Schedule for 2018
1. 海洋底序説2. 海洋リソスフェアと大構造 10/33. 沈み込み帯(1)海溝域の構造と沈み込みダイナミクス 10/104. 沈み込み帯(2)前弧の構造と多様性 10/175. 沈み込み帯(3)島弧火成活動と物質循環6. 中央海嶺(1)海洋性地殻の生成7. 中央海嶺(2)中央海嶺プロセスと基本構造8. 中央海嶺(3)海洋性地殻の多様性9. 海洋トランスフォーム断層と断裂帯12. 背弧拡大系13. プレート内火成活動:海山と巨大海台10. 海底下流体 (1)冷湧水系 12/1911. 海底下流体 (2)熱水系 1/9 」
Introduction2. Oceanic lithosphere3. Subduction zones (1) trenches and subduction dynamics4. Subduction zones (2) forearc5. Subduction zones (3) arc magmatism and mass flux6. Mid-ocean ridges (1) formation of oceanic crust7. Mid-ocean ridges (2) basic process and architecture8. Mid-ocean ridges (3) structure of oceanic crust and its variabillity9. Oceanic transforms and fracture zones12. Backarc spreading system13. Intra-plate volcanism : seamounts and oceanic plateau10. Hydrogeology (1) cold seeps11. Hydrogeology (2) hydrothermal systems
沈み込み帯(3回シリーズ)Subduction zones dynamics
•3.海溝域の構造と沈み込みダイナミクス(10/10)•4.前弧の構造と多様性(10/17)•5.島弧火成活動:物質循環
---------------------------------• 海底下流体(1)冷湧水系• 海底下流体(2)熱水系
海洋底ダイナミクス2018冬学期 理1-739
•Trench system structure and subduction dynamics •Forearc structure and diversity•Arc volcanism and material recycling
Today’s contents
•Geometry & morphology•Physics – How it goes down?•Where is the fluid loaded? – A new aspect•Fate of sediments•Reading: Peacock 1999
海洋底ダイナミクス2018冬学期
Subduction zone= destructive / convergent boundary
(Understanding Earth, Sliver&Jordan 2003)
Type of convergent boundaries
(Understanding Earth, Sliver&Jordan 2003)
• ocean- ocean boundaries, where oceanic plate is subducted beneath oceanic plate• ocean-continent boundaries, where oceanic plate is subducted beneath continental lithosphere• continent-continent boundaries, where two continental plates collide
衝突帯 Collision zone North America Plate
衝突帯 Collision zone
沈み込み帯Subduction zone
World topography
plate convergent rate 65 ~ 106 mm/year
( An introduction to our dynamic planet, Rogers ed., 2008)
age of down-going slab
( An introduction to our dynamic planet, Rogers ed., 2008)
World’s seismicityMost earthquakes occur in subduction zones
Trench-Arc-Backarc system海溝ー島弧ー背弧系
島弧海溝系の例:伊豆小笠原The Izu-Bonin-Mariana (IBM) Trench-arc system
伊豆島弧
四国背弧海盆
伊豆小笠海溝
Global seismic tomography-fate of subducting slab-
(Fukao et al., 2001)
670km440km
Schematic section through the center of the Earth
(Stern, 2002)
area of previous slide(410km)
Tomographic image and seismicity across IBM and Ryukyu Trench
(Widiyantoro et al., 1999 EPSL)
Intraplate seismicity and thermal structure across NE Japan
Seismicity cross section across Japan trenchhttp://www.eps.s.u-tokyo.ac.jp/epenv/members/Shimidzu.html
( An introduction to our dynamic planet, Rogers ed., 2008)
angle of Wadati-Benioff zone @ 20°S
first occurrence of > 400km events= 400km from trench
dip angle =~ 45°
( Hayes et al., 2012)
( SLAB1.0 : Hayes et al., 2012)
Global 3D slab model
( SLAB1.0 : Hayes et al., 2012)
slab subduction angle
• Mariana• Lesser Antilles• Tonga• Kurile• Northern Chile
max. depth660km discontinuity
(Uyeda and Kanamori, 1978)
Key ParametersCurvatureConvergent rateAge
Measuring the onset of locking in the Peru–Chile trench with GPS and acoustic measurements
(Gagnon et al., 2005 Nature)
“measured horizontal surface motion perpendicular to the trench is consistent with a model having no slip along the thrust fault between 2 and 40 km depth.”
•Physics of subduction – How (why) it goes down?
• The driving force - Negative buoyancy • Cold, high density down-going plate
• Oceanic plate subducts beneath the continental plate• Chile Trench : Nazca vs. South America
• Old oceanic plate subducts beneath the young oceanic plate• Izu-Bonin-Mariana Trench: Pacific vs. Philippine Sea
• Phase transition @ ~90km • Eclogite formation
Rough estimation: Average density of old oceanic plate
old 90km-thick lithosphere with 7km crust
@ 0km
@ lithosphere asthenosphere boundary
100x109500-50-100
Mass Balance
80km
60
40
20
0
Plate thickness
3400320030002800Density (kg/m^3)
Minimum lithosphere thickness for subduction to occur.
• Plate can subduct if the average density of the plate is larger than that of asthenosphere.
• In this case, subduction cannot occur if the lithosphere is thinner than 73.2 km.
Average plate density > Asthenosphere density
Average plate density < Asthenosphere density
Oceanic crust
crust
Asthe
nosp
here
(Subduction Top to Bottom, 1996)
Phase transition due to eclogite formation with dehydration
Fluid imported in the oceanic crust -Hydrothermal circulation near spreading centers-
temperature
numbers = density
P= ρgz = 3300*9.8*depth ~ 3e4*depth100km 1e5*3e4=3e9 Pa = 3GPa
100 km
50 km
Phase transition due to eclogite formation, making the lithosphere heavy.
pressure
eclogite
depth
(Peacock and Wang, 1999 Science)
Eclogite
PM
Eclogite
Eclogite
PM
Eclogite
Low V
Low V
NE: Blueschist(dehydration) Eclogite110->160 km depthSW: Greenschist.amphibolite Ecolgite: 50-60 km depth
PM
Possible Cause for difference in low-velocity zone & arc volcanism
Solid line, top of subducting oceanic crustDashed line, base of subducting oceanic crust.
(Peacock et al., 2005 PEPI)
Another Case in the Middle America Trench
PM
Partial melting and dehydration at 60-120 km depth
Where is the fluid loaded? – A new aspect
• Loading of fluid in the oceanic crust is considered to occur mostly at spreading centers through vigorous hydrothermal circulation within a newly-formed, fractured basalt.
• Recently, another candidate is proposed for the fluid loading into the oceanic crust & upper mantle.
• The trench outer-rise area has normal faults, caused by the bending-triggered extension.
海溝外縁隆起帯 アウターライズ outer rise
海溝の海側にゆるやかな高まりが存在
Bending-related faulting and mantle serpentinization at the Middle America trench
(Ranero et al., 2003 Nature)https://sfb574.geomar.de/subproject-a5.html
1)Trench-outer rise seaward of the trench when the plate is forced to subduct
- to carry chemically-bound water into the deep subduction zone2) As fluids are released from the downgoing plate and infiltrate the
mantle wedge. - Seismogenesis as serpentines are weak; thus, they limit the size of the seismogenic megathrust fault and hence earthquake hazards.
Fault-induced seismic anisotropy by hydration in subducting oceanic plates (Faccenda et al., 2008Nature)
• Seismic anisotropy strongly depends on the preferred orientation of hydrated faults in the subductingoceanic plate.
• It originates from crystallographic preferred orientation of anisotropic hydrous minerals (serpentine / talc). (滑石=かっせき)
Schemtic diagram of tectonic/compositional structure of the slab and inferred S-wave splitting behaviour.(Faccenda et al., 2008Nature)
Oceanic plate hydration and dehydration model (Faccenda et al., 2012 G3)
• slab bending produces … subhydrostatic pressure gradients; … fluids are pumped … forming hydrated and serpentinized rocks in the slab.
• At intermediate-depths, slab dehydration during unbending favors fluid flow toward the inner portion of the slab … the formation of a double hydrated zone.
http://147.162.183.151/personal/faccenda-manuele