Central Aleutian Arc Research Articles

Double Reservoirs Imaged Below Great Sitkin Volcano, Alaska, Explain the Migration of Volcanic Seismicity

AbstractVolcanic seismicity provides essential insights into the behavior of an active volcano across multiple time scales. However, to understand how magma moves as the eruption cycle develops, better knowledge of the geometry and physical properties of the magma plumbing system is required. In this study, using full‐wave ambient noise tomography, we image the three‐dimensional (3‐D) crustal shear‐wave velocity structure below Great Sitkin Volcano in the central Aleutian Arc. The velocity model reveals two low‐velocity anomalies correlating with the migration of volcanic seismicity. With a bulk melt fraction of about 2.5%–9%, these low‐velocity anomalies are interpreted as mushy magma reservoirs. We propose a six‐stage eruption cycle to explain the migration of seismicity and the alternating eruption of the two reservoirs with different recharging histories. These findings have broad implications for the dynamics of magma plumbing systems and the structural control of eruption behaviors.

Seismic character and progression of explosive activity during the 2016–2017 eruption of Bogoslof volcano, Alaska

Bogoslof volcano, in the central Aleutian arc, experienced a major eruption between December 2016 and August 2017 that was characterized by explosive activity (Volcanic Explosivity Index 2 to 3) and the extrusion of lava domes. The Alaska Volcano Observatory tracked the activity in real time using seismicity observed on distant stations as well as infrasound, lightning, satellite data, and occasional visual observations. In this study, we measure the duration of seismic signals associated with individual explosive events to track their progression during the two explosive phases of the eruption. Seismic recordings of Bogoslof explosions show complex waveforms that suggest both individual explosive events and sequences of several explosions separated by lower amplitude tremor. The lack of local seismic monitoring (stations at distances of 1 km to 15 km distance) unfortunately limits our ability to closely observe seismicity and to interpret changing conditions at the vent such as position, the presence of a lava dome or plug, and the role of seawater associated with the eruption. We use the rate of explosive activity, seismic waveform character, and repose time between explosions to infer the conditions within the conduit.

The Calc-Alkaline Hidden Bay and Kagalaska Plutons and the Construction of the Central Aleutian Oceanic Arc Crust

Abstract Calc-alkaline plutons are the major crustal building blocks of continental margins, but are rarely exposed in oceanic island arcs. Two of the best examples are the ∼10 km wide Hidden Bay and Kagalaska plutons that intrude Eocene mafic volcanic–sedimentary rocks on Adak and Kagalaska islands in the central Aleutian arc. Twenty new Ar/Ar and U/Pb ages, coupled with published ages, show that the Hidden Bay pluton was intruded in multiple stages from ∼34·6 to 30·9 Ma, whereas the Kagalaska pluton was intruded at ∼14 Ma. The plutons largely consist of medium- to high-K2O hornblende-bearing cumulate diorite (53–55 wt % SiO2) and hornblende–biotite granodiorite (57–64 wt %), with lesser amounts of gabbro (50–52 wt % SiO2), leucogranodiorite (67–69 wt % SiO2) and aplite (76–77 wt % SiO2) that can generally be linked to each other by crystal fractionation. The compositions of these plutons are generally similar to those of continental plutons, except for more oceanic-like large ion lithophile element and isotopic signatures (87Sr/86Sr = 0·703–0·7033; ɛNd = 9·4–7·7) that reflect oceanic- rather than continental-type crustal contaminants. Chemical similarities between the Hidden Bay homogeneous gabbros and high-Al basalts in Adak Pleistocene-Holocene volcanoes indicate little temporal evolution in the general character of the mantle-derived basalts. Rather than a unique arc setting and distinctive magmas, formation of the Aleutian calc-alkaline plutons seems to require a sufficient crustal thickness (∼37 km) and a high enough water content to stabilize pargasitic hornblende amphibole in a relatively closed magma system that favors increasing K, Ti and H2O at the end of a magmatic cycle. This termination of magmatism coincides with a northward migration of the magmatic front that is inferred to be associated with fore-arc subduction erosion. In accord with Adak region crustal architecture based on seismic data, crystallization models for the plutons suggest that mantle-generated hydrous arc basalts fractionated olivine and clinopyroxene in the lower crust to form high-Al basaltic composition magmas that rose into the mid-crust, where gabbro and diorite crystallized to form the magmas that buoyantly rose into the upper crust and crystallized to form the volumetrically dominant granodiorite (58–63 wt % SiO2). The most important temporal changes in chemistry can be explained by fore-arc crust incorporated into the mantle wedge by fore-arc subduction erosion creating ‘adakitic’ signatures at times of northward arc migration and a change to a more continental subducted sediment component at the time of Plio-Pleistocene glaciation.

Age models and tephrostratigraphy from two lakes on Adak Island, Alaska

Radiocarbon-based age models from two lakes on Adak Island, Alaska, were compared using correlated tephra to check the accuracy of the radiocarbon ages and to develop a regional tephrochronology. Andrew and Heart Lakes are located adjacent to and 5 km south of Mt. Moffett on the northern part of Adak Island in the central Aleutian Arc. The downcore trends in radiocarbon ages of macrofossils, the 1963 spike in plutonium (239+240Pu) activity, and a lead (210Pb) profile from Andrew Lake were modeled with smooth spline fits. Tephras were correlated based on magnetic susceptibility, particle morphology, and internal stratigraphy including color and particle size. Of the 19 pure tephras in Heart Lake and 21 in Andrew Lake, ten could be confidently correlated between the two lakes. The composite tephrostratigraphy from the two lakes includes at least 30 unique tephras deposited over 9600 years. Radiocarbon ages obtained from mixed aquatic and terrestrial macrofossils with ages hundreds of years too old were inferred for Heart Lake by correlated tephra ties to the Andrew Lake age model. The eroding tephra-soil deposits that crop out along the banks of Heart Lake, and its large shallow platform, may facilitate remobilization of previously stored sediments. Previous studies on Adak Island of archeological sites and tephra-soil outcrops described five tephras that are correlated with a subset of tephras from the lakes based on stratigraphic succession, inferred age, and physical description. Age-depth models for Heart and Andrew Lakes as well as tephrochronology for Adak Island provide geochronological context for future studies.

Seismic reflection imaging of ultradeep roots beneath the eastern Aleutian island arc

Seismic reflection data show that the eastern Aleutian Arc is characterized by reflectors that extend continuously from the lower arc crust to >50 km depth, which is considerably deeper than the crustal thickness of 27–35 km previously inferred from coincident wide-angle seismic surveys. Because the upper mantle is commonly homogeneous, and therefore nonreflective, relative to the overlying crust, we interpret these reflectors to be gabbro, garnet gabbro, and pyroxenite intrusions within two 50-km-wide roots that represent a >25-km-thick heterogeneous transition from mafic lower crustal rocks to ultramafic mantle rocks. We suggest that the reflectivity is linked to repeated differentiation and intrusion of mantle-derived melts into the subarc lithosphere, and that the depth of these roots shows that fractionation of arc crust can extend well below the seismically determined Moho. Because these deep roots are not evident beneath the central Aleutian Arc, either the roots form sporadically, perhaps as a consequence of an elevated magmatic supply, or such roots ultimately founder into the underlying mantle due to their relatively high mass density.

Influence of the Amlia fracture zone on the evolution of the Aleutian Terrace forearc basin, central Aleutian subduction zone

During Pliocene to Quaternary time, the central Aleutian forearc basin evolved in response to a combination of tectonic and climatic factors. Initially, along-trench transport of sediment and accretion of a frontal prism created the accommodation space to allow forearc basin deposition. Transport of sufficient sediment to overtop the bathymetrically high Amlia fracture zone and reach the central Aleutian arc began with glaciation of continental Alaska in the Pliocene. As the obliquely subducting Amlia fracture zone swept along the central Aleutian arc, it further affected the structural evolution of the forearc basins. The subduction of the Amlia fracture zone resulted in basin inversion and loss of accommodation space east of the migrating fracture zone. Conversely, west of Amlia fracture zone, accommodation space increased arcward of a large outer-arc high that formed, in part, by a thickening of arc basement. This difference in deformation is interpreted to be the result of a variation in interplate coupling across the Amlia fracture zone that was facilitated by increasing subduction obliquity, a change in orientation of the subducting Amlia fracture zone, and late Quaternary intensification of glaciation. The change in coupling is manifested by a possible tear in the subducting slab along the Amlia fracture zone. Differences in coupling across the Amlia fracture zone have important implications for the location of maximum slip during future great earthquakes. In addition, shaking during a great earthquake could trigger large mass failures of the summit platform, as evidenced by the presence of thick mass transport deposits of primarily Quaternary age that are found in the forearc basin west of the Amlia fracture zone.

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

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Seismic swarm associated with the 2008 eruption of Kasatochi Volcano, Alaska: Earthquake locations and source parameters

[1] An energetic seismic swarm accompanied an eruption of Kasatochi Volcano in the central Aleutian volcanic arc in August of 2008. In retrospect, the first earthquakes in the swarm were detected about 1 month prior to the eruption onset. Activity in the swarm quickly intensified less than 48 h prior to the first large explosion and subsequently subsided with decline of eruptive activity. The largest earthquake measured as moment magnitude 5.8, and a dozen additional earthquakes were larger than magnitude 4. The swarm exhibited both tectonic and volcanic characteristics. Its shear failure earthquake features were b value = 0.9, most earthquakes with impulsive P and S arrivals and higher-frequency content, and earthquake faulting parameters consistent with regional tectonic stresses. Its volcanic or fluid-influenced seismicity features were volcanic tremor, large CLVD components in moment tensor solutions, and increasing magnitudes with time. Earthquake location tests suggest that the earthquakes occurred in a distributed volume elongated in the NS direction either directly under the volcano or within 5–10 km south of it. Following the MW 5.8 event, earthquakes occurred in a new crustal volume slightly east and north of the previous earthquakes. The central Aleutian Arc is a tectonically active region with seismicity occurring in the crusts of the Pacific and North American plates in addition to interplate events. We postulate that the Kasatochi seismic swarm was a manifestation of the complex interaction of tectonic and magmatic processes in the Earth's crust. Although magmatic intrusion triggered the earthquakes in the swarm, the earthquakes failed in context of the regional stress field.

Magmatic Differentiation at an Island-arc Caldera: Okmok Volcano, Aleutian Islands, Alaska

Okmok volcano is situated on oceanic crust in the central Aleutian arc and experienced large ( 15 km) caldera-forming eruptions at 12 000 years BP and 2050 years BP. Each caldera-forming eruption began with a small Plinian rhyodacite event followed by the emplacement of a dominantly andesitic ash-flow unit, whereas effusive interandpost-caldera lavashavebeenmorebasaltic.Phenocryst assemblages are composed ofolivineþ pyroxeneþ plagioclase Fe^Ti oxides and indicate crystallization at1000^11008Cat 0 1^0 2GPa in thepresence of 0^4% H2O.The erupted products follow a tholeiitic evolutionary trend and calculated liquid compositions range from 52 to 68 wt % SiO2 with 0 8^3 3 wt%K2O.Major and trace element models suggest that the more evolved magmas were produced by 50^60% in situ fractional crystallization around the margins of the shallow magma chamber. Oxygen and strontium isotope data ( O 4 4^4 9o, Sr/ Sr 0 7032^0 7034) indicate interaction with a hydrothermally altered crustal component, which led to elevated thorium isotope ratios in some caldera-forming magmas. This compromises the use of uranium^thorium disequilibria [(Th/U)1⁄4 0 849^0 964] to constrain the time scales of magma differentiation but instead suggests that the age of the hydrothermal system is 100 ka. Modelling of the diffusion ofstrontiuminplagioclase indicates thatmanyevolved crystal rims formed less than 200 years prior to eruption.This addition of rim material probably reflects the remobilization of crystals from the chamber margins following replenishment. Basaltic recharge led to the expansion of the magma chamber, which was responsible for the most recent caldera-forming event.

Interferometric synthetic aperture radar study of Okmok volcano, Alaska, 1992–2003: Magma supply dynamics and postemplacement lava flow deformation

Okmok volcano, located in the central Aleutian arc, Alaska, is a dominantly basaltic complex topped with a 10‐km‐wide caldera that formed circa 2.05 ka. Okmok erupted several times during the 20th century, most recently in 1997; eruptions in 1945, 1958, and 1997 produced lava flows within the caldera. We used 80 interferometric synthetic aperture radar (InSAR) images (interferograms) to study transient deformation of the volcano before, during, and after the 1997 eruption. Point source models suggest that a magma reservoir at a depth of 3.2 km below sea level, located beneath the center of the caldera and about 5 km northeast of the 1997 vent, is responsible for observed volcano‐wide deformation. The preeruption uplift rate decreased from about 10 cm yr−1 during 1992–1993 to 2 ∼ 3 cm yr−1 during 1993–1995 and then to about −1 ∼ −2 cm yr−1 during 1995–1996. The posteruption inflation rate generally decreased with time during 1997–2001, but increased significantly during 2001–2003. By the summer of 2003, 30 ∼ 60% of the magma volume lost from the reservoir in the 1997 eruption had been replenished. Interferograms for periods before the 1997 eruption indicate consistent subsidence of the surface of the 1958 lava flows, most likely due to thermal contraction. Interferograms for periods after the eruption suggest at least four distinct deformation processes: (1) volcano‐wide inflation due to replenishment of the shallow magma reservoir, (2) subsidence of the 1997 lava flows, most likely due to thermal contraction, (3) deformation of the 1958 lava flows due to loading by the 1997 flows, and (4) continuing subsidence of 1958 lava flows buried beneath 1997 flows. Our results provide insights into the postemplacement behavior of lava flows and have cautionary implications for the interpretation of inflation patterns at active volcanoes.

Inferring crustal structure in the Aleutian island arc from a sparse wide‐angle seismic data set

Compressional seismic travel times from a relatively sparse wide‐angle data set hold key information on the structure of a 800 km long section of the central Aleutian arc. Since the source and receiver locations form a swath along the arc crest that is ∼50 km wide, we trace rays in 3‐D for a collection of 8336 seismic refraction and reflection arrivals. We investigate variations in seismic velocity structure parallel to the Aleutian arc, assuming that our result represents average crustal structure across the arc. We explore seismic velocity models that consist of three crustal layers that exhibit smooth variations in structure in the 2‐D vertical plane. We consider the influence of additional constraints and model parameterization in our search for a plausible model for Aleutian arc crust. A tomographic inversion with static corrections for island stations reduces the data variance of a 1‐D starting model by 91%. Our best model has seismic velocities of 6.0–6.5 km/s in the upper crust, 6.5–7.3 km/s in the middle crust, and 7.3–7.7 km/s in the lower crust and a total crustal thickness of 35–37 ± 1 km. A resolution analysis shows that features having a horizontal scale less than 20 km cannot be imaged, but at horizontal length scales of ∼50 km most model features are well resolved. The study indicates that the Aleutian island arc crust is thick compared to other island arcs and strongly stratified and that only the upper 60% of the arc crust has seismic velocities that are comparable to average seismic velocities in continental crust.

Variable Impact of the Subducted Slab on Aleutian Island Arc Magma Sources: Evidence from Sr, Nd, Pb, and Hf Isotopes and Trace Element Abundances

Major and trace element compositions and Sr, Nd, Pb, and Hf isotope ratios of Aleutian island arc lavas from Kanaga, Roundhead, Seguam, and Shishaldin volcanoes provide constraints on the composition and origin of the material transferred from the subducted slab to the mantle wedge. 40 Ar/ 39 Ar dating indicates that the lavas erupted mainly during the last � 400 kyr. Along-arc geochemical and isotopic variations are consistent with variable degrees of fluid input to the mantle wedge. Addition of bulk sediment, partially melted sediment, or a combination of sediment and fluid components may also explain the major and trace element and isotopic compositions of some Aleutian lavas. Mass-balance modeling suggests that the fluid is derived from subducted sediment (10–25%) and underlying oceanic crust (75–90%). Hf–Nd isotope data suggest that relative to Nd, little Hf is transferred to the mantle wedge via fluid. Lavas from Seguam Island in the central Aleutian arc have distinctly elevated B/La, U/Th, 87 Sr/ 86 Sr, and 207 Pb/ 204 Pb ratios, which probably reflect a large volume of fluid released from serpentinized oceanic crust plus the overlying layer of subducted sediment. We propose that the Amlia Fracture Zone, which was subducted beneath Seguam Island in the past 1 Myr, contains excess sediment and larger quantities of H2O-rich serpentine near the surface of the Pacific plate, and hence more fluid was available for transfer into the wedge in this section of the arc. The degree of partial melting of the mantle, modeled from the incompatible trace element contents of the lavas, correlates with the estimated mass of fluid fluxing of the mantle wedge. Seguam lavas, which show the largest quantity of fluid addition, have compositions that can be matched by a 22% partial melt of a fluid-modified mantle source, whereas Shishaldin and Roundhead lava compositions are consistent with an order of magnitude less partial melting of the mantle wedge.

Petrologic constraints on the spatial distribution of crustal magma chambers, Atka Volcanic Center, central Aleutian arc

The Atka volcanic center, the largest modern, central Aleutian magmatic complex, covers 360 km2 with an estimated eruptive volume of ~200 km3. The oldest exposed rocks, the Old Harbor Series, are a suite of basaltic andesite to dacite flows, pyroclastic rocks and lahars of presumed Tertiary age. The youngest phase of activity on Atka probably began 1–2 Ma ago near the center of the present volcanic field, and consists of basaltic and basaltic andesite flows which produced a large shield volcano, Atka Volcano. Subsequent to this activity, satellite vents formed along the margin of this structure. Collapse of the ancestral Atka Volcano produced a caldera 5 km in diameter and was accompanied by the eruption of a large dacitic flow (Big Pink). After caldera formation, four major volcanic cones (Korovin, Mount Kliuchef, Sarichef and Konia) grew around the margins of Atka caldera. This study focuses on the Atka Volcano, Korovin and Mount Kliuchef volcanic centers. Atka lavas range from basalt through dacite, with all major volcanic vents erupting lavas spanning similar compositional ranges. They are porphyritic with phenocrysts of plagioclase (25–36%), olivine (0.4–3.6%) or orthopyroxene (2.8%), clinopyroxene (1–12%) and a single Fe–Ti oxide (0–2%). Hydrous phases are lacking even in the most evolved lavas. Two generations of plagioclase occur in the Korovin suite but not in lavas from Mount Kliuchef. Korovin plagioclase also displays resorption surfaces and inclusion zones which are either absent in the phenocrysts of Mount Kliuchef lavas or very rare. Similarly, clinopyroxene in Korovin has reaction rims and abundant glass inclusions which are absent in the Mount Kliuchef lavas. Lavas erupted in the Atka volcanic field vary from 47 to 66% SiO2, although the complex is dominated volumetrically by basaltic lavas. On major-element Harker diagrams, mafic and intermediate Atka Volcano lavas form linear trends but the most siliceous lavas do not fall along these trends. By contrast, Mount Kliuchef lavas are compositionally bimodal whereas Korovin samples range from 51 to 62 wt% silica with intermediate lavas clustering around 55% SiO2. Incompatible trace-element abundances of Korovin lavas do not increase with silica content but those of Atka Volcano and Mount Kliuchef do. On incompatible-incompatible element diagrams, the three suites behave very differently. Differences also exist between the REE characteristics of the three Atka suites. The petrographic and geochemical differences between the three suites suggest formation by markedly different crustal magmatic processes. The characteristics of the Atka Volcano suite suggest formation of the basalt to dacite suite by crystal fractionation of an anhydrous phenocryst assemblage. Incompatible-incompatible element trends suggest the mafic Mount Kliuchef lavas were also produced by crystal fractionation, a conclusion supported by quantitative mass balance calculations. The difference in slope between the Atka Volcano and Mount Kliuchef suites indicates, however, that there were significant differences in these fractionation schemes. Modeling as well as geochemical characteristics suggest that the Mount Kliuchef dacites are not the product of crystal fractionation. They may represent (1) crustal melts, (2) complete melts of siliceous lava produced during earlier magmatic stages, or (3) residual liquids from the Atka Volcano magmatic system. In contrast to these suites, the Korovin lavas could not have been formed by crystal fractionation. Rather the andesites are probably the result of mixing basaltic and dacitic magmas. The Atka data suggest: (1) large volcanic centers may be supplied by a series of non-communicating, crustal magma chambers; (2) local factors may be important in determining liquid-lines-of-descent; and (3) very different magmatic processes may occur in adjacent magma chambers. If the complexity of the Atka suite is a common feature of arc magmatic plumbing systems, reconstructing differentiation processes requires analytical sample sets which are well constrained spatially and temporally.

Distinguishing melt and fluid subduction components in Umnak Volcanics, Aleutian Arc

Plate processing at convergent margins plays a central role in the distribution of elements among major earth reservoirs. The mechanisms by which this distribution occurs, however, have remained poorly constrained. This paper provides new constraints through a detailed isotope and trace element study of volcanic rocks from Umnak Island, Central Aleutian Arc. The data require the addition of three distinct subduction components to the subarc mantle, which are characterized and quantified: (1) a hydrous fluid from subducted oceanic crust, with mid‐ocean ridge basalt (MORB)‐like isotopic compositions but high Pb/Nd; (2) a hydrous fluid from subducted sediment, with sediment‐like isotopic compositions and an enrichment in fluid‐mobile elements; (3) a sediment partial melt, with sediment‐like isotopic compositions and high Th/Nd and Th/Nb compared to both regional and global sediments. The sediment melt is depleted in fluid‐mobile elements, indicating loss of fluid prior to melting. The high Th/Nb of the sediment melt indicates presence of a Ti‐rich residual phase such as rutile during partial melting. The observation that sediment fluid and sediment melt can be distinguished in different volcanic rocks suggests that they arrive separately at the sites of arc magma formation. This indicates release of multiple discrete fluid and melt phases from sediment to the overlying mantle wedge, which can be viewed as a natural consequence of progressive metamorphism of the subducting slab.

Seismic cycles along the Aleutian arc: Analysis of seismicity from 1957 through 1991

We catalog and relocate Aleutian arc seismicity. Between 1957 and 1991, two great earthquakes ruptured the same 250‐km‐long portion of the central Aleutian arc: the 1957 Aleutian Islands earthquake and the 1986 Andreanof Islands earthquake. Because accurate estimates of the moment distribution of the 1957 earthquake are not available, the spatial distribution of aftershocks for each of these events is compared and tested against models describing the modes of occurrence of great subduction zone earthquakes. Earthquake relocations are based on P wave arrival times published in the International Seismological Summary, the Bureau Central International Seismologique, and the International Seismological Centre bulletins and include corrections for the near‐source velocity structure associated with the down‐going slab. Magnitude estimates are extracted from bulletins and prior to 1964 are estimated by us from microfilmed records. Our catalog is complete above magnitude 5.5. Aftershocks associated with the 1957 and 1986 earthquakes appear to occur in different areas. East of the main shock epicenters, aftershock locations are anticorrelated. West of the main shock epicenter, aftershocks of the 1986 earthquake tended to concentrate along the updip edge of aftershock clusters associated with the 1957 earthquake. If we assume aftershocks rim the distribution of seismic moment release associated with each event, these observations imply that the moment distribution of the 1986 earthquake was different from that of the 1957 earthquake. This suggests that we should use caution in identifying mechanically strong portions of a fault, asperities, by simply mapping the moment distribution of a single great earthquake. A fundamental tenet of the asperity model, that rupture always occurs on the strongest portions of the fault with weaker portions rupturing either aseismically or dynamically as a result of rupture on a strong fault patch, may in the case of the central Aleutian arc not be correct. Thus observing the moment distribution from a single great earthquake may tell us little about what the distribution of moment release will look like during the next earthquake.

A petrologic re-investigation of the Adak volcanic center, central Aleutian arc, Alaska

The Adak volcanic center is located in the central part of the Aleutian arc and consists of three main volcanic vents. Andrew Bay Volcano, the oldest center, has been mostly removed by erosion. The next youngest vent, Mount Adagdak, was built in three major volcanic stages whereas Mount Moffett, the largest volcanic edifice, consists of a main cone and a parasitic cone each with several magmatic phases. Adak is unique compared to other modern Aleutian volcanic centers in that it contains two xenolith suites (Conrad and Kay, 1984; Debari et al., 1987). One suite consisting predominantly of mafic xenoliths occurs on Mount Moffett whereas an assemblage of ultramafic and mafic xenoliths is found on Mount Adagdak. Lavas erupted at Adak span the compositional range from 48.4 to 65.0 wt.% SiO2 and are characterized by significant variations in Al2O3, MgO, Sr, Ni and Cr. On Harker diagrams, this variability produces compositional trends with significant scatter. The Adak suite has total REE contents that vary from 32 to 154 ppm but do not correlate systematically with silica. (LaYb)n ratios range from 2.41 to 21.72 with the majority of lavas between 2.41 and 6.06. On process identification diagrams, the Adak suite plots as steeply sloping trends that contrast with the horizontal patterns of most other Aleutian centers. Measured isotopic ranges are large and nearly equal to those for the entire arc. Although they span similar silica ranges, subtle geochemical and isotopic differences distinguish the different volcanic vents of Adak. On Mount Moffett, a geochemically and isotopically distinct group of andesites (55.5–57.9 SiO2), the mafic andesites, occur on its NE flank. These lavas have elevated MgO, Ni and Cr but are depleted in Al2O3 relative to other Mount Moffett andesites with similar silica. They also have more heterogeneous REE abundances and isotopic ratios than most of the other andesites. Significant compositional differences exist between Adak and the other volcanic centers of the central Aleutian arc. Although these differences are characteristic of all geochemical systems, they are greatest for major and rare-earth elements and isotopic ratios. The lack of coherent relationships on major- and trace-element Harker diagrams, the isotopic variability, as well as the steeply sloping trends on REE process identification diagrams suggest that the Adak volcanic suite was not formed predominantly by closed-system crystal fractionation, but must be the product of a complex open-system process(es). The significant isotopic variability displayed by the suite suggests that contamination by an isotopically distinct contaminant must also have been an important petrologic component in the evolution of the suite. REE data are also suggestive of a role for magma mixing. Such a complex petrologic evolution is consistent with an immature lithospheric plumbing system. Based on REE systematics, the xenolith suites of Adak cannot, as previously proposed, be related to the host lavas or the rest of the Adak suite through crystal fractionation schemes. Rather they are probably accidental fragments derived from various depths along lithospheric conduits. In light of their relation to xenolith-bearing units, the mafic andesites of Adak presumably represent hybrid magmas formed during the interaction of ascending magmas with lithospheric wall rock. They are, therefore, characteristic of immature volcanic centers and unlikely to be related directly to the magmatic processes responsible for the generation of primary arc magmas. Because of the close proximity of the vents and the subtle compositional differences between their lavas, the Adak volcanic center was probably supplied by a single, deep lithospheric plumbing system that fed separate crustal magma chambers. The absence of historic volcanic activity on Adak suggests this plumbing system was abandoned before complete conduit development. This decline in magmatism may reflect a re-adjustment of volcano spacing within this part of the Aleutian arc.

Mid‐Pleistocene basalt from the Seguam Volcanic Center, central Aleutian Arc, Alaska: Local lithospheric structures and source variability in the Aleutian Arc

Recent geophysical and tectonic studies make it possible to examine the relation between local structural and tectonic evolution of the modern Aleutian Arc and the sources and processes that produce the broad spectrum of Aleutian basaltic lavas. Mid‐Pleistocene basalt from the Seguam Island volcanic center in the central Aleutian Arc erupted through strongly extended arc crust, contains 4–8% MgO, is low in K2O (<0.5%), high in Al2O3 (>18%), and is distinguished from Aleutian basalt erupted through less extended crust by a low abundance of incompatible and rare earth elements (REE) and low Sr and Pb content. The Ba/La (47–72) and Pb/La (0.7–1.5) ratios are high, incompatible element ratios are variable, and REE patterns cross one another. The 87Sr/86Sr ratio (0.70365 ± 0.00003) is the highest value obtained from the Aleutian islands, also 206Pb/204Pb (18.91 ±0.02), 207Pb/204Pb (15.60 ±0.02), and 208Pb/204Pb (38.51 ±0.04) ratios are among the most radiogenic values known, whereas the 143Nd/144Nd ratio (0.51297 ±0.00003) is among the lowest measured. Low abundances of incompatible elements, high Ba/La and Pb/La ratios, uniform Sr, Nd, and Pb isotope ratios, and high eruptive temperatures (∼1160° C) suggest that basalt erupted at Seguam ascends through extended subarc lithosphere without extensive chemical exchange with the upper mantle or lower crust. Partial melting of geochemically distinct portions of an isotopically uniform source region best explains observed geochemical characteristics. Trace element abundances rule out an eclogite source and suggest instead a peridotite‐dominated source with ∼5–10% of a subducted sediment component. This peridotite may reside in the mantle wedge or in the Amlia Fracture Zone (AFZ) of the Pacific plate that was subducted beneath Seguam ∼1 Ma. In addition, the AFZ possibly enhanced the subduction of terrigenous sediment into the source region beneath Seguam and this extra terrigenous component may explain the Ba/La, Pb/La and Sr, Nd, and Pb isotope ratios. The distinctive composition of Seguam basalt reflects a complex set of physical conditions, including fracture zone subduction and intraarc extension, that affected both the source region and thermo‐chemical history of magma ascent. Local structural and tectonic factors impact profoundly the nature of island arc magma sources and processes. Models that propose a common parental arc magma type or common processes on a global or arcwide scale are too generalized to explain the diversity actually observed in the Aleutian Arc.

Mid-Pleistocene lavas from the Seguam volcanic center, central Aleutian arc: closed-system fractional crystallization of a basalt to rhyodacite eruptive suite

In contrast to adjacent volcanic centers of the modern central Aleutian arc, Seguam Island developed on strongly extended arc crust. K-Ar dates indicate that mid-Pleistocene, late-Pleistocene, and Holocene eruptive phases constitute Seguam. This study focuses on the petrology of the mid-Pleistocene, 1.07–07 Ma, Turf Point Formation (TPF) which is dominated by an unusual suite of porphyritic basalt and basaltic andesite lavas with subordinate phenocryst-poor andesite to rhyodacite lavas. Increasing whole-rock FeO*/MgO from basalt to dacite, the anhydrous Plag+Ol+Cpx±Opx±Mt phenocryst assemblage, groundmass pigeonite, and the reaction Ol+Liq=Opx preserved in the mafic lavas indicate a tholeiitic affinity. Thermometry and comparison to published phase equilibria suggests that most TPF basalts crystallized Plag+Ol+Cpx±Mt at ≥1160°C between about 3–5 kb (±1–2% H2O), andesites crystallized Plag+Cpx+Opx±Mt at ≥1000°C between 3–4 kb with 3–5% H2O, and dacites crystallized Plag +Cpx±Opx±Mt at 1000°C between 1–2 kb with 2–3% H2O. All lavas crystallized at f o 2 close to the NNO buffer. Mineral compositions and textures indicate equilibrium crystallization of the evolved lavas; petrographic evidence of open-system mixing or assimilation is rare. MgO, CaO, Al2O3, Cr, Ni, and Sr abundances decrease and K2O, Na2O, Rb, Ba, Zr, and Pb increase with increasing SiO2 (50–71%). LREE enrichment [(Ce/Yb)n=1.7±0.2] characterizes most TPF lavas; total REE contents increase and Eu anomalies become more negative with increasing SiO2. Relative to other Aleutian volcanic centers, TPF basalts and basaltic andesites have lower K2O, Na2O, TiO2, Rb, Ba, Sr, Zr, Y, and LREE abundances. 87Sr/86Sr ratios (0.70361–0.70375) and ratios of 206Pb/204Pb (18.88–18.97), 207Pb/204Pb (15.58–15.62), 208Pb/204Pb (38.46–38.55) are the highest measured for any suite of lavas in the oceanic portion of the Aleutian arc. Conversely, eNd values (+5.8 to+6.7) are among the lowest from the Aleutians. Sr, Nd, and Pb ratios are virtually constant from basalt through rhyodacite, whereas detectable isotopic heterogenity is observed at most other Aleutian volcanic centers. Major and trace element, REE, and Sr, Nd, and Pb isotopic compositions are consistent with the basaltic andesitic, andesitic, dacitic, and rhyodacitic liquids evolving from TPF basaltic magma via closed-system fractional crystallization alone. Fractionation models suggest that removal of ∼80 wt% cumulate (61% Plag, 17% Cpx, 12% Opx, 7% Ol, and 3% Mt) can produce 20 wt% rhyodacitic residual liquid per unit mass of parental basaltic liquid. Petrologic and physical constraints favor segregation of small batches of basalt from a larger mid-crustal reservoir trapped below a low-density upper crustal lid. In these small magma batches, the degree of cooling, crystallization, and fractionation are functions of the initial mass of basaltic magma segregated, the thermal state of the upper crust, and the magnitude of extension. Tholeiitic magmas erupted at Seguam evolved by substantially different mechanisms than did calc-alkaline lavas erupted at the adjacent volcanic centers of Kanaga and Adak on unextended arc crust. These variable differentiation mechanisms and liquid lines of descent reflect contrasting thermal and mechanical conditions imposed by the different tectonic environments in which these centers grew. At Seguam, intra-arc extension promoted eruption of voluminous basalt and its differentiates, unmodified by interaction with lower crustal or upper mantle wallrocks.

Intra-arc extension and magmatic evolution in the central Aleutian arc, Alaska

New geologic, petrologic, and Sr isotope data from Seguam Island, Alaska, reveal substantial variation in the composition of magmas of the central Aleutian arc. The data indicate that variable magmatic processes occur along the strike of the arc. Seguam is situated on extended arc crust and has erupted voluminous basalt and dacite. In contrast to basalt from adjacent volcanic centers atop unextended crust, which evolved by polybaric open-system crystallization, Seguam lavas crystallized at shallow levels under closed-system conditions. Seguam basalt therefore better reflects the character of its source materials than basalt erupted through unextended arc crust. The authors conclude that along-arc differences in the nature of crystallization were facilitated by changes in the thermal and density structure of the subarc lithosphere caused by intra-arc extension.

Teleseismically recorded seismicity before and after the May 7, 1986, Andreanof Islands, Alaska, earthquake

The May 7, 1986, Andreanof Islands earthquake (Mw 8.0) is the largest event to have occurred in that section of the Aleutian arc since the March 9, 1957, Aleutian Islands earthquake (Mw 8.6). Teleseismically well‐recorded earthquakes in the region of the 1986 earthquake are relocated with a plate model and with careful attention to the focal depths. The data set is nearly complete for mb≥4.7 between longitudes 172°W and 179°W for the period 1964 through April 1987 and provides a detailed description of the space‐time history of moderate‐size earthquakes in the region for that period. Additional insight is provided by source parameters which have been systematically determined for Mw≥5 earthquakes that occurred in the region since 1977 and by a modeling study of the spatial distribution of moment release on the mainshock fault plane. A technically significant component of oblique convergence in the central Aleutian arc results in its breakup into clockwiserotating and westward translating blocks. The western part of the Andreanof block is distinct from and stronger than flanking regions. The greater strength of this block segment and strong coupling along the main thrust zone result in the accumulation of high levels of shear stress, which give rise to great earthquakes near its eastern boundary. The occurrence of the 1986 rupture only 29 years after the 1957 earthquake may indicate that in the central Aleutians M 7+ earthquakes ordinarily do represent the predominant mode of strain release. Segmentation of the main thrust zone into upper and lower planes is supported by spatial and temporal patterns of seismicity and by focal mechanism data. This and other lines of evidence indicate a downdip increase in fault strength and possibly in heterogeneity within the main thrust zone in subduction zone environments. Aftershocks of the 1986 earthquake were bounded to the west by the Andreanof block boundary (Adak Canyon) and to the east by an aseismic segment of the main thrust zone near the subducted extension of the Amlia fracture zone. The aftershock distribution was bounded to the south by a forearc shear zone and to the north by the base of the main thrust zone in the Hawley Ridge segment and by the downdip edge of the upper plane of the main thrust zone in the eastern segment. Aftershocks which occurred near the volcanic line at shallow crustal depths in the upper plate were triggered by the mainshock and manifest a partial decoupling of oblique slip in this region along a west‐striking right‐lateral fault with low shear strength. Aftershock clustering along the main thrust zone was very similar to the distribution of prior seismicity, suggesting a continuation of long‐term processes and the existence of areas with distinct mechanical properties. Interconnecting regions of low seismicity during both the premainshock and aftershock periods coincided with areas of major moment release during the mainshock. Seismicity data prior to the 1986 mainshock gave few clues about the location of the mainshock nucleation point, the mainshock size, and its time of occurrence. However, a large part of the mainshock moment release did coincide with a zone of seismic quiescence monitored by the Adak local seismograph network.