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Imaging mantle structure of the central Mariana subduction-arc-back arc system using marine magnetotellurics. 1 Kobe Univ., 2 JAMSTEC, 3 Flinders Univ., 4 WHOI, 5 ERI, Univ. of Tokyo, 6 Univ. of Adelaide. - PowerPoint PPT Presentation

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  • Imaging mantle structure of the central Mariana subduction-arc-back arc system using marine magnetotelluricsN. Seama 1, 2, A. White 3, A. D. Chave 4, K. Baba 5, T. Goto 2, T. Matsuno 1, R. L. Evans 4, G. Boren 3, A. Yoneda 5, H. Iwamoto 1, R. Tsujino 1, Y. Baba 5, H. Utada 5, G. Heinson 6, and K. Suyehiro 2 1 Kobe Univ., 2 JAMSTEC, 3 Flinders Univ.,4 WHOI, 5 ERI, Univ. of Tokyo, 6 Univ. of AdelaideContents:* Observation & Data analysis* 2-D resistivity models & their interpretations

  • Observation Line: New data from KR05-17 deployment and KR06-12 recovery cruises (Kairei, JAMSTEC): Previous study (Filloux, 1983; Goto et al., 2003; Baba et al., 2005; Seama et al., 2007)Pacific PlateTrenchFore ArcBack Arc Spreading AxisRemnant ArcVolcanic Arc

  • Observation sites near spreading axis12 sites in 55km -line (1-6km sites spacing)spreading axis

  • Australian OBEMAustralian OBM (Type 1)Australian OBM (Type 2)US OBEMUS OBE

  • ERI-OBEM (Type 2)IFREE/JAMSTEC-OBEMERI-OBEM (Type 1)Kobe OBEM

  • Data Analysis1) Clean up the raw time series data (9 months)2) Estimate the magnetotelluric impedance tensor responses (MT responses) from the time series data# BIRRP(Chave and Thomson, 2003, 2004)3) Correct the MT responses for the effect of 3-D seafloor bathymetry# Nolasco et al. (1998), Matsuno et al., (2007)# FS3D (Baba and Seama, 2002)4) Estimate 2-D resistivity (or conductivity) structure models to fit the corrected MT responses

  • Inversion methods for estimating 2-D resistivity models1) Data Space Occam inversion (Siripunvaraporn and Egbert, 2000)We modified this algorithm for the MT responses at ocean bottom.

    2) Anisotropic inversion (Baba et al., 2006)

    These inversion algorithms find optimally smooth sets of resistivity models that fit the corrected MT responses to a desired level of misfit.

  • Sites used for 2-D inversions: New data (26 sites): Previous study (8 sites)

  • 2-D Resistivity Model

    with Hypocenters (Shiobara, personal comm.)

  • 2-D Resistivity Model Slab lithosphereImposedResistivity: 3000Ohm-mThickness:60kmbased on the results from EPR (Baba et al., 2006)

  • Fitting the corrected MT responsesData (dots) Model (red lines)RMS misfit (tm app): 1.94RMS misfit (all): 1.77

  • 2-D Resistivity Model

  • Resistivity Model

  • Forward Modeling Test (1)Low resistivity beneath the fore-arc

  • Resistivity value of the low resistivity region beneath the fore-arc20 Ohm-m

  • Resistivity value: 20 Ohm-m54321Extent of the low resistivity region beneath the fore-arcLow resistivity can be due to:1) high water contents2) existence of melt3) high temperature4) low resistivity rock

  • Forward Modeling Test (2)Low resistivity beneath the volcanicarc

  • Resistivity value of the low resistivity region beneath the volcanic arc20 Ohm-m

  • Low resistivity region beneath the volcanic arcLow resistivity can be due to:1) high water contents2) existence of melt3) high temperature4) low resistivity rock?Conder , personal comm.Takahashi et al., 2007

  • Forward Modeling Test (3)Connection between the slab and the volcanicarc

  • Resistivity value of the region connected between the slab and the volcanic arcNot enough resolution?

  • Resistivity Model

  • Anisotropic models beneath the back-arc basinConder , personal comm.Standard Olivine 2 (SO2 model; Constable et al., 1992)Dry Olivine

  • Characteristic features of the low resistivity region beneath the spreading axis# Anisotropic featureExistence of melt

  • Characteristic features of the low resistivity region beneath the spreading axisspreading axis# Asymmetric features+ Location+ ShapeExistence of melt

  • Asymmetric features of the low resistivity region beneath the spreading axisspreading axisConder et al., 2002 (Lau back-arc spreading)MBA: Kitada et al., 2006

  • 100km Dry Wet Anisotropic Anisotropic layered resistivity structure beneath the back-arc basin

  • 60km100kmBaba et al., 2006Mariana vs EPR Dry Wet Anisotropic

  • Resistivity profile with depthBlack: IsotropicBlue: Parallel to spreading directionGreen: vertical directionRed: Perpendicular to spreading directionPT=1300C3000H/106Si=0.02wt%Modified from Seama et al., 2007Melt beginning depthGrey: Olivine with different water contents

  • Summary (our results are initial, but probably show the first order of the nature)# Existence of the low resistivity region beneath the fore-arc (probably due to water from the slab)# Existence of the low resistivity region beneath the volcanic arc (probably due to low resistivity of the volcanic arc crust and of the upper most mantle)# Existence of the asymmetric low resistivity region beneath the back-arc spreading axis (probably due to melt affected by the dynamics)# Existence of the anisotropic layered resistivity structure beneath the back-arc basin (probably due to differences in water contents affected by the dynamics)

  • Forward Modeling Test (4)Low resistivity beneath the back-arc spreading axis

  • Resistivity value of the low resistivity region beneath the spreading axis10-30 Ohm-m

  • Isotropic models using different inversion algorithmsData Space Occam inversion (Siripunvaraporn and Egbert, 2000)Anisotropic inversion (Baba et al., 2006)

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