Ridge Interaction Research Articles

Laboratory study of the breaking and energy distribution of internal solitary waves over a ridge

The breaking and energy distribution of mode-1 depression internal solitary wave interactions with Gaussian ridges are examined through laboratory experiments. A series of processes, such as shoaling, breaking, transmission and reflection, are captured completely by measuring the velocity field in a large region. It is found that the maximum interface descent ( $a_{max}$ ) during wave shoaling is an important parameter for diagnosing the type of wave–ridge interaction and energy distribution. The wave breaking on the ridge depends on the modified blockage parameter $\zeta _m$ , the ratio of the sum of the upper layer depth and $a_{max}$ to the water depth at the top of the ridge. As $\zeta _m$ increases, the interaction type transitions from no breaking to plunging and mixed plunging–collapsing breaking. Within the scope of this experiment, the energy distribution can be characterized solely by $\zeta _m$ . The transmission energy decreases monotonically with increasing $\zeta _m$ , and there is a linear relationship between $\zeta _m^2$ and the reflection coefficient. The value of $a_{max}$ can be determined from the basic initial parameters of the experiment. Based on the incident wave parameters, the depth of the upper and lower layers, and the topographic parameters, two new simple methods for predicting $a_{max}$ on the ridge are proposed.

Seismic structure of Iceland revealed by ambient noise Rayleigh wave tomography

As it is an ideal location for studying plume–ridge interactions, a clear image of the Icelandic upper mantle structure is necessary. We collect continuous seismic records from 164 stations and extract Rayleigh wave dispersion curves via the frequency-Bessel (F-J) transform method. Based on ambient noise tomography, we provide a new shear-wave velocity model of the Icelandic crust and uppermost mantle, extending to a depth of 120 km. The model is validated by the waveform simulation method and reveals extensive crustal low-velocity zones (LVZs) across both the neovolcanic and nonvolcanic zones of Iceland. These crustal LVZs may be attributed to elevated temperatures, partial melting, and lithological variations. A distinct LVZ beneath a depth of 60 km, mainly on the North American Plate, may correspond to Icelandic plume material. Additionally, hot plume material may be delivered to the crust through low-velocity conduits beneath the spreading mid-ocean ridge. There is a clear contrast between the uppermost mantle low-velocity zones (UMLVZs) in the western region and the uppermost mantle high-velocity zones in the eastern region, which may indicate asymmetric tectonic plates on both sides of the mid-ocean ridge. This asymmetry may be attributed to the multiple eastward jumps of the ridge systems. The eastern high-velocity body, meaning a cooler uppermost mantle than that of the western region, may act as a barrier to obstruct the eastward plume flow. Under plume–ridge interactions, plume material can affect crustal accretion and feed volcanic activity on the surface along the spreading Mid-Atlantic Ridge.

Geochemistry and magmatic petrology of meta-ophiolites from the Bajgan Complex (Makran Accretionary Prism, SE Iran): new insights on the nature of the Early Cretaceous Middle East Neotethys

The Bajgan Complex in the North Makran Domain (Makran Accretionary Prism) comprises disrupted meta-ophiolitic sequences originating from oceanic crust protoliths. They include ultramafic and mafic cumulates, isotropic gabbros, plagiogranites and basalts. Ultramafic–mafic cumulates and plagiogranites exhibit compositions akin to rocks formed in mid-ocean ridge settings. Isotropic gabbro and basalt protoliths can be subdivided into three distinct geochemical types. Type-1 rocks are sub-alkaline (Nb/Y < 0.1) with low Th, Nb and Ta contents and La N /Yb N ratios <1, resembling those of normal mid-ocean ridge basalts (N-MORB). Type-2 rocks display slight enrichment in Th, Ta, Nb (Nb/Y = 0.36–0.45) and La N /Yb N = 2.12–3.20, resembling the chemistry of enriched (E-) MORB. Type-3 basalts show an alkaline nature (Nb/Y = 0.88–1.82), significant Th, Ta and Nb enrichment, and high La N /Yb N ratios (7.01–20.08), resembling the chemistry of alkaline basalts (ocean island basalts, OIB). Petrogenetic modelling indicates that N-MORB protoliths originated from a depleted MORB mantle source, whereas E-MORB and OIB protoliths were generated from partial melting of sub-oceanic depleted sources that underwent varying degrees of OIB-type enrichment. The Bajgan meta-ophiolitic protoliths were formed within a Late Jurassic to Cretaceous oceanic basin influenced by mantle plume activity and plume–ridge interaction. Supplementary material : Field photographs, micrographs, rocks sampled, analytical methods and results, and supplementary tables are available at https://doi.org/10.6084/m9.figshare.c.7193937 Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists

Speculations on the Paleozoic legacy of Gondwana amalgamation

Using Gondwana as an example, we show how the geological record can be interrogated to detect significant changes in mantle convection patterns at critical junctures in Earth’s evolution. Evidence of major changes in mantle circulation in the aftermath of late Neoproterozoic-early Paleozoic Gondwana assembly is provided by widespread (i) plume-related magmatism around Gondwana’s periphery, (ii) ironstone deposits related to mantle plume-ocean ridge interaction and enhanced hydrothermal activity, and (iii) super-mature clastic deposits that reflect epeirogenic uplift triggered by mantle upwelling beneath Gondwana combined with deep tropical weathering.In our model, Gondwana assembled above a region of mantle downwelling in which subducted slabs between the converging Gondwanan continents descended to the core-mantle boundary. Renewed subduction along Gondwana’s periphery yielded early arc magmas. But as downwelling beneath Gondwana evolved into upwelling as a result of the ponding of subducted slabs at the base of the mantle, mantle plumes rose from the margins of the nascent upwelling to interact with the edges of Gondwana, where they penetrated the peripheral subduction zones via slab windows, tears and transform faults to generate voluminous calc-alkalic crustal melts in hydrated arc regions and A-type magmas in dry back-arc regions. The plumes also underplated oceanic lithosphere and interacted with adjacent ocean ridges, thereby enhancing hydrothermal activity and the flux of bioessential nutrients, leading to the recurrence of marine iron-rich sedimentary rocks in the geological record. At the same time, upwelling beneath a tropical to equatorial Gondwana led to epeirogenic uplift, deep weathering and erosion, resulting in the production of widespread super-mature clastic deposits. We contend that major changes in mantle convection patterns were encoded into the geological record of Gondwana assembly, influenced global-scale mantle convection patterns, and should be incorporated into geodynamic models for the assembly of Pangea.

Magmatism of Shatsky Rise controlled by plume–ridge interaction

Magmatism of Shatsky Rise controlled by plume–ridge interaction

The Shatsky Rise oceanic plateau formed through plume–ridge interaction

The Shatsky Rise oceanic plateau formed through plume–ridge interaction

Plume–ridge interactions: ridgeward versus plate-drag plume flow

Abstract. The analysis of mid-ocean ridges and hotspots that are sourced by deep-rooted mantle plumes allows us to get a glimpse of mantle structure and dynamics. Dynamical interaction between ridge and plume processes have been widely proposed and studied, particularly in terms of ridgeward plume flow. However, the effects of plate drag on plume–lithosphere and plume–ridge interaction remain poorly understood. In particular, the mechanisms that control plume flow towards vs. away from the ridge have not yet been systematically studied. Here, we use 2D thermomechanical numerical models of plume–ridge interaction to systematically explore the effects of (i) ridge-spreading rate, (ii) initial plume head radius and (iii) plume–ridge distance. Our numerical experiments suggest two different geodynamic regimes: (1) plume flow towards the ridge is favored by strong buoyant mantle plumes, slow spreading rates and small plume–ridge distances; (2) plume drag away from the ridge is in turn promoted by fast ridge spreading for small-to-intermediate plumes and large plume–ridge distances. We find that the pressure gradient between the buoyant plume and spreading ridge at first drives ridgeward flow, but eventually the competition between plate drag and the gravitational force of plume flow along the base of the sloping lithosphere controls the fate of plume (spreading towards vs. away from the ridge). Our results highlight that fast-spreading ridges exert strong plate-dragging force, which sheds new light on natural observations of largely absent plume–lithosphere interaction along fast-spreading ridges, such as the East Pacific Rise.

The off-axis plume–ridge interaction model: Confirmation from the mineral chemistry of Cretaceous basalts of the Ontong Java Plateau

The Ontong Java Plateau (OJP) is the largest known igneous province on Earth. The proposal that its origins involved off-axis plume–ridge interactions remains highly controversial. Here we present new in-situ major (EPMA), trace element (LA-ICP-MS) and Sr isotopic (LA-MC-ICP-MS) characterization of clinopyroxene and plagioclase phases present in OJP tholeiitic basalts from Sites 1183, 1186, 1185, 1187. Site 1187 and Upper Site 1185 have yielded olivine tholeiites containing olivine, Ca-rich augite, Ca-deficient pigeonite, and plagioclase, inferred to be derived from higher degrees of partial melting of dominantly peridotitic mantle and reflecting relatively low crystallization temperatures. Sites 1183 and 1186 and Lower Site 1185 yielded tholeiitic basalts consisting mainly of plagioclase and Ca-rich augite without olivine, interpreted to originate from lower degrees of partial melting of pyroxenite- and peridotite-derived melts with relatively high clinopyroxene and plagioclase crystallization temperatures. Site 1187 and Upper Site 1185 plagioclase, clinopyroxene, and equilibrium melts have much lower REE contents but slightly higher in-situ Sr isotopic ratios compared to those phases at Sites 1183 and 1186 as well as Lower Site 1185. Sites 1186 and 1187 plagioclases and clinopyroxenes display normal zoned textures which are attributed to the process of fractional crystallization and extensive magma mixing beneath the heterogeneous mantle source. In-situ Sr isotopic disequilibrium between plagioclase phenocrysts and plagioclase clusters further indicates an enriched compositional heterogeneity. Due to the strong tensile forces expected at the spreading ridge, the OJP off-axis mantle plume is inferred to be drawn towards the ridge axis, leading to shifts in mineral compositions, degrees of partial melting, and crystallization temperatures inferred for the volcanics of Sites 1183, 1186, 1185, and 1187 as a function of distance from the spreading ridge. The enriched invariability of in-situ Sr isotopic compositions is likely due to the relatively long half-life of the radiogenic elements. These mineral chemistries are consistent with off-axis ridge–plume interaction.

Aseismic ridge subduction focused late Cenozoic exhumation above the Peruvian flat slab

Subduction of aseismic ridges and flat slab subduction are important processes that punctuate Cordilleran orogenesis and may enhance exhumation and rock uplift in the overriding plate. Distinguishing between the two drivers is often challenging, as many modern flat slabs spatially coincide with subducting buoyant ridges. The Peruvian flat slab is the largest region of active flat slab subduction on Earth, spanning over 1300 km of the subducting Nazca plate along the western margin of South America. The flat slab is associated with two seafloor ridges: the Nazca Ridge at the southern terminus and the fully subducted Inca Plateau in the north. These aseismic ridges are spatially confined with respect to the flat slab, allowing assessment of the relative roles of aseismic ridge interactions and flat slab subduction in driving upper plate exhumation. We present: (1) a regional compilation of geochronologic ages of Andean igneous rocks, which track the spatio-temporal evolution of Neogene magmatic arc cessation and hence slab flattening; (2) calculated geomorphic indices, which document landscape perturbations and climatic or lithologic changes, (3) a summary of erosion rates from river catchments on the western Andean slope, and; (4) a regional synthesis of thermochronologic ages that reflect the timing and magnitude of upper crustal cooling.Thermochronometric cooling ages systematically track the progressive passage of the Nazca Ridge, suggesting that the buoyant ridge focused exhumation in the overriding plate. Geomorphic indices demonstrate enhanced topography and steeper channels closer to the position of the subducted ridge. The spatial progression of basement block uplifts in Peru also coincides with the timing of ridge passage. In hinterland regions, >2 km of exhumation occurred since ca. 15 Ma above the Peruvian flat slab. For individual locations within the orogen, active rock uplift declines after ridge passage, suggesting that increased coupling is not maintained across the entirety of the flat slab. We argue that above broad zones of flat slab subduction, focused areas of aseismic ridge subduction concentrate upper-plate exhumation and uplift. These observations may be relevant to other flat slab systems, which exhibit a broad zone of arc shutoff with corridors of focused exhumation.

Seismic and thermo-compositional insights into the uppermost mantle beneath the Northern Andes magmatic arc

The mantle wedge beneath the Northern Andes magmatic arc is expected to be latitudinally heterogeneous as suggested by the highly variable composition of recovered xenoliths, a discontinuous pattern of low seismic velocity zones, and unusually high Vp/Vs ratios below volcanic centers. Such heterogeneity might be controlling the development of the modern arc which currently shows a narrow northern volcanic segment between ∼4° and 6°N (<75 km wide), a volcanic gap of hundreds of km in its central portion (∼2°-4°N), and a wider southern volcanic expression between ∼2°S and 2°N (>120 km wide). Through an analysis of Pn and Sn inter-station wave speed estimates, the seismic structure dissimilarity and latitudinal thermo-compositional insights were constrained within the uppermost mantle beneath the arc. Results, integrating wave speed estimates, anisotropy, and thermo-compositional inferences obtained by wave velocity comparison using physical properties of minerals and phase equilibria modeling, suggest a seismically slower structure, higher anisotropy, and warmer conditions beneath the northern region (>4°N), whereas a faster, less anisotropic, and colder mantle towards the south (<2°N). We interpret the warmer and higher degree of anisotropy in the northern uppermost mantle as influenced by the Caldas tear, prompting hot mantle influx. Beneath the gap region, seismic speeds are similar to those in the north, yet a colder thermal state is suggested. We interpret the colder temperatures and the abrupt reduction in surface volcanism as reflecting an absence, or a reduced amount of magma pounding within the Moho vicinity. The southern upper mantle is seismically faster and colder compared to the northern portions of the arc. We interpret the fast velocities, and a wider volcanic expression on the surface, as influenced by the Carnegie ridge interaction prompting a shallower subduction with respect to the north. Our wave speed estimate and thermo-compositional inferences draw an increase in temperature and seismic anisotropy in the uppermost mantle from south to north along the Northern Andes magmatic arc. The main controlling factor of the general anisotropy below the entire arc seems to be the preferred orientation of olivine and pyroxene rather than an aligned melt fabric.

Selenium and tellurium in Reykjanes Ridge and Icelandic basalts: Evidence for degassing-induced Se isotope fractionation

Selenium behaves as a chalcophile and moderately volatile element during planetary accretion and magmatic processes on Earth. Together with the geochemically similar S and Te, Se is more volatile than most other moderately volatile elements and thus potentially becomes a new tracer to constrain the mechanism of volatile depletion in the Earth’s mantle and other planetary bodies. As previously observed for several volatile elements, stable isotopes of Se are expected to fractionate upon eruptive outgassing of magmas. To understand the degassing behavior of Se and associated isotope fractionation, we report on Se and Te contents and Se isotope compositions (δ82Se) of submarine glasses across a range of distant ridge depth intervals along the Reykjanes Ridge and subglacial/subaerial basalts on Iceland (51–65°N; N = 22). Selenium (150–399 ng/g) and Te (2.61–14.5 ng/g) contents of the submarine glasses display progressive enrichment along the Reykjanes Ridge towards Iceland. This can be explained either by enhanced mantle melting towards Iceland or by enrichment of Se–Te contents in the mantle source due to the Icelandic plume–Reykjanes Ridge interaction. Both scenarios are equally plausible. The δ82Se values of submarine Reykjanes Ridge glasses range between −0.20 ± 0.08‰ and −0.08 ± 0.08‰ (on average −0.15 ± 0.07‰; 2SD, N = 15), which are unaffected by the Icelandic plume contribution and remain strictly within the previously reported average for depleted MORBs. These new data combined with literature δ82Se for depleted MORBs define a highly homogeneous depleted mantle composition δ82Se = −0.15 ± 0.11‰ (2SD, N = 44). On the other hand, we observed degassing of Se and Te to a variable extent (~40–95%) in submarine Reykjanes glasses at depths shallower than ~250 m and in subaerial/subglacial basalts on Iceland. Degassing-induced Se isotope fractionation shifted δ82Se of subaerial lavas towards heavier values (by up to ~0.44‰) well outside the range of submarine MORBs. However, when Se outgassing is associated with subglacial eruption, the outer glass rim of basalt preserves the primary/undegassed Se isotopic signature; whereas the pillow interior experiencing further Se loss (~50%) is enriched in heavier isotopes (−0.01 ± 0.12‰) relative to the outer glass rim (−0.22 ± 0.08‰). These observations are complemented by measurements of BHVO-2G from high-temperature heating experiments of the natural Hawaiian basalt BHVO-2, which show evaporation of 40% Se and 85% Te and resulting shift in δ82Se by ~+0.74‰. Degassing-induced Se isotope fractionation during volcanic eruption may be well explained by a simple Rayleigh distillation model with an empirical fractionation factor α of ~0.9998 (between volcanic gas and silicate melt). The results presented here show that Se isotope and Se–Te systematics can potentially contribute further constraints on degassing of chalcophile and volatile elements during terrestrial volcanism and large-scale planetary processes.

Middle–late Permian mantle plume/hotspot–ridge interaction in the Sumdo Paleo-Tethys Ocean region, Tibet: Evidence from mafic rocks

The Sumdo Paleo-Tethys Ocean was the southernmost part of the Paleo-Tethys Ocean and was located on the Qinghai–Tibet Plateau. Doubts exist as to the internal dynamics and evolution of the Sumdo Paleo-Tethys Ocean region during the Permian, owing to insufficient knowledge regarding the remnants of oceanic crust. Here, we report results for petrology, laser ablation–inductively coupled plasma–mass spectrometry zircon UPb dating, whole-rock geochemistry, and zircon Hf and whole-rock SrNd isotopes of mafic rocks in the Sumdo area. Geochemical characteristics of these mafic rocks allow them to be divided into two groups, A and B. Group A rocks are characterized by high SiO2 and TiO2 contents, low Al2O3 contents, and Na2O > K2O. Their chondrite-normalized rare-earth-element (REE) and primitive-mantle-normalized trace-element patterns are similar to those of N-MORB. The samples exhibit enrichment in U and depletion in Th, depletion in light REEs, and high 143Nd/144Nd values. Group B rocks have high SiO2, TiO2, and Al2O3 contents. Their chondrite-normalized REE patterns and primitive-mantle-normalized trace-element patterns are similar to those of E-MORB. Group B rocks are enriched in Nb and Ta, depleted in Th and U, and show high 87Sr/86Sr and low 143Nd/144Nd values. Zircon UPb dating of five meta-gabbro rock samples yielded ages of 268.9 ± 1.6 Ma (STN66), 273.8 ± 2.2 Ma (STN79), 265.1 ± 4.1 Ma (STE26), 258.7 ± 4.4 Ma (STE89), and 261.0 ± 3.1 and 263.7 ± 5.4 Ma (STE41). STN79 shows N-MORB geochemical characteristics and has εHf(t) values of +12.4 to +14.6, whereas STE41 displays E-MORB geochemical characteristics with εHf(t) values of −2.6 to +0.4. The Sumdo mafic rocks were not contaminated by subducted sediments and/or slab-derived fluids, and formed in a mid-ocean ridge setting. Combining results of previous studies with our new geochemical data for mafic rocks in the Sumdo area, we infer the occurrence of plume/hotspot–ridge interaction in the Sumdo Paleo-Tethys Ocean during the middle–late Permian. Mixing of mantle plume/hotspot melt and depleted asthenospheric mantle explains the origin of basaltic magma that formed rocks with the observed geochemical and isotopic characteristics in the Sumdo area.

Oceanic crustal flow in Iceland observed using seismic anisotropy

Understanding accretion and deformation processes at mid-ocean ridges is crucial as they control the resulting oceanic crustal structure, which covers two-thirds of Earth’s surface. The most common tool for observing such dynamic processes within the Earth is seismic anisotropy. Iceland, which is uplifted by a convective mantle plume and has an active spreading ridge system exposed above sea level, offers a unique opportunity for studying this phenomenon. Here we use a high-resolution dataset of Love and Rayleigh wave speeds to constrain the seismic anisotropy in the Icelandic crust. We show that seismic anisotropy in the lower crust is controlled by crystal preferred orientation, providing a direct observation of lower crustal flow. Furthermore, since shear is needed to align the crystals, our results reveal that crustal flow cannot be a simple translation of mass and requires internal deformation. This finding suggests that crustal flow plays an important role in oceanic crustal accretion and deformation where thick, hot oceanic crust is formed, such as at volcanic rifted margins and where there are mantle plume–ridge interactions. The lower oceanic crust beneath Iceland is flowing and internally deforming, according to constraints on seismic anisotropy in the Icelandic crust from an analysis of seismic surface waves.

Early Cretaceous Plume–Ridge Interaction Recorded in the Band-e-Zeyarat Ophiolite (North Makran, Iran): New Constraints from Petrological, Mineral Chemistry, and Geochronological Data

The North Makran domain (southeast Iran) is part of the Makran accretionary wedge and consists of an imbricate stack of continental and Neo-Tethyan oceanic tectonic units. Among these, the Band-e-Zeyarat ophiolite consists of (from bottom to top): ultramafic cumulates, layered gabbros, isotropic gabbros, a sheeted dyke complex, and a volcanic sequence. Sheeted dykes and volcanic rocks are mainly represented by basalts and minor andesites and rhyolites showing either normal-type (N) or enriched-type (E) mid-ocean ridge basalt affinities (MORB). These conclusions are also supported by mineral chemistry data. In addition, E-MORBs can be subdivided in distinct subtypes based on slightly different but significant light rare earth elements, Th, Nb, TiO2, and Ta contents. These chemical differences point out for different partial melting conditions of their mantle sources, in terms of source composition, partial melting degrees, and melting depths. U-Pb geochronological data on zircons from intrusive rocks gave ages ranging from 122 to 129 Ma. We suggest that the Band-e-Zeyarat ophiolite represents an Early Cretaceous chemical composite oceanic crust formed in a mid-ocean ridge setting by partial melting of a depleted suboceanic mantle variably metasomatized by plume-type components. This ophiolite records, therefore, an Early Cretaceous plume–ridge interaction in the Makran Neo-Tethys.

Geochemistry of Cretaceous basalts from the Ontong Java Plateau: Implications for the off-axis plume–ridge interaction

Ontong Java Plateau (OJP) is the largest known igneous province preserved on Earth. The hypotheses for the origin of the OJP volcanic activities remain controversial. Here we report new major, trace-element, and Sr–Nd–Pb isotopic compositions from volcanic rocks from Ocean Drilling Program Sites 1183, 1185, 1186, and 1187 of the OJP. Samples from the OJP are tholeiitic in composition and can be divided into low- and high-Ti groups, display flat rare-earth elements (REEs) distributions that are generally similar to normal mid-ocean-ridge basalt (N-MORB) patterns with slightly positive Nb, Ta, Th, U, and Ti anomalies and negative K, Pb, and Sr anomalies. However, OJP samples have significantly enriched 87Sr/86Sr (0.703641–0.704629), 143Nd/144Nd (0.512941–0.512972), and 206Pb/204Pb (18.493–18.940) radiogenic isotopic ratios, indicating that at least three distinct FOZO-type, EM1-type, and HIMU-type enriched components are involved. Combining with the method of principal component analysis, the special geochemical compositions with depleted N-MORB-incompatible elements and enriched isotopes of OJP volcanic samples could be explained by the off-axis ridge–plume interaction model. Due to the influence of strong ridge tensile forces, the off-axis plume magmatic mantle would flow to the ridge axis and undergo decompression melting, resulting in the depletion of the highly incompatible elements but the invariability of the isotopic compositions due to the relatively long half-life of the radiogenic elements. We speculate that these isotopically enriched N-MORB-type basalts may be an exclusive product of an off-axis ridge–plume interaction.

Development of environmental contours for first-year ice ridge statistics

Ice ridges represent a major threat to ships and offshore structures in areas with sea ice but no icebergs, since they frequently determine and govern the structural design loads. This work focuses on the development of environmental contours for first-year sea ice ridge statistics, which are able to represent the key parameters that will influence the extreme loads that would be acting on ice-capable vessels sailing in Arctic regions. Based on the inverse first order reliability method (IFORM), the development of environmental contours to be applied for the reliability-based design of ice-capable vessels in Arctic regions is elaborated. The number of relevant parameters and hence the dimension of the space containing the environmental contour will depend on the particular ice-vessel interaction model that is to be applied. Furthermore, the influence from the degree of correlation between the environmental parameters as well as from the number of ship-ice ridge interaction during the voyage on the environmental contour shapes are studied.

Tokoro Belt (NE Hokkaido): an exhumed, Jurassic – Early Cretaceous seamount in the Late Cretaceous accretionary prism of northern Japan

AbstractThe Tokoro Belt exposed in NE Hokkaido (Japan) represents part of a Late Cretaceous accretionary complex, which includes variously metamorphosed volcanic rocks that are interbedded with chert, lenticular limestone and some fore-arc sedimentary rocks. The Tokoro Belt is notably different from other Late Cretaceous accretionary complexes around the Pacific Rim because of widespread occurrence of basalts and volcaniclastic rocks in it. The Nikoro Group, characterized by widespread occurrence of volcanic rocks, is divided into western, eastern and southern sections based on the internal structure, geochemical affinities and metamorphic grades of their volcanic lithologies. OIB (ocean island basalt)-type volcanic rocks with low-grade metamorphic overprint predominate in the western and southern sections, whereas MORB (mid-ocean ridge basalt)- and OIA (ocean island alkaline basalt)-type rocks in the eastern section with partly high-pressure metamorphism make up the northern part of the eastern section. Trace element patterns display transitional trends from MORB to OIA geochemical affinities. OIB-type rocks display trace element characteristics similar to those of shield volcano lavas on Hawaii, rather than small and mainly alkaline, Polynesian hotspot lavas; furthermore, they show significant HREE (heavy rare earth element) enrichment probably caused by plume–ridge interaction. Widespread OIBs in the Tokoro Belt represents tectonic slices of a large (&gt;80 km wide) Hawaiian-style, seamount shield volcano on the Izanagi oceanic plate that was accreted into the continental margin of Far East Asia in the Late Cretaceous.

Dem Simulation of First-Year Ice Ridge Interactions With Moored Structures

First-year ice ridges determine the design load of moored structures in many Arctic and sub-Arctic regions. Their interactions with moored structures were simulated with discrete element method (DEM), considering the complex ... | Find, read and cite all the research you need on Tech Science Press

Magnetic anomaly map of Ori Massif and its implications for oceanic plateau formation

Many oceanic plateaus have been emplaced at or adjacent to mid-ocean ridges. To explain plateau volume and thickened crust compared to normal oceanic crust, hotspot–ridge interaction is commonly assumed, but the manner of interaction remains unclear. The Shatsky Rise oceanic plateau is a large volcanic mountain that formed at a triple junction during Late Jurassic and Early Cretaceous time. Recent drilling and seismic investigations suggest that the intermediate edifice in the rise, Ori Massif, is a central volcano. Paradoxically, magnetic lineations were traced across parts of Ori Massif, implying formation at a spreading ridge. In this study, we examined magnetic anomalies over and around Ori Massif to obtain insights about the formation of this volcanic edifice. Magnetic data from 21 cruises were corrected, combined, and gridded to construct a magnetic anomaly map. Forward and inverse magnetic modeling was done to investigate the magnetic structure of Ori Massif. The results imply that this large volcanic edifice is predominantly characterized by linear magnetic anomalies resulting from alternating normal and reversed polarity magnetization blocks, analogous to magnetic anomalies recorded by spreading-ridges. This magnetic structure is not expected for a central volcano that was built by long runout lava flows, implying that Ori Massif eruptions must have been constrained near the ridge axis. Magnetic bights on the north and south boundaries of Ori Massif imply that it was bracketed by triple junctions, indicating complex ridge tectonics during the formation of Shatsky Rise. The surprising finding that Ori Massif is traversed by coherent linear magnetic anomalies indicates that oceanic plateaus can record seafloor spreading magnetic anomalies despite large crustal thickness. Other oceanic plateaus also record linear magnetic anomalies, implying a link between divergent plate boundaries and oceanic plateau volcanism.

Delivery of deep-sourced, volatile-rich plume material to the global ridge system

The global mid-ocean ridge (MOR) system represents a major site for outgassing of volatiles from Earth's mantle. The amount of H2O released via eruption of mid-ocean ridge basalts varies along the global ridge system and greatest at sites of interaction with mantle plumes. These deep-sourced thermal anomalies affect approximately one-third of all MORs – as reflected in enrichment of incompatible trace elements, isotope signatures and elevated ridge topography (excess melting) – but the physical mechanisms involved are controversial. The “standard model” involves solid-state flow interaction, wherein an actively upwelling plume influences the divergent upwelling generated by a mid-ocean ridge so that melting occurs at higher pressures and in greater amounts than at a normal spreading ridge. This model does not explain, however, certain enigmatic features including linear volcanic ridges radiating from the active plume to the nearby MOR. Examples of these are the Wolf–Darwin lineament (Galápagos), Rodrigues Ridge (La Réunion), Discovery Ridge (Discovery), and numerous smaller ridge-like structures associated with the Azores and Easter–Salas y Gómez hot spots. An important observation from our study is that fractionation-corrected MORB with exceptionally-high H2O contents (up to 1.3 wt.%) are found in close proximity to intersections of long-lived plume-related volcanic lineaments with spreading centres.New algorithms in the rare-earth element inversion melting (INVMEL) program allow us to simulate plume–ridge interactions by mixing the compositions of volatile-bearing melts generated during both active upwelling and passively-driven corner-flow. Our findings from these empirical models suggest that at sites of plume–ridge interaction, moderately-enriched MORBs (with 0.2–0.4 wt.% H2O) result from mixing of melts formed by: (i) active upwelling of plume material to minimum depths of ∼35 km; and (ii) those generated by passive melting at shallower depths beneath the ridge. The most volatile-rich MORB (0.4–1.3 wt.% H2O) may form by the further addition of up to 25% of “deep” small-fraction plume stem melts that contain >3 wt.% H2O. We propose that these volatile-rich melts are transported directly to nearby MOR segments via pressure-induced, highly-channelised flow embedded within a broader “puddle” of mostly solid-state plume material, spreading beneath the plate as a gravity flow. This accounts for the short wavelength variability (over 10s of km) in geochemistry and bathymetry that is superimposed on the much larger (many 100s of km) “waist width” of plume-influenced ridge.Melt channels may constitute a primary delivery mechanism for volatiles from plume stems to nearby MORs and, in some instances, be expressed at the surface as volcanic lineaments and ridges. The delivery of small-fraction hydrous melts from plume stems to ridges via a two-phase (melt-matrix) regime implies that a parallel, bimodal transport system is involved at sites of plume–ridge interaction. We estimate that the rate of emplacement of deep-sourced volatile-rich melts in channels beneath the volcanic lineaments is high and involves 10s of thousands of km3/Ma. Since mantle plumes account for more than half of the melt production at MORs our findings have important implications for our understanding of deep Earth volatile cycling.