Western Aleutian Arc Research Articles

Gas Emissions From the Western Aleutians Volcanic Arc

The Aleutian Arc is remote and highly active volcanically. Its 4,000 km extent from mainland Alaska to Russia’s Kamchatka peninsula hosts over 140 volcanic centers of which about 50 have erupted in historic times. We present data of volcanic gas samples and gas emission measurements obtained during an expedition to the western-most segment of the arc in September 2015 in order to extend the sparse knowledge on volatile emissions from this remote but volcanically active region. Some of the volcanoes investigated here have not been sampled for gases before this writing. Our data show that all volcanoes host high-temperature magmatic-hydrothermal systems and have gas discharges typical of volcanoes in oceanic arcs. Based on helium isotopes, the western Aleutian Arc segment has minimal volatile contributions from the overriding crust. Volcanic CO2fluxes from this arc segment are small, compared to the emissions from volcanoes on the Alaska Peninsula and mainland Alaska. The comparatively low CO2emissions may be related to the lower sediment flux delivered to the trench in this part of the arc.

40Ar/39Ar ages and bulk-rock chemistry of the lower submarine units of the central and western Aleutian Arc

In order to further constrain the timing of the Aleutian Arc initiation as well as its early evolution, an extensive 40Ar/39Ar dating and geochemical (major and selected trace elements) campaign (40 samples) of the lower units of the Aleutian ridge has been carried out on samples dredged from deep fore-arc canyons and rear-arc tectonic structures. The new dataset slightly increases the minimum inception age for the Aleutian system, with the two oldest samples dated at 46.1 ± 3.3 Ma and 47.80 ± 0.57 Ma. Both mid Eocene ages were obtained on tholeiitic mafic volcanic rocks from the western section of the arc. The new data also support the occurrence of three distinct periods of enhanced magmatic activity (magmatic pulses) during the pre-Quaternary evolution of the arc (at 38–27, 16–11 and 6–0 Ma), as previously suggested based on a more limited and dominantly subaerial dataset. Moreover, the data refine the duration of the first pulse of activity, which ended 2 Ma later than previous estimates. The first and last pulses may be associated with rotations of the subducting plates while the second pulse might result from regional tectonic changes. The significant overlap between the age distribution of the submarine and subaerial samples suggests that much of the earlier parts of the arc may have been uplifted and subaerially exposed. The expected crustal growth associated with the pulses is unlikely to have significantly impacted magmatic residence times, since no variation in the degree of differentiation of the rocks can be observed during or after the pulses. On the other hand, the type of magmas erupted may have changed during the arc evolution. Prior to the first pulse, activity appears to have been dominantly tholeiitic. On the other hand, the first pulse was characterized by coeval tholeiitic, transitional and calc-alkaline magmas, with calc-alkaline activity increasing after the first ~3 Ma. Subsequently, a dominantly calc-alkaline period occurred from 29 to 8 Ma, followed by a progressive return of coeval tholeiitic, transitional and calc-alkaline activity. These temporal changes in magma types correspond to likely variations in arc crustal thickness beneath the active front, and could therefore be a response to physical changes of the overriding plate.

Generation of Calc-Alkaline Magmas during Crystallization at High Oxygen Fugacity: An Experimental and Petrologic Study of Tephras from Buldir Volcano, Western Aleutian Arc, Alaska, USA

Abstract Despite agreement that calc-alkaline volcanism occurs at subduction zones and is responsible for the genesis of continental landmasses, there is no consensus on the source of the Fe-depleted signature hallmark to calc-alkaline volcanism. In this study, we utilize mafic tephras collected from Buldir Volcano to address the genesis of strongly calc-alkaline volcanic rocks (those with a low Tholeiitic Index; ≤0·7) in a segment of the western Aleutian Arc to determine if the eruptions are plausibly part of a liquid line of descent, if they are mixtures of crustal melts and parental magmas, or if they are mixtures of melts of the mantle and the subducting slab. We conducted a series of H2O-saturated phase equilibrium experiments (1175–1000°C; 100 MPa) in a rapid-quench cold-seal (MHC) apparatus on the most primitive natural lava from Buldir (9·34 wt % MgO) at oxidizing conditions near the Re–ReO2 buffer. We confirmed that all experiments equilibrated 0·3 ± 0·23 log units above the Re–ReO2 buffer (ΔQFM ∼ +2·8) using X-ray Absorption Near Edge Structure (XANES) spectroscopy. Chromite is the liquidus phase, followed by olivine, then plagioclase, then clinopyroxene, and finally hornblende. Once clinopyroxene saturates, spinel composition shifts to magnetite. We compared our experimental results to the major element geochemistry and petrology of six tephras (51·9–54·8 wt % SiO2) from Buldir collected during the 2015 field season of the GeoPRISMS shared platform. Tephras contain olivine + plagioclase + clinopyroxene + spinel ± hornblende; plagioclase comprises most of the crystalline volume, followed by either olivine or hornblende. Spinel is ubiquitous; with Cr-rich spinel inclusions in olivine and hornblende, and magnetite in the groundmass. Variations in phenocryst assemblages and compositions between samples can be attributed to differences in pre-eruptive temperatures, where hotter samples are devoid of hornblende, and contain Fo-rich olivine and plagioclase with lower An-contents, owing to the position of the mineral-in curves at fluid-saturated conditions. Experimental glasses match the depletion in FeOT observed in the tephra whole rock compositions. The continuous depletion in FeOT is attributable to saturation of spinel as a liquidus phase (initially as chromite) and continuous crystallization through the experimental series (changing to magnetite at colder temperatures). In contrast to the natural samples, the experiments show enrichment in TiO2 with decreasing MgO, suggesting that differentiation did not occur at 100 MPa on Buldir. The TiO2 depletion in volcanic rocks from Buldir can be accounted for if hornblende crystallization occurs close to the liquidus of a parental magma; a condition that is met at higher pressures and hydrous conditions. The emerging picture for Buldir Island is that (1) oxidizing conditions are required to drive the observed depletions in FeOT via crystallization of spinel, and (2) elevated H2O contents and high pressures are required to saturate hornblende close to the liquidus to reproduce the entire suite of major elements. Our study provides a mechanism to generate the calc-alkaline trends observed at Buldir without requiring mixing of slab and mantle melts. We conclude that calc-alkaline volcanic rocks with extremely low Tholeiitic Indices (0·7), like those from Buldir, cannot be generated in absence of high oxygen fugacity, even at high pressure and/or elevated water pressures.

Os isotopic composition of western Aleutian adakites: Implications for the Re/Os of oceanic crust processed through hot subduction zones

Constraining the behaviour of Re and Os during eclogite melting is required to understand the Re and Os budget and 187Os/188Os of recycled slabs produced at warm subduction zones. It is particularly relevant to early Earth history, a period during which slab melting could have prevailed over dehydration due to higher mantle temperatures. There are however currently few constraints on Re and Os mobility during slab melting. Accordingly, we measured Os, Re and 187Os/188Os in primitive submarine lavas (Mg# ˃ 0.6) from the western Aleutian Arc. These include strongly adakitic rocks shown to be derived from eclogite melting (high-Mg# andesite, dacites and rhyodacites), as well as non-adakitic rocks (high-Mg# andesites, basaltic andesites and basalts) with variable sediment and fluid-derived slab contributions for comparison.The 187Os/188Os of the adakitic and non-adakitic volcanic rocks vary significantly but largely overlap. In both groups, the most radiogenic values occur in samples with the lowest Os concentrations, thus implicating crustal assimilation as the main cause of Os isotope variations. Adakitic and non-adakitic rocks least affected by crustal assimilation have overlapping 187Os/188Os of 0.141–0.149. We show that the source of the adakites is very unlikely to comprise significant eclogite-derived Os, which suggests no or minimal mobilization of Os during eclogite melting. Eclogitic Os is inferred to be retained in sulphides or replacement phases formed upon sulphide breakdown for which Os has high affinity, such as a platinum-group minerals (PGMs). The small Os budget of the adakites is most likely derived from limited reaction with the mantle wedge during ascent. Degassing has reduced Re contents in most samples, but not for end-member adakites (SiO2 > 67% and Sr/Y > 200; n = 4) that were erupted at seafloor depths > 2500 m. These undegassed samples have elevated Re concentrations (0.8–1.5 ppb) that are positively correlated with Sr/Y and so are interpreted to be primary magmatic concentrations resulting from the mobilization of Re from the slab. Re could either be derived from the eclogites or from the serpentinite-derived fluids fluxing eclogites during melting. The former scenario would produce recycled residual crusts with lower Re/Os than in unmelted eclogites while the latter would result in Re/Os ranging from similar to higher than prior to melting. In both cases, the Re/Os and therefore the time-integrated 187Os/188Os of residual crust produced at warm subduction zones involving slab melting are likely to be different from that processed at cooler typical modern subduction zones. Therefore, if slab melting was an important process during the early Earth, the use of Re and Os partitioning in modern subduction zones to model the source of magmas comprising old recycled oceanic crust, such as the HIMU (high μ = 238U/204Pb) ocean island basalts (OIBs), might lead to erroneous interpretations.

3D S-velocity model of the lithosphere-asthenosphere system beneath the Bering Sea

The 3D S-velocity structure for the Bering Sea region is determined from dispersion analysis (Rayleigh waves), from which the most conspicuous features of the crust and upper mantle (from 0 to 400 km depth) will be revealed. In the depth range from 0 to 5 km, this model shows the distribution of the sedimentary basins present in the study area, in terms of S-velocity. For the Bering Shelf, the S-velocity values decrease southward, indicating the presence of deep sedimentary basins in the southern Bering Shelf. In the depth range from 5 to 30 km, the S-velocity model shows clearly the division of the Bering Sea basin in three sub-basins: the Aleutian Basin, the Bowers Basin and the Komandorsky Basin, produced by the Bowers Ridge and the Shirshov Ridge. In this model, the low S-velocity pattern determined for the Bering Shelf confirms that its crust is similar to a continental-type crust, no an oceanic crust. The Moho map determined in the present study, from the 3D S-velocity model, is the first Moho map calculated for the Bering Sea region. In this map, the crust beneath the Bering Shelf shows thicker thickness than a typical oceanic crust, from which is concluded that this crust must be considered as a transitional crustal structure. In the depth range from 30 to 100 km, the Aleutian Basin shows a pattern of high S-velocity that can be correlated with the origin of this basin, because this basin is considered as formed by the entrapment of a piece of Pacific plate. In the depth range from 45 to 60 km, the low S-velocity pattern shown for the eastern Aleutian arc is associated to the active arc volcanoes present in this region. From the S-velocity model, the asthenosphere beneath the study area is firstly determined from ~ 100 to ~ 180 km depth. For the depth range from 80 to 400 km, the high S-velocity pattern determined for the eastern Aleutian arc, allows the modeling (imaging) of the subducting Pacific slab, in terms of S-velocity. This pattern of high S-velocity is not visible beneath the western Aleutian arc, because the Pacific plate is not subducting beneath the Bering Sea in the western Aleutian arc.

The earliest Jurassic A-type rhyolites and high-Mg andesites–dacites in southern Jiangxi Province, southeast China: Evidence for delamination of a flat-slab?

Early Jurassic igneous rocks are abundant in the Nanling Range, southeastern China. They are closely related with economically significant deposits, but the tectonic-magmatic controls on their formation are still controversial. This paper reports zircon U–Pb–Hf–O isotopes for the rhyolites, and whole-rock major–trace element and SrNd isotope compositions for the rhyolites and andesites/dacites in the Changpu and Dongkeng basins located in the southern part of Jiangxi Province. The Changpu and Dongkeng rhyolites have high K2O contents (4.1–7.6 wt%) and K2O/Na2O ratios (> 1). Both groups are enriched in HFSE, LREE and show pronounced Nb–Ta–Eu–Sr–P–Ti negative anomalies. Geochemically, they can be classified as A-type and are similar to A2-type granitoids globally. Their highly evolved whole-rock SrNd isotopes and elevated zircon δ18O (Changpu rhyolites: (87Sr/86Sr)i = 0.7140 to 0.7156, εNd(t) = −7.2 to −8.7, δ18O = 8.76 to 9.93‰; Dongkeng rhyolites: (87Sr/86Sr)i = 0.7149 to 0.7162, εNd(t) = −6.4 to −7.9, δ18O = 7.04 to 9.78‰) are consistent with their generation dominated by remelting of exisiting crustal materials in an extensional environment. The slightly more radiogenic zircon Hf isotopes of the Dongkeng rhyolites, εHf(t) = −2.1 to −7.1 as compared with −6.1 to −8.3 of Changpu rhyolites, likely signify some mantle contribution. The Changpu high-Mg andesites–dacites are geochemically similar to high-Mg andesites (HMAs) of the Setouchi arc volcanic belt in Japan and the Piip Volcanos in the western Aleutian Arc. They have very low εNd(t) from −7.8 to −11.3 and highly radiogenic (87Sr/86Sr)i values from 0.7143 to 0.7154. Similar to HMAs in Setouchi, we suggest that they may be derived from melting of subducted sediments and followed by melt-mantle interaction.SIMS zircon UPb dating yielded extrusive ages of ~190 Ma for both Changpu and Dongkeng rhyolites. The new dating results constrain the minimum age of extrusion of rhyolites and andesites-dacites from the same volcanic sequence to be the earliest Jurassic period. Igneous rocks of this age have very limited distribution in south China. While those found in the coastal regions show features similar to arc rocks, these in the hinterland are rift related. We propose that formation of the Early Jurassic A-type rhyolites and high-Mg andesites–dacites may provide evidence for foundering/delamination of a previously subducted oceanic plateau beneath southeastern China.

Plate coupling and strain in the far western Aleutian arc modeled from GPS data

AbstractThe Aleutian and Kuril‐Kamchatka arcs meet at a triple junction of the Pacific (PAC), Bering (BER), and North American (NAM) plates. We invert GPS observations from the westernmost Aleutian (Komandorsky) Islands and Kamchatka for the fault locking depth and block motion in the far western Aleutian transform boundary. Three boundary models were considered: (1) only the Aleutian thrust fault without a trench‐normal component, (2) only a strike‐slip fault in the back arc north of the Komandorsky Islands, and (3) a rigid Komandorsky sliver bounded by the Aleutian and back‐arc faults. Observed velocities prefer Model 3, with a secular westward sliver velocity of 51 mm/a relative to NAM (two thirds of the total PAC‐NAM motion). The observed velocities are ~10% slower because of elastic strain from boundary faults. The best fitting locking depth of faults bounding the sliver is 12 km, which is similar to depths observed in diverse tectonic environments.

Advent of Continents: A New Hypothesis.

The straightforward but unexpected relationship presented here relates crustal thickness to magma type in the Izu-Ogasawara (Bonin) and Aleutian oceanic arcs. Volcanoes along the southern segment of the Izu-Ogasawara arc and the western Aleutian arc (west of Adak) are underlain by thin crust (10–20 km). In contrast those along the northern segment of the Izu-Ogasawara arc and eastern Aleutian arc are underlain by crust ~35 km thick. Interestingly, andesite magmas dominate eruptive products from the former volcanoes and mostly basaltic lavas erupt from the latter. According to the hypothesis presented here, rising mantle diapirs stall near the base of the oceanic crust at depths controlled by the thickness of the overlying crust. Where the crust is thin, melting occurs at relatively low pressures in the mantle wedge producing andesitic magmas. Where the crust is thick, melting pressures are higher and only basaltic magmas tend to be produced. The implications of this hypothesis are: (1) the rate of continental crust accumulation, which is andesitic in composition, would have been greatest soon after subduction initiated on Earth, when most crust was thin; and (2) most andesite magmas erupted on continental crust could be recycled from “primary” andesite originally produced in oceanic arcs.

Bowers Ridge (Bering Sea): An Oligocene-Early Miocene island arc

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Geology and 40Ar/39Ar geochronology of the medium- to high-K Tanaga volcanic cluster, western Aleutians

We used geologic mapping and geochemical data augmented by 40 Ar/ 39 Ar dating to establish an eruptive chronology for the Tanaga volcanic cluster in the western Aleutian arc. The Tanaga volcanic cluster is unique in comparison to other central and western Aleutian volcanoes in that it consists of three closely spaced, active, volumetrically significant edifices (Sajaka, Tanaga, and Takawangha), the eruptive products of which have unusually high K 2 O contents. Thirty-five new 40 Ar/ 39 Ar ages obtained in two different laboratories constrain the duration of Pleistocene–Holocene subaerial volcanism to younger than 295 ka. The eruptive activity has been mostly continuous for the last 150 k.y., unlike most other well-characterized arc volcanoes, which tend to grow in discrete pulses. More than half of the analyzed Tanaga volcanic cluster lavas are basalts that have erupted throughout the lifetime of the cluster, although a considerable amount of basaltic andesite and basaltic trachyandesite has also been produced since 200 ka. Major- and trace-element variations suggest that magmas from Sajaka and Tanaga volcanoes are likely to have crystallized pyroxene and/or amphibole at greater depths than the older Takawangha magmas, which experienced a larger percentage of plagioclase-dominated fractionation at shallower depths. Magma output from Takawangha has declined over the last 86 k.y. At ca. 19 ka, the focus of magma flux shifted to the west beneath Tanaga and Sajaka volcanoes, where hotter, more mafic magma erupted.

Slab shape in subduction zones beneath the Kurile–Kamchatka and Aleutian arcs based on regional tomography results

A 3-D model of the seismic heterogeneities of P- and S-velocities has been constructed down to 1100 km beneath the Kurile–Kamchatka and Aleutian subduction zones on the basis of the regional tomographic inversion of data from global seismic catalogs. Particular attention is paid to verifying the data by different tests. A clear image of a classic subducting oceanic slab is observed along the entire Kurile–Kamchatka arc, which coincides in the P- and S-models and with the distribution of deep seismicity. These data served as a basis for a parametric model of the upper and lower slab boundaries beneath the Kurile–Kamchatka arc. According to this model, the slab has various thicknesses and maximum penetration depths in different arc segments. In the southern part of the arc, between depths of 600 and 700 km, the slab moves horizontally and does not penetrate the lower mantle. Beneath the North Kuriles and southern Kamchatka, it subducts down to 900 km. These data suggest that the subducting slab becomes a viscous and nonelastic body and the changes in its shape may be due to phase transitions with increasing temperature and pressure. We attribute its gentler dipping and thickening beneath the South Kuriles to the oceanic “pushing” mechanism. The lithospheric thinning, steeper subsidence, and penetration into the lower mantle beneath the North Kuriles are due to the predominant “gravity sinking,” or “slab pull,” mechanism. Unlike some other researchers, we have obtained a high-velocity anomaly beneath the western Aleutian arc (not as clear as beneath the Kurile–Kamchatka arc, yet quite reliable). It suggests the presence of a slab subducting down to 200–250 km. In the eastern Aleutian arc, we clearly observe the Pacific slab subducting down to 500–600 km (somewhat deeper than in the previous studies).

Volcano collapse along the Aleutian Ridge (western Aleutian Arc)

Abstract. The Aleutian Ridge, in the western part of the Aleutian Arc, consists of a chain of volcanic islands perched atop the crest of a submarine ridge with most of the active Quaternary stratocones or caldera-like volcanoes being located on the northern margins of the Aleutian Islands. Integrated analysis of marine and terrestrial data resulted in the identification and characterization of 17 extensive submarine debris avalanche deposits from 11 volcanoes. Two morphological types of deposits are recognizable, elongate and lobate, with primary controls on the size and distribution of the volcanic debris being the volume and nature of material involved, proportion of fine grained material, depth of emplacement and the paleo-bathymetry. Volume calculations show the amount of material deposited in debris avalanches is as much as three times larger than the amount of material initially involved in the collapse, suggesting the incorporation of large amounts of submarine material during transport. The orientation of the collapse events is influenced by regional fault systems underling the volcanoes. The western Aleutian Arc has a significant tsunamigenic potential and communities within the Aleutian Islands and surrounding areas of the North Pacific as well as shipping and fishing fleets that cross the North Pacific may be at risk during future eruptions in this area.

Submarine hydrothermal activity and mineralization on the Kurile and western Aleutian island arcs, N.W. Pacific

The Kurile Arc consists of at least 100 submarine volcanoes and 5 submarine caldera located mainly in the rear arc. The arc is seismically very active, particularly in the south, and is characterized by extensive volcanism and hydrothermal activity. At least one subaerial volcano on the arc (Medvezhy located on Iturup island) has an extremely shallow magma chamber and is intensely active. Three types of submarine hydrothermal deposit were recovered from this area, hydrothermal manganese crusts, nontronite and hydrothermal manganese crusts overlain by hydrogenous manganese oxides. In addition, submarine hydrothermal Fe oxyhydroxides enriched in P were sampled from the submerged caldera of Kraternaya Bight in the central part of the arc. These deposits appear to be analagous to deposits from Santorini caldera on the Aegean Arc. Piip submarine volcano located in the western Aleutian Arc is characterized by two shallow summit craters. Both intermediate-temperature (anhydrite, gypsum, barite, amorphous silica, pyrite, calcite and aragonite) and low-temperature (nontronite, Fe oxyhydroxides, hisingerite and ferromanganese crusts) hydrothermal minerals were recovered from this seamount together with bacterial mats and giant clams. The maximum measured temperature of the hydrothermal fluids was 133 °C, although the actual temperature may have reached 250 °C. Both the Kurile and Aleutian Arcs have the potential to host major submarine hydrothermal systems. Detailed exploration of these poorly studied areas using modern geophysical, geochemical and sampling techniques is therefore strongly recommended.

Slab portal beneath the western Aleutians

Tomographic images of the distribution of shear-wave speed beneath the northwestern Pacific delineate the configuration of the subducted oceanic lithosphere beneath the western Aleutian arc. At ;100 km depth, a fast shear-wave speed anomaly is beneath the Aleutian arc everywhere east of 1738E. Between 1648E and 1738E, however, seismic velocities at this depth are slow relative to the surrounding mantle. The lateral termination of the fast shear-wave speed anomaly at depth coincides with a gap in deep seismicity beneath the Aleutians. The absence of these two distinctive traits of subducting slabs leads us to conclude that this segment of the Aleutian arc overlies a very large window in the otherwise continuous lithospheric slab that we term a ‘‘slab portal.’’ This portal is likely to facilitate the production of distinctive volcanic rocks (adakites or high-Mg# andesites) by partially melting the adjacent edges of the slab. The Miocene age of most adakites in the westernmost Aleutians where no slab is present may indicate that the portal formed relatively recently. The chemistry of western Aleutian adakites approximates that of typical continental crust, so their genesis and subsequent lateral transport toward Kamchatka is a plausible mechanism for new continent formation.

Seismic evidence for catastrophic slab loss beneath Kamchatka.

In the northwest Pacific Ocean, a sharp corner in the boundary between the Pacific plate and the North American plate joins a subduction zone running along the southern half of the Kamchatka peninsula with a region of transcurrent motion along the western Aleutian arc. Here we present images of the seismic structure beneath the Aleutian-Kamchatka junction and the surrounding region, indicating that: the subducting Pacific lithosphere terminates at the Aleutian-Kamchatka junction; no relict slab underlies the extinct northern Kamchatka volcanic arc; and the upper mantle beneath northern Kamchatka has unusually slow shear wavespeeds. From the tectonic and volcanic evolution of Kamchatka over the past 10 Myr (refs 3-5) we infer that at least two episodes of catastrophic slab loss have occurred. About 5 to 10 Myr ago, catastrophic slab loss shut down island-arc volcanic activity north of the Aleutian-Kamchatka junction. A later episode of slab loss, since about 2 Myr ago, seems to be related to the activity of the world's most productive island-arc volcano, Klyuchevskoy. Removal of lithospheric mantle is commonly discussed in the context of a continental collision, but our findings imply that episodes of slab detachment and loss are also important agents in the evolution of oceanic convergent margins.

The role of an H2O-rich fluid component in the generation of primitive basaltic andesites and andesites from the Mt. Shasta region, N California

This paper presents analyses of the trace element abundances and isotopic compositions in primitive lavas from the Mt. Shasta region, N California. These data are combined with estimates of pre-eruptive H2O contents and constraints from experimental petrology to develop a model of subduction zone magmatism. These lavas share geochemical characteristics of high-Mg andesites from the Setouchi volcanic belt in SW Japan and Adak-type high-Mg andesites of the western Aleutian arc. Estimates of the pre-eruptive water contents of the Shasta region lavas range from 8 wt % H2O. The pre-eruptive H2O content and an inferred melt of a harzburgitic residue are used to carry out a mass balance for the relative contributions from a mantle-derived melt and slab-derived fluid-rich component. We assume that elements are contributed either from melting of mantle peridotite or from a subduction-related fluid-rich component. Estimated fluid-rich component compositions are characterized by strong light rare earth element (LREE) enrichments ([La/Gd]N=3 to 7) and variable heavy rare earth element (HREE) depletions ([Dy/Yb]N=1 to 3). Sr and Ba abundances vary by approximately a factor of 2.5 in the fluid compositions calculated for the Mt. Shasta region lavas and large ion lithophile element (LILE) abundances are similar to those calculated by Stolper and Newman (1994) and Eiler et al. (2000). The major elements in the fluid-rich component are H2O (~55–68 wt%), Na2O (~25–33 wt%) and K2O (~5–13 wt%). This composition may be that of a supercritical fluid or a low-degree melt of the slab that has reacted with the overlying mantle wedge. Although the slab beneath Mt. Shasta is inferred to be hot (~600 – 650 °C), the calculated fluid-rich components do not resemble a pure slab melt. The calculated isotopic composition of the fluid-rich component is bimodal. One component has 87Sr/86Sr=0.7028 and eNd=+8, and is most similar to a MORB source. The second component has more radiogenic 87Sr/86Sr=0.7038 and eNd=+1 and is most similar to a sediment. These fluid-rich components probably represent a mixture of fluids and melts from the slab (serpentinized mantle, altered basalt, and sediment).

Structure of an active arc-continent collision area: the Aleutian–Kamchatka junction

The plate boundary between the North American and Pacific plates along the westernmost portion of the Aleutian arc is formed by several active, dextral strike-slip fracture zones. Fracturing is expressed by fault escarpments visible in reflection seismic profiles, and narrow zones of high seismicity. The seismicity changes from the Aleutian arc, where shallow earthquakes occur, to intermediate to deep earthquakes in the Kamchatka–Aleutian junction area and to the south. In the vicinity of this junction, the distribution of earthquakes follows the traces of major Aleutian fracture zones down to a depth of 100 km. South of the Kronotsky Peninsula, the distribution of earthquakes changes and is similar to other subduction zones. The fracture zones cut the western Aleutian arc into individual blocks, which move along strike-slip faults to the west and collide with Kamchatka. Related transtension and transpression has led to the development of pull-apart basins and block tilting. The fracture zones splay into numerous strike-slip faults to the west, where they meet the Kamchatka margin. The main faults of the Aleutian fault system can be traced on the Kamchatka Mys Peninsula, where deformed Pleistocene sediments, high uplift rates, recent thrust faulting, and changes in the drainage system provide evidence for neotectonic activity. The fault pattern onshore and offshore in the junction area supports an indentation tectonic model of the collision of Aleutian arc slivers with Kamchatka.

Sea birds as proxies of marine habitats and food webs in the western Aleutian Arc

ABSTRACTWe propose that ocean conditions of the Near Islands in the western Aleutian Arc mimic those of the shallow continental shelf of the eastern Bering Sea to the extent that the marine community, including assemblages of forage fishes and their avian predators, has distinctly coastal characteristics. In contrast, marine avifauna and their prey at neighbouring Buldir Island are distinctly oceanic. For example, at the Near Islands, the ratio of thick‐billed to common murres, Vria lomvia and U. aalge, is low and black‐legged kittiwakes, Rissa tridactyla, but not red‐legged kittiwakes, R. brevirostris, nest there. Diets of murres and kittiwakes are dominated by sand lance, Ammodytes hexapterus, an abundant coastal species. At Buldir Island, thick‐billed murres greatly outnumber common murres, red‐legged kittiwakes and black‐legged kittiwakes are both abundant, and diets of the birds consist primarily of oceanic squid and lantern‐fish (Myctophidae). This mesoscale difference in food webs is apparently a consequence of the local physiography. A broad escarpment on the Near physiographic block creates a comparatively expansive, shallow, shelflike habitat around the Near Islands, where a pelagic community typical of coastal regions flourishes. Buldir Island is the only emergent feature of the Buldir physiographic block, with little shallow water surrounding it and, apparently, little opportunity for other than oceanic species to exist. Patterns in the distribution of fishes, and thus of sea birds, throughout the Aleutian Islands might be largely explained by the presence or absence of shelf‐like habitat and the relationship between physical environments and food webs. In the larger context of fisheries oceanography, this model for the Aleutian Islands improves our ability to interpret physical and biological heterogeneity in the ocean and its relationship to regional community dynamics and trends in the abundance and productivity of individual species at higher trophic levels.

Magnesian andesite in the western Aleutian Komandorsky region: Implications for slab melting and processes in the mantle wedge

Research Article| May 01, 1995 Magnesian andesite in the western Aleutian Komandorsky region: Implications for slab melting and processes in the mantle wedge G. M. Yogodzinski; G. M. Yogodzinski 1Institute for the Study of the Continents and Department of Geological Sciences, Cornell University, Ithaca, New York 14850 Search for other works by this author on: GSW Google Scholar R. W. Kay; R. W. Kay 1Institute for the Study of the Continents and Department of Geological Sciences, Cornell University, Ithaca, New York 14850 Search for other works by this author on: GSW Google Scholar O. N. Volynets; O. N. Volynets 2Institute of Volcanic Geology and Geochemistry, Petropavlovsk, Kamchatka 683006, Russia Search for other works by this author on: GSW Google Scholar A. V. Koloskov; A. V. Koloskov 2Institute of Volcanic Geology and Geochemistry, Petropavlovsk, Kamchatka 683006, Russia Search for other works by this author on: GSW Google Scholar S. M. Kay S. M. Kay 3Institute for the Study of the Continents and Department of Geological Sciences, Cornell University, Ithaca, New York 14850 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1995) 107 (5): 505–519. https://doi.org/10.1130/0016-7606(1995)107<0505:MAITWA>2.3.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation G. M. Yogodzinski, R. W. Kay, O. N. Volynets, A. V. Koloskov, S. M. Kay; Magnesian andesite in the western Aleutian Komandorsky region: Implications for slab melting and processes in the mantle wedge. GSA Bulletin 1995;; 107 (5): 505–519. doi: https://doi.org/10.1130/0016-7606(1995)107<0505:MAITWA>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The role of the subducting lithospheric slab in the genesis of mantle-derived (primitive) magmas is investigated through a study of volcanic rocks formed in the tectonically strike-slip–dominated western Aleutian arc. Two types of chemically and petrologically distinctive primitive andesites have been found among the Miocene–late Pleistocene–age volcanic rocks in the western Aleutians. These are termed the “Adak-type” and “Piip-type” magnesian andesites. Trace element and isotopic characteristics indicate that Adak-type magnesian andesites (adakites) formed principally as small percentage melts of the basaltic portion of the subducting oceanic crust, leaving a clinopyroxene-garnet-rutile residual mineralogy. The resulting slab melt signature (high La/Yb, Sr) distinguishes Adak-type magnesian andesites from all other Aleutian volcanic rocks. Primitive characteristics (high Mg#, Cr, Ni) and intermediate compositions (∼59% SiO2) of Adak-type magnesian andesites were acquired by interaction with peridotite and/or basalt in the mantle wedge. The absence of olivine phenocrysts from Adak-type magnesian andesites indicates that they were not equilibrated with peridotite and so are unlike Piip-type magnesian andesites, which appear to have equilibrated under low pressure and hydrous conditions in the subarc mantle. Piip-type magnesian andesites also contain a slab melt component, but reaction-equilibration with peridotite has lowered La/Yb and Sr to levels like those of common Aleutian volcanic rocks.Miocene-age calc-alkaline rocks of the Komandorsky Islands have chemical characteristics transitional between those of Adak-type magnesian andesites and common Aleutian volcanic rocks from the central and eastern arc. In a source mixture of depleted mantle wedge, slab melt, and sediment, the Komandorsky rocks have a relatively large contribution from the slab melt endmember. The strong slab melt signature among western Aleutian rocks is attributed to highly oblique convergence that produced a slow subduction path into the subarc mantle. Geochemically, the slab melt provided a high Sr, La/Yb, La/Ta, and low Ti/Hf endmember to the western Aleutian source mixture. The enhanced role for slab melting in the western Aleutians may be like that predicted for Archean systems and for modern systems where the subduction zone is warm. In this regard, Adak-type magnesian andesites are probably the appropriate analog to sanukitoids and other primitive andesitic rocks of Archean age. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The Aleutian Arc: Tectonic blocks, episodic subduction, strain diffusion, and magma generation

A relocated earthquake sequence in the western Aleutian arc lasting from February 1965 through December 1968 is shown to map an episode of subduction of the Pacific plate. The February 4, 1965, Rat Island main shock (M/sub s/ + 8.1) was a complex rupture involving greatest underthrusting at lock zones beneath the Aleutian arc transverse canyons. The primary tectonic consequences of the main shock rupture were (1) the downslab diffusion of a compressional pulse, (2) the oceanward diffusion of an extensional pulse in the oceanic lithosphere, and (3) the slow rebound of the continental plate. The velocities of these strain pulses (actual plate motions) are determined by the viscosity of the constraining mantle material; observed strain velocities indicate a Newtonian mantle viscosity of 6 x 10/sup 19/ P. The extensional pulse that propagated oceanward is shown to determine the locations and normal-faulting mechanisms of the trench earthquakes of this sequence. It is inferred that the entire subducting oceanic lithosphere contains a sequence of near-vertical faults that strike parallel to the trench axis, reflecting former trench-related normal-faulting earthquakes. This condition is supported by considerable data and suggests a slabbing, escalatorlike descent mechanism for the upper 100 km of the subducting Pacificmore » plate. A working hypothesis is presented that relates the primaryfeatures of the typical volcanic arc and interarc basin to episodic downslab compressional pulses. This hypothesis involves a sequence of high-loading-rate mantle compression, viscoelastic rebound, pressure reduction, increased partial melt concentrations, and counterflow in a thin tabular zone above the subducting plate.« less