T44A-01 INVITED
A New Insight Into Mantle Flow Beneath the NE Japan And Kurile Arcs Inferred From Comprehensive Seismological Observations
The northeastern (NE) Japan arc is one of the most investigated arcs in the world through various seismological approaches, and recent comprehensive observations have provided crucial constraints on mantle dynamics. Here we introduce our recent results and propose a model for the mantle flow with implication for arc magmatism. Seismic tomography studies have revealed an inclined low-velocity zone in the mantle wedge of NE Japan (Tohoku) and southern Kurile (Hokkaido) down to depths of ~150 km [Zhao et al., 1992; Nakajima et al., 2001; Nakajima and Hasegawa, 2005]. The zone is considered to be an upwelling flow portion of the secondary convection mechanically induced by slab subduction [Hasegawa et al., 1991; Hasegawa and Nakajima, 2004]. Shear-wave splitting observed in Tohoku and Hokkaido [Nakajima and Hasegawa, 2004; Shimizu, 2004] suggests that the upwelling flow is generated sub-parallel to the maximum dip-direction of the subducting slab, not to the relative plate motion. For NE Japan, a quantitative analysis of the inclined low-velocity zone shows that the upwelling flow contains melt inclusions with volume fractions of 0.1-3 % [Nakajima et al., 2005], suggesting that the upwelling flow is the main source of arc magmas. Interestingly, the inclined low-velocity zone (upwelling flow) shows an along-arc variation in its S-wave velocity reduction rate: very low velocity regions periodically occur about every 80 km along the strike of the arc. Clustering of Quaternary volcanoes and topographic highs at the surface are located immediately above these very low-velocity areas in the mantle wedge, and low-frequency microearthquakes, perhaps caused by rapid movements of fluids in the lower crust, also occur right above them [Hasegawa and Nakajima, 2004]. These observations suggest that the upwelling flow plays an important role on arc magmatism and the 3D magma supply system is working beneath NE Japan.
T44A-02
A 3-D P wave velocity model of the crust and uppermost mantle of the Mariana volcanic arc
As part of the Izu-Bonin-Mariana subduction factory project, a 3-D seismic refraction survey was acquired in 2002 over the Mariana volcanic arc between 14.5 and 18.5 degrees N using a combination of ocean-bottom seismometers, Reftek receivers on land, and large airgun shots from the R/V Maurice Ewing. First arrival travel times have been combined with similar data from an approximately east-west 2-D airgun refraction profile across the arc acquired by the JAMSTEC R/V Kaiyo using ocean-bottom seismometers spaced at intervals of approximately 5 km. We inverted these 277,000 travel times for a 3-D subsurface P wave velocity model using isotropic iterative first arrival seismic tomography (FAST). Forward travel times were calculated through a 500 m model grid, and the inverted model velocities were recovered in 1000 m cubic cells. The starting velocity model did not vary laterally beneath a rugged seafloor interface, which was fixed in subsequent iterations, and a 0-1250 m-thick sediment layer that was introduced in areas where the water depth exceeded 1000 m. The sediment layer simulated well deposits both on the flanks of the volcanoes and in the deep ocean basin, was updated in subsequent iterations of the inversion, and served to remove unrealistic small-scale structure in the final model. After nine iterations of the non-linear inversion, the RMS traveltime residual was reduced from 0.672 s to 0.127 s, equivalent to a normalised chi-squared value of 1.0. First arrival tomography does not typically recover sharp velocity contrasts at deep interfaces such as the Moho. Therefore in our preliminary interpretations we have employed the 7.6 km/s isovelocity contour as a proxy for the location of the Moho. The thickness of the igneous forearc crust decreases from 14 km in the north of the survey area to 9 km in the south. The Eocene arc, which is no longer active, exhibits an igneous crustal thickness of 21-24 km with much of this variability associated with the topography of the volcanic edifices. An 18-22 km thick igneous crust characterizes the 3-4 Ma active Mariana arc, which is located approximately 40 km west of the inactive Eocene arc. P wave velocities within the upper crust of the active arc appear to be systematically lower than in the inactive arc, by approximately 450 m/s. At a depth of 15 km, in contrast, velocities are around 350 m/s higher in the active arc. These results suggest an evolution of arc structure with increasing age: closure of fractures and porosity in the upper crust through hydrothermal circulation and a reduction in the mafic character of the mid-lower crust, presumably as a result of some degree of crustal differentiation.
T44A-03
Along Arc Structural Variation in the Izu-Bonin Arc and its Implications for Crustal Evolution Processes
A continental-type middle crust having Vp = 6.1 - 6.3 km/s has been imaged at several oceanic island arcs (e.g. northern Izu, Mariana, Tonga, Kyushu-Palau ridge) since Suyehiro et al. (1996) has found a felsic middle crust in the northern Izu arc. A high velocity lower crust (Vp > 7.3 km/s) underlying the felsic middle crust has been also underlined as a characteristic structure in the northern Izu arc. A bulk composition of the crust in the Izu arc may indicate more mafic than that of a typical continental crust due to a large volume of the high velocity lower crust. Since a crust becomes more mature toward the north along the Izu-Bonin arc, investigating structural variation along the volcanic front has been believed to provide a fundamental knowledge for a crustal evolution process. In 2004 and 2005, Japan Agency for Marine-Earth Science and Technology has conducted two along arc wide-angle seismic surveys from the Sagami-bay to the Kita-Iwo jima, a total profile length of about 1000 km. Although data from the Bonin-part of the profile which were acquired this year has not been processed yet, a result from the Izu-part, from the Sagami-bay to Tori shima, shows significant structural variations along the volcanic front. The crustal thickness are varied with a wavelength of several tens of km, i.e., thickened up to 25-30 km around the volcanoes (the Miyake jama, Hachijo jima, Aoga sima, Sumisu jima), while thinned down to 20 km between them. The fine seismic velocity image obtained by refraction tomography as well as a wide-angle reflection migration shows that the variation of the crustal block having 6.0 - 6.7 km/s, which is a typical continental crustal velocity, is mainly responsible for the observed variation of the crustal thickness. The thickness of the high velocity lower crust is not significantly varied along the arc. Therefore, an average crustal seismic velocity (varied 6.6 to 7.0 km/s) represents a higher velocity that that of a typical continental crust (6. 4 km/s), and a negative correlation between the thickness of the 6.0 - 6.7 km/s block and the average crustal seismic velocity is recognized. In conclusion, the continental-type of the crust efficiently grow at the Quaternary volcanoes along Izu arc, but even at those areas the bulk composition of the entire crustal section shows more mafic than a continental crust due to the uniformly existing high velocity lower crust. A delamination process may be necessary to form a continental crust form the Izu island arc crust
T44A-04
An Overview of Results from the 2003-2004 Mariana Subduction Factory Passive Imaging Experiment
We investigate the seismological structure and seismicity of the Mariana subduction zone and backarc basin using data from the Mariana Subduction Factory Imaging Experiment. The passive component of this experiment consisted of 20 broadband seismographs deployed on the island chain and 58 ocean-bottom seismographs (OBS) across the arc and in the backarc. This dense seismic network recorded from May-June, 2003 until April-May, 2004 and 68 instruments returned useful data. We obtained the 3D P-wave velocity structure of the Mariana mantle wedge from a tomographic inversion of P-wave arrivals from local earthquakes, arrival times from large teleseismic earthquakes determined by multi-channel cross correlation, and arrivals from Mariana earthquakes to stations around the world reported by the PDE bulletin. These results clearly show a high velocity slab extending to depths of ~600 km. A slow velocity anomaly of ~8% is present within the upper 150 km beneath the Mariana back-arc spreading center. Slow velocity anomalies are also found beneath the active volcanic arc and in the uppermost forearc mantle. We also investigate the anisotropy of the Mariana region using shear wave splitting from S waves propagating from intermediate and deep earthquakes in the Mariana slab to the OBS and land seismic stations. The northern part of the arc system shows approximately arc-parallel fast directions for intermediate depth events. Fast directions surrounding the spreading center also show N-S orientation, whereas raypaths sampling west of the spreading center show NW-SE and E-W fast directions, or approximately parallel to the the absolute plate motion of the subducting Pacific Plate. This overall pattern could be due to either the predominance of along-strike flow near the slab and slab induced counterflow in the far backarc, or to a different relationship between LPO and mantle flow near the water rich slab. Slab seismicity is characterized by a double seismic zone in the best-imaged part of the arc near 18N, with the lower zone initiating in the forearc at a depth of 60 km and extending to depths of about 200 km. Planes of the double seismic zone are initially separated by about 40 km, but the separation decreases with increasing depth. The forearc seismicity is dominated by a highly seismic region of the shallow thrust zone at depths of 30-40 km, containing several extremely active patches with diameters of about 20 km. The outer forearc, in the region of the serpentinite seamounts, is mostly aseismic, particularly immediately beneath the seamounts. We suggest that the lack of large (Mw > 7.0) thrust earthquakes in Mariana results from serpentinization of the outermost mantle forearc and from the the extreme heterogeneity of the shallow thrust interface, as revealed by the patches of high seismicity.
T44A-05 INVITED
Spatial and Temporal Variation of Sub-arc Mantle in the Southern Izu-Bonin arc
We present spatial and temporal variation of sub-arc mantle in the southern Izu-Bonin arc and discuss the processes controlling it in conjunction with the tectonic evolution of this area. The southern Izu-Bonin arc is characterised by an Eocene fore-arc massif (Bonin Ridge), an Oligocene rift basin (Ogasawara Trough), active submarine volcanoes and intra-arc rift basins. The Quaternary lavas are mostly low-K tholeiitic basalt and basaltic andesite. Exceptions are highly alkaline shoshonitic rocks from Iojima and the surrounding volcanoes. A southward decrease in $^{87}$Sr/$^{86}$Sr and increase in $^{206}$Pb/$^{204}$Pb is observed in the northern section of the arc and this continues into the southern arc as far as 27$^{o}$N. In Pb-Pb isotope space volcanoes plot systematically closer to the NHRL from north to south. The decoupled behavior of Sr and Pb isotopes led us to propose an along-arc mantle wedge heterogeneity prior to the addition of the slab-derived component. South of 27$^{o}$N, the along-arc isotopic trend changes dramatically. Sr isotopic ratio increases southward from 27$^{o}$N, and the $^{206}$Pb/$^{204}$Pb becomes highly radiogenic (~19.5). This isotopic signature requires involvement of a slab-derived component with high $^{206}$Pb/$^{204}$Pb with low _elta8/4. Lack of correlation between Nd isotopic composition and Th enrichment implies a major contribution of fluid as a slab component in 27-25$^{o}$N, while clear correlation between the two implies a significant melt contribution south of 25$^{o}$N (in the vicinity of the Iojima). The estimated slab component south of 27$^{o}$N is different from altered MORB-like oceanic crust and pelagic sediment, and this could be derived from the seamounts on the Pacific Plate. New isotopic data for the subducting seamount chains on the Pacific plate demonstrate that one of the chains in the Marcus-Necker seamount chain shows a O'HIMU-likeO_L character, i.e., $^{206}$Pb/$^{204}$Pb>20, _elta8/4<0 and low $^{87}$Sr/$^{86}$Sr and significant variation occurs among these chains. This isotopic character and heterogeneity of the subducting seamounts could cause large along-arc variation of the slab component in this region. Variation in geometry of the subducting slab (e.g., Miller et al., 2004, 2005) and PT condition in mantle wedge in the southern Izu-Bonin arc (e.g., higher slab surface temperature south of 25$^{o}$N may induce melting of sediment) may cause change in the transporting media of slab component. New data from Eocene-Oligocene lavas from the Southern Izu-Bonin arc demonstrate different isotopic characteristics from Quaternary frontal arc volcanoes and remarkable temporal variation in sub-arc mantle composition occurred during this period. Eocene lavas from the Bonin Ridge show higher $^{206}$Pb/$^{204}$Pb (>18.6), while Oligocene lavas from the fore-arc of the southern Izu arc and Kyushu-Palau Ridge show lower $^{206}$Pb/$^{204}$Pb and relatively small along-arc variation (similar to the Quaternary northern Izu-arc in most cases). These results imply that the composition and/or geometry of the subducting plate is clearly different among Eocene, Oligocene and Quaternary arcs.
T44A-06
Composition and Evolution of the IBM Arc Crust
The IBM oceanic arc is characterized by the occurrence of the middle crust with P-wave velocity of 6.0- 6.5 km/s and probably having andesitic compositions. It may be thus suggested that this arc could represent the initial stage of formation of the andesitic continental crust. Knowledge of arc crust composition and its temporal variation is crucial to understanding the evolution from the arc to the continental crust. We herein estimate the composition of the IBM arc crust based on the seismic structure of the arc and compositions of IBM arc magmas including the Tanzawa tonalite that would represent the obducted portion of the IBM middle crust. Two different processes of andesitic crust formation are assumed: (1) 30% melting of the pre-existing basaltic crust that was created by 20% fractionation of olivine from an inferred IBM primary basalt magma; (2) mixing of the differentiated basalt magma and the felsic magma produced by 15% melting of the basaltic crust, Both processes can reasonably explain compositions of the Tanzawa tonalite. Seismologically determined volume of the IBM middle crust and the inferred andesite formation processes provide estimates for volume of dunitic cumulate and gabbroic restites. The calculated volume of the restite layer is identical to that determined by seismic observation, suggesting that the dunitic cumulate would form a part of the upper mantle, not of the arc crust. It may be thus inferred that the bulk IBM arc crust possesses basaltic, not andesitic, compositions. In order for the IBM arc crust to evolve into an andesitic continental crust, further separation of mafic component from the arc crust such as delamination of the lower crust is needed.
T44A-07
Relationship Between the West Philippine Basin and the Early Izu-Bonin-Mariana Arc: Arc Initiation and Ancestry
The early Izu-Bonin-Mariana (IBM) arc formed during the Eocene along a margin of the West Philippine Basin (WPB). At that time, the WPB had at least one active spreading center, the Central Basin Ridge (CBR) and it was rimmed by fragments of older, Early Cretaceous to Paleocene island arc and ocean floor lithosphere. Published models of IBM arc initiation suggest that subsidence and ultimately subduction of old, dense oceanic lithosphere beneath the young oceanic lithosphere near the CBR, which was either an active mid-ocean ridge or a back-arc basin spreading center, triggered melting to form boninite and primitive arc tholeiite. Published ages of rocks from the CBR show that magmatism continued until at least 28 Ma, which implies that CBR and early IBM arc magmatism overlapped for almost 20 Ma. Geochemical data for rocks from the WPB and from surrounding older arcs and basins, including the Amami Plateau, Daito Ridge and Huatung basin, indicate that WPB lithosphere was diverse in composition. This diversity probably influenced the along-strike character of the IBM arc as it evolved from its initiation at 48 Ma to the first episode of arc rifting and opening of the Shikoku and Parece Vela back-arc basins. In contrast, there is little geochemical evidence that the newly subducting plate influenced the composition of magma generated concurrently in the WPB.
T44A-08
Tracing slab inputs along the Izu-Bonin-Marianas subduction zone: results from volatile emissions
The Izu-Bonin-Mariana (IBM) arc system extends 2800 km from the island of Honshu, Japan to Guam and is a type example of an intra-oceanic convergent margin. Subduction began 45 Ma ago and IBM subducts the oldest seafloor on Earth. A number of parameters vary systematically along the strike of the arc: the slab is steeply plunging in the S and gently dipping in the N; the age of the subducted crust varies from Mid-Jurassic in the S to Mid-Cretaceous in the N. Other parameters remain constant: crustal thickness (~20 km); no accretionary prism; no sediment fill in the trench. The sediment outboard of the arc is characterized based on ODP sites 801 (Marianas) and 1149 (Izu islands). 200 m of volcaniclastics are overlain by a 100 m of pelagic clay and chert in the S. In the N, volcaniclastics are lacking and the 400 m sediment sequence is dominated by 200 m of cherts, overlain by 40 m of pelagic clay and 120 m of volcanic ash and diatom/radiolarian clay. There is also a distinct layer (3 m) of hydrothermally altered MORB in the S. Thus, the IBM system is an ideal location to study the inputs and outputs of the subduction factory and to understand the processes occurring within the factory itself. We collected hydrothermal gas samples from 4 volcanic centers in the Marianas (Alamagan, Pagan, Agrigan, Uracas) and 6 centers in the Izu arc (Aogashima, Hachijojima, Niijima, Shikinejima, Oshima, Hakone). With the exception of Uracas (140C) and a well on Hachijojima (170C), all gas discharges were at or below the boiling temperature of water. As is typical for arc-related samples, the major gases are dominated by H2O, CO2 and S species. We see the following variations in N2/Ar and N2/He ratios of non-air contaminated samples along the arc: Agrigan clearly shows a mantle wedge signature of low N2/Ar (70) and N2/He (210) and negative δ15N (- 2.0 ‰). All other centers have N2/He ratios characteristic of that resulting from the addition of N from subducted sediments (1000 to 2500). Most Izu samples also show N2/Ar ratios higher than air (up to 210). Helium isotopes of Mariana samples are MORB-like (7.4 to 7.9 R$_{A}$), whereas CO2/$^{3}He$ varies from 10.1 to 10.7 x 109 with δ13C between -0.5 to - 0.7 $permil$. Based on N-CO2-He-Ar sytematics, the Izu section of the arc has a signature characteristic of subducted sediment derived fluids. The Mariana section (Agrigan in particular), shows a volatile signature that suggests contribution dominantly from the altered oceanic basement. This is in contrast to studies based on trace elements and radiogenic isotopes that identify Agrigan as the 'sediment endmember' of the Mariana arc. Analyses of stable and noble gas isotopes of the samples are currently underway to further constrain the source of volatiles discharging along the arc.