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Water Input Into the Mariana Subduction Zone Estimated from Ocean-Bottom Seismic Data Chen Cai1*, Douglas A

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Water Input Into the Mariana Subduction Zone Estimated from Ocean-Bottom Seismic Data Chen Cai1*, Douglas A LETTER https://doi.org/10.1038/s41586-018-0655-4 Water input into the Mariana subduction zone estimated from ocean-bottom seismic data Chen Cai1*, Douglas A. Wiens1, Weisen Shen1,2 & Melody Eimer1 The water cycle at subduction zones remains poorly understood, Because a long-term net influx of water to the deep interior of although subduction is the only mechanism for water transport Earth is inconsistent with the geological record8, estimates of water deep into Earth. Previous estimates of water flux1–3 exhibit large expelled at volcanic arcs and backarc basins probably also need to variations in the amount of water that is subducted deeper than 100 be revised upwards9. kilometres. The main source of uncertainty in these calculations The Mariana subduction zone has long been cited as a water-rich is the initial water content of the subducting uppermost mantle. system owing to the prevalence of forearc serpentinite mud volcanoes10, Previous active-source seismic studies suggest that the subducting a serpentinized mantle wedge11, and hydrous arc and backarc lavas12,13. slab may be pervasively hydrated in the plate-bending region near However, the initial amount of water within the subducting mantle is the oceanic trench4–7. However, these studies do not constrain the unknown. The subducting Pacific plate in this region is among the depth extent of hydration and most investigate young incoming oldest sections of oceanic lithosphere worldwide14 (about 150 Myr). plates, leaving subduction-zone water budgets for old subducting The incoming plate displays widespread plate-bending normal-fault plates uncertain. Here we present seismic images of the crust and scarps and earthquakes15,16 (Fig. 1b), making it an excellent place to uppermost mantle around the central Mariana trench derived from investigate the depth extent of faulting-induced hydration in an old Rayleigh-wave analysis of broadband ocean-bottom seismic data. oceanic plate. The potential hydration of old, cold oceanic plates is These images show that the low mantle velocities that result from particularly important for the water cycle because the thermal structure mantle hydration extend roughly 24 kilometres beneath the Moho of the plates permits temperature-sensitive hydrous minerals to occur discontinuity. Combined with estimates of subducting crustal water, throughout a thicker region2. these results indicate that at least 4.3 times more water subducts We used data collected by broadband ocean-bottom seismographs than previously calculated for this region3. If other old, cold and island seismographs deployed around the central Mariana trench subducting slabs contain correspondingly thick layers of hydrous (Fig. 1a). The deployment covers a sufficiently large region to inves- mantle, as suggested by the similarity of incoming plate faulting tigate structure variations in the subducting plate before and after across old, cold subducting slabs, then estimates of the global water subduction. We derived local Rayleigh-wave group- and phase- flux into the mantle at depths greater than 100 kilometres must be velocity dispersion curves using two surface-wave methods (Methods). increased by a factor of about three compared to previous estimates3. A Bayesian Monte Carlo algorithm was then used to determine a −10 −8 −6 −4 −20 Bathymetry (km) a 146° E 148° E 150° E b 148° E Trench 18 N Agrigan ° M22 M23 M24 Pagan Big Blue 18° N C seamount C′ Alamagan B B′ Guguan Turquoise Sarigan A seamount A′ 17° N Celestial seamount Anatahan 16° N APM Saipan Fig. 1 | Distribution of seismic stations and bathymetry. a, Station locations in Fig. 2a–c. The arrow labelled APM indicates the direction of distribution. Red circles show ocean-bottom seismographs deployed absolute plate motion. Thin solid white lines show magnetic lineations from January 2012 to February 2013. White squares represent the (M22, M23, M24)30. b, High-resolution bathymetry for the outer-rise temporary island-based stations. Red squares indicate stations from the region of the Mariana trench (indicated by the black rectangle in a) and US Geological Survey (USGS) Northern Mariana Islands Seismograph relocated earthquake locations from the Global Centroid Moment Tensor Network used in our study. Open triangles show the locations of three catalogue15 (black and white focal projections). Magnetic lineations are large serpentine seamounts within the study area. The dashed white shown as in a. line is the trench axis. Thick solid white lines show the cross-section 1Department of Earth and Planetary Sciences, Washington University in St Louis, St Louis, MO, USA. 2Department of Geosciences, Stony Brook University, Stony Brook, NY, USA. *e-mail: [email protected] 15 NOVEMBER 2018 | VOL 563 | NATURE | 389 © 2018 Springer Nature Limited. All rights reserved. RESEARCH LETTER three-dimensional structure to greater depth and avoids biases caused 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 by possible azimuthal anisotropy. In addition, it images shear velocities V (km s–1) 17 a AATurquoise sv ′ that are more sensitive to hydration than are compressive velocities . serpentine seamount 0 The resulting azimuthally averaged velocity model of vertically 0 10 polarized S waves shows systematic changes in the incoming plate and 10 subducting slab along the direction normal to the trench (Fig. 2a–c). 3.9 Bathymetry (km) 3.6 20 4.2 At distances far from the trench axis (more than 100 km seaward), the subducting plate has a typical structure of old oceanic lithosphere18 30 at depths of more than 20 km, with velocities greater than 4.5 km s−1. Depth (km) 40 − 4.2 A lower-velocity (about 4.2 km s 1) layer with a thickness of about 50 10 km is observed immediately beneath the Moho, consistent with 60 active-source seismic results19. This suggests that the very shallow 160 120 80 40 0 –40 –80 –120 part of the incoming plate starts to be altered far from the trench, as b BB′ observed in other subduction zones4. A thicker region of low velocities 0 begins about 80 km seaward of the trench axis and deepens towards 0 10 the trench, with velocities as low as 3.8 km s−1. At the trench axis, 10 the bottom of this low-velocity layer reaches 30 ± 5 km beneath the 3.9 Bathymetry (km) 4.2 20 3.6 seafloor, 24 ± 5 km into the upper mantle. (The errors quoted here and 30 elsewhere correspond to the variations in thickness that result from different assumptions about the velocity that bounds the serpentine Depth (km) 40 4.2 layer.) This low-velocity layer persists after the Pacific plate is subducted 50 as a (30 ± 5)-km-thick low-velocity layer atop the fast slab mantle 60 between 100 km and 160 km west of the trench axis. Synthetic data 160 120 80 40 0 –40 –80 –120 tests show that a velocity anomaly of this large magnitude and extent Big Blue c CCserpentine seamount ′ cannot result from the effect of limited resolution of an approximately 0 6-km-thick layer of low-velocity oceanic crust (Methods, Extended 0 10 Data Fig. 1). 10 We also solved for the azimuthal anisotropy of the phase velocity. 3.6 3.9 Bathymetry (km) 20 4.2 These results show trench-parallel fast axes in the incoming Pacific plate for periods of 12–21 s, with a maximum magnitude as large as 9% 30 reached between 14 s and 16 s. By contrast, for periods of more than Depth (km) 40 4.2 27 s, the fast directions rotate to be oblique to the trench strike, close 50 to the palaeo-spreading direction, which is normal to the magnetic 60 lineations (Fig. 3). 160 120 80 40 0 –40 –80 –120 The region of the incoming plate where we observe a reduction in Distance from the trench (km) mantle velocity and large azimuthal anisotropy coincides with the d plate-bending region, as characterized by substantial normal faulting 15 16 0 seismicity and large extensional seafloor fault scarps (Fig. 1b). There Crust 10 100 MPa is a clear spatial association between plate-bending-induced faulting ) PS + PW 20 300 °C and velocity reduction. Velocities within the incoming plate begin to UM 0 Mpa 30 decrease sharply 80 km from the trench, at about the same distance at 16 600 °C which intense seismicity and faulting begins on the seafloor . In this Depth (km 40 PS region, the fault scarps and the earthquake fault planes strike approx- 50 UM imately north–south, subparallel to the trench axis (Fig. 1b). This is 60 160 120 80 40 0 −40 −80 −120 3.0 3.5 4.0 4.5 consistent with the observations of the azimuthal anisotropy of the –1 Distance from the trench (km) Vsv (km s ) phase velocity at 12–21 s, which primarily sample depths down to about 25 km below the seafloor (Fig. 3). These results reveal trench-parallel ′ Fig. 2 | Vertical profiles and interpretation. a–c, Cross-sections A–A (a), fast directions, as would be expected for pervasive trench-parallel B–B′ (b) and C–C′ (c) showing the azimuthally averaged velocity of water-filled faults or zones of alteration. The flexure model that best vertically polarized S waves (VSV; colour scale). Thick dashed white lines 15 show the location of the forearc Moho31. Thick solid white lines are fits the bathymetry of the Pacific plate seaward of the trench axis projected 6-km-thick slab crust (combining an active-source reflection predicts a neutral plane at around 30 km (Fig. 2d), which suggests that result16 for depths of less than 30 km and the Slab 1.0 model32 for greater brittle normal faulting can extend nearly 30 km into the plate. This depths).
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