DOCSLIB.ORG

Ocean Drilling Program Initial Reports Volume

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Ocean Drilling Program Initial Reports Volume Tamaki, K., Pisciotto, K., Allan, J., et al., 1990 Proceedings of the Ocean Drilling Program, Initial Reports, Vol. 127 1. BACKGROUND, OBJECTIVES, AND PRINCIPAL RESULTS, ODP LEG 127, JAPAN SEA1 Shipboard Scientific Party2 INTRODUCTION ized core-mantle interactions and heat transfer (Miyashiro, 1986); (2) convective flow induced in the asthenosphere by the sub- Leg 127 of the Ocean Drilling Program accomplished the ducting slab (Sleep and Toksöz, 1971; Andrews and Sleep, 1974; first successful penetration of the volcanic rocks which underlie Toksöz and Bird, 1977; Toksöz and Hsui, 1978); and (3) sea- the deep basins of the Japan Sea. The basaltic rocks recovered ward migration of the trench hinge line (Molnar and Atwater, at three of the four drilling sites provide valuable new con- 1978; Chase, 1978; Wu, 1978; Uyeda and Kanamori, 1979); re- straints on the style and timing of volcanism and, by inference, treat of the back-arc plate from the volcanic arc (Dewey, 1980); on the origin of these basinal areas (Fig. 1). In addition, this and intracontinental deformation such as the case of the Hima- drilling effort marks the first recovery of complete sedimentary layan convergence (Tapponier et al., 1982; Jolivet, 1986; Ki- sequences in this marginal sea. The vertical and lateral progres- mura and Tamaki, 1986; Lallemand and Jolivet, 1986). sions of the sedimentary facies within these sequences provide Within this context, no single process can easily account for the most updated view of how this marginal sea developed, the formation of all these western Pacific marginal basins. In from inception to the present day. Lastly, the Leg 127 drilling re- fact, evidence gained thus far by drilling and geophysical inves- sults unequivocally date the uplift of Okushiri Ridge and pro- tigations demonstrates that the marginal basins formed by a va- vide a minimum estimate of the initiation of compression along riety of processes, including single and multiaxial spreading and a zone of possible incipient subduction in the eastern Japan rifting of volcanic and continental arcs, changes in subduction Sea. This active zone has been the site of large magnitude earth- polarity due to arc-arc and arc-continent collisions, transcur- quakes for the past 20 yr. rent pull-aparts and displacement of continental borderlands, In this chapter, we provide information regarding the back- and entrapment of parts of larger ocean basins (Taylor and ground and objectives of this drilling expedition as well as a Karner, 1983; Jolivet et al., 1989). In each case, the data needed comparative summary of the key findings at all the sites. More for unraveling the basin origin are the same, namely: (1) crustal extensive and specific coverages of each site are in the ensuing structure; (2) magnetic anomaly patterns; (3) ages, composi- chapters. Leg 127 was the first of two scientific drilling ventures tions, and variability of basement rocks; (4) structural style and conducted by the Ocean Drilling Program in the Japan Sea dur- trends; (5) seismicity; and (6) sedimentary history. ing the summer and fall of 1989. Our drilling sites were mostly The Japan Sea is perhaps the most studied of the marginal in basinal areas. In contrast, the succeeding drilling effort by basins in the western Pacific region. Over the years, the area has Leg 128 focused on high-standing blocks and on geophysical ex- been probed based on a variety of geophysical methods and periments. Together, the drilling and geophysical results of these geological samples (see summary in Tamaki, 1988). It is gener- two cruises provide crucial new data needed for an understand- ally agreed that the basin formed some time in the mid-Tertiary ing' of the tectonic, sedimentological, and oceanographic devel- and is floored in part by oceanic-type crust flanked by found- opment of this marginal sea. In a broader context, these find- ered and rifted continental blocks. Beyond this point, interpre- ings aid greatly in our perceptions of the timing and mecha- tations diverge and a host of theories have been proposed con- nisms of formation of marginal basins in this region. cerning the exact timing and mechanism of basin development (see summary in next section). Common to all these scenarios BACKGROUND are two critical gaps in the database, namely: (1) the composi- tion, age, and variability of the basement rocks which underlie General the basinal areas of the Japan Sea; and (2) detailed documenta- The back-arc basins and marginal seas of the western Pacific tion of the sedimentation history. These two needs can only be (Fig. 2) are the keys to understanding the processes of continen- addressed by drilling and they provided the focus for our efforts tal dismemberment, accretion, and growth in a convergent mar- on Leg 127. gin setting. These basins lie between two of the most dynamic In the following sections, we briefly summarize some of the regions of the earth's lithosphere, the subduction zones of the key background information related to the Japan Sea in order to western Pacific and the Himalayan convergence. The interaction provide a perspective for our drilling results. More specific back- between these two regions is primarily responsible for the com- ground data pertaining to each site can be found in the site plex stress fields which shaped these basins (Jolivet et al., 1989). chapters which follow. The proposed mechanisms for back-arc spreading and marginal sea formation include: (1) mantle diapirism spawned by shear Tectonics, Structure, and Age strain heating of the upper surface of the underthrust lithosphere Four major plates and several microplates and tectonic blocks (Karig, 1971), by dewatering of the slab and partial melting of converge in the area of the Japanese islands and eastern Japan the overlying mantle (Karig, 1974), or by deep-seated and local- Sea (Fig. 2). The Japan Sea lies on the eastern margin of the Eurasian Plate and is separated from the Philippine, Pacific, and North American plates by a complex border (Chapman and Solomon, 1976; Nakamura, 1983; Kobayashi, 1983; Savostin et 1 Tamaki, K., Pisciotto, K., Allan, J., et al., 1990. Proc. ODP, Init. Repts., 127: College Station, TX (Ocean Drilling Program). al., 1983; Tamaki and Honza, 1985; Lallemand and Jolivet, 2 Shipboard Scientific Party is as given in the list of participants preceding the 1986; Jolivet et al., 1989). To the southeast, the Nankai Trough contents. is a subduction zone which marks the plate boundary between SHIPBOARD SCIENTIFIC PARTY 45°N 40c 130°E 135C 140c Figure 1. Map of the Japan Sea showing the locations of Leg 127 ODP Sites 794-797 and DSDP Leg 31 Sites 299-302. the Eurasian and Philippine Plates. To the northeast the bound- the underlying acoustic basement is also smooth and flat, par- ary bifurcates with one arm transferring to the Japan Trench, a ticularly in the central areas of the Yamato and Japan Basins. part of the western Pacific subduction zone. The other arm runs This character is thought to represent interbedded sediments inboard or westward of this trench, along the eastern margin of and volcanic flows or sills which overlie a rougher basement sur- the Japan Sea, and coincides with a north-trending zone charac- face (Ludwig et al., 1975; Tokuyama et al., 1987). To date, no terized by active thrust and reverse faults, large (M > 7) com- basement samples have been recovered from these basinal areas, pressional earthquakes, and en-echelon seafloor ridge and basin other than Miocene volcanic rocks dredged from the seamounts morphology typical of transcurrent fault zones (Fukao and Furu- (Tamaki, 1988; Kaneoka et al., in press). As a result, the only moto, 1975; Lallemand and Jolivet, 1986; Tamaki and Honza, information regarding the age and nature of the basement rocks 1985). Although this boundary has a rather distinctive transcur- that underlie these basins comes from these samples, and from rent zone morphology, the fault types and earthquake focal seismic data and magnetic anomaly patterns. mechanisms and focal depths suggest that it may presently rep- The Japan Basin is the deepest and largest of the basinal resent a young zone of incipient eastward subduction between a areas of the Japan Sea (Fig. 3). The seafloor lies at 3.0-3.7 km narrow projection of North American Plate and the Eurasian and the depth to acoustic basement is 4.2-5 km below sea level Plate (Nakamura, 1983; Kobayashi, 1983; Tamaki and Honza, (Hilde and Wageman, 1973; Ludwig et al., 1975; Tamaki, 1988). 1985). If this interpretation is correct, then the eastern margin Based on refraction and gravity data, this basin is underlain by of the Japan Sea is the site of a developing plate boundary. oceanic-type crust about 11-12 km thick and by a thin litho- Patterns of faulting and the deep and shallow structure of sphere which is only about 35 km thick (Fig. 4; Abe and Kana- the Japan Sea are well constrained by seismic reflection and re- mori, 1970; Ludwig et al., 1975). In addition, the detailed ma- fraction data (Ludwig et al., 1975; Gnibidenko, 1979; Tamaki, rine magnetic anomaly pattern in the western part of this basin 1988; Hirata et al., 1987, 1989; Tokuyama et al., 1987; Katao, suggests complex pseudofault patterns similar to those gener- 1988). At least three structural and physiographic provinces ex- ated by propagating spreading ridges (Tamaki and Kobayashi, ist and have significance for crustal structure (Fig. 3): (1) deep 1988). Preliminary correlations with the marine magnetic anom- basinal areas, such as the Japan and Yamato Basins; (2) block- aly time scale suggests a maximum age of about 26 Ma for this faulted ridges, typified by the Yamato Rise and Korea Rise; and part of the basin (Fig.
Load more

Recommended publications
  • Low-Frequency Tremor and Slow Slip Around the Probable Source Region
    Proceeding of the UJNR 5th Earthquake Research Panel Meeting, 2004 1 Low-frequency tremor and slow slip around the probable source region of the Tokai Earthquake { A new indicator for the Tokai Earthquake prediction provided by unified seismic networks in Japan { Noriko Kamaya (Seismological and Volcanological Department, JMA), Akio Katsumata (Meteorological College, JMA) and Yuzo Ishigaki ( Seismological and Volcanological Department, JMA) Abstract Enhancement of seismic networks in the Japan Islands of these years revealed occurrence of deep low-frequency continuous tremors of a belt distribution in the southwest Japan, where the subducting Philippine Sea plate reaches depths of 25{40 km. The source region of the tremor is assumed to correspond to boundary region between oceanic and island crusts close to mantle wedge. The east end of the belt tremor distribution is adjacent to the probable source region of the Tokai Earthquake. A slow-slip event has continued in the west of the source region of the Tokai Earthquake since 2000. Synchronous activation of the tremor and slow-slip speed was observed in 2003. This suggests some relationship between the slow-slip and the tremor activity. We expect that the tremor activity could be a new valuable indicator for the Tokai Earthquake prediction. The long duration of the tremor indicates existence of fluid relating to its generation. The most probable fluid is considered to be water produced by dehydration of chlorite and formation of clinopyroxene in the oceanic crust on the basis of data from high pressure and temperature experiments. The northern border of the belt distribution is possibly rimmed by the edge of the mantle wedge because serpentine formation absorbs fluid water.
    [Show full text]
  • Constraints on the Moho in Japan and Kamchatka
    Tectonophysics 609 (2013) 184–201 Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto Review Article Constraints on the Moho in Japan and Kamchatka Takaya Iwasaki a, Vadim Levin b,⁎, Alex Nikulin b, Takashi Iidaka a a Earthquake Research Institute, University of Tokyo, Japan b Rutgers University, NJ, USA article info abstract Article history: This review collects and systematizes in one place a variety of results which offer constraints on the depth Received 1 July 2012 and the nature of the Moho beneath the Kamchatka peninsula and the islands of Japan. We also include stud- Received in revised form 12 November 2012 ies of the Izu–Bonin volcanic arc. All results have already been published separately in a variety of venues, and Accepted 22 November 2012 the primary goal of the present review is to describe them in the same language and in comparable terms. Available online 3 December 2012 For both regions we include studies using artificial and natural seismic sources, such as refraction and reflec- tion profiling, detection and interpretation of converted-mode body waves (receiver functions), surface wave Keywords: Kamchatka dispersion studies (in Kamchatka) and tomographic imaging (in Japan). The amount of work done in Japan is Japan significantly larger than in Kamchatka, and resulting constraints on the properties of the crust and the upper- Crustal structure most mantle are more detailed. Upper-mantle structure Japan and Kamchatka display a number of similarities in their crustal structure, most notably the average Moho crustal thickness in excess of 30 km (typical of continental regions), and the generally gradational nature of the crust–mantle transition where volcanic arcs are presently active.
    [Show full text]
  • ITM International Tectonics Meeting
    NUMO-TR-16-04 TOPAZ Project Long-term Tectonic Hazard to Geological Repositories Toward practical application of the ITM-TOPAZ methodology February 2017 Nuclear Waste Management Organization of Japan (NUMO) 2017 年 2 月 初版発行 本資料の全部または一部を複写・複製・転載する場合は，下記へ お問い合わせください。 〒108-0014 東京都港区芝 4 丁目 1 番地 23 号 三田 NN ビル 2 階 原子力発電環境整備機構 技術部 電話 03-6371-4004（技術部） FAX 03-6371-4102 Inquiries about copyright and reproduction should be addressed to: Science and Technology Department Nuclear Waste Management Organization of Japan Mita NN Bldg. 1-23, Shiba 4-chome, Minato-ku, Tokyo 108-0014 Japan ©原子力発電環境整備機構 (Nuclear Waste Management Organization of Japan) 2017 NUMO-TR-16-04 TOPAZ Project Long-term Tectonic Hazard to Geological Repositories Toward practical application of the ITM-TOPAZ methodology February 2017 Nuclear Waste Management Organization of Japan (NUMO) 各章の和文要約 1 序論 本報告書は，国際プロジェクトメンバーが作成した原稿を NUMO が編集したもの である。NUMO は，地層処分システムに著しい影響を及ぼす火山活動や断層活動な どの自然現象の発生可能性を確率論的に評価するための手法として，ITM 手法1を開 発した。続いて，プレート運動の安定性を必ずしも前提にできない超長期の評価に 向けて，ITM-TOPAZ 手法2の開発を進めた。東北地方および中国地方を対象とした ケーススタディを通じて，ITM-TOPAZ 手法の日本の地質環境への基本的な適用性を 確認してきた。今回，ITM-TOPAZ 手法の適用性のさらなる向上を目指し，再び東北 地方を対象に，火山活動・断層活動に加えて隆起・侵食の変遷に関するシナリオの 拡充，それらの影響の評価方法の高度化などを行った。 2 広域変遷シナリオ（RES: Regional Evolution Senarios）の拡充 これまでに例示した将来 100 万年までのプレート運動の変遷に関する四つのシナ リオ（RES1: 現在の傾向が継続，RES2: プレート収束速度が増加，RES3: プレート運 動の方向が変化，RES4: プレート収束速度が減少）における，東北地方の背弧域 （日本海側），脊梁山地，前弧域（太平洋側）での広域的な火山活動，断層活動， 隆起・侵食の傾向について，追加情報に基づき記述を補強した。さらに，最近の科 学的知見の進展を踏まえたシナリオの拡充の可能性について述べた。 3 サイト変遷シナリオ（SES: Site Evolution Senarios）の設定 背弧域（日本海側），脊梁山地，前弧域（太平洋側）を代表する数十
    [Show full text]
  • Evolution History of Gassan Volcano, Northeast Japan Arc
    Open Journal of Geology, 2018, 8, 647-661 http://www.scirp.org/journal/ojg ISSN Online: 2161-7589 ISSN Print: 2161-7570 Evolution History of Gassan Volcano, Northeast Japan Arc Ryo Oizumi1, Masao Ban2*, Naoyoshi Iwata2 1Graduate School of Science and Technology, Yamagata University, Yamagata, Japan 2Faculty of Science, Yamagata University, Yamagata, Japan How to cite this paper: Oizumi, R., Ban, Abstract M. and Iwata, N. (2018) Evolution History of Gassan Volcano, Northeast Japan Arc. Evolution history of the volcano is essential not only to characterize the vol- Open Journal of Geology, 8, 647-661. cano, but also consider magma genesis beneath the volcano. Most of the stra- https://doi.org/10.4236/ojg.2018.87038 tovolcanoes in northeast Japan follow a general evolutional course: cone building, horse-shoe shaped caldera forming collapse, and post-caldera stages. Received: June 4, 2018 However, the detailed history of each stage is not well investigated. We inves- Accepted: July 10, 2018 Published: July 13, 2018 tigated evolution history of young edifice of Gassan volcano, representative stratovolcano in rear side of northeast Japan arc. Most of the products are la- Copyright © 2018 by authors and vas, which are divided into two groups by geomorphologic and geologic fea- Scientific Research Publishing Inc. tures. The former (Gassan lower lavas) is composed of relatively thin and This work is licensed under the Creative Commons Attribution-NonCommercial fluidal lavas, whose original geomorphology remains a little, while the latter International License (CC BY-NC 4.0). (Gassan upper lavas) is composed of relatively thick and viscous lavas, whose http://creativecommons.org/licenses/by-nc/4.0/ original geomorphology is moderately preserved.
    [Show full text]
  • Workshop Reports on Bend-Fault Serpentinization (BFS) and Bend-Fault Hydrology in Old Incoming Plate (H-ODIN)
    Workshop reports on Bend-fault serpentinization (BFS) and Bend-Fault Hydrology in Old Incoming Plate (H-ODIN) 19-21 June, 2016, University of London, Royal Halloway Jason P. Morgan, Royal Holloway, University of London, UK Gou Fujie, JAMSTEC, Japan Ingo Grevemeyer, GEOMAR, Germany Tim Henstock, University of Southampton, UK Tomoaki Morishita – Kanazawa University, Japan Damon Teagle, University of Southampton, UK 1. Introduction and workshop goals Crustal hydration at mid-ocean ridges by hydrothermal circulation has been considered to be the first-order control on the degree of the oceanic plate hydration (see ‘Primary Hydration’ region on Fig. 1). Previous ocean drilling projects have aimed to reveal hydration processes and their extent of oceanic crust at spreading centers (e.g., Alt et al., 1996; Bach et al., 2003; Wilson et al., 2006). Recently, hydration due to plate bending-induced normal faults (bend-faults) in the region between the trench axis and outer rise (outer rise hereafter) has also drawn considerable attention (see ‘Rehydration’ region on Fig. 1). During the last decade, multiple independent geophysical structure studies have revealed that plate bending-induced normal faults in outer rise regions around the world are associated with significant hydration along (e.g., Grevemeyer et al., 2007; Ivandic et al., 2008; Key et al., 2012; Fujie et al., 2013). This bend-fault-linked hydration and Bend-Fault Serpentinization (BFS), with its associated physical and chemical changes is one of the most significant geological discoveries of the last 15 years. It has the potential to reshape our understanding of Earth’s deep water and carbon cycles, the ecology and evolution of species in deep-sea chemosynthetic environments, and even the fundamental mechanism by which slabs bend and unbend, thereby driving Plate Tectonics.
    [Show full text]
  • Threedimensional Seismic Velocity Structure and Configuration of The
    JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, B09315, doi:10.1029/2007JB005274, 2008 Three-dimensional seismic velocity structure and configuration of the Philippine Sea slab in southwestern Japan estimated by double-difference tomography Fuyuki Hirose,1 Junichi Nakajima,2 and Akira Hasegawa2 Received 18 July 2007; revised 23 June 2008; accepted 17 July 2008; published 26 September 2008. [1] Three-dimensional seismic velocity structure in and around the Philippine Sea plate subducting beneath southwestern (SW) Japan is determined by applying double- difference tomography method to arrival time data for earthquakes obtained by a dense nationwide seismic network in Japan. A region of low S wave velocity and high Vp/Vs of several kilometers in thickness is recognized immediately above the region of intraslab seismicity in a wide area from Tokai to Kyushu. This characteristic layer dips shallowly in the direction of slab subduction. Compared with the upper surface of the Philippine Sea slab based on seismic reflection and refraction surveys on seven survey lines, we interpret that the low-Vs and high-Vp/Vs layer corresponds to the oceanic crust of the Philippine Sea slab. On the basis of the position of the low-Vs and high-Vp/Vs layer and the precisely relocated hypocenter distribution of intraslab earthquakes, the upper surface of the Philippine Sea slab is reliably determined for the entire area of SW Japan. Nonvolcanic deep low-frequency earthquakes that occurred associated with the subduction of the Philippine Sea slab are distributed along the isodepth contour of 30 km in SW Japan, except for the Tokai district where the depth of deep low-frequency earthquakes becomes gradually deeper toward northeast.
    [Show full text]
  • Spatial Variations of Incoming Sediments at the Northeastern
    https://doi.org/10.1130/G46757.1 Manuscript received 14 July 2019 Revised manuscript received 4 February 2020 Manuscript accepted 10 February 2020 © 2020 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 30 March 2020 Spatial variations of incoming sediments at the northeastern Japan arc and their implications for megathrust earthquakes Gou Fujie1, Shuichi Kodaira1, Yasuyuki Nakamura1, Jason P. Morgan2, Anke Dannowski3, Martin Thorwart4, Ingo Grevemeyer3 and Seiichi Miura1 1 Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokohama 236-0001, Japan 2 Department of Ocean Science and Engineering, Southern University of Science and Technology (SUSTech), Shenzhen 518055, China 3 GEOMAR–Helmholtz-Centre for Ocean Research Kiel, Kiel 24148, Germany 4 Institute of Geosciences, Christian-Albrechts University Kiel, Kiel 24118, Germany ABSTRACT subduction) show that the origin of smectite The nature of incoming sediments is a key controlling factor for the occurrence of megath- at the plate boundary fault is from pelagic red- rust earthquakes in subduction zones. In the 2011 Mw 9 Tohoku earthquake (offshore Japan), brown clay within the incoming sediments (Ka- smectite-rich clay minerals transported by the subducting oceanic plate played a critical role in meda et al., 2015; Moore et al., 2015). Thus, the development of giant interplate coseismic slip near the trench. Recently, we conducted inten- the composition of incoming sediments is also sive controlled-source seismic surveys at the northwestern part of the Pacific plate to investigate a key factor shaping the occurrence of mega- the nature of the incoming oceanic plate.
    [Show full text]
  • Spatiotemporal Variations in the Stress Field In
    Maeda et al. Earth, Planets and Space (2020) 72:117 https://doi.org/10.1186/s40623-020-01245-8 EXPRESS LETTER Open Access Spatiotemporal variations in the stress feld in the northeasternmost part of the NE Japan arc: constraints from microearthquakes Sumire Maeda1* , Toru Matsuzawa2 , Tomomi Okada2, Hiroshi Katao3, Takeyoshi Yoshida2 , Masahiro Kosuga4 and Makoto Otsubo1 Abstract We determined focal mechanism solutions of microearthquakes and examined the stress feld in the low-seismicity region from southern Hokkaido to eastern Aomori, NE Japan. The stress felds determined in this study comprise (1) a reverse faulting stress regime in southern Hokkaido with the axis of maximum compressional stress (σ1) being sub- horizontal and trending WNW–ESE, and (2) a stress regime in eastern Aomori to Tsugaru Strait that shows temporal variations and diferential stress of less than tens of MPa. The spatiotemporal variation in stress from eastern Aomori to Tsugaru Strait might refect the efects of the upper-plate bending and the 2011 Mw 9.0 Tohoku-Oki earthquake. It also indicates that the compressional stress caused by the descending Pacifc plate is relatively weak, which is similar to other areas in eastern parts of the NE Japan arc. Keywords: Focal mechanism, Stress feld, Low-seismicity area, Microseismicity, Forearc, Northeastern Japan, Pacifc plate subduction, Kurile arc Introduction in low-seismicity areas. However, to properly understand Te stress feld in inland areas of NE Japan is generally earthquake generation mechanisms, it is necessary to recognized as being associated with a WNW–ESE com- investigate the stress feld in low-seismicity areas as well pressional reverse-faulting stress regime (e.g., Townend as in areas with high seismicity.
    [Show full text]
  • Melting and Seismic Structure Beneath the Northeast Japan Arc
    GEOPHYSICAL RESEARCH LETTERS, VOL. 27, NO. 3, PAGES 425-428, FEBRUARY 1, 2000 Melting and seismic structure beneath the northeast Japan arc Hikaru Iwamori Geological Institute, University of Tokyo, Tokyo, Japan Dapeng Zhao Department of Earth Sciences, Ehime University, Matsuyama, Japan Abstract. Transportation of H20 associatedwith subduc- of the area [Zhao et al., 1992] (Fig.2). The NE Japan arc is tion of the Pacific plate beneath the northeast Japan arc one of the best test field of the model: (1) a denseseismic is modeled to predict distribution of aqueous solution and network with active seismicity have revealed the geometry melt, and the consequentP-wave velocity structure. The and velocity structures of the subducting Pacific plate, and observed velocity structure coincides well with the model the overlyingmantle wedge-crustsystem (e.g., [Zhao et al., for equilibrium transport of H20: most of H20 subductedis 1992, 1994]), (2) the geometricalrelationship between the brought down to 150-200 km depth along the slab, then the plate motion, trench and arc is simple, i.e., the Pacific plate aqueous fluid generated ascendsto initiate melting, which subducts nearly perpendicular to the Japan trench and the explainsthe low velocity regions(-•6% reduction) observed activevolcanic front (Fig.2), allowingus to apply the across- beneath the backarc, rather than beneath the volcanic front. arc 2-D modeling. Based on the comparison, we propose a If equilibrium transport of H20 occurs, initiation of melting model which accounts for the observations, including the beneath the backarcwith deep subductionof H20 is likely to regions with 6% reduction in P-wave velocity beneath the be the case also for other subduction zones with slabs older backarc, and discussthe fluid processesin subduction zones.
    [Show full text]
  • The Cause of the East–West Contraction of Northeast Japan
    Bulletin of the Geological Survey of Japan, vol. 68 (4), p. 155–161, 2017 Article The cause of the east–west contraction of Northeast Japan Masaki Takahashi1,* Masaki Takahashi (2017) The cause of the east–west contraction of Northeast Japan. Bull. Geol. Surv. Japan, vol. 68 (4), p. 155–161, 5 figs. Abstract: Northeast (NE) Japan, where the Pacific Plate is subducted to the west, frequently suffers large earthquakes not only along the Japan Trench but also along the Japan Sea side. Those occurred in the former area (subduction–zone earthquake) such as the 2011 off the Pacific coast of Tohoku Earthquake can easily be understood as a releasing process of accumulated stress along the boundary between the subducting Pacific Plate and the overlying plate. On the contrary, those in the latter area (inland earthquake), which occur at relatively shallow depth (<20 km), cannot be explained by such a simple dislocation model. Here I show, the cause of such inland earthquakes can be identified by considering the plate kinematics around the Japanese Islands on the basis of three dimensions, not conventional two dimensions, and the cause of the present E–W contractive tectonics of NE Japan is not the Pacific Plate motion itself but the northwestward–moving Philippine Sea Plate. Keywords: tectonics, Japanese Islands, inland earthquake, triple junction, Philippine Sea Plate, Pacific Plate 1. Introduction 40 million years (Harada and Hamano, 2000). Therefore, suppose the E–W contraction of NE Japan is caused by It is well known that the Japanese Islands are currently the Pacific Plate motion itself, the Japanese Islands should situated under E–W compression stress field (Terakawa have been contracted at least for 15 million years after and Matsu’ura, 2010).
    [Show full text]
  • Stress and Displacement Fields in the Northeastern Japan Island Arc As Evaluated with Three-Dimensional Finite Element Method and Their Tectonic Interpretations
    Stress and Displacement Fields in the Northeastern Japan Island Arc as Evaluated with Three-Dimensional Finite Element Method and Their Tectonic Interpretations 著者 Sato Kachishige 雑誌名 The science reports of the Tohoku University. Fifth series, Tohoku geophysical journal 巻 31 号 3-4 ページ 57-100 発行年 1988-06 URL http://hdl.handle.net/10097/45308 Tohoku Geophys. Journ. (Sci. Rep. TOhoku Univ., Ser. 5), Vol. 31, No. 3, 4, pp. 57-99, 1988 Stress and Displacement Fields in the Northeastern Japan Island Arc as Evaluated with Three- Dimensional Finite Element Method and Their Tectonic Interpretations KACHISHIGE SAT 0 International Latitude Observatory of Mizusawa, Mizusawa, Iwate Pref. 023 (Received March 25, 1988) Abstract : Numerical experiments for evaluation of the stress and displacement fields within the northeastern Japan island arc were performed by using three-dimensional finite element method. We study mainly the accumulation process of the stress within the arc by imposing boundary conditions as realistically as possible upon the plate-interaction models. Since this arc has very different structures in its southern and northern parts, calculations were separately made for these two parts. They roughly correspond to the Kanto and Tohoku districts, respectively. The patterns of calculated vertical displacement fields at the earth's surface are generally similar to the observed ones in the models of both parts. As to the stress, in the southern part, it is large not only in the shallower portion of the thrust zone between the subducting and overriding plates, but also in the deeper portion where two subducting slabs collide beneath the land area.
    [Show full text]
  • Frequency-Dependent Shear Wave Splitting Beneath the Japan and Izu-Bonin Subduction Zones
    Physics of the Earth and Planetary Interiors 181 (2010) 141–154 Contents lists available at ScienceDirect Physics of the Earth and Planetary Interiors journal homepage: www.elsevier.com/locate/pepi Frequency-dependent shear wave splitting beneath the Japan and Izu-Bonin subduction zones Erin Wirth ∗, Maureen D. Long Department of Geology and Geophysics, Yale University, New Haven, CT, United States article info abstract Article history: Despite its importance for our understanding of physical processes associated with subduction, the geom- Received 19 November 2009 etry of mantle flow in subduction zones remains poorly understood, particularly in the mantle wedge Received in revised form 18 May 2010 above subducting slabs. Constraints on mantle flow and deformation can be obtained by measurements of Accepted 30 May 2010 shear wave splitting, a valuable tool used to characterize the geometry and strength of seismic anisotropy. Edited by: G. Helffrich A complete characterization of shear wave splitting is particularly important for understanding the man- tle wedge beneath Japan, which overlies multiple subduction zones with complex slab morphologies; previous studies indicate that the upper mantle beneath Japan exhibits complicated anisotropy that Keywords: Shear wave splitting manifests itself in complex splitting patterns. To characterize better the geometry of mantle anisotropy Seismic anisotropy beneath Japan, we analyzed direct S waves from local earthquakes originating in the subducting slabs for Japan evidence of shear wave splitting using data from 54 broadband seismic stations in Japan’s F-net array. In Subduction dynamics addition, both local S and teleseismic SKS phases were examined using data from four F-net stations in the Mantle wedge Izu-Bonin arc.
    [Show full text]