The Izu-Ogasawara Arc is divided into two parts, the northern and southern, at its centralpart. There are notable differences between the two parts in submarine topographic feature, rock chemistry, earthquake hypocenter distribution, etc. The two parts of the arc originallyindependent arcs which have different development histories and joined together at the bendingpart of pre-existing arc. They were rifted by the wedge of oceanic crust which is caused by the Parece Vela Basin opening. The different development histories are reflected in the abovementioned differences between them.
Late Quaternary sedimentation and faulting in the Sumisu Rift, an active rift in the Izu-Ogasawara Arc, were studied by relating existing seismic profiles to recently acquired ODP cores. High resolution, single-channel seismic profiles collected by the Geological Survey of Japan show that the surface sediments (about 0.4 or 0.6s) are composed of an alternation of transparent layers, 0.05 to 0.2 s thick, and packages of continuous reflections. The transparent layers correlate with five thick pumiceous beds recovered by ODP drilling (Leg 126) at two sites in the northern part of the south Sumisu Basin. These transparent layers extend to the southeast onto the northwestern flank of Torishima Island, indicating that the volcanic debris in the transparent layers was supplied from volcanoes in and around this island. Near Torishima Island, the transparent layers show both irregular tops and bases and hummocky internal reflections, suggesting a slump or slide origin; the same transparent layers below the basin floor are characterized by flat boundaries except where faulted or tectonically tilted. On the basin floor, the transparent layers were probably transported by high-concentration sediment gravity flows that would tend to cover and smooth bottom irregularities formed by faulting and tilting. Thus, the offset of the top of the thick transparent layer along the faults represents the amount of displacement on the faults after the deposition of the layers. The two transparent layers that correlate with pumiceous beds II and IV of ODP leg 126 are thick and are believed to have had enough volume to blanket the entire basin. Rough estimates of their volume exceed 60 km3. Based on an age of 31, 000 and 131, 000 B. P. for beds II and V, respectively, the displacement rates on the faults are estimated at about 0.15 to 0.3 cm/y. If the displacement rate has been constant since the beginning of the formation of the Sumisu Rift, about 2, 000 m vertical offset of the rift basement along the major fault zone of the eastern boundary of the rift is estimated to have formed in a period of 0.5 to 1 million years.
Philippine Sea has been thought to be formed by repeated back-arc spreading. The Kyusyu-Palau Ridge is thought to be remnant arc and is divided by the opening of the Shikoku and Parece Vela basins from 30 to 15 Ma. Its eastern half is presumed to be the West Mariana Ridge and the Nishi-Shichito Ridge. The Nishi-Shichito Ridge is topographically divided into two parts (northern and southern parts). The northern part consists of several echelon mountain ridges. The southern part consists of N-S trending seamounts array. The age data so far reported from this Ridge are Pliocene or younger. However, recently, the Miocene data have been obtained from the Tenpo Seamount based on the foraminiferal assemblages of the calcareous sediments covering the top of the seamount. The Tenpo Seamount, being located on the southern part of the Nishi-Shichito Ridge, was formed before 15 Ma, by the time of cessation of the Shikoku Basin opening. This fact suggests that this seamount is the remnant arc and the Kyusyu-Palau Ridge is equivalent counterpart. The magnetic anomaly of the Tenpo Seamount is large positive and it is not recognized on any other seamounts in the Nishi-Shichito Ridge. The Ohmachi Seamount, situated on the Shichito-Iwojima Ridge, has similar magnetic anomaly and the K-Ar age of the volcanic rocks from this seamount is Oligocene. These facts probably indicate that a part of the remnant arc will extend further south to this seamount. The seamounts on the Nishi-Shichito Ridge except the Tenpo Seamount were formed as the present arc after the spreading of the Shikoku Basin ceased. The seamounts of the northern part of the Nishi-Shichito Ridge have obviously dipole anomaly corresponding with each seamount. On the other hand, magnetic anomalies of the seamounts in the south part of it are obscure and faint. There is difference in tectonics and age of formation between the northern and the southern part. Based on the above mentioned observations, the Nishi-Shichito Ridge is thought to be a complex arc.
Sediments and rocks cored at Sites 792 and 793 (Fore Arc basin), Ocean Drilling Program, Izu-Bonin, Leg 126, are described with focus on hydrothermal minerals. About 500 samples were used for X-ray powder diffractions, scanning electron microscopy and transmission electron microscopy studies provide data to describe alteration processes. Mineralogical variations through time indicating abundant smectite, zeolites and gypsum occur in volcanic, pelagic and hemipelagic sediments. Microscopic observation shows the alteration processes of volcanic glass and feldspar to smectite and zeolite which form cementing materials at marginal rims or around primary mineral fragments. TEM study of altered feldspar revealed flaky smectite formation on the surface. Various zeolites were observed in vein and cavities of sandstone and andesite. Wairakite is important, for it indicates that temperatures of alteration may have been quite high in excess of 200°C. The mineralogical variations were strongly related to chemical and physical characteristics in the sediments and fluids. Particularly SO42-, SiO2, Mg2+, Ca2+ and Cl- in the pore water of the sediments were incorporated in secondary minerals and circulating fluids.
A review of the seismicity in Izu-Ogasawara region is presented. In this region, earthquakes occur associated with the interaction of the subducting Pacific Plate and the overriding Philippine Sea Plate. The Wadati-Benioff zone, the deep seismic zone of the descending salb, is steep in this region. The Japan Trench subduction zone, which is the northern extension of Izu-Ogasawara arc has a less steep dip angle. The Mariana slab, the southern extension of the Izu-Ogasawara slab dips almost vertically. Within the Izu-Ogasawara slab, the dip angle becomes steeper towards the Mariana arc. The seismic activity differs at three depth ranges. The top zone, shallower than 100 km depth, is active probably due to plate interaction at subduction. The middle zone between the depths of 100 km and 300 km is low in activity. The bottom zone is an active seismic zone, outstanding in the western Pacific region. This bottom active zone deepens toward the south. At the southern deepest end of the slab, however, the slab is suggested to be bent horizontally. No earthquakes occur deeper than 600 km. Earthquake mechanisms in this region can be characterized by locations in the plate. There are several groups of different mechanisms indicating the regional stress field: normal type caused by plate bending seaward of trench; thrust type were two plates contact; high angle normal fault type within the descending Pacific Plate; down dip compression type near bottom of the top active zone; and tensile type in the back arc region suggesting an east to west extensional field. Finally, a peculiar tsunami earthquake in 1984 that suggests a magma intrusion in the back arc region is described.
The Izu-Ogasawara (Bonin) arc is along the eastern boundary of the Philippine Sea plate, where the Pacific plate is subducted from east. It is one of the typical intra-oceanic island arcs, and consists of the following units from the oceanic side; the “forearc ophiolite”, forearc made of volcanic island arc of old age, active island arc volcanics, rift (“back-arc depression”), and back arc basin with several inactive volcanics. On land in the southern Kanto to the Southern Fossa Magna regions and surroundings, there are many lines of evidence for the ancient relicts of the Paleo-Izu materials and other oceanic materials, which have been accreted by various ways since Oligocene. Before the accretion of island arc materials, ophiolitic rocks, composed chiefly of MORB with alkali basalt and harzburgite, were jammed within the terrigenous accretionary complex in the Shimanto belt. These rocks might be the obducted materials from the Pacific side. After that abundant volcanic island arc suites have been accreted to Honshu, sometimes mixed with Honshu-derived terriginous materials, sometimes island arc materials only. The Fossa Magna might be formed between the stages of ophiolite obduction and island arc com-mencement, around 17 MaBP. Several accretionary prisms composed of the Izu forearc mate-rials are developed in the present Miura-Boso Peninsulas. The volcanic arc materials, consisting remarkably of ash or lapilli fall deposits near the volcanic front, were offscraped and accreted to the Honshu arc side on the Sagami Trough. This reflecs a particular tectonics around the trench-trench-trench triple junction off the Boso Peninsula, which arrived at around the present position at about 17 MaBP.
The Early to Middle Miocene Emi Group is well exposed along the coast line in the Emi area, southern part of the Boso Peninsula. It is composed mainly of tuffaceous siltstone with tuffaceous and volcaniclastic sandstones. The strata are highly faulted, folded and disrupted. The minor structures were analyzed by precise mapping and careful observation of various styles of deformations under semi-lithified conditions. First, dish structures and other dewatering structures including web structures were formed. Next, bedding parallel or slighly oblique thrust faults occurred to make a possible duplex structures. Abundant thrust faults cut the previous structures to bring all the strata horizontally shortened. Assuming the thrusting occurred when the bedding was still nearly horizontal, NWSE horizontal compression was inferred. Strike slip faults of NE trend at last occurred. Such series of deformation indicate that the Izu forearc tuffaceous sediments were accreted along the proto-Sagami trough during Early to Middle Miocene subduction of the Philippine Sea plate to Honshu.
Central Japan has been tectonically situated at a triple juncture among Izu-Bonin, Northeast Japan, and Southwest Japan Arcs. Neotectonics of central Japan is geohistorically reexamined with the special reference to the results of ODP Leg126 transect of Izu-Bonin Arc. Three major points are claimed as follows: (1) The tectonic belt along eastern margin of Japan Sea (EMJS) is characterized by severe compressional deformation including thrusts and folds have developed within the Miocene rifted trough, Uetsu sedimentary basin, by a tectonic reversal which occurred at the end of Miocene around 6 Ma. Since then, the belt have behaved as a newly formed plate boundary between Eurasia and North America Plates. (2) During the period of 2.8-1.4 Ma, a bimodal volcanism occurred at the both flanks of the southern Hida Mountain Range. Area of such explosive acidic volcanism was bounded by a rift-like depression zone called “Omine Rift” was formed along the north-central segment of Itoigawa-Shizuoka Tectonic Line. This means that backarc rifting of Izu-Bonin arc forced the above colliding boundary between Northeast Japan and Southwest Japan to expand. (3) As for plate tectonic frame-work in central Japan, two possibilities are pointed out based on contemporal changes throughout central Japan. a) An eastward motion of Southwest Japan (EUR) has started at about 6 Ma, which immediately caused the jump of plate boundary from central Hakkaido to the EMJS following the tectonic reversal in the inner Northeast Japan arc. b) Philippine Sea plate has changed the direction of its movement from North-northeast to West-southwest around 3.5 Ma, which activated the backarc rifting of Izu-Bonin arc.
The Ryukyu Arc-Okinawa Trough system is one of several arc-backarc systems alongthe margin of the western Pacific and east Asia. Recent results of seismic refractionand reflection studies have revealed that the origin of the trough is the continental rifting, and the trough is in a state of the active rifting stage (e. g. Hirata et al., 1991; Furukawa et al., 1991). Basement rocks and sediments to appraise for the age of the trough, however, have notbeen sampled by deep sea drilling. Then the formation age of the trough was mainlyexamined by the geological studies of the Ryukyu Arc and Okinawa Trough areas andpaleomagnetic stud -ies of the Ryukyu Arc. Geological sequence of the Ryukyu Arc is generally divided into two units, thepre-Miocene basement complex and post-middle Miocene sediments (e. g. Kizaki, 1986). Thenorth and central Ryukyu Arcs represent the geological continuation of the Outer Beltof the south west Japan Arc, composed of the Mesozoic to Eocene sedimentary sequences, whereas thesouth Ryukyu Arc is characterized by high-pressure metamorphic rocks and the Eocenevolcanics and limestone. The geological and structural contrasts between the north-central andsouth Ryukyu Arcs are conspicuous before the late Miocene. These observations suggest that theformation of the Okinawa Trough have been originated since the Miocene, and the Ryukyu Archave been established since then. Paleomagnetic studies for the the Eocene to Pliocene volcanics and sedimentscollected from the Ryukyu Arc and northeast Taiwan suggest that the south Ryukyu Arc rotatedclockwise 19° with respect to the central Ryukyu Arc and Taiwan between 10 Ma and 4 Ma.Paleomagnetic studies attribute this rotation to the opening of the southern part of theOkinawa Trough during 10 and 4 Ma (Miki et al., 1990; Miki, 1991). The acoustic sequence in the trough can be divided into three units. And the eachunit are correlated to the Pliocene to early Pleistocene, middle to late Pleistocene, and late Pleistocene to Recent sediments in descending stratigraphic order. Widespreaddeformation by normal faulting is especially developed in the lowest unit which displays horst andgraben structures. The deformation would have originated at about the Plio-Pleistoceneboundary and formed the prototype of the trough (Furukawa et al., 1991). Synthesizes of the above studies have resulted in interpretativedevelopment of the trough as follows; a major phase of formation of the trough occurred during 10 and4 Ma, and reactivation of the formation of the trough occurred after 2 Ma. Recent studies fortherifting of several continental margins, however, suggest that the intra-arc rifts evolved 10times as first as major intra-continental rifts, and the life time of the rapid rifting is onlyabout 3 to 5 Myr (Yamaji and Takeshita, 1989). The middle Pleistocene reefal limestone dredged fromsome knolls in the Okinawa Trough indicate that the subsidence of the trough was about500-1, 000m after the middle Pleistocene. As a preliminary conclusion, therefore, the age ofthe trough could be estimated to be roughly 2 Ma. The present age estimation is againstrecently proposed clockwise rotation of the south Ryukyu Arc due to the opening of thetrough from 10 Ma to 4 Ma which was reported by the paleomagnetic study.
Heat flow measurements in the Philippine Sea area have been recentlyconducted not only across trench-arc-backarc systems, but also in backarc basins wherehydrothermal activity has been found and around accretionary prisms in order to clarify theirlocalized thermal features. In most trench-arc-backarc systems, heat flow is commonly low fromtrench axis to forearc areas, whereas it is higher on backarc side. The Yap Arc system has asimilar heat flow profile to other arc systems, suggesting that its subducting activity is notyetceased. In the Nankai Trough, heat flow is locally high near the deformation front anddecreases landward. It is attributed to pore fluid flow in sediment layer and from deeper part.Anomalously high and variable heat flow along a fault at the western margin of Sagami Baycanbe explained by outflow of pore fluid driven by shallow heat source below. Heat flow distribution in backarc basins is influenced by past orpresent hydrothermal activities, depending on their age. The relatively older Shikoku and ParaceVela basins have average heat flow values lower than those estimated from their age inthinly sedimented areas, whereas in the eastern half of the Parace Vela Basin they are uniformand concordant with those estimated from its age. The Mariana Trough and Izu-Bonin backarcdepressions are thought to be generally in a backarc spreading stage, and activehydrothermal venting with high and variable heat flow distribution was discovered near the axialpart. In the Okinawa Trough area which is presumed to be in a nascent stage of backarcrifting, hydrothermal ativities with active vents have also been found. A speculated pattern ofhydrothermal circulation is the discharge at topographic highs and the recharge of sea waterintosediment around the topographic low where heat flow values are low. Comparison of heat flow distribution among these backarc basins forvarious spatial scales would be useful for better understanding of various kinds of geophysicalphenomena, such as cooling of lithosphere or hydrothermal activities.
The results of the recent seismic refraction surveys using an Ocean Bottom Seismograph (OBS) array have clarified fine crustal structures of the trench=island-arc= back-arc systems around the Japan Islands, namely the Kurile, the Northeastern Japan, the Southwestern Japan, the Izu-Ogasawara and the Ryukyu. The present paper tries to look over theseismic velocity structures of the crust and of the uppermost mantle beneath the trenches, the island arcs and the back-arc basins revealed by the OBS studies in the last decade. New OBS data have shown regional variations in the crustal structures of the subduction zones. The crustal structures around trenches vary not only from a trench to another but also along one trench. The dip angle of a subducting oceanic lithosphere, the spatial extent of an accretionary prism and the shallow structure of a continental slope characterize each subduction zone and are deeply related to its tectonic evolution. Interesting similarity and dissimilarity are found between the structures of the Bonin Trough, a intra-arc rift, and of the Okinawa Trough, a back-arc rift, providing a clue to understanding of the mechanism of the rifting in the island-arc crust. We also note the crustal structure of the Yamato Basin, a part of the Sea of Japan (an inactive back-arc basin), a curious structure in which the crust has a composition similar to the normal oceanic crust but has a twice thickness.
The sources of clay minerals in surface sediments of the Philippine Sea are not only from the mainland of China, the Philippine islands, and the Japanese islands bu t also from other volcanic arcs and the ridge basalt. Broadly speaking the distribution of clay minerals in surface sediments of the Philippine Sea are influenced by the Kuroshio current and its cou nterclockwise current. Clay mineralogy in drilled core samples from the Philippine Sea revealed the sedimentary history of clay minerals since the early Cenozoic era. The clay mineral composition in the drilled core samples has been changed by the factors influencing such as geodynamics, volcanic activity, climatic change, and oceanic current throughout the Cenozoic era
The deepsea cores recovered from about 50 drilling sites in the Philippine Sea, equally distributed in marginal basins, remnat arcs, present arcs and others, during the DSDP/IPOD/ODP offer significant geotectonic information. Of these cores of the drillsites, sediment accumulation rates, lithologic changes and frequency of tephras were reviewed in the light of the recent advanced nannofossil biostratigraphy of the sediment cores. Sediment accumulation rate curves of these sites were classified into two major types, A and B types, respectively. A type has rapid accumulation rates just above the arc basement and then decreasing pattern. In contrast, B type has rather constant accumulation rate throughout the cores. Rapid accumulation rates imply volcanogenic debris flow and volcaniclastic turbidite sequences derived from arcs which represent activity of magmatic arc consisting of tholeiitic and calc -alkalic volcanic rocks. On the contrary, low sediment accumulation rates imply biogenic materials instead of volcaniclastic rocks. This means the termination of intense are volcanism. Frequency of volcanic ash layers deduced from these cores has maxima just after the rapid sediment accumulation stage of A type curves. As for the remnant arcs such as the Kyushu-Palau and the Daito Ridge, tephra maxima exist at late Eocene to early Oligo cene time and the present arc such as the Izu-Bonin Arc, there are two major maxima at Eorene-Oligocene and Pliocene-Pleistocene time, respectively. Explosive volcanism may take place when oceanic arc develops as shallo w as the pressure compensation level (PCL). If this is the case, we may draw the volcanic history of oceanic island arc. At the incipient stage in Fig. 6, style of volcanism is quiet resultant formation of pillow lavas and hyaloclastite. On the contrary, volcanism takes place very intense with form ation marine tephras at the explosive stage, and at the subareial stage, large amount of tephras are exhausted from the summit of the volcanoes. No volcanisms were happened instead continuous rapid subsidence of the arc at the remnant stage. These stages of the evolution of oceanic island arc are quite similar to those of the volcanic islands such as the Hawaiian volcanoes. This study presents the images of the evolution of oceanic island arcs by the compilation of the sediment accumulation rates, lithologies and frequency of the volcanic ash layers of the cores recovered during the Deep Sea Drilling Programs.
Structural characteristics of Recent forearc regions can be divided into three types (shelf type, terrace type, slope type) in terms of the morphology of forearc basins and the topography of basements. The differences in forearc structure seems to correspond with the difference in deformation style of forearc lithospher by subduction. Supply and/or subduction of sediments at trench are secondary factor of forearc deformation. High plate convergence rate leads to widely uplifting of island arc less subsidence of forearc basin (shelf type). Conversely, low plate convergence rate leads to one-sided subsidence of trench side and increasing in undulation of forearc topography (slope type). Sediments from forearc basin can reconstruct the subsidence rate of forearc basin and the vertical movement of island arc and by use of planktonic microfossils and benthic for eminifera which determine the changes in sedimentation rate and paleobathymetry. Thus, forearc basins are tectonic recorder to estimete the arc evolution and the change in subduction styles.
A brief review of paleomagnetic studies in the Philippine Sea and Izu-Bonin arc including the recent Ocean Drilling Program (ODP) results was made to clarify the present status and problems of the study on the tectonic evolution of the Philippine Sea plate. Paleomagnetic directions observed from the onland and marine areas on the Philippine Sea plate consistently show progressive increase of inclinations with time since the Eocene, and show large (30°-100°) clockwise deflections of declination in Eocene to middle Miocene rocks. These results suggest large clockwise rotation and northward drift of the whole Philippine Sea plate. This paper critically reviews all the models which have been proposed for the tectonic evolution of the Philippine Sea plate and proposes two possible reconstructions which can explain the paleomagnetic results.