Carbonate mineral precipitation associated with pore fluid venting requires direct transfer of Ca 2+ from the oceanic basement to the accretionary complex. Such a mechanism leads us to propose that the accretionary complexes of the global plate subduction zones are a major sink for crustal Ca 2+. A global flux of crustal Ca 2+ that is removed by carbonate mineral precipitation may be as muc3 as the hydrothermal Ca-input. This significant Ca-flux, not previously considered in the global geochemical budget, implies that pore fluid venting in subduction zones may also act as a global sink or source for other elements.

Non‐double‐couple earthquake mechanisms in the Nazca Plate Subduction Zone

Earthquakes that occur within the Nazca plate subduction zone tend to fall into two populations: the generally smaller, nearly pure double‐couple events, and the usually larger, non‐double‐couple earthquakes. In fact, there appears to exist a direct relationship between the proportion of the non‐double‐couple component in the earthquake solution and the scalar moment. A similar trend is not observed on the global scale. Furthermore, most of the earthquakes with large non‐double‐couple components occur in the locations where the greatest amount of structural complexity is expected. We therefore hypothesize that the non‐double‐couple mechanisms in this region are, in general, related to the break‐down of the assumption of slip on a planar fault‐surface, one of the fundamental assumptions in obtaining moment tensor source mechanisms in accord with the double‐couple model. This failure is, in turn, related to the extreme structural complexity in this region.

87:6366 Non-double-couple earthquake mechanisms in the Nazca plate subduction zone: Kubas, Agnieszka and S.A. Sipkin, 1987. Geophys. Res. Letts, 14(4):339–342

87:6366 Non-double-couple earthquake mechanisms in the Nazca plate subduction zone: Kubas, Agnieszka and S.A. Sipkin, 1987. Geophys. Res. Letts, 14(4):339–342

Analysis of seismic instability of the Vancouver Island lithoprobe transect

Seismic refraction and reflection surveys and gravity measurements over Vancouver Island, British Columbia, Canada, can constrain a finite-element model of the geodynamics of the subduction zone. Stress estimates obtained from this model have been combined with rock failure criteria to yield a probability measure of seismic risk that assumes seismic events start from a dilute distribution of Griffith cracks. The results are in agreement with the observed seismicity and lead to the suggestion that the dominant mechanism of this oceanic plate subduction zone is gravitational ridge push and mantle convection.

An anomalous low velocity region above the deep earthquakes in the Japan Subduction Zone

P wave travel time residuals for local intermediate and deep focus earthquakes are studied to infer a velocity model of the asthenosphere beneath the southern Okhotsk Sea and the western Japan Sea. Tectonically, both regions belong to plate subduction zones. Large positive travel time anomalies are observed at stations above the deepest earthquakes. Consideration of source, path, and receiver effects leads to a conclusion that within the asthenosphere there is a lower velocity region below about 300 km depth with velocities about 5–10% below the Jeffreys‐Bullen velocity model.

Tectonic Control of Eocene Arkosic Sediment Deposition, Oregon and Washington: ABSTRACT

Chronostratigraphic and geographic studies of Eocene arkosic sandstones suggest deposition during a volcanically quiet interval resulting from the westward jump of the Farrallon-Kula plate subduction zone in Oregon and Washington. The Eocene arkosic sandstones were deposited as part of a broad fluvial plain-coastal plain-shelf margin basin complex extending throughout Oregon and Washington between uplands End_Page 413------------------------------ of Mesozoic rocks. Feldspathic-quartzose sediments were transported from the east by river systems draining granitic terrains perhaps as far away as the Idaho Batholith. Local volcanism within the drainage system, along the coastal plain, and on volcanic islands to the west, added variable amounts of volcanic rock fragments to the feldspathic-quartzose sediments. Chronostratigraphic correlations suggest that the arkosic sandstones were deposited along the margins of the depositional system during the early Eocene, prograded westward during the middle Eocene, and then regressed during the latest Eocene and Oligocene simultaneously with the influx of abundant pyroclastic debris. The pyroclastic material was derived from ancestral Cascade Mountain Range volcanism. Onset of Cascade volcanism, and volcanic-lithic grain dilution of the feldspathic-quartzose sandstones, began in the south during the early middle Eocene and extended northward reaching Washington in the early late Eocene. The Eocene tectonic history of western Oregon and Washington provides a framework for understanding the occurrence of the feldspathic-quartzose rocks in a dominantly graywacke fore-arc provinence. During the early Eocene, a northwest-southeast seamount chain was extruded on the Farallon and Kula plates west of an eastward-dipping subduction zone. Subduction of the oceanic plates moved the seamount chain obliquely toward the subduction zone. In middle Eocene time--49 to 40 m.y.b.p.--the seamount chain reached the subduction zone creating instability in the subduction system and resulting in the westward jump of the underthrust boundary between the Farallon-Kula and North American plates. The westward jump of the underthrust boundary resulted in both the accretion of the seamount chain as part of a newly formed fore-arc accretionary prism, and a decrease in magnetic arc volcanism. The relative decrease in arc volcanism occurred during the interval after the consumption of the detached eastern subduction plate and the onset of magma generation from the newly formed western subduction zone. Coincident with and continuing after the subduction zone jump and seamount accretion, eastwardly derived arkosic sediments prograded across Oregon and Washington spilling into the new fore-arc basin and enveloping the seamounts. Basaltic intrusion within the fore-arc basin occurred along tensional fault systems within the accretionary prism. As the western subduction zone developed, a new, more western magmatic arc formed along the axis of the modern Cascade Mountains. Beginning in the late Eocene, the fore-arc basin subsided and the sea transgressed eastward depositing fine-grained arc-derived tuffaceous sediment over the entire basin. End_of_Article - Last_Page 414------------

Ukinrek Maars, Alaska, I. April 1977 eruption sequence, petrology and tectonic setting

Abstract During ten days of phreatomagmatic activity in early April 1977, two maars formed 13 km behind the Aleutian arc near Peulik volcano on the Alaska Peninsula. They have been named “Ukinrek Maars”, meaning “two holes in the ground” in Yupik Eskimo. The western maar formed at the northwestern end of a low ridge within the first three days and is up to 170 m in diameter and 35 m in depth. The eastern maar formed during the next seven days 600 m east of West Maar at a lower elevation in a shallow saddle on the same ridge and is more circular, up to 300 m in diameter and 70 m in depth. The maars formed in terrain that was heavily glaciated in Pleistocene times. The groundwater contained in the underlying till and silicic volcanics from nearby Peulik volcano controlled the dominantly phreatomagmatic course of the eruption. During the eruptions, steam and ash clouds reached maximum heights of about 6 km and a thin blanket of fine ash was deposited north and east of the vents up to a distance of at least 160 km. Magma started to pool on the floor of East Maar after four days of intense phreatomagmatic activity. The new melt is a weakly undersaturated alkali olivine basalt (Ne = 1.2%) showing some transitional character toward high-alumina basalts. The chemistry, an anomaly in the tholeitic basalt-andesite-dominated Aleutian arc, suggests that the new melt is primitive, generated at a depth of 80 km or greater by a low degree of partial melting of garnet peridotite mantle with little subsequent fractionization during transport. The Pacific plate subduction zone lies at a depth of 150 km beneath the maars. Their position appears to be tectonically controlled by a major regional fault, the Bruin Bay fault, and its intersection with cross-arc structural features. We favor a model for the emplacement of the Ukinrek Maars that does not link the Ukinrek conduit to the plumbing system of nearby Peulik volcano. The Ukinrek eruptions probably represent a genetically distinct magma pulse originating at asthenospheric depths beneath the continental lithosphere.