seismological studies on mantle upwelling in ne japan: implications for the genesis of arc magmas
DESCRIPTION
IBM Workshop in Honolulu. Seismological studies on mantle upwelling in NE Japan: Implications for the genesis of arc magmas. Junichi Nakajima & Akira Hasegawa Research Center for Prediction of Earthquakes & Volcanic Eruptions Graduate School of Science, Tohoku University, JAPAN. - PowerPoint PPT PresentationTRANSCRIPT
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Seismological studies on mantle upwelling in NE Japan:
Implications for the genesis of arc magmas
Junichi Nakajima & Akira Hasegawa
Research Center for Prediction of Earthquakes & Volcanic Eruptions
Graduate School of Science, Tohoku University, JAPAN
November 8, 2007
IBM Workshop in Honolulu
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Outline1. Review previous
results on mantle-wedge structure in 2000’s and their interpretations.
2. Introduce recent (preliminary) results of velocity structure in Tohoku and Hokkaido
Miller et al. (2006)
Study Study areaarea
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Seismological observations in 1990’s
SeismologyHasegawa et al. [1991]Zhao et al., [1992-1994]
Hasegawa et al. [1991]
Zhao et al. [1992]
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Travel-time tomography in NE Japan
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Seismic tomography study - Eqs. and stations(Nakajima et al., 2001, JGR)
•169,712 P-wave arrivals & 103,993 S-wave arrivals
•Method : Zhao et al. [1992, JGR]
•Grid separation: 15-25 km in both horizontal and vertical directions
Neqs. = 4338
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Inclined low-Vs zones in mantle wedge
Low-F eventVolcano
Pacific plate
Nakajima et al. [2001]
Inclined low-V zone ~50 km above the slab
Velocity reductions of 4-6 % in Vp and 6-10 % in Vs
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Velocity structure at a 40 km depth (below the Moho)
Nakajima et al. JGR, [2001]
dVp Vp/VsdVs
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(Eberle et al., PEPI, 2002)
Flow pattern
Numerical simulation
Inclined low-V zone
= upwelling flow induced by slab subduction
Predicted low-V zone is consistent with the observation.
Flow pattern (wedge)
Upward flow (high-T) is generated in the mantle wedge.
Karato [1993, GRL]
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Question
What causes an inclined low-velocity zone ?
- thermal heterogeneity?- melts?- chemical heterogeneity?
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Qp structure in NE Japan
(Tsumura et al., 2000)
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Conversion from Qp to Temperature[Nakajima and Hasegawa, GRL, 2003]
]/)(exp[),,( 00*000
10 RTPHfPTfQ
]/)(exp[),,( *1 RTPHfPTfQ
( f: frequency [Hz] , P : pressure [GPa], T: temperature[K], H*: activation enthalpy [kJ/mol] )
1
10
1
*0
*0
*
ln)(
1
)(
)(
Q
Q
PH
R
TPH
PHT
Simple relationship between Q, temperature, pressure and frequency [e.g., Karato, 2004]
Given T0, P0 and Q0 as reference values….
References
T0 : 1025℃ ( 40 km depth ) [Kushiro, 1987]
Q0-1 = 0.0035 [Tsumura et al., 2000]
a=0.20, H*(P)=500 + 16×P kJ/mol, H0*=500 kJ/mol [Karato,
2004]
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Thermal structure [Nakajima and Hasegawa, GRL, 2003]
Wet solidus of peridotite
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Correction of thermal effect
Observed low-velocity anomalies-> 4-6 % in Vp and 6-10 % in V
sExpected velocity reductions from therm
al anomalies-> 1-2 % in Vp and 2-3 % in Vs
Residuals of velocity anomalies-> -dlnVp=0.03-0.04
-dlnVs=0.04-0.07 dlnVp/dlnVs = 1~2
2
*1,
,,
),()(
lnln
RT
HPTQ
F
T
V
T
V
sp
anh
spsp
Karato (1993)
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Takei’s model ( Takei, JGR, 2002 )
dlnVs/dlnVp( Velocity reduction rate )
Aspect ratio ( α )
Volume fraction from dlnVs ( φ )
Takei (2002)
physical properties of fluids
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Melt distribution in low-V zoneNakajima, Takei and Hasegawa (2005, EPSL)
Partial melting with fractions of 0.3-5 vol% in low-V zone.
Depth (km) Aspect ratio Melt fraction (%)
40 0.01-0.1 ~1
65 0.001-0.05 0.05-1
90 0.1-0.2 3-5
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Interpretation of depth variation in pore shapes
dike/crack
dike/crack
GeneratioGeneration?n?
MigrationMigration??
Accumulation below Accumulation below Moho?Moho?
Depth variation in aspect ratio of melt-filled pores
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Which direction does mantle upwelling flow?
Seismic velocity/attenuation structures are the present-day snap shot and do not provide the direction of mantle flow.
Shear-wave splitting could provide an important and independent information on mantle dynamics.
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Results of shear-wave splitting(Nakajima and Hasegawa, EPSL, 2004)
Assuming A-type olivine in back arc, flow direction is inferred to be EW.
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A model of return flow in NE Japan
Hasegawa & Nakajima (2004)
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Summary
1. An inclined-low-velocity zone in the mantle wedge sub-parallel to the slab
2. Temperatures in the mantle wedge of 1000-1300 C
3. Depth variation in aspect ratio of melt-filled pores and melt fractions of 0.05-5 vol% in the low-velocity zone
4. Flow direction parallel to the slab dip
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Recent tomographic results in NE Japan
@ Update previous results by Nakajima et al. (2001)
@ Obtain clearer images of inclined low-velocity zone
@ Understand whole fluid circulation
Kawakatsu & Watada (2007)
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Data set
Zhao Nakajima This study
Eqs. 450 4400 8700
Stations ~50 150 >300
Grid int. 20-30km 15-25 km 10-20 km
P arrivals 16,000 160,000 600,000
S arrivals 5,000 100,000 350,000
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Comparison with Nakajima et al. (2001)
This studyThis study Nakajima et al. (2001)Nakajima et al. (2001)
Central part of Tohoku
dVp
dVs
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Results
Sheet-like low-velocity zoneSheet-like low-velocity zone
Larger velocity reductions in S wave than P waveLarger velocity reductions in S wave than P wave
( -dlnVp=3-6%, -dlnVs=5-10%)
Thickness of low-velocity zone of 10-30 km with an along-arc variation (seThickness of low-velocity zone of 10-30 km with an along-arc variation (seems to be thinner in C and D)ems to be thinner in C and D)
dd VVpp
dd VVss
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Low-velocity zone beneath back-arc volcanoes
Diapirs from the upwelling?
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Path of fluids from slab to mantle
Low-V zone at a dept of 150 km -> Supply of fluids from slab to mantle?
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Summary of recent results
1: Low-velocity zone corresponding to oceanic crust down to a depth of 100 km (Tsuji et al., unpublished).
2~3: Low-velocity zone at a depth of ~150 km. Supply of fluids to mantle there?3~4: Sheet-like low-velocity zone
-dlnVs > -dlnVp . Thickness of 10-30 km with along-arc variation5: Segregated diapirs from upwelling?. Source of magmas of back-arc volcanoes?
11
2233
4455
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Low-velocity zone in mantle wedge
Hasegawa and Nakajima (2004), AGU Geophys. Monog.
NE Japan : Zhao et al. (1992), Nakajima et al. (2001)
Alaska & Aleutian : Abers (1994), Zhao et al. (1995)
Kamchatka : Gorbatov et al. (1999)
Tonga : Zhao et al. (1997)
Inclined low-V zone (from back-arc to the VF)
Hokkaido : Wang and Zhao (2006)
Kyushu : Wang and Zhao (2006)
New Zealand : Reyners et al. (2006)
Alaska : Eberhart-Phillips et al. (2006)
Tonga : Conder and Wiens (2006)
After 2004After 2004
Is inclined low-V zone a common feature in subduction zones?
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S-wave velocity structure -HOKKAIDO