Kuril-Kamchatka Island Arc Research Articles

The Role of the Back-arc Basin in the Formation of Slab Heterogeneity and the Origin of Volcanism in the Kuril–Kamchatka Island Arc

The Role of the Back-arc Basin in the Formation of Slab Heterogeneity and the Origin of Volcanism in the Kuril–Kamchatka Island Arc

Coseismic and Tectonic Time-Scale Deformations of an Island Arc Based on Studies of East Coast of Kamchatka Peninsula (Far East, Russia)

Geologic structure of the late Holocene accumulative marine terrace on the coast of Kamchatka Bay (Kamchatka Peninsula) has been studied. The ages and relative hypsometric position of beach ridges composing the terrace allowed us to identify two types of vertical movements of the coast: periodic fast (coseismic) and slow time-scale uplift or subsidence. Large-amplitude vertical coseismic movements (up to 1‒2 meters) occur on average once in about 1200‒1300 years, and slow movements occur at an average rate from a fraction of a millimeter to about 2 mm per year. Coseismic motions represent relaxation of elastic deformations accumulated during the interseismic interval of the seismic cycle, neither exceed them nor accumulate. Slow motions set the general trend of vertical deformations of the coast. It is assumed that the subsiding central parts of the eastern bays of Kamchatka Peninsula (Avachinsky, Kronotsky and Kamchatsky) and depressions between the eastern peninsulas (Kronotsky, Shipunsky) and the main massif of Kamchatka form an arc-parallel extension zone, which is nearest to the deep-water trench, and that extension is caused by the migration of the subducted part of the Pacific plate toward the Pacific Ocean. Under the eastern Shipunsky and Kronotsky peninsulas, the arc-normal extension of the earth’s crust of the Kamchatka segment of the Kuril-Kamchatka island arc is replaced by a zone of transverse compression.

О ПОПЕРЕЧНОЙ ПЕТРОГЕОХИМИЧЕСКОЙ ЗОНАЛЬНОСТИ МЕЗОЗОЙСКИХ ВУЛКАНИТОВ В РУДНЫХ ПОЛЯХ ЗАБАЙКАЛЬСКОГО СЕГМЕНТА МОНГОЛО-ОХОТСКОГО СКЛАДЧАТОГО ПОЯСА

Актуальность работы обусловлена необходимостью изучения закономерностей формирования оруденения в Забайкальском сегменте Монголо-Охотского складчатого пояса. Цель работы состоит в установлении условий образования, выявлении наличия поперечной геохимической зональности в мезозойской вулканической дуге Забайкальского сегмента Монголо-Охотского складчатого пояса. Методы. Определение элементного состава пород производилось рентгенфлуоресцентным методом (ГИН СОРАН, г, Улан-Удэ). Концентрации редкоземельных элементов определялись методом сорбционно-атомноэмиссионного анализа с индуктивно-связанной плазмой (ГИН СОРАН, г, Улан-Удэ). Силикатный анализ проводился стандартным методом «мокрой химии» (ГИН СОРАН, г, Улан-Удэ). Результаты. Установлена поперечная геохимическая зональность мезозойских вулканитов Забайкальского сегмента Монголо-Охотского складчатого пояса, выраженная в увеличении концентраций от фронтальной части вулканической дуги Монголо-Охотского складчатого пояса к тыловым ее частям концентраций K, Rb, Sr, Zr, Sb, Ba, Th, U, Ce отношений K/Na, Th/U, Rb/Sr, ∑TR. Отмечена пространственная приуроченность золоторудных месторождений к фронтальной части мезозойской вулканической дуги, полиметаллических и оловополиметаллических месторождений – к тыловым ее частям. Вулканиты полиметаллических месторождений характеризуются более значительной степенью окисленности железа и меньшей степенью дифференциации магматических очагов. Окисленность железа (мера окислительных условий), соотнесенная к кремнекислотности пород в вулканитах золоторудных месторождений, варьирует от 0,19 до 0,90 (среднее значение – 0,53); в вулканитах полиметаллических месторождений – от 0,32 до 0,93 (среднее – 0,58). Такая закономерность типична для вулканитов островных дуг зон перехода континент–океан. Выявлено, что по мере удаления от фронтальной части вулканической дуги увеличиваются глубины генерации магм вулканитов Забайкальского сегмента Монголо-Охотского складчатого пояса. Такая закономерность характерна для Курило-Камчатской островной дуги. Это объясняется увеличением глубин зоны генерации магм по мере удаления от фронтальной части зоны субдукции. Глубины генерации магм вулканитов золоторудных месторождений, по данным Rb-Sr соотношений, составляют 15–20 км и более, полиметаллических и оловополиметаллических месторождений – 20–30 км и более.

Long-Term Earthquake Prediction (LTEP) for the Kuril–Kamchatka island arc, June 2019 to May 2024; Properties of Preceding Seismicity from January 2017 to May 2019. The Development and Practical Application of the LTEP Method

This paper is concerned with results from ongoing work using the method of long-term earthquake prediction (LTEP) for the Kuril–Kamchatka island arc based on the seismic gap and seismic cycle patterns. We highlight the most important lines of research in the LTEP method during the preceding decade. A long-term forecast is provided using the basic method for the next 5 years, June 2019 through May 2024, for the most active part of the earthquake-generating zone in the region. The 20 segments of that zone have forecasts for the next 5 years, including the phases of the seismic cycle, the normalized rate of low-magnitude earthquakes (А10), the magnitudes of moderate-size earthquakes that are expected to occur with probabilities of 0.8, 0.5, and 0.15, the maximum expected magnitudes and probabilities of great (М ≥ 7.7) earthquakes. This study continues the well-known work of S.A. Fedotov by examining space–time features of the regional seismic process for the period beginning 2017, including the great Near Islands (Aleutian) earthquake of July 17, 2017, M = 7.7. The results, as obtained here, corroborate a close relationship between the seismic process occurring in the segments that pose the highest earthquake hazard according to the LTEP and the great earthquakes in the region itself and in the adjacent seismic areas, as well as emphasizing a very high earthquake hazard for several areas at the Kuril–Kamchatka island arc; Thus it is necessary to continue and expand the ongoing work in earthquake strengthening and enhancing the level of seismic safety in those areas under the highest hazard, including the administrative center of Kamchatka Krai, the town of Petropavlovsk-Kamchatsky.

Structure, substance and near-surface magmatic chambers of Mutnovsky and Gorely volcanoes (Mutnovsky geothermal region, Kamchatka). I. Geological position of the volcanoes

Geological position of two close active volcanoes in Southern Kamchatka – Mutnovsky and Gorely perspective for geotechnological development as sources of energy and raw materials is presented. Multicone ridged volcanoes edifices of a general linear range are localized respectively in front and near rear zones of the Kuril-Kamchatka island arc system in the zone of crust decompression with different depth of their basement. The characteristics of the volcanoes positions are considered as the reasons for the differences of compositions and evolution features of magma feeding them.

Deep-focus earthquakes: spatial patterns, possible causes and geodynamic consequences

The spatial analysis was conducted to analyze the positions of earthquakes hypocenters in the transit zone of the upper mantle and the focal mechanisms of the strongest earthquakes in the subduction slabs of theOkhotskSeasegment of the Kuril-Kamchatka island arc and theJapanSeasegment of the Japanese island arc. It revealed a significant difference in the morphology of these slabs, as well as in the positions of the earthquake hypocenters relative to the active and stagnating parts of the slabs and the forces that caused the earthquakes. Based on the seismic data presented in the article, it is confirmed that there are two types of subduction of the oceanic lithospheric plates in the mantle. The article discusses relationships between the subduction and various geological processes at the upper–lower mantle boundary. It considers possible causes (including those related to phase transitions) of deep-focus earthquakes, in case of which splitting of the oceanic lithospheric plates takes place at depths near the upper–lower mantle boundary. Subduction of the oceanic lithospheric plates and their splitting predetermine a possibility for the crustal elements to penetrate into the lower mantle and deeper into the D″ layer, wherein new plumes arise and transport the deep magma together with the recycled substance into the crust. Deep-focus earthquakes are a necessary link in the mechanism providing for the recycling of chemical elements in the crust – mantle – D″ layer system and thus leading to the formation of a wide range of mineral deposits.

Holocene Tsunamis in Avachinsky Bay, Kamchatka, Russia

This article presents results of the study of tsunami deposits on the Avachinsky Bay coast, Kurile-Kamchatka island arc, NW Pacific. We used tephrochronology to assign ages to the tsunami deposits, to correlate them between excavations, and to restore paleo-shoreline positions. In addition to using established regional marker tephra, we establish a detailed tephrochronology for more local tephra from Avachinsky volcano. For the first time in this area, proximal to Kamchatka’s primary population, we reconstruct the vertical runup and horizontal inundation for 33 tsunamis recorded over the past ~ 4200 years, 5 of which are historical events − 1737, 1792, 1841, 1923 (Feb) and 1952. The runup heights for all 33 tsunamis range from 1.9 to 5.7 m, and inundation distances from 40 to 460 m. The average recurrence for historical events is ~ 56 years and for the entire study period ~ 133 years. The obtained data makes it possible to calculate frequencies of tsunamis by size, using reconstructed runup and inundation, which is crucial for tsunami hazard assessment and long-term tsunami forecasting. Considering all available data on the distribution of historical and paleo-tsunami heights along eastern Kamchatka, we conclude that the southern part of the Kamchatka subduction zone generates stronger tsunamis than its northern part. The observed differences could be associated with variations in the relative velocity and/or coupling between the downgoing Pacific Plate and Kamchatka.

ОПЫТ ГЕОРАДИОЛОКАЦИОННОГО ЗОНДИРОВАНИЯ ПОЧВЕННО-ПИРОКЛАСТИЧЕСКОГО ЧЕХЛА ЮГО-ВОСТОЧНОЙ ЧАСТИ о. МАТУА (КУРИЛЬСКИЕ ОСТРОВА)

Matua is a volcanic island located in the central part of the Kurile-Kamchatka island arc. Its greater north-western part is occupied by the edifice of Sarychev Peak Volcano, which is one of the most active volcanoes in the Kuriles. The south-eastern part of the island is an ancient abrasion-accumulative terrace, which base is formed by weakly dislocated pliocene-eopleistocene volcanic-aqueous complexes overlapped with marine deposits and solid soil-pyroclastic cover. Besides, Matua Island is an area with a unique anthropogene landscape shaped by the Japan army as a result of the construction of elaborated military fortifications in the first half of the 20th century. The paper provides the results of GPR investigation of the soil-pyroclastic cover on Matua Island. The survey included detection of hidden posterns, large-scale cavities and a GPR inspecting of fortifications areas. The results of GPR sounding are compared to geological profile data, which provide records for the explosive activity on the island over the period late Pleistocene to Holocene.

5. Geochemical Types,Petrology and the Problems of Genesis of Late Cenozoic Volcanic Rocks from Kurile-Kamchatka Island Arc System

5. Geochemical Types,Petrology and the Problems of Genesis of Late Cenozoic Volcanic Rocks from Kurile-Kamchatka Island Arc System

The long-term earthquake prediction for the Kuril–Kamchatka island arc for the April 2016 through March 2021 period, its modification and application; the Kuril–Kamchatka seismicity before and after the May 24, 2013, M 8.3 deep-focus earthquake in the Sea of Okhotsk

This paper discusses results from ongoing research on long-term earthquake prediction for the Kuril–Kamchatka island arc based on the concepts of seismic gaps and the seismic cycle. We developed a forecast for the next 5 years (April 2016 through March 2021) for all segments of the earthquake-generating zone along the Kuril–Kamchatka arc. The 20 segments of the arc were analyzed to develop forecasts of the appropriate phases of the seismic cycle, a normalized parameter of the rate of small earthquakes (A10), the magnitudes of moderate earthquakes that are expected with probabilities of 0.8, 0.5, and 0.15, the maximum expected magnitudes, and the probabilities of great (M ≥ 7.7) earthquakes. We discuss the seismic process in the Kuril–Kamchatka earthquake-generating zone before and after the deep-focus May 24, 2013 M 8.3 earthquake in the Sea of Okhotsk. The results corroborate the high seismic hazard in the area of Petropavlovsk-Kamchatskii and the urgent need for continuing with and expanding the ongoing work of seismic retrofitting and seismic safety enhancement. We quote practical results from applications of the method during 30 years.

Tectonic stress accumulation in the region of the Kuril–Kamchatka Island Arc system

The time variations in the tidal response of the medium in 2011–2015 according to the measurements at the global seismographic network (GSN) in Kamchatka are considered. Based on the data from the horizontal pendulums recording the eastward tilts at the station, it is established that there was a linear growth in the tidal tilt amplitudes up to May 24, 2013 due to the changes in the elastic moduli caused by tectonic stress accumulation. The growth phase was followed by the decline in the tidal tilt amplitudes induced by the release of stresses after the Sea-of-Okhotsk earthquake.

Melt inclusions in quartz phenocrysts: The acid rocks of the Bannaya–Karymshina area, Kamchatka

The Bannaya–Karymshina area is situated in southern Kamchatka west of the East Kamchatka Volcanic Belt in the backarc part of the Kuril–Kamchatka island arc. The area is unique in that it contains abundant ejecta of calc-alkaline, acid, mostly ignimbrite, volcanism for a period of 4 Ma. Three rock complexes can be identified with rhyolitic and rhyodacitic compositions: Middle Pliocene ignimbrites, crystalloclastic tuffs of Eopleistocene age that fill in the Karymshina caldera, and Early Pleistocene intrusions. All of these are composed of rocks with normal total alkalinity, while the concentration of potassium places them at the boundary between moderate and high-potassium rocks. We sought to determine the composition of primary acid melts by studying the composition of the silicate phase in homogeneous melt inclusions that were conserved in quartz phenocrysts hosted by volcanic rocks of varying ages. Practically all the melt inclusions we analyzed show increased total alkalies and are in the class of trachyrhyodacites and trachyrhyolites, with the varieties of the highest alkali content being alkaline rhyolites and comendites; the concentration of K2O classifies them as subalkaline rocks; one also notes the increased alumina of the acid melts. The compositions and spatial locations of the melt inclusions in quartz phenocrysts provide evidence of a three-phase crystallization in magma chambers at different depths. According to the experimental data, the quartz phenocrysts crystallized in a water-saturated melt at pressures of 0.1 to 3.5 kbars.

Volcanic-glacial interactions: GIS applications to the assessment of lahar hazards (case study of Kamchatka)

On the Kamchatka peninsula, lahars or volcanogenic mudflows arise as a result of intensive snow melting caused by incandescent material ejected by volcanoes onto the surface. Such flows carrying volcanic ash and cinders together with lava fragments and blocks move with a speed up to 70 km/h that can result in significant destructions and even human victims. Formation of such water flows is possible during the whole year. Large-scale GIS «Hazards of lahars (volcanogenic mudflows)» has been developed for some volcano group as well as for individual volcanoes on the peninsula in framework of the GIS «Volcanic hazard of the Kuril-Kamchatka island arc». Main components of this database are the following: physic-geographical information on region of active volcanism and adjacent areas, on human settlements; data on the mudflow activity; data on distribution of the snow and ice reserves. This database is aimed at mapping of surrounding territories and estimating a hazard of lahars. For illustration the paper presents a map of the lahar hazards, results of calculations of the distances of ejects and maximal area of ejected material spreading in dependence on a character and power of an eruption. In future we plan to perform operational calculations of maximal possible volumes of such flows and areas of their spreading. The calculations will be made on the basis of the GIS «Volcanic hazard of the Kuril-Kamchatka island arc». A volume of hard material carried by lahars onto slopes and down to foot of the Kluchevskaya volcanic massif is estimated on the basis of data on the snow and ice reserves on volcano slopes. On the average for many years, the snow accumulation in zones of the mudflow formations their volume often reaches 15–17 millions of cubic meters. Depending on the snowfall activity in different years this value may vary within 50% relative to the norm. Further on, calculations of maximal possible volume of such flows will be performed in a real time regime with regard for a character of power of a volcano eruption and reserves of snow and ice in the mudflow-forming basins. The main objective of this work is an effort to reduce a risk of a mudflow formation and to prevent possible damages.

Andaman-Sumatra island arc: III. Time evolution of seismogenic activation of the arc since the beginning of the 21st century

The spatiotemporal evolution of the intense burst in seismogenesis within the Andaman-Sumatra island arc in 2000–2010 is analyzed. The onset of seismogenic activation was marked by two strong (MS ∼ 7.9, MS ∼ 7.8) quakes that occurred in the lithosphere of the South Sumatra region on June 4, 2000 and by the strongest (MW = 7.3) earthquake of July 25, 2004 that took place in the lower part of the focal zone. These seismic events were the foreshocks of the main episode of seismogenic activation of the island arc—the catastrophic earthquake of December 26, 2004, with its source near the northern coast of Sumatra. The large shocks (MS ∼ 7.7–8.4) that occurred from March 28, 2005 to October 25, 2010 between the source of the Sumatran earthquake and epicentral zone of the foreshocks of June 4, 2000, are the aftershocks of the Sumatran event. The spatiotemporal evolution of seismogenic activation of the Andaman-Sumatra island arc at the beginning of the 21st century is compared to the seismogenic activation of the Kuril-Kamchatka arc in the middle of the 20th century. The positions, geological conditions, and focal parameters of the strongest Sumatran earthquakes of 2000–2010 are determined. The interpretation of the sources relies on the (1) complex analysis of all the manifestations of the earthquakes of 2000–2010 and (2) the established regularities of the occurrence of the earthquakes in the island arcs. The sources of the earthquakes of 2005–2010 are the steep longitudinal (trending along the arc) reverse faults of a backthrust type, which have a bedding depth of about a few dozen kilometers. The reverse-fault sources of the earthquakes of March 28, 2005, September 12, 2007 (11:10 UT), and October 10, 2010 are located in the zone of the Outer Range, and the earthquakes of September 12, 2007 (23:49 UT) fall in the Mentawai Trough. The strike-slip reverse fault, which cuts the island arc, is likely to be the focal mechanism of the earthquake of June 4, 2000. The sources of the events of March 28, 2005 and September 12, 2007 did not coincide with the sources of the historical devastating earthquakes of 1861 and 1833, respectively. The system of seismic sources in the form of a series of subparallel arc-parallel steep reverse faults off the western coast of Sumatra is similar to the system of seismic sources in the Kuril-Kamchatka island arc.

The long-term earthquake forecast for the Kuril-Kamchatka island arc for the September 2013 to August 2018 period; the seismicity of the arc during preceding deep-focus earthquakes in the sea of Okhotsk (in 2008, 2012, and 2013 at M = 7.7, 7.7, and 8.3)

Work was performed in 2008–2014 on the long-term prediction of great earthquakes at the Kuril-Kamchatka arc based on patterns that are observable in the locations of the rupture zones of great earthquakes (seismic gaps) and in the seismic cycle for such earthquakes. The prediction method has been successfully developed and applied to the region since 1965 (the LTEP method). This paper presents a long-term seismic forecast for the September 2013 to August 2018 period for the Kuril-Kamchatka arc earthquake-generating zone. The forecast predicts the following quantities for 20 constituent segments: phases of the seismic cycle, the normalized rate of small earthquakes A10, the magnitudes of moderate-size earthquakes to be expected with probabilities 0.8, 0.5, and 0.15, the maximum possible magnitudes and probabilities of occurrence for great (M ≥ 7.7) earthquakes in the most active depth interval, viz., 0–80 km. We also discuss the seismic process in this region for the 2008–2013 period, namely, the patterns that are observable in the locations of earthquake swarms, the values of and changes in several characteristics of the seismic process in the Kuril-Kamchatka seismic zone during the period 2008–2013, which occurred in relation with great shallow earthquakes beneath the Sea of Okhotsk (July 5, 2008, MW = 7.7; August 14, 2012, MW = 7.7; May 24, 2013, MW = 8.3, and the giant Tohoku earthquake of March 11, 2011 with MW = 9.0). The results corroborate the high seismic hazard for the area of Petropavlovsk-Kamchatskii and the urgent necessity of continuing and intensifying the ongoing work on seismic retrofitting and seismic safety.

Geochemical evolution of volcanism of Matua Island in the Central Kurils

The first comprehensive isotope-geochemical study of volcanogenic rocks of different ages from Matua Island made it possible to distinguish the general stages in the magmatic evolution of the subduction system. The petrological similarity of the rocks from the central and northern chains of the Kuril island arc testifies that they were formed above “hot” geochemically enriched but isotopically depleted lithospheric mantle. The compositional change of the volcanic rocks of Matua Island in the Pleistocene-Holocene suggests a geodynamic transition at that time. Taking into account the similar tendency previously established in the southern Kuril Range (Kunashir Island) and for the Mutnovsky and Gorelyi volcanoes of southern Kamchatka, a global Pleistocene-Holocene tectonic event may be proposed in the evolution of the Kuril-Kamchatka island arc. The finding of the “adakite-like” tephra on Matua Island indicates the presence of felsic adakite-like melts among the eruption products of Sarychev Peak Volcano.

Effects of hydrothermal alterations on physical and mechanical properties of rocks in the Kuril–Kamchatka island arc

The hydrothermal systems of the Kuril–Kamchatka island arc are hosted in volcanic formations of Neogene–Quaternary age. Thermal water alters volcanic rocks and transforms them into hydrothermal rocks such as secondary quartzites, various propylites, zeolitic rocks, argillic rocks, clays, opalites, and quartz–feldspar metasomatites. Mineralogical alteration of rocks contributes to changes of their physical and mechanical properties. The relationship between type of hydrothermal alteration and rock properties is quite variable and depends on a number of factors including parent rocks, РТ-conditions, chemical composition and type of fluid, and duration of fluid–rock interaction. The study showed that high-temperature fluids cause consolidation and strengthening of rock, decrease in their porosity and permeability, and remove hygroscopic moisture. This tendency is observed independently of fluid chemical composition. Property variations caused by low-temperature subsurface fluids are more complicated and diverse but their “deterioration” prevails i.e. a decrease in density, strength and elastic modulus, formation of secondary porosity, high hygroscopy, softening and swelling in water saturated environment.Special attention is given to the near-surface alteration zones which affects the selection of sites for power plant construction and can impact the integrity of their foundation. The subsurface horizon of hydrothermal clays is the most problematic zone due to high porosity, plasticity, hygroscopy, compressibility and occasionally swelling.

The upper mantle of Kamchatka in isotopic-geochemical and geophysical anomalies: The role of asthenospheric diapirism

Data of isotopic-geochemical, seismotomographic, and gravimetric studies of the mantle characteristics of the Kamchatka and Bering Sea regions are compared. It is shown that the revealed isotopicgeochemical anomalies are well verified by the geophysical materials. A hypothesis is put forward that the anomalies of the Central Kamchatka and Bering Sea regions are a consequence of the penetration of local diapirs into the lithosphere; their activity is associated with the appearance of rocks belonging to the intraplate geochemical type. The juncture region of the Kuril-Kamchatka and Komandor-Aleutian island arc systems is characterized by the higher participation of crustal material in the composition of the volcanic rocks of the mantle genesis and by a combination of manifestations of the island arc and intraplate types.

The evolutionary stages and petrology of the kekuknai volcanic massif reflecting the magmatism in the backarc zone of the kuril-kamchatka island arc system. Part II. petrologic and mineralogical features, petrogenesis model

The Kekuknai massif was formed in the course of tectono-magmatic activity that involved the origin of a shield volcano and a caldera depression with associated emplacement of extrusions that terminated in intense post-caldera areal volcanism. The mineralogical compositions of the massif’s rocks have been considered in detail. The use of previously known and newly developed indicator properties of rock-forming minerals allowed the reconstruction of the general picture of the magmatic melt evolution and conditions of rock crystallization (various fluid and water saturation levels, as well as the oxidation state of the system). Essentially island-arc or intraplate characteristics of the massif’s rock compositions are found at different stages of development of a single fluid-magmatic system. Decompression evolution of the parent deep-seated basanitic magma occurred via occurrence in intermediate magma chambers of daughter magmas of trachybasalt (pre-caldera stage) or hawaiite (areal volcanism) composition. Subsequent emanate-magmatic differentiation of these melts, combined with crystallization differentiation under changing P-T-conditions, resulted in the formation of the entire diversity of the Kekuknai rocks.

Allivalites as indicators of fractional crystallization of the island-arc calc-alkaline low-K series

Allivalites are particularly distinguished by their composition and morphology among the wide spectrum of inclusions in the volcanic rocks of the Kurile-Kamchatka island arc. This work reports new chemical and mineral data of a comparative study of the allivalites and host rocks using modern analytical facilities. On the one hand, it was confirmed that allivalites are genetically related with host volcanics. On the other hand, it was established for the first time that allivalites were formed in several stages: from the deepest stage of their origination at the generation depth of primary melts to the crustal and subsurface conditions of the formation of magmatic chamber. Hence, allivalites (olivine-anorthite inclusions) bear valuable information on the evolution of deep island-arc magmatism.