1 Kobe Univ., 2 JAMSTEC, 3 Flinders Univ.,

1. 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 N. Seama1, 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 Contents: * Observation & Data analysis * 2-D resistivity models & their interpretations

2. Observation Line Back Arc Spreading Axis Trench Fore Arc Remnant Arc Volcanic Arc Pacific Plate ●: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)

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

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

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

6. 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

7. 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.

8. X Y Sites used for 2-D inversions ●: New data (26 sites) ●: Previous study (8 sites)

9. 2-D Resistivity Model with Hypocenters (Shiobara, personal comm.)

10. 2-D Resistivity Model Slab lithosphereImposedResistivity: 3000Ohm-mThickness:60kmbased on the results from EPR (Baba et al., 2006)

11. Fitting the corrected MT responses Data (dots) Model (red lines) RMS misfit (tm app): 1.94 RMS misfit (all): 1.77

12. 2-D Resistivity Model

13. Resistivity Model

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

15. Resistivity value of the low resistivity region beneath the fore-arc 20 Ohm-m

16. Extent of the low resistivity region beneath the fore-arc 3 4 5 1 2 Resistivity value: 20 Ohm-m Low resistivity can be due to: 1) high water contents 2) existence of melt 3) high temperature 4) low resistivity rock

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

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

19. 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 ? Takahashi et al., 2007 Conder , personal comm.

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

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

22. Resistivity Model

23. Anisotropic models beneath the back-arc basin Dry Olivine Standard Olivine 2 (SO2 model; Constable et al., 1992) x Conder , personal comm. y z

24. Characteristic features of the low resistivity region beneath the spreading axis x y Existence of melt # Anisotropic feature z

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

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

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

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

29. Resistivity profile with depth Grey: Olivine with different water contents Black: Isotropic Blue: Parallel to spreading direction Green: vertical direction Red: Perpendicular to spreading direction Melt beginning depth PT=1300C 3000H/106Si=0.02wt% Modified from Seama et al., 2007

30. Summary(our results are initial, but probably show the first order of the nature) # Existence of the low resistivityregion 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 crustand of the upper most mantle) # Existence of the asymmetriclow 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)

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

33. Resistivity value of the low resistivity region beneath the spreading axis 10-30 Ohm-m

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