9. 沈み込み帯（2/3）：前弧の構造と多様性Subduction zones 2 of 3: Structure & diversity of forearcs

•先週の演習

• Fate of sediments• State of the subduction zones• Thermal structure / Stress State / Pore pressure• Clift論文の解説• ------------------------• Frontal wedge dynamics• Seismogenic zone of megathrust earthquakes• Arc magmatism• Recycling

海洋底ダイナミクス2018秋学期

Trench-Arc-Backarc system海溝ー島弧ー背弧系

海側 Seaward side陸側 Landward side

前 Fore-後 Back-

Fate of sediments entering the trench

• Incoming hemi-pelagic sediments• 海洋地殻の上に載って海溝軸に向かう，半遠洋性堆積物

• Trench wedge turbidite• 海溝軸付近で，もともと沈み込もうとしている堆積物（半遠洋性など）の上にwedge状に堆積した，タービダイト（混濁流堆積物；砂泥の級化構造を持つ）

• Accretion/erosion at the trench - Frontal thrusts / Subducting sediments• 付加・浸食 － 前縁断層・沈み込む堆積物

• Decollement 水平な断層面 滑り面• OOST(Out of sequence thrust) 順序外断層，in-sequence thrustに対する呼び名

• Accretion/erosion at the base – Underplating / basal erosion• （堆積物の）底付け作用 （付加体）底面の浸食

• Slope sediments / Forearc basin deposits• 斜面堆積物 前弧海盆堆積物

• Methane Hydrate メタンハイドレート

地震波速度構造からみた前弧の構造Structure of forearc portion of subduction zone

地球の内部構造 The Earth’s structure

海洋地殻：海嶺でマントルから分化化学組成Oceanic Crust

リソスフェア：温度により硬さが変化物性変化Lithosphere=Plate

地震波速度と物質（状態）の関係

地震波速度と物質（状態）の関係

Incoming sediments

• Pelagic clay• Siliceous mud• Terrigenous-volcaniclastic mud• Turbidites

(J. C. Moore et al., 2015 Geosphere)

turbidity current / turbidites

(Press et al., Understanding of Earth)

陸源堆積物が，台風や地震などの折に一気に堆積する一番下に砂，上方級化構造を持つ．“砂泥互層”

(Clift and Vannucchi, 2004)

海溝軸の堆積物の行く末：付加体 vs. 構造浸食Fate of incoming sediments: Accretion vs. Erosion

沈み込むプレート上の堆積層が厚い：

その上部は沈み込めずに陸側プレートに付加する（陸側が肥え太る）Thick sediment above subducting plate case:Shallow sediment is accreted to the landward plate.

堆積層が薄い：

沈み込むプレートが上盤を下から削り取る（陸側がやせ細る）Thin sediment case:Subducting plate erodes the hanging wall block.

Origin and fate of the continental crust

• magmatic productivity at mid-ocean ridges exceeds that in the magmatic arcs that are developed along convergent plate boundaries

• oceanic lithosphere is usually destroyed by subduction, while tectonic and geochemical evidence indicates that active margins are likely the principal source of the continental crust.

• subduction zones represent the only significant pathway by which continental material can be returned to the upper mantle.

• involvement of subducted sediments in the generation of arc magmas• tectonic processes may remove significant volumes of continental crust at

subduction zones and transport them to great depths in the Earth• Quantifying how much crust is subducted to the roots of volcanic arc systems,

and even back into the upper mantle, is important

• If significant volumes of crust are lost at modern convergent margins, then new crust must be generated at faster rates if the current volume of the continental crust is to be sustained.

Mass flux through the major subduction zones

• Convergent margins either accrete continental material delivered by the subducting plate or, to subduct the trench sediment pile and even erode the basement of the overriding forearc.

• Accretionary margins … oceanic and trench sedimentary rocks are off scraped• Oceanic and trench sediments are subducted & removed from the overriding

plate

• Drilling reveal that sediment accretion was by no means a ubiquitous feature of convergent margins

• volcaniclastic, volcanic, intrusive, and serpentinized/fresh mantle peridotite rocks from the trenches … no sediment accretion was occurring … losing parts of their crystalline basement.

• long-term landward retreat of the trench along the Peru and Honshu• even in accretionary active margins ~70% of the sediment is likely

subducted to great depths below the forearc

Accretion vs. erosion?

• in any given system both processes may be occurring, either switching through time or at the same time in different parts of the subduction zone

• It is common to find small accretionary complexes in the trench of subduction zones where tectonic erosion is dominating under the forearc (e.g., the Aleutians and Chile)

Accretionary vs. erosive margins

付加体が発達するのは、＿＿＿＿＿＿＿＿＿

場合である

The accretionary prism develops if the subducting plate has a thick sediment and is subducting slowly.

(Clift and Vannucchi, 2004)

付加体Accretionary complexAccretionary prism

• Horizontal compression to incoming sediments

• Hanging-wall of the weak decollement deforms

• Initial weak deformation as a short-wavelength Imbricate thrusts

• As sediments harden, larger Out-Of-Sequence Thrust develops, cutting the pre-existing thrusts

• OOST as a boundary between active (seaward) and inactive (landward) zones

付加体の構造 (室戸沖南海トラフの例）Domains of accretionary complex (Nankai Trough)

室戸沖南海トラフの変形開始の場所‘Deformation front’ of Nankai accretionary prism off Muroto

デコルマ： もぐりこむ海側プレートの上面（滑り面）Decollement as a slip interface 四国海盆プレート（玄武岩）の上面

Top of oceanic crust

圧縮に伴う断層（フロンタルスラストの形成） Frontal thrust

付加体形成の仕組みを理解することは、日本列島の生い立ちや、巨大地震発生の理解には不可欠であるUnderstanding how the accretionary prism develops is a key for creation of Japan land and megathrust earthquakes

3D Seismic volume off Muroto

(ODP Leg146 Initial Results)

北西太平洋オレゴン沖カスカディア付加体Cascadia accretionary prism off Oregon

カリブ海バルバドス付加体Barbados accretionary prism in Carribean Sea

(Maltman et al., 1997; SR156)

(156 Science Party,1995; IR156)

構造浸食Subduction Erosion / Tectonic Erosion

堆積層が薄い：沈み込むプレートが上盤を下から削り取る（陸側がやせ細る）Thin sediment case:Subducting plate erodes the hanging wall block.

コスタリカ沖中米海溝 浸食型縁辺Erosional margin off Costa Rica

Subduction-Erosion in the middle America Trench

(Von Huene et al., 2004 Geology)

沈み込む堆積層が薄いプレート境界で，上盤物質が削り取られるThin sediment above the subducting plateHanging-wall materials are eroded.

〇Basal erosion〇Surface erosion

• 堆積物が薄い（陸から遠い） Thin sediments

• 沈み込む太平洋プレートの表面が凸凹（ホルスト＆グラーベン構造）Subducting Pacific plate is fractured with horst-graben structure

• 上盤の先端部には付加体あり その後ろは白亜紀などの非常に古い地層Accretionary wedge seaward of backstopinterface

(Nakamura et al., 2014EPS)

日本海溝付近の地層断面Cross section across Japan Trench

（Kodaira et al., 2012)

日本海溝軸付近の詳細な構造Close-up section across Japan Tranch

• ホルスト・グラーベン構造がそのまま沈み込んでいる Horst & Graben structure starts subduction here

• その上の堆積層の厚さがH&Gに影響を受けているOverlying sediment thickness is obviously affected by H&G geometry

• 3.11東北地震で滑った「先端部」がここ．The very tip of ‘rupture to the trench’ at 3.11 Tohoku EQ

Slope sediment / Forearc Basin deposits• 前弧斜面のWedge堆積物の上に堆積したもの．

• Recently deposited on the forearc accretionary prism sediment

• Wedge堆積物が隆起ー浸食による再堆積したものと，陸源堆積物の2種類がある

• Two origins – reworked wedge (accretionary prism) sediments and terrigenous sediments

• MTDと呼ばれる大規模混濁流による堆積

• Set in place as Mass Transport Deposit, a large-scale turbidity flow

（Strasser et al., 2011 GC)

State of the subduction zones

• Thermal structure

• Stress State

• Pore pressure & fluid transport

熱伝導方程式による温度場の推定Thermal structure estimated from thermal conduction equation

• 浅部の温度場を規定する要因

ATTcvtTc +Δ=∇+

∂∂ λρρ

Heat Capacity Advection velocity

Thermalconductivity

Internal heat production

Advectiveterm

Conductiveterm

(Hyndman and Wang, 1995)

南海（室戸沖） 東南海（熊野沖） 日本海溝(Hamamoto et al., 2011) (Kimura et al., 2012)

沈み込むプレートの熱構造が重要な役割海洋地殻中の熱水循環の影響

State of the subduction zones

• Thermal structure

• Stress State

• Pore pressure & fluid transport

Stress Indicators (World Stress Map)

In the World Stress Map different types of stress indicators are used to determine the tectonic stress orientation. They are grouped into four categories:

• Earthquake focal mechanisms (断層滑りは，その場の応力場を反映する）

• Well bore breakouts and drilling-induced fractures（掘削時の孔の破壊状況から差応力の向きを実測）

• In-situ stress measurements (overcoring, hydraulic fracturing, borehole slotter)

• Young geologic data (from fault-slip analysis and volcanic vent alignments)（地層やコアの観察から応力場復元）

http://dc-app3-14.gfz-potsdam.de/pub/introduction/introduction_frame.html

• 覆瓦上スラストや順序外スラスト，どちらも逆断層のセンスなのだから，応力場も逆断層的であるべきだ

• Both the imbricate thrust & OOST should indicate a thrust-faulting stress regime….

付加体の構造 (室戸沖南海トラフの例）Domains of accretionary complex (Nankai Trough)

3 Principal stresses and earthquake (fault rupture)

Sv

Sv

正断層レジームSv > SHmax > Shmin

Sv

Sv

逆断層レジームSHmax > Shmin > Sv

• 浅部では が最大， 断層レジーム

• 深くなるに従い， が増加

• 水平主応力2成分のうち，プレート運動方向の成分が大きくなる（深度＋時間の関数） が最大， 断層レジームに．

• クーロン破壊条件を満たすときに逆断層＝地震発生．

Normal faulting

Strike-slipfaulting

Thrust faulting

Dep

th/O

verb

udre

n(S

v)Sh

ear s

tress

Normal stress

Sv (Overburden)

SHmax

Mohr’s circle

Shmin

Insitu stress of Nankai Forearc

In situ stresses (Sv, Sh, and SH) and pore pressure (Pp) at Site C0002.

The black solid curve represents the overburden stress (Sv) calculated from the density profile. The green line shows Sh, interpolated based on leak-off and stepped-rate injection test results (green squares). Hydrostatic pore pressure (λ* = 0) and pore pressure ratio of λ* = 0.5 are shown for reference (black dashed lines, as noted); the lithostatic pressure (λ* = 1; Pp = Sv) is represented by the black curve for Sv.

The annular pressure (AP) measured during drilling is also shown for reference. The AP of a nonriserhole (Hole C0002A) down to ~1,400 mbsf probably reflects the formation pressure, whereas the AP in Holes C0002F, C0002N, and C0002P arises from the controlled mud weight during riser drilling operation.

(Kitajima et al., 2017 GRL)

What controls the wedge shape –Critical Taper model

• “The critical taper is the shape for which the wedge is on the verge of failure under horizontal compression everywhere, including the basal decollement.” (Davis et al., 1983)

• The shape (dip of the slope and decollement) is basically controlled by the force balance: • If it is too steep (beyond the angle of repose 安息角), its slope sediment fails and slope

becomes gentler (gravitational instability)• If it is too gentle, tectonic compression (or coseismic rupture propagation) will generate the

fold & thrust structure, causing horizontal shortening and vertical uplift (and making the slope steeper)

NOTE: The critical taper is not just a single solution; it has some ‘play’ before it is readjusted.

Dynamic Coulomb Wedge Theory

(Wang and Hu, 2006)

Interseismic Slip/deformation

(Bilek and Lay, 2002)

When and how does this slip occur?What if it occurs coseismically?

State of the subduction zones: Summary

• Thermal structure• 沈み込むプレートの熱構造が重要な役割• 海洋地殻中の熱水循環の影響

• Stress State• 浅部：ノンテクトニック応力場（σ1=Sv；正断層レジーム）• 深部：Tectonic loadingによる水平圧縮卓越（σ1＝SH＝プレート運動方向）

• Pore pressure & fluid transport in accretionary prisms• 重力＋テクトニックによる圧密 →間隙率減少• 浅部：排水 → 静水圧を保つ• 深部：非排水発生 → 間隙水圧増加 →強度低下• 断層面の有効摩擦はかなり小さい → 地震時のせん断応力全開放も