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Controlling Factor of Incoming Plate Hydration at the North-Western Pacific Margin

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Controlling Factor of Incoming Plate Hydration at the North-Western Pacific Margin ARTICLE DOI: 10.1038/s41467-018-06320-z OPEN Controlling factor of incoming plate hydration at the north-western Pacific margin Gou Fujie1, Shuichi Kodaira1, Yuka Kaiho1, Yojiro Yamamoto 1, Tsutomu Takahashi1, Seiichi Miura1 & Tomoaki Yamada2 Hydration of the subducting oceanic plate determines the amount of water transported from Earth’s surface into its interior, and plate bending-related faulting (bend faulting) just prior to 1234567890():,; subduction is considered to promote hydration. Bend faulting shows significant spatial var- iation, but its contribution to hydration is still poorly understood. Here we present the results of controlled-source seismic surveys around the junction of the Japan and Kuril trenches. We found structural changes caused by bend faulting before subduction differed distinctly between both trenches and were well correlated with plate hydration after subduction, suggesting the bend faulting controls spatial variations in plate hydration. Differences in bend faulting are closely related to the angle between the current trench and the ancient spreading ridge, and the hydration is more extensive where this trench-ridge angle is oblique in the study area. Thus, we propose this angle is a major factor controlling plate hydration. 1 Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 3173-25, Showa-machi, Kanazawa-ku, Yokohama, 236-0001 Kanagawa, Japan. 2 Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, 113-0032 Tokyo, Japan. Correspondence and requests for materials should be addressed to G.F. (email: [email protected]) NATURE COMMUNICATIONS | (2018) 9:3844 | DOI: 10.1038/s41467-018-06320-z | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-06320-z s a part of global mantle convection, water is transported ferences between the trenches (Fig. 1). In the Japan Trench, the Aby subducting oceanic plates from Earth’s surface into its throw of the bend faults is more than 800 m, and the fault spacing interior, and the degree of hydration of the subducting (horizontal distance between two horsts) is 10–15 km (Fig. 2c). In oceanic plate determines the amount of transported water. In contrast, in the Kuril Trench, the throw is less than half (at most subduction zones, dehydration occurring within the subducting 400 m), and the spacing is much less (roughly 5 km) (Fig. 2d). oceanic plate induces intra-slab intermediate-depth earthquakes, These differences make this trench junction a good place to and the expelled water leads to arc magmatism, promotes investigate the contribution of bend faulting and its spatial var- metamorphism, lowers the slab temperature, and consequently iations to plate hydration. affects interplate coupling1,2. Thus, the degree of hydration of the In this study, we show the seismic structural changes caused by oceanic plate before subduction is key to understanding these bend faulting before subduction are well correlated with plate subduction zone phenomena as well as the global water cycle. hydration after subduction. Differences in bend faulting between The first-order control on oceanic plate hydration has pre- these two trenches are closely related to the angle between the viously been considered to be hydrothermal circulation and current trench axis and the ancient spreading ridge axis. These alteration at the spreading ridge. Recently, seismic and electro- findings indicate that the trench-ridge angle is a major controlling magnetic structure studies in subduction zones around the world factor of plate hydration in these subduction zones. have revealed that plate bending-related normal faulting near the subduction trench also promotes hydration of the oceanic Results plate3-11. Unlike hydrothermal circulation near the spreading fi Tomographic imaging. To reveal the structural changes caused ridge, which is con ned to the oceanic crust, bend faulting near by bend faulting, we conducted extensive seismic surveys in the the subduction trench can potentially cut across the entire crust outer trench areas of these subduction zones (Fig. 1). We and reach to the upper mantle (e.g., the 1933 Sanriku earth- 12,13 deployed Ocean Bottom Seismometers (OBSs) at intervals of quake ). Thus, if bend faults act as pathways for water 6 km along each survey line, and fired a large airgun array at 200- penetration into the mantle, the amount of water transported m intervals for OBSs and at 50-m intervals for a 6-km-long, 444- by the oceanic plate might be much larger than previously 14,15 channel hydrophone streamer. The data quality was good (Sup- thought . plementary Figs. 1-3). We succeeded in obtaining P-wave velocity Bend faulting shows remarkable variation among subduction (Vp) and S-wave velocity (Vs) models by tomographic traveltime trenches. However, the actual contribution of bend faulting and inversion of both OBS and MCS data (see Methods). its spatial variations to plate hydration remains poorly under- The resultant Vp models show a simple layered oceanic plate stood because plate hydration is affected by various hydration and structure in which the oceanic crust is 7 km thick (Fig. 3). The dehydration processes from the spreading ridge to the subduction topmost mantle Vp outside of the bend faulting area was trench and isolating the contribution of each process is not ~8.5 km s−1 in the Kuril Trench profile (A2) but ~7.9 km s−1 in straightforward. the Japan Trench profile (A3). This large difference is well At the north-western Pacific margin off Japan, the old oceanic fi – explained by seismic anisotropy due the alignment of olivine Paci c plate, which formed at 120 130 Ma at a fast-spreading crystals in response to mantle flow at the spreading-ridge ridge, is subducting beneath the north-eastern Japan arc at the 17,18 16 axis and is not relevant to the bend faulting. On the other junction of the Japan and Kuril trenches . Although the same hand, the reduction of V and increase of the V /V ratio in the oceanic plate is subducting with a similar curvature at each p p s fl vicinity of the trench axis are considered to be related to the trench, horst and graben structures, which are sea oor topo- bend faulting (Fig. 3, Supplementary Figs 4-8). A reduction of graphic features related to bend faulting, exhibit remarkable dif- Vp is widely observed in the outer trench area of subduction a b c 140° 142° 144° 146° 148° 150° 152° 143° 144° 145° 146° 147° 143° 144° 145° 146° 147° 44° 42° 0246 Kuril Trench (degree) Kuril Trench 42° Kuril Trench 41° –6000 –6000 A2 40° 40° A3 Japan Trench –6000 39° 38° Japan Trench Japan Trench –6000 –6000 –6000 –6000 45°N 38° 36° –6000 35°N 37° 140°E 150°E –8000 –6000 –4000 –2000 0 34° Curvature of seafloor bathymetry Slope of seafloor bathymetry Fig. 1 Study area. a Bathymetric map of the north-western Pacific margin off Japan around the junction of Kuril and Japan trenches (location in inset). Solid red lines A2 and A3 are seismic survey lines. White circles represent OBSs located in the vicinity of the Japan Trench axis, and green circles represent other OBSs. No OBSs were deployed in the vicinity of the Kuril Trench axis. b Curvature (Laplacian) of the seafloor topography in the outer trench area. c Slope gradient of the seafloor in the outer trench area. Dashed red lines in b, c are magnetic anomaly lineations16 2 NATURE COMMUNICATIONS | (2018) 9:3844 | DOI: 10.1038/s41467-018-06320-z | www.nature.com/naturecommunications NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-06320-z ARTICLE a 140° 142° 144° 146° 148° 150° b 44° 5000 (J4) (2) 5500 – 6000 (1) –6000 6000 42° (3) Kuril Trench K1 K2 6500 K3 Depth (m) A2 K4 7000 J1 40° (4) Seafloor topography –6000 J2 Smoothed topography 7500 J3 0 20 40 60 80 100 –6000 J4 A3 J5 38° Japan Trench J6 (5) d Kuril Trench J7 400 m 36° (K1) c Japan Trench (K2) 400 m (J1) (K3) Residual depth (m) Residual (J2) (K4) (J3) 0 20 40 60 80 100 Distance from trench axis (km) (J4) e Frequency of intermediate-depth earthquakes (after Kita et al. 2006) 700 Residual depth (m) Residual 600 (J5) 500 400 (J6) 300 Kuril 200 (J7) of upper seismic plane Trench The number of earthquakes 100 Japan Trench 0 0 20 40 60 80 100 (5) (4) (3) (2) (1) Distance from trench axis (km) Fig. 2 Seafloor topography. a Location map. Solid lines K1-K4 and J1-J7 are the locations of the seafloor topographic profiles shown in b–d, and boxes 1-5 are the areas for evaluating the frequency of the earthquakes in e. b Seafloor topography along profile J4: raw data (solid) and smoothed data (dashed). Smoothing was performed with a moving average spatial filter. c Seafloor topography after removing the trend, defined by difference between raw and smoothed data, in the Japan Trench. The horizontal axis is the distance from the trench axis. The throw of horst and graben reaches more than 800 m near the trench axis. d Seafloor topography after removing the trend in the Kuril Trench. The throw is <400 m. e Frequency of dehydration-induced intermediate-depth earthquakes after subduction24. The number of earthquakes was calculated for each box (1)–(5). The vertical axis shows the peak of the upper plane earthquake frequency. The activity of dehydration-induced earthquakes is remarkably higher in the Japan Trench than in the Kuril Trench zones worldwide, where it is interpreted to be a consequence of themantle,theincreaseoftheVp/Vs ratio within the crust fracturing, water penetration, and hydration5,6,8,10,19. A reduc- indicates that the water content became higher in the bend tion of Vp in the continental rifting area is similarly interpreted faulting areas.
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