The bulk composition of the continental crust throughout geological history is thought by most previous workers to be andesitic. This assumption of an andesitic bulk composition led to an early hypothesis by Taylor (1967 ) that the continental crust was created by arc magmatism. This hypothesis for the origin of continental crust was challenged by several authors because: (i) the mean rate of arc crust addition obtained by Reymer and Schubert (1984 ) is too small to account for some certain phases of rapid crustal growth; and (ii) the bulk composition of ocean island arcs, the main contributor to the Archean and early Proterozoic crust, is basaltic rather than andesitic ( Arculus 1981 ; Pearce et al. 1992 ). New data from the Northern Izu–Bonin arc are presented here which support the Taylor (1967 ) hypothesis for the origin of the continental crust by andesitic arc magma. A geological interpretation of P wave crustal structure obtained from the Northern Izu–Bonin arc by Suyehiro et al. (1996 ) indicates that the arc crust has four distinctive lithologic layers: from top to bottom: (i) a 0.5–2‐km‐thick layer of basic to intermediate volcaniclastic, lava and hemipelagite (layer A); (ii) a 2–5‐km‐thick basic to intermediate volcaniclastics, lavas and intrusive layer (layer B); (iii) a 2–7‐km‐thick layer of felsic (tonalitic) rocks (layer C); and (iv) a 4–7‐km‐thick layer of mafic igneous rocks (layer D). The chemical composition of the upper and middle part of the northern Izu–Bonin arc is estimated to be similar to the average continental crust by Taylor and McLennan (1985 ). The rate of igneous addition of the Northern Izu–Bonin arc since its initial 45‐Ma magmatism was calculated as 80 km 3 /km per million years. This rate of addition is considered to be a reasonable estimate for all arcs in the western Pacific. Using this rate, the global rate of crustal growth is estimated to be 2.96 km 3 /year which exceeds the average rate of crustal growth since the formation of the Earth (1.76 km 3 /year). Based on this estimate of continental growth and the previously documented sediment subduction and tectonic erosion rate (1.8 km 3 /year, von Heune & Scholl 1991 ), several examples of growth curves of the continental crust are presented here. These growth curves suggest that at least 50% of the present volume of the continental crust can be explained by arc magmatism. This conclusion indicates that arc magmatism is the most important contributor to the formation of continental crust, especially at the upper crustal level.
Revised interpretation of the basement geology of the Japanese island arcs which has emerged in the last 20 years or so, indicates that they are mostly composed of two geological belts: volcanics (greenstone)-granitoid belt (VGB) and turbidite-granitoid belt (TGB). The VGB exhibits thrust-bounded thick sequences of basic volcanics and associated granitoid plutons and was formed by arc-arc collision process. The TGB is composed of turbidite and melange units and was formed by progressive growth of trench accretionary prism and later intrusion of granitoids. Both belts represent a style of crustal growth in a convergent margin. In these orogenic belts, involvement ot many oceanic island arcs and formation of the VGB were a major mechanism of juvenile crust, in addition to the continental crust. The formation of the TGB played a major role in the reworking and recycling of the continental crust which resulted in a long-term secular compositional change of the upper crust. Une nouvelle interpréntation de la géologie du socle des arcs insulaires du Japon, dont l'émergence date d'environ 20 ans, indique que ceux-ci sont, pour la plupart, composés de deux ceintures, l'une dite VGB formée de granites et greenstones (matériaux volcuniques), l'autre dite TGB constitutée de granites ct turbidites. La ceinture VGB muntre d'épaisses séries charriées de matériaux volcaniques basiques et de plutons grunitiques associés; elle s'est formée par un prncessus de collinsion arc-arc. La ceinture TGB est composée d'unités de turbidites et de mélange; elle s'est formée par croissance progressive d'un prisme d'accréium affecté ultérieurement par des intrusions granitiques. Les deux centures représentent style de croissance crustale dans une marge convergente. Dans ces cointures orogénitiques, l'implication de nombreux arcs insulaires océaniques, ainsi que la formation de la ceinture (TGB) ont joué un rôle majeur dans le rernaniement et le recytlage de la croûte continentale.
DONET (Dense Oceanfloor Network system for Earthquakes and Tsunamis) has been developed and installed around Nankai Trough, which is motivated by the 2004 Sumatra-Andaman Earthquake. DONET contains pressure gauges as well as seismometers, which is expected to detect crustal deformations driven by peeling off subduction plate coupling process. From our simulation results, leveling changes are different sense among at the DONET points even in the same science node. On the other hand, oceanic fluctuations such as melting ice masses through the global warming has so large scale as to cause ocean bottom pressure change coherently for all of DONET points especially in the same node. This difference suggests the possibility of extracting crustal deformations component from ocean bottom pressure data by differential of stacking data. However, this operation cannot be applied to local-scale fluctuations related to ocean mesoscale eddies and current fluctuations, which affect ocean bottom pressure through water density changes in the water column (from the sea surface to the bottom). Recently, Kuroshio current path has been changed drastically, which significantly affect ocean bottom pressures at DONET station points. Therefore, we need integral analysis by combining seismology, ocean physics and tsunami engineering so as to decompose into crustal deformation, oceanic fluctuations and instrumental drift, which will bring about high precision data enough to find geophysical phenomena. Since DONET has been and will be connected to long-term borehole observatories constructed in the Nankai Trough under the Integrated Ocean Drilling Program (IODP) by using the deep-sea drilling vessel Chikyu, we have to discuss the best way to do simultaneous observation from seafloor to atmosphere by taking advantage of this chance.
The Taiwan Chelungpu‐Fault Drilling Project was undertaken in 2002 to investigate the faulting mechanism of the 1999 M w 7.6 Taiwan Chi‐Chi earthquake. Hole B penetrated the Chelungpu fault, and core samples were recovered from between 948.42‐ and 1352.60‐m depth. Three major zones, designated FZB1136 (fault zone at 1136‐m depth in hole B), FZB1194, and FZB1243, were recognized in the core samples as active fault zones within the Chelungpu fault. Nondestructive continuous physical property measurements, conducted on all core samples, revealed that the three major fault zones were characterized by low gamma ray attenuation (GRA) densities and high magnetic susceptibilities. Extensive fracturing and cracks within the fault zones and/or loss of atoms with high atomic number, but not a measurement artifact, might have caused the low GRA densities, whereas the high magnetic susceptibility values might have resulted from the formation of magnetic minerals from paramagnetic minerals by frictional heating. Minor fault zones were characterized by low GRA densities and no change in magnetic susceptibility, and the latter may indicate that these minor zones experienced relatively low frictional heating. Magnetic susceptibility in a fault zone may be key to the determination that frictional heating occurred during an earthquake on the fault.
Abstract The National Gas Hydrate Program Expedition 02 was conducted in early 2015 using the Drilling Vessel Chikyu in the western part of the Bay of Bengal, India. During drilling off Vishakhapatnam, NE India, some bottom-simulating reflectors were penetrated, and numerous mass-transport deposits (MTDs) were identified. The recovered cores were composed of post-late Miocene muddy slope deposits containing the late Miocene–Pliocene hiatus that is widespread in that region. Based on detailed visual core descriptions and calcareous nannofossil biostratigraphy, two major MTD-rich intervals were identified: the Pleistocene interval above the hiatus, and the middle–late Miocene interval below it. Although the MTDs in both intervals are composed of variously coloured clay–silt blocks in an olive-black or olive-grey silty clay matrix (muddy MTDs), the Pleistocene MTDs consist of larger-sized blocks (mostly less than a few metres but with some >10 m) without clear shear fabrics, whereas the Miocene MTDs contain smaller blocks (<0.1 m) with asymmetrical shear fabrics. The muddy blocks are composed of older components (Pliocene–Cretaceous) compared with the depositional ages of the MTDs. The high abundance of MTDs above the hiatus and the depositional ages of the interbedded coherent layers indicate that large-scale MTDs occurred repeatedly during the Pleistocene. Such repeated MTDs contributed to maintaining the high sedimentation rate in this area and potentially provided stable pressure and temperature conditions for the formation of gas hydrates.
Abstract Detailed reconstruction of Indian summer monsoons is necessary to better understand the late Quaternary climate history of the Bay of Bengal and Indian peninsula. We established a chronostratigraphy for a sediment core from Hole 19B in the western Bay of Bengal, extending to approximately 80 kyr BP and examined major and trace element compositions and clay mineral components of the sediments. Higher δ 18 O values, lower TiO 2 contents, and weaker weathering in the sediment source area during marine isotope stages (MIS) 2 and 4 compared to MIS 1, 3, and 5 are explained by increased Indian summer monsoonal precipitation and river discharge around the western Bay of Bengal. Clay mineral and chemical components indicate a felsic sediment source, suggesting the Precambrian gneissic complex of the eastern Indian peninsula as the dominant sediment source at this site since 80 kyr. Trace element ratios (Cr/Th, Th/Sc, Th/Co, La/Cr, and Eu/Eu*) indicate increased sediment contributions from mafic rocks during MIS 2 and 4. We interpret these results as reflecting the changing influences of the eastern and western branches of the Indian summer monsoon and a greater decrease in rainfall in the eastern and northeastern parts of the Indian peninsula than in the western part during MIS 2 and 4.
Abstract Seismic reflections across the accretionary prism of the North Sulawesi provide excellent images of the various structural domains landward of the frontal thrust. The structural domain in the accretionary prism area of the North Sulawesi Trench can be divided into four zones: (i) trench area; (ii) Zone A; (iii) Zone B; and (iv) Zone C. Zone A is an active imbrication zone where a decollement is well imaged. Zone B is dominated by out‐of‐sequence thrusts and small slope basins. Zone C is structurally high in the forearc basin, overlain by a thick sedimentary sequence. The subducted and accreted sedimentary packages are separated by the decollement. Topography of the oceanic basement is rough, both in the basin and beneath the wedge. The accretionary prism along the North Sulawesi Trench grew because of the collision between eastern Sulawesi and the Bangai–Sula microcontinent along the Sorong Fault in the middle Miocene. This collision produced a large rotation of the north arm of Sulawesi Island. Rotation and northward movement of the north arm of Sulawesi may have resulted in southward subduction and development of the accretionary wedge along North Sulawesi. Lateral variations are wider in the western areas relative to the eastern areas. This is due to greater convergence rates in the western area: 5 km/My for the west and 1.5 km/My for the east. An accretionary prism model indicates that the initiation of growth of the accretionary prism in the North Sulawesi Trench occurred approximately 5 Ma. A comparison between the North Sulawesi accretionary prism and the Nankai accretionary prism of Japan reveals similar internal structures, suggesting similar mechanical processes and structural evolution.
The iron content and the asymmetry of iron and magnesium ions in chlorites are examined for the Chelungpu Fault in Taiwan, which is a seismogenic fault.The samples are collected from the cores drilled for the Taiwan Chelungpu Fault Drilling Project (TCDP, borehole B).Three fault zones are recognized as candidates for the source of seismogenic materials.The fault zones are composed of fractured-damaged rocks, breccia, gray gouge, black gouge, and black material.Chlorite from each type of rock was analyzed by using X-ray diffraction (XRD).The iron content and asymmetry of the iron and magnesium ions in the Correspondence to: Y. Hashimoto (
