Imaging mantle structure of the central Mariana subduction-arc-back arc system using marine magnetotellurics N. 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 Adelaide Contents: * 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 Plate Trench Fore Arc Back Arc Spreading Axis Remnant ArcVolcanic Arc

Observation sites near spreading axis 12 sites in 55km -line (1-6km sites spacing) spreading axis

Australian OBEM Australian OBM (Type 1) Australian OBM (Type 2) US OBEM US OBE

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

Data Analysis 1) 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 models 1) 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.

X Y 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 lithosphere Imposed Resistivity: 3000Ohm-m Thickness: 60km based on the results from EPR (Baba et al., 2006)

Fitting the corrected MT responses Data (dots) Model (red lines) RMS misfit (tm app): 1.94 RMS 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-arc 20 Ohm-m

Resistivity value: 20 Ohm-m Extent of the low resistivity region beneath the fore-arc Low resistivity can be due to: 1) high water contents 2) existence of melt 3) high temperature 4) low resistivity rock

Forward Modeling Test (2) Low resistivity beneath the volcanic arc

Resistivity value of the low resistivity region beneath the volcanic arc 20 Ohm-m

Low resistivity region beneath the volcanic arc Low resistivity can be due to: 1) high water contents 2) existence of melt 3) high temperature 4) low resistivity rock ? Conder, personal comm. Takahashi et al., 2007

Forward Modeling Test (3) Connection between the slab and the volcanic arc

Resistivity value of the region connected between the slab and the volcanic arc Not enough resolution?

Resistivity Model

Anisotropic models beneath the back-arc basin z y x Conder, 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 feature Existence of melt z y x

Characteristic features of the low resistivity region beneath the spreading axis spreading axis # Asymmetric features ?????? + Location + Shape Existence of melt z y x

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

100km Dry Wet Anisotropic Wet Anisotropic Anisotropic layered resistivity structure beneath the back-arc basin z y x

60km 100km Baba et al., 2006 Mariana vs EPR Dry Wet Anisotropic Wet Anisotropic

Resistivity profile with depth Black: Isotropic Blue: Parallel to spreading direction Green: vertical direction Red: Perpendicular to spreading direction PT=1300C 3000H/10 6 Si=0.02wt% Modified from Seama et al., 2007 Melt beginning depth Grey: 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 axis Ohm-m

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