Arc-trench System Research Articles

Implications from the seismic crustal structure of the northern Izu–Bonin arc

The results of a controlled source seismic reflection–refraction experiment carried out in 1992 reveal the following characteristics of the northern Izu–Bonin (Ogasawara) oceanic island arc–trench system. (1) The crust rapidly thickens from the Shikoku back‐arc basin to the arc, is thickest beneath the active rifts, and then gradually thins to the forearc. The thickness of the crust beneath the arc rift zone and the back‐arc basin are ∼ 20 km and 8 km, respectively. (2) The Moho vanishes beneath the forearc. Velocities rapidly decrease eastwards beneath the inner trench wall. (3) The velocity of the lower crust of the arc and the back‐arc basin is 7.1–7.3 km/s. This velocity is higher than the typical oceanic lower crust whose velocity is ∼ 6.7 km/s. (4) The velocity of the middle crust of the arc is ∼ 6 km/s. This layer does not exist beneath the back‐arc basin. (5) A slight difference in the velocity gradient of the middle crust exists between the arc rift zone and the forearc. Based on these findings and previous studies, it is inferred that: (i) the middle crust is probably granitic rock and formed in more than two episodes; (ii) the lower crust formed by igneous underplating which may also have affected part of the back‐arc basin; and (iii) the root of the serpentinite diapir on the inner trench wall is a low‐velocity mantle wedge that was probably caused by large amounts of water released from the subducting Pacific plate at depths shallower than 30 km.

Embaluh Group turbidites in Kalimantan: evolution of a remnant oceanic basin in Borneo during the Late Cretaceous to Palaeogene

Turbidites crop out in large parts of NW Borneo, including Kalimantan and Sarawak. The strata predominantly comprise the Embaluh Group in Kalimantan and lateral equivalents the Rajang Group in Sarawak (collectively known as the Rajang–Embaluh Group). Previous interpretation of these rocks as the product of deep marine deposition and deformation as part of an accretionary prism during Late Cretaceous to Palaeogene subduction implies south-dipping thrusts, northward stratigraphic younging, the existence of a complete arc to trench system, and significant deformation and metamorphism of the turbidites. However new fieldwork described here established consistent southward stratigraphic younging in Kalimantan, no evidence for southward-dipping thrusts and a lack of metamorphism and associated accretionary complex-related deformation features. Furthermore all available palaeontological and radiometric age data indicate that deposition of the bulk of the Rajang–Embaluh Group postdates in-board subduction-related magmatism and thus a complete arc-trench system cannot be demonstrated. Consequently the Rajang–Embaluh Group turbidites are re-interpreted as having formed in either a post-collisional foreland basin or a remnant ocean basin. The lack of a identifiable mountain belt and linked thrust system, and probable oceanic aYnity of crust beneath the Rajang–Embaluh Group basin favour the latter interpretation. Borneo occupies a pivotal position within southeast Asia. This new model of a remnant oceanic basin setting for extensive turbidite units of Borneo therefore has far-reaching implications for the tectonic evolution of Borneo, the South China Sea and SE Asia. The new model is compared with older, Palaeozoic orogenic belts which also contain thick turbidites sequences.

Evolutionary model for convergent margins facing large ocean basins: Mesozoic Baja California, Mexico

Mesozoic rocks of the Baja California Peninsula form a convergent-margin complex that is one of the best-preserved and longest-lived convergent-margin complexes in the world. It shows a three-phase evolutionary trend that we propose is typical of arc systems facing large ocean basins. The trend progresses from phase 1, highly extensional intraoceanic-arc systems, to phase 2, a mildly extensional fringing-arc system, to phase 3, a compressional continental-arc system. This trend is largely due to the progressively decreasing age of lithosphere that is sub- ducted. The modern Earth is strongly biased toward long-lived arc-trench systems, which are compressional, and so evolutionary models for convergent margins must be constructed from well-preserved ancient examples like Baja California.

Relationships among the tectonic components of the island arcs and their plate tectonic implications

Relationships among the tectonic components of the simple (non-circum Philippine plate) east-facing island arcs of the western Pacific are as follows: positive relationship between strike (strike of the subduction zone) and latitude (Euler and geographic), radius of curvature and latitude, slab dip and latitude, radius of curvature and slab dip, strike and radius of curvature, dip and strike of the slab, and negative relationship between the dip of the slab and convergence rate. In the east-facing island arcs, the slab dip is inversely related to the convergence rate, whereas in subduction zones which are from various orientations, a similar relationship has not been recognized. Plots of the latitudinal variation of tectonic components of the island arcs as proposed in this study can be used to forecast the geographic position of the possible arc-trench systems in the tectonically unknown regions of the western Pacific. These plots can also be used to forecast the geometrical and kinematic characteristics of the newly explored subduction zones in this region. In regions where the slope of Benioff-Wadati zone is known, plot of slab dip vs convergence rate proposed here, can be used to estimate the rate of convergence. Variations in radius of curvature with convergence rate indicate that convergence rate is another controlling factor which determines the radius of curvature, and provides a means for estimating the former convergence rate of extinct arcs.

Backarc spreading, rifting, and microplate rotation, between transform faults in the Manus Basin

The Manus Basin in the eastern Bismarck Sea is a fastopening backarc basin behind the New Britain arc-trench system. Within the basin, motion between the Pacific and Bismarck plates about a pole located at 11° S, 145° E, occurs along three major leftlateral transform faults and a variety of extensional segments. We interpret SeaMARC II sidescan and other geophysical data to show that a Brunhes age plate reorganization created new extensional boundaries and a microplate between the NW-trending Willaumez, Djaul, and Weitin transforms. Two linked spreading segments formed in backarc basin crust between the Willaumez and Djaul transforms: the ESE-trending extensional transform zone (ETZ) in the west and the Manus spreading center (MSC) in the east. Positively magnetized crust on the MSC forms a wedge varying in width from 72 km at its southwest end to zero at its northeast tip, with corresponding Brunhes spreading rates varying from 92 mm/yr to zero. The MSC forms the northwestern boundary of the 100 km-scale Manus microplate and opens at 51°/m.y. about a pole near its apex at 3°02′S, 150°32′E. Opposite the MSC, bordering the arc margin of New Britain, the microplate is bound by a zone of broadly distributed strike slip motion, extension, and volcanism. Within this area, the Southern Rifts contain a series of grabens partially floored by lava flows. Left-lateral motion between the Pacific and Bismarck plates appears to drive the counterclockwise pivoting motion of the Manus microplate and the complementary wedge-like opening of the MSC and the Southern Rifts. The pivoting motion of the microplate has resulted in compressional areas along its NE and SW boundaries with the Pacific and Bismarck plates respectively. East of the microplate, between the Djaul and Weitin transforms and within the arc margin of New Ireland, another zone of broad extension referred to as the Southeast Rifts takes up opening in a pull-apart basin. There, en echelon volcanic ridges may be the precursors of spreading segments, but erupted lavas include calcalkaline volcanics. Kinematic modeling and marine geophysical observations indicate that the responses to similar amounts of extension in the eastern Manus Basin have varied as a function of the different types of pre-existing crust: arc crust tectonically stretched over a broad area whereas backarc crust underwent relatively little stretching before accommodating extension by seafloor spreading.

Crustal Structure of the Northeastern Taiwan Area From Seismic Refraction Data and Its Tectonic Implications

The southwestern part of the Ryukyu arc-trench system to the west of126°E is obviously bent northwards horizontally.The mechanism of thishorizontal bending has been discussed in the literature,but it is still ambiguous.The West Philippine Basin(WPB)is generally believed to be produced by spreading along the Central Basin Ridge(CBR).However,theCBR does not extend to the northwestern corner of the WPB.Previousgeomagnetic and geomorphological studies suggested that the CBR terminates or is displaced at its northwestern end.However,the detailed pictureis still unclear.In this paper,we have examined bathymetry,gravity andseismicity in and around the northwestern corner of the WPB,and discovered a NE-trending right-handed strike-slip fault system which can be usedto interpret the horizontal bending of the southwestern part of the Ryukyuarc-trench system.A plausible model showing how the right-handed faultsystem results in the bending of the Ryukyu arc-trench system is demonstrated.As the southeastern part of the right-handed strike-slip fault system,the Luzon-Okinawa Fault Zone,which is an obvious fault zone between the Luzon and Okinawa islands,is described in terms of bathymetriccontours and profiles along some cruise lines.The Luzon-Okinawa FaultZone is the main cause of the termination of the CBR.

Tectonic evolution of two paleo arc-trench systems in Hokkaido, northern Japan

Los paleo-sistemas de arco-trinchera del Japón y Kuriles que se desarrollaron del Cretácico Tardío al Paleoceno Temprano han sido estudiados usando trabajos paleomagnéticos y de petrología sedimentaria en la zona axial y en la parte occidental de Hokkaido en el noreste de Japón. Las rocas sedimentarias en la parte sur del Cinturón Hidaka definen tres petroprovincias (zonas I a III). Las características petrológicas de los sedimentos de la zona I son similares a aquellas de los depósitos de la cuenca antearco en la región del paleoarco de las Kuriles. La componente modal de las areniscas de la zona III es similar a la de los depósitos de antearco de la región de paleoarco del Japón. La zona II tiene propiedades intermedias entre la zona I y III. Los sedimentos del cinturón Hidaka fueron derivados entonces. de dos sistemas de arco diferentes durante el Paleoceno. Los datos paleomagnéticos sugieren que una rotación de más de 70° en el sentido de las manecillas del reloj pudo haber ocurrido en Hokkaido oriental desde el Eoceno Tardio. En este trabajo se discuten posibles reconstrucciones de placas en relación con el esparcimiento en la Cuenca de las Kuriles.doi: sin doi

Structure and morphotectonics of the accretionary prism along the Eastern Sunda-Western Banda Arc

Seismic reflection profiles across the accretionary prism along the Eastern Sunda-Western Banda Arc reveal variation in structure that relates to the incipient collision with Australia. The morphology of the arc-trench system changes from ridged, south of Bali, Lombok and Sumbawa, to sloped south of Sumba. East of Sumba, the accretionary wedge is backthrust over the forearc basin, incorporating forearc sediments and basement. Frontal accretion in the Java Trench is characterized by imbricate thrusting of a “thin” pelagic sediment cover at the toe of the accretionary wedge. The morphology near the toe of the wedge appears to be controlled by faults in the subducting oceanic plate that strike parallel to the deformation front and may have originated in the outer-trench swell. The prism incorporates oceanic basement by trenchward verging thrusts, which cut across the accretionary prism and extend into the subducting slab. A comparison between the volume of the accretionary prism and the amount of sediment delivered to the Java Trench in the past 30 Ma shows that probably little of the sediment has been subducted. The decrease in width of the prism from Bali to Sumbawa corresponds to an eastward younging trend of the arc-trench system from late Oligocene to early Miocene. South of Sumba the width of the accretionary prism increases considerably, due to the accretion of thick continental margin carbonates which deform by thrust-bounded folds. Buoyancy of the partially subducted marginal Scott plateau increases basal shear stresses, adding to the growth of a large accretionary wedge. Further east, the subduction of thick continental crust results in even higher basal shear stresses that are distributed throughout the accretionary wedge. They cause the progressive development of backthrusts and internal deformation, leading to shortening and thickening of the wedge.

The East Siberia Transect

The new East Siberia transect, constructed by synthesizing recently analyzed geological and geophysical data, runs in a broken line through the vicinities of the towns of Nizhneangarsk, Chita, and Borzya, traversing the Siberian Platform margin and the Baikal and Mongolia-Okhotsk fold areas. The transected region encompasses a number of terranes, most of which once were arc-trench systems with fore-arc and back-arc basins. The island arcs involved Early Precambrian blocks that were sialic cores of some of their islands. The terranes have complex spatial relations in the present tectonic framework, with most of the rocks belonging to magmatic arcs, whereas the associated basins comprise allochthons thrust over continental margins. Accretion of the terranes to the Siberian continent occurred in two stages, the Early Proterozoic and the Early Paleozoic. Middle Paleozoic and Late Paleozoic–Early Mesozoic subduction on the periphery of the Mongolia-Okhotsk ocean, which actually was an enormous guK of the Pacif...

Basic Maps of the Sea in Continental Shelf Areas (the East of Nansei-syoto) at a scale of 11/1, 000, 000: Characteristics of Arc Trench systems and Back arc Basins in the Southern waters of Japan

Basic Maps of the Sea in Continental Shelf Areas (the East of Nansei-syoto) at a scale of 11/1, 000, 000: Characteristics of Arc Trench systems and Back arc Basins in the Southern waters of Japan

Progressive arcward contraction of a Mesozoic-Tertiary fore-arc basin, southwestern Sacramento Valley, California

The modern Sacramento Valley intermontane basin is the successor to a deformed Mesozoic-Tertiary fore-arc basin. Based on analysis of seismic reflection profiles, drill hole data, and surface mapping, we propose a model for progressive arcward contraction of the fore-arc basin in the Rumsey Hills region, southwestern Sacramento Valley, beginning in Cretaceous and continuing episodically through Cenozoic time. We present our interpretation in the form of a kinematically restorable forward model. The major tectonic and depositional events include (1) mid-Cretaceous or older shortening of fore-arc basin strata along imbricate, west-dipping thrust faults; (2) erosion of the deformed fore-arc strata; (3) onlap of Cenomanian and younger fore-arc strata onto deformed older strata and crystalline Sierran basement; (4) renewed shortening in latest Cretaceous to early Tertiary, accommodated by delamination of Upper Cretaceous strata from the mid-Cretaceous angular unconformity, tectonic wedging, and west-directed backthrusting; (5) erosion of the deformed Upper Cretaceous strata and deposition of the Eocene Capay Formation; (6) uplift and east-directed shortening from Eocene to late Neogene, with local erosion of the Capay Formation over the ancestral Rumsey Hills; (7) uplift of the northern Coast Ranges beginning ca. 3.4 Ma, and deposition of the Pliocene-Pleistocene Tehama Formation in the western Sacramento Valley as a syntectonic molasse; and (8) fault-propagation folding of the Tehama Formation beginning ca. 0.5–1.0 Ma. With the exception of Quaternary shortening, progressive arcward contraction of the western basin margin occurred during east-dipping subduction and plate convergence beneath western California. The deformation of the ancestral Sacramento Valley fore-arc basin is analogous to arcward contraction of the Tobago Trough in the Lesser Antilles arc-trench system, and suggests that tectonic wedging may be a relatively common process in the evolution of fore-arc regions. Late Quaternary shortening has occurred in a transpressional setting and suggests a continuity in structural style despite a change in plate boundary kinematics from convergence to transpression during the past 3–5 m.y.

High rates of arc consumption by subduction processes: Some consequences

New estimates of long-term consumption of Japanese and Peruvian continental fore-arc margins together with recent drilling data (Ocean Drilling Program) on some other erosional island arcs such as Izu-Bonin, Mariana, or Tonga allow the calculation of a mean rate of 7 ± 3 km/m.y. landward migration of the arc-trench system on both east and west sides of the Pacific basin during Cenozoic time. Such relative arcward retreat counterbalances seaward migration of the slab hinge line to accommodate opening of the Atlantic Ocean. Retreat can thus be interpreted as a delay in seaward or retrograde migration due to upper mantle resistance to contraction of the Pacific basin. Mass transfers in subduction zones must be reconsidered in the light of these new estimates, especially the balance between net growth and loss of continental crust in recent geologic time. A 50 m.y. period is long enough to erode the initial width of the volcanic-arc and fore-arc massif of any subduction zone, assuming a mean trench–volcanic-arc landward migration of 5 km/m.y. and a mean trench–volcanic-arc distance of 250 km. This circumstance thus raises the question of whether the Izu-Bonin-Mariana and Tonga arcs are older than their reported Eocene age.

Formation of the Abitibi greenstone belt by arc-trench migration

Volcanic and plutonic rocks of Archean greenstone belts in the Abitibi subprovince become younger southward; plutonic rocks are ∼10–30 m.y. younger than volcanic rocks. The Abitibi greenstone belt has an upper crust of ∼2.75–2.70 Ga oceanic supracrustal assemblages and a middle crust of ∼2.70–2.68 Ga granitic and gneissic rocks. The greenstone belt upper crust is greenschist facies with upright structures and low reflectivity, and the middle crust is amphibolite facies with moderate to shallowly dipping structures and strong reflectivity. We propose that the age progression and crustal layering resulted from accretion of oceanic assemblages followed by extensive mid-crustal magmatism linked to southward migration of an arc-trench system. Thermal softening of the middle crust during magmatism caused ductile flow now represented by strong, mid-crustal reflectivity. Upward heat flow following magmatism caused late-tectonic (2.67–2.65 Ga) low-pressure metamorphism observed in the upper crust.

Slope-shelf-nearshore depositional sequences with transgressive conglomerate in a Miocene backarc and arc-junction setting, central Japan

Several third-order slope-shelf-nearshore depositional sequences are found in the Middle to Late Miocene Aoki and Ogawa Formations in the southern part of the Northern Fossa Magna region, central Japan. Each depositional sequence consists of a transgressive systems tract, varying in thickness from several tens of centimetres to about 10 m, a falling stage systems tract several tens of metres in thickness and a highstand systems tract, 20–80 m thick. The highstand systems tract and falling stage systems tract are bounded by the regressive surface of marine erosion. Each depositional sequence mainly consists of coarse-grained sediments. The materials of the transgressive conglomerate beds were derived from the gravelly foreshore and upper shoreface by transgressive shoreface erosion. The thin and conglomeratic transgressive systems tract indicates that these slope-shelf-nearshore sequences were deposited on the steep slope area under a rapid transgression. Therefore, these sequences were formed in response to excess sediment supply and a rapid rise in relative sea-level at an active margin of the backarc side of the arc-trench system.

Consolidation of the American Cordilleras

The continental margin orogenic systems of the western Americas are enormous features that formed along the Pacific margins of the North and South American plates during late Mesozoic through Cenozoic time. There has been considerable debate concerning their origin, and they are often compared with intra-oceanic fringing arc-trench systems more typical of the Australasian margins of the Pacific Ocean, in that both involve the subduction of oceanic lithosphere, often with similar convergent relative motion vectors. The onset of orogenesis in the two Cordilleras, as shown in reversal of sedimentary polarity from sources generally on the continent to sources along the Pacific margin, seems to date from shortly after emplacement of the oldest oceanic crust in that part of the Atlantic Ocaen east of each continent — i.e., about 170 Ma, or Middle Jurassic, in the case of the Central Atlantic, and about 135 to 100 Ma, or Early to mid-Cretaceous, in the case of the South Atlantic. These ages also seem to mark the onset of westward motion of the two continents over the Pacific Ocean basin and subsequent crustal thickening and uplift, with development of thrust belts, foreland basins, and foredeeps. Prior to this prolonged westward drift, both margins had been convergent for at least several hundred million years, but no massive mountain building had taken place. Instead, the margins were tectonically “neutral”, with typically submarine fringing arc-trench systems or shallow marine to continental margin arcs which stood “outboard” of shallow marine platformal shelves or basins whose main sedimentary polarity was from the continent. Although accretion of “suspect” terranes, high rates of convergence, and age of subducting lithosphere all may have influenced particularly local tectonic response and/or phases of orogenic activity in the two chains, the absolute motion of the two continental margins over the Pacific Ocean basin is considered to have been the dominant factor in Cordilleran tectonic evolution.

Tectono-sedimentary development of backarc continental basin in Yunnan, southern China

Sedimentological analyses of the strata occurring in the Cretaceous-Paleogene continental basin of western and central Yunnan enabled the writers to reconstruct the sedimentary history of the basin within the framework of the regional tectonics of the Yunnan-Burma region. The sedimentary record in the basin is characterized by diachroneity within the various sedimentary facies. This diachroneity resulted from increasing uplift in the west that produced a steadily increasing drainage gradient toward the east. The record of uplift is especially reflected in the changes within the sandstones. Granitic batholiths extend north-south through the full length of the uplifting hinterland along the Yunnan-Burma border region. Sedimentation in the continental basin exhibits a close relationship with a multiphase granitic emplacement history revealed through radiometric dating. A close relationship is also evident with an ophiolite-bearing flysch basin exposed today in the Indo-Burma Ranges of western Burma. The writers interpret the Cretaceous-Paleogene sequences as follows. The Yunnan-Burma region was the site of a westward-facing continental arc-trench system with a forearc basin and a backarc basin developed above continental crust. Tectonic and sedimentologic data suggest that the development of the backarc continental basin was controlled by asymmetric arching of the continental arc, in which structural vergence was toward the west and granitic magmatism took place along its axis.

Sedimentary petrology and paleotectonic analysis of the arc—arc junction: the Paleocene Nakanogawa Group in the Hidaka Belt, central Hokkaido, Japan

Two arc-trench systems have been recognized by using paleomagnetic data in the Hokkaido Central Belt, northeast Japan, during Late Cretaceous to Early Paleogene: the Paleo-Japan and the Paleo-Kuril arc-trench systems. The Hidaka Belt is composed mainly of Paleocene turbidite facies, with a small amount of hemipelagic sediment and melange facies. These sediments accumulated near the trench area, later composed accretionary bodies in the two arc-trench systems. The Nakanogawa Group is typically exposed on the southern side of the Hidaka Belt. This group is divided into three petroprovinces; zones I–III, from south to north on the basis of sedimentary, petrological and paleocurrent analyses. The modal component of sandstones from zone I indicate that they are characteristically rich in volcanic rock fragments (intermediate to basic composition), clinopyroxene, hornblende and poor in quartz. The characteristics of zone I are similar to those of forearc to slope basin sediments in the Paleo-Kuril arc-trench system. The modal component of sandstones from zone III consists predominantly of monocrystalline quartz, K-feldspar, acidic volcanic fragments, radiolarian chert and tectonite fragments, similar to those of forearc basin sediments of the Paleo-Japan arc-trench system. The modal component of rocks from zone II are regarded as being intermediate between zones I and III. However, clinopyroxene and chromian spinel chemical data suggest that the sandstones of zones I and II were derived from the Paleo-Kuril arc region. The sediments of the Hidaka Belt were thus derived from two different arc-trench systems and deposited in the arc—arc junction area during the Paleocene.

A kinematic model for evolution of island arc-trench systems

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

Geochemistry and heat transfer processes in Quaternary rhyolitic systems of The Taupo Volcanic Zone, New Zealand

The Taupo Volcanic Zone (TVZ) is a Quaternary volcanic arc associated with subduction at the southern end of the Taupo-Kermadec arc-trench system. At least 10,000 km 3 (dense equivalent) of volcanic rocks have been extruded during the last 2 Ma along the 200 km long segment of the exposed arc. These consist dominantly of rhyolite lavas and pyroclastics (80% by volume) with subordinate andesite, dacite and basalt. The TVZ's crustal heat transfer of about 5650 MW is anomalously high and is a result of production of large volumes of dominantly rhyolitic magma by crustal anatexis. About 20% of this magma reaches the surface. The erupted rhyolites have high-Si metaluminous compositions showing only small variations between and within eruption centres. Magma batches of essentially homogeneous composition have been identified which feed single eruption sequences as well as multiple sequences with timespans of up to 20 Ka. The chemical composition of the TVZ rhyolites indicates that they are partial melts derived from an apparently homogeneous lower crust. The TVZ has been classified in the past as a back-arc basin heated by ponded mafic melts associated with a concealed, spreading asthenosphere plume (exogenous crustal heating). However, geochemical and geophysical arguments indicate that there is little evidence for any asthenosphere plume beneath the TVZ. An alternative heat source is proposed in which crustal heating is produced by plastic deformation of the ductile crust concentrated within a narrow tectonic hinge line (endogenous crustal heating). Such deformation promotes crustal fusion and leads to fractionation of the lower crust into an upper “felsic layer” and a lower mafic to ultramafic “restite layer”.

中部北海道，滝川地域の中新世―鮮新世玄武岩のＫ‐Ａｒ年代と主成分化学組成 東北日本弧―千島弧，島弧会合部の玄武岩単成火山群

Cenozoic volcanism in the Takikawa district of the central Hokkaido, located at the junction of the Kuril and the northeastern Japan arcs, is unique in arc-trench system. It consists of Takikawa monogenetic volcano group (TMV) and four polygenetic volcanoes (Ofuyu-dake, Etai-dake, Shokanbetsu and Irumukeppu volcanoes). TMV are widely distributed in the district and mainly made up of alkali olivine basalt and olivine tholeiitic basalt. Whereas, voluminous polygenetic volcanoes associated with the TMV are mainly composed of calc-alkaline andesite and dacite. New data of K-Ar ages for the rocks show that the volcanic activity of TMV continued from late Miocene to early Pleistocene, with coeval polygenetic volcanism during Pliocene. The presence of TMV and the geological evidence for the development of sedimentary basin suggest that the junction area of the two arcs has been under extensional stress field during late Miocene and Pliocene. The SiO2 contents of nearly primitive basalts (FeO/MgO<1.3) in the Takikawa district exhibit an ellipse shaped zonation in space. More silica-saturated basalts are distributed in the center of the district. This feature is not concordant with across-arc compositional zonation in arc volcanics. Major element chemistry and their spatial zonation of these rocks from Takikawa district suggest that the magmatism should not be explained by the steady state subduction-related one but by mantle upwelling under extensional stress field in the arc-arc junction.