Sedimentary Basins related to Volcanic Arcs

M08353 Basin AnalysisM08353 Basin Analysis

Reading - start with:

Reading, H.G.: Sedimentary EnvironmentsReading, H.G.: Sedimentary Environments

– 2nd edition. Tectonics & Sedimentation chapter by Mitchell & 2nd edition. Tectonics & Sedimentation chapter by Mitchell &

ReadingReading

– 3rd edition. Volcaniclastics chapter by Orton, p. 549-3rd edition. Volcaniclastics chapter by Orton, p. 549-

Volcanic arcs may develop...

within oceanic lithosphere, where ocean floor within oceanic lithosphere, where ocean floor subducts beneath ocean floor, and an subducts beneath ocean floor, and an island arcisland arc results, e.g. Lesser Antilles arcresults, e.g. Lesser Antilles arc

or at the edge of a continent, where oceanic or at the edge of a continent, where oceanic lithosphere subducts beneath continental lithosphere subducts beneath continental lithosphere, and a lithosphere, and a ccontinental margin magmatic arcontinental margin magmatic arc forms, e.g. Andesforms, e.g. Andes

Basins related to volcanic arcs

fore-arcfore-arc back-arcback-arc intra-arcintra-arc

All may be either submarine or All may be either submarine or subaerial, or may have marine & subaerial, or may have marine & subaerial parts subaerial parts

Much sediment is supplied from Much sediment is supplied from active arc. active arc.

Fore-arc basins

Lie in the Lie in the arc-trench gaparc-trench gap, between , between volcanic arc and submarine trenchvolcanic arc and submarine trench

range from small basins on range from small basins on trench trench slopeslope to large basins (50 to 100 km to large basins (50 to 100 km wide, and > 500 km long) with thick wide, and > 500 km long) with thick fills (several km)fills (several km)

Basins tend to become wider and Basins tend to become wider and shallower with time, partly because of shallower with time, partly because of accretionaccretion at trenches at trenches

fore-arc basins

Sediment sources:Sediment sources: volcanic arcvolcanic arc outer arcouter arc longitudinally fromlongitudinally from

a continenta continent

Tectonic style varies:Tectonic style varies: compressionalcompressional extensionalextensional strike-slipstrike-slip

Back-arc basins

lie behind the magmatic arclie behind the magmatic arc often the site of extension & thinning of crustoften the site of extension & thinning of crust may overlie either ocean or continental crustmay overlie either ocean or continental crust oceanic back-arc basins are eventually oceanic back-arc basins are eventually

subducted and destroyed, or preserved in subducted and destroyed, or preserved in thrust complexes related to ocean closure.thrust complexes related to ocean closure.

back-arc basins on continental crust - more back-arc basins on continental crust - more varied facies, because of terrigenous input; varied facies, because of terrigenous input; higher preservation potential.higher preservation potential.

Intra-arc basins

Sedimentary basins within magmatic arcs, Sedimentary basins within magmatic arcs, between volcanoes, or between older and between volcanoes, or between older and younger belts of the arcyounger belts of the arc

Some are fault-bounded and subside rapidly. Some are fault-bounded and subside rapidly. Faulting due to extension within arc, or Faulting due to extension within arc, or flexure of lithosphere due to weight of flexure of lithosphere due to weight of volcano. volcano.

With time, position of the arc migrates, and With time, position of the arc migrates, and basins may change between intra-arc, back-basins may change between intra-arc, back-arc and fore-arc.arc and fore-arc.

Sediment supply and transport

Sediment supply varies according to volcano Sediment supply varies according to volcano behaviour, governed by behaviour, governed by magma viscositymagma viscosity and and gas contentgas content..

In deep water, explosive activity is suppressed In deep water, explosive activity is suppressed by by hydrostatic pressurehydrostatic pressure..

More silicic magmas in more evolved arcs - More silicic magmas in more evolved arcs - therefore greater explosive activity, more supply therefore greater explosive activity, more supply of pyroclastic sediment.of pyroclastic sediment.

Sediment transport and deposition is controlled by:

topography - both subaerial and topography - both subaerial and submarinesubmarine

volcanic processes, especially eruption volcanic processes, especially eruption column height, direction of pyroclastic column height, direction of pyroclastic flowsflows

sediment transport systems - e.g. rivers, sediment transport systems - e.g. rivers, prevailing windsprevailing winds

Subduction Zones

Subduction zones

also termed convergent or consuming plate marginsalso termed convergent or consuming plate margins occur where adjacent plates move toward each other occur where adjacent plates move toward each other

and relative motion is accommodated by one plate and relative motion is accommodated by one plate over-riding the other. over-riding the other.

These zones are classified as either oceanic or These zones are classified as either oceanic or subcontinental, depending on the overriding plate.subcontinental, depending on the overriding plate.

If the "subducting" plate is continental, subduction If the "subducting" plate is continental, subduction will cease and a mountain belt will form within a will cease and a mountain belt will form within a collision zone.collision zone.

Where do subduction zones occur?

along the "Ring of Fire" around the Pacific Ocean. along the "Ring of Fire" around the Pacific Ocean. Two short subduction zones occur at the Lesser Two short subduction zones occur at the Lesser

Antilles, at the eastern side of the Carribean plate Antilles, at the eastern side of the Carribean plate and the South Sandwich Island plate. and the South Sandwich Island plate.

Three short segments of the Alpine Himalayan Three short segments of the Alpine Himalayan system involve subduction of oceanic lithosphere.system involve subduction of oceanic lithosphere.

– the Calabrian and Aegean boundaries in the Mediterranean the Calabrian and Aegean boundaries in the Mediterranean SeaSea

– Makran boundary along the SW boundary of the Iran plate.Makran boundary along the SW boundary of the Iran plate.

Physiography

Outer SwellOuter Swell Outer Trench WallOuter Trench Wall TrenchTrench Forearc Forearc (Arc-Trench Gap) (Arc-Trench Gap) Volcanic Arc Volcanic Arc Back-ArcBack-Arc

Physiography 2

Outer swellOuter swell– Low topographic bulge (a few hundred meters of relief) Low topographic bulge (a few hundred meters of relief) – develops just outboard of where subducting plate develops just outboard of where subducting plate

bends down into the mantle. bends down into the mantle.

Outer Trench Wall Outer Trench Wall – Slope on ocean floor between the outer swell and the Slope on ocean floor between the outer swell and the

trench floor. trench floor. – Slope dip is typically -5 degrees Slope dip is typically -5 degrees

TrenchTrench– DeepDeep valley that develops at the plate boundary. valley that develops at the plate boundary. – Continuous for 1000s of km Continuous for 1000s of km – typically 10 - 15 km deep (5 - 10 km below typically 10 - 15 km deep (5 - 10 km below

surrounding ocean floor.)surrounding ocean floor.)

Forearc (Arc-Trench Gap) – Consists of region between trench and the arc. Consists of region between trench and the arc. – steep steep inner trench wall inner trench wall (lower trench slope)(lower trench slope)

dips of - 10 degdips of - 10 deg– flattens into a gentle slope termed the flattens into a gentle slope termed the forearc basin forearc basin (upper (upper

trench slope). trench slope). – The inner trench wall is usually separated from the forearc by The inner trench wall is usually separated from the forearc by

the the outer ridge. outer ridge. – The accretionary prism underlies the inner trench wall, the The accretionary prism underlies the inner trench wall, the

outer ridge and part of the forearc basin. outer ridge and part of the forearc basin.

Volcanic Arc

Active Active arc built on a topographically high region arc built on a topographically high region of older rocks, the arc basementof older rocks, the arc basement

may be a shallow marine platform or an may be a shallow marine platform or an emergent region of older rocks. emergent region of older rocks.

In continental arcs, the basement is continental In continental arcs, the basement is continental crust standing a few kms above sea level.crust standing a few kms above sea level.

Volcanoes in island arcs are usually 1 - 2 km Volcanoes in island arcs are usually 1 - 2 km above sea level. Volcano elevation in above sea level. Volcano elevation in continental arcs is strongly influenced by continental arcs is strongly influenced by continental crust thickness.continental crust thickness.

Back-Arc AreaArea behind the volcanic arc. behind the volcanic arc. In island arcs this region consists of basins with In island arcs this region consists of basins with

oceanic crustal structure and abyssal water oceanic crustal structure and abyssal water depths. depths.

Sometimes remnant arcs are preserved behind Sometimes remnant arcs are preserved behind the island arcs. the island arcs.

On continents this region is the continental On continents this region is the continental platform which may be subaerially exposed, or platform which may be subaerially exposed, or the site of a shallow marine basin. the site of a shallow marine basin.

Gravity Typically, similar free-air gravity profilesTypically, similar free-air gravity profiles

– 50 mGal gravity high associated with the outer bulge50 mGal gravity high associated with the outer bulge

– 200 mGal low associated with the trench and accretionary prism 200 mGal low associated with the trench and accretionary prism

– 200 mGal high associated with the arc.200 mGal high associated with the arc.

Isostatic anomalies have the same polarity as the free-air Isostatic anomalies have the same polarity as the free-air gravitygravity

Suggests that the gravity anomalies are caused by the Suggests that the gravity anomalies are caused by the dynamic equilibrium imposed by the system by compression. dynamic equilibrium imposed by the system by compression.

Compressional forces cause the trench to be deeper and the Compressional forces cause the trench to be deeper and the arc to have less of a root than they would be if only isostatic arc to have less of a root than they would be if only isostatic forces were at work.forces were at work.

Structure from Earthquakes

Subduction zones are characterized by Subduction zones are characterized by dipping seismic zones termed Benioff zones dipping seismic zones termed Benioff zones or Wadati-Benioff zonesor Wadati-Benioff zones

Result from deformation of the downgoing Result from deformation of the downgoing lithospheric slab. The zones have dips lithospheric slab. The zones have dips ranging from 40 to 60 degranging from 40 to 60 deg

Because, the slab is colder and more dense Because, the slab is colder and more dense than surrounding asthenosphere, it's than surrounding asthenosphere, it's position can be mapped seismically as high position can be mapped seismically as high velocity anomalies and as high "Q" (little velocity anomalies and as high "Q" (little attenuation of seismic waves) zones in the attenuation of seismic waves) zones in the mantle. High Q, and high velocity are mantle. High Q, and high velocity are thought to correspond to relatively high thought to correspond to relatively high density, cold materialdensity, cold material

earthquake hypocenters related to their position within

the slab Shallow depthsShallow depths predominantly thrust faults within the upper predominantly thrust faults within the upper

part of the downgoing plate or in the adjacent part of the downgoing plate or in the adjacent overriding plate. overriding plate.

Down to depths of 400 km, down-dip extension.Down to depths of 400 km, down-dip extension.– In some slabs, down-dip extension is found in the upper In some slabs, down-dip extension is found in the upper

part of the slab, accompanied by down-dip compression part of the slab, accompanied by down-dip compression at the base of the slab. The extension probably results at the base of the slab. The extension probably results from the lithosphere being pulled into the mantle by the from the lithosphere being pulled into the mantle by the weight of the downgoing portion. weight of the downgoing portion.

Deep slabs usually show down-dip compressionDeep slabs usually show down-dip compression– may result from increased viscous resistance at depth. may result from increased viscous resistance at depth. – deeper part of the slab will feel a push from the weight of the shallower deeper part of the slab will feel a push from the weight of the shallower

portion of the slab. portion of the slab.

Between 70 - 300 km, faulting may occur due to dehydration Between 70 - 300 km, faulting may occur due to dehydration of serpentinite. of serpentinite.

From 300 - 700 krn may also be due to the sudden phase From 300 - 700 krn may also be due to the sudden phase change of olivine to spinel which may be accommodated by change of olivine to spinel which may be accommodated by rapid shearing of the crystal lattice along planes on which rapid shearing of the crystal lattice along planes on which minute spinel crystals have grown. minute spinel crystals have grown.

Structural Geology- Trenches

Trenches normally contain flat-lying Trenches normally contain flat-lying turbidites deposited by currents flowing turbidites deposited by currents flowing down into the trench from the overriding down into the trench from the overriding plate or along the axis of the trench. The plate or along the axis of the trench. The outer swell is probably caused by elastic outer swell is probably caused by elastic bending of the subducting plate.bending of the subducting plate.

Forearc

may be underlain either by the accretionary prism may be underlain either by the accretionary prism or arc basement rocks covered by a thin veneer of or arc basement rocks covered by a thin veneer of sediments or both. sediments or both.

Where there is little sediment accumulation on the Where there is little sediment accumulation on the subducting plate, island arc or continental basement subducting plate, island arc or continental basement may extend all the way to the lower trench slope and may extend all the way to the lower trench slope and little or no accretionary prism may occur. little or no accretionary prism may occur.

Forearc basement may draped by a thin veneer of Forearc basement may draped by a thin veneer of sediment, and is commonly cut by normal faults sediment, and is commonly cut by normal faults toward the trench.toward the trench.

Accretionary Prism

wedge of deformed sedimentary rocks wedge of deformed sedimentary rocks the main locus of crustal deformation the main locus of crustal deformation Rocks are typically cut by numerous imbricate Rocks are typically cut by numerous imbricate

thrust faults that dip in the same direction as thrust faults that dip in the same direction as the subduction zone. the subduction zone.

As more material is added to the toe of the As more material is added to the toe of the wedge, the thrusts are moved upwards and wedge, the thrusts are moved upwards and rotate arcwards. rotate arcwards.

Rocks within the accretionary prism are derived Rocks within the accretionary prism are derived from the downgoing and/or overriding plates.from the downgoing and/or overriding plates.

Accretionary Prism

At the toe of the wedge, sediments are At the toe of the wedge, sediments are added thru added thru offscraping offscraping

propagation propagation of the basal thrust into of the basal thrust into undeformed sediments on the undeformed sediments on the subducting plate. subducting plate.

This process results in progressive This process results in progressive widening of the wedge, and eventually a widening of the wedge, and eventually a decrease in dip on the subduction zone.decrease in dip on the subduction zone.

Accretionary Prism

When sediments on the downgoing plate are When sediments on the downgoing plate are subducted without being disturbed they can subducted without being disturbed they can still be added to the prism thru still be added to the prism thru underplatingunderplating

propagation propagation of the basal thrust into the of the basal thrust into the downgoing undeformed sediments to form a downgoing undeformed sediments to form a duplex beneath the main part of the prism.duplex beneath the main part of the prism.

Subduction Erosion

erosion erosion and subsequent subduction of and subsequent subduction of rocks from the toe of the prismrocks from the toe of the prism..

Sediment on the subducting plate that is not Sediment on the subducting plate that is not added to the overriding plate thru these added to the overriding plate thru these processes may descend into the mantle and processes may descend into the mantle and contribute to the generation of arc magmas.contribute to the generation of arc magmas.

Forearc Basin

Wide sedimentary basin Wide sedimentary basin – develops above irregular basement on the upper part of the arc-trench develops above irregular basement on the upper part of the arc-trench

gap. gap.

– Sediments from the active arc or arc basement rocksSediments from the active arc or arc basement rocks deposited by turbidity currents traveling along the basin axis or deposited by turbidity currents traveling along the basin axis or

perpendicular to the arc.perpendicular to the arc.

asymmetric basin asymmetric basin – inner part of the upper slope basin subsides inner part of the upper slope basin subsides

– outer edges rises due to accretion at the toe of the wedge.outer edges rises due to accretion at the toe of the wedge.

high-P, low-T metamorphismhigh-P, low-T metamorphism– increases in grade toward the inner forearc region increases in grade toward the inner forearc region

– in the direction of subductionin the direction of subduction

Arc Arc basement Arc basement

– older more deformed and metamorphosed rocks in platform on which older more deformed and metamorphosed rocks in platform on which the modem arc is built. the modem arc is built.

– oceanic rocksoceanic rocks

– On the continents, complex continental basement.On the continents, complex continental basement.

Volcanic arcVolcanic arc– a chain of largely andesitic stratovolcanoes spaced at fairly regular a chain of largely andesitic stratovolcanoes spaced at fairly regular

intervals of 70 km. intervals of 70 km.

– The structural environment of these arcs is commonly extensional The structural environment of these arcs is commonly extensional (numerous normal faults)(numerous normal faults)

– volcanoes in grabens termed volcanic depressions.volcanoes in grabens termed volcanic depressions.

– underlain by large plutonic bodies (e. g. the Sierra Nevada).underlain by large plutonic bodies (e. g. the Sierra Nevada).

Arcs

Metamorphism Metamorphism – common and suggest a high geothermal gradient. common and suggest a high geothermal gradient.

– Much of the lower crust may be at the melting Much of the lower crust may be at the melting temperature of granite.temperature of granite.

Sediments Sediments – debris from active volcanoes. debris from active volcanoes.

– deposited as turbidites. deposited as turbidites.

– In tropics, settings these volcanogenic sediments may In tropics, settings these volcanogenic sediments may interfinger with carbonate reefs. interfinger with carbonate reefs.

– In continental arcs, sediments are often deposited subaerially.In continental arcs, sediments are often deposited subaerially.

Back-arc

extensional tectonics and subsidence. extensional tectonics and subsidence. In oceans arc-derived sediments are In oceans arc-derived sediments are

deposited in an ocean basin behind the arc deposited in an ocean basin behind the arc termed the termed the back-arc basin. back-arc basin.

In continents, sediments are deposited into In continents, sediments are deposited into basins on the continental platform and are basins on the continental platform and are termed termed foreland basins foreland basins or or retro-arc basins.retro-arc basins.

Foreland Fold and Thrust Belts

Relation between foreland fold and thrust belts Relation between foreland fold and thrust belts and subduction not understoodand subduction not understood

not all continental arcs display these features. not all continental arcs display these features. Possible explanations if there is a relationPossible explanations if there is a relation

– Thrust belt caused by compression at margin of Thrust belt caused by compression at margin of overriding plate due to subduction of hot, buoyant overriding plate due to subduction of hot, buoyant lithosphere. lithosphere.

– Thrust belt associated with shallow dip of a downgoing Thrust belt associated with shallow dip of a downgoing slab.slab.

– Thrust belt associated with subduction of an aseismic ridge.Thrust belt associated with subduction of an aseismic ridge.

Models of thermal processes in subduction zones

Rate of Subduction Rate of Subduction – The The faster the descent of the slab, the less time it has to faster the descent of the slab, the less time it has to

absorb heat from the mantle. absorb heat from the mantle.

Slab Thickness Slab Thickness – TheThe thicker the descending slab, the more time it takes thicker the descending slab, the more time it takes

to come into equilibrium with the surrounding to come into equilibrium with the surrounding lithosphere. lithosphere.

Frictional HeatingFrictional Heating– occursoccurs at top of slab due to friction as slab descends at top of slab due to friction as slab descends

and is resisted by the lithosphere. and is resisted by the lithosphere.

Conduction Conduction – heat into slab from the asthenosphere heat into slab from the asthenosphere

Adiabatic Heating Adiabatic Heating – associatedassociated with compression of slab with increased pressure at depth. with compression of slab with increased pressure at depth.

Heat of Radioactive Decay Heat of Radioactive Decay – decaydecay of radioactive minerals in the oceanic crust (minor) of radioactive minerals in the oceanic crust (minor)

Latent Heat of Mineral Phase Transitions Latent Heat of Mineral Phase Transitions – olivine-spinel transition at 400 km is exothermic. Spinel-oxide transition at olivine-spinel transition at 400 km is exothermic. Spinel-oxide transition at

670 km could be either exothermic or endothermic. 670 km could be either exothermic or endothermic.

All thermal models show that the All thermal models show that the downgoing slab maintains its thermal downgoing slab maintains its thermal identity to great depths (e. g. contrasts identity to great depths (e. g. contrasts of 700 deg C can still exist at 700 krn of 700 deg C can still exist at 700 krn depth).depth).

If the slab is so cold, how do we get enough heating to cause arc

magmatism?

Melting of Slab in Presence of Water Melting of Slab in Presence of Water – Partial Partial melting may take place at lower temperatures melting may take place at lower temperatures

due to presence of water as slab dehydrates. Water is due to presence of water as slab dehydrates. Water is released by transition of amphibolite to ecologite, and released by transition of amphibolite to ecologite, and dehydration of serpentinite at depths of - 100 km. dehydration of serpentinite at depths of - 100 km.

Corner Flow and Melting of Mantle Corner Flow and Melting of Mantle – DDowngoing slab may cause flow of hot mantle into the owngoing slab may cause flow of hot mantle into the

comer of the overriding mantle where it impinges on the comer of the overriding mantle where it impinges on the downgoing slab. This may provide enough heat to downgoing slab. This may provide enough heat to cause melting. cause melting.

Origins of back-arc basins Entrapment of previous oceanic crustEntrapment of previous oceanic crust

– Change Change of plate motion may lead to abandonment of a fragment of of plate motion may lead to abandonment of a fragment of oceanic crust behind the arc. (e.g., Aleutian Basinoceanic crust behind the arc. (e.g., Aleutian Basin and West and West Philippines Basin )Philippines Basin )

Formation of new crust - behind Formation of new crust - behind the arc. 3 modelsthe arc. 3 modelso Spreading caused by forceable injection of a diapir rising from the Spreading caused by forceable injection of a diapir rising from the

downgoing slab. downgoing slab.

o Spreading induced in the overriding plate by the viscous drag in the Spreading induced in the overriding plate by the viscous drag in the mantle wedge caused by the motionmantle wedge caused by the motion of the downgoing plate (comer of the downgoing plate (comer flow). flow).

o Spreading induced by the relative drift of the overriding plate away Spreading induced by the relative drift of the overriding plate away from the downgoing slab (slab fixed with respect to mantle). This is from the downgoing slab (slab fixed with respect to mantle). This is also termed also termed roll-back.roll-back.

Ocean-ocean plate convergent boundary.

Structure of a continent-ocean convergent boundary.

Continent-continent collision.

Mid-ocean ridge divergent boundary showing transform

faults.

Two stages of an opening rift.

slab-pull due to its greater density

Reconstruction @ 250 million years ago

Concept of a Basin

Three dimensional architecture of basin fill. Three dimensional architecture of basin fill. Affected by spatial and temporal pattern of Affected by spatial and temporal pattern of

tectonic subsidence: tectonic subsidence: – Lithospheric deformation process. Lithospheric deformation process. – Three basic causes of subsidence: Three basic causes of subsidence:

Loading and flexure (like an elastic plate). Loading and flexure (like an elastic plate). Thermal and density changes - isostasy. Thermal and density changes - isostasy. Faulting - isostasy. Faulting - isostasy.

Sea level changes. Sea level changes. Sediment supply rates and source position Sediment supply rates and source position

(drainage basin outlets). (drainage basin outlets).

Basin Classification

Extensional:Extensional: – Half graben/graben - Rift Basin, e.g. Basin and Range, Half graben/graben - Rift Basin, e.g. Basin and Range,

U.S.A. U.S.A. – Mature oceanic speading - e.g. Atlantic margin (Passive Mature oceanic speading - e.g. Atlantic margin (Passive

Margin). Margin). – Syn-rift, post-rift megasequences. Syn-rift, post-rift megasequences.

Compressional:Compressional: – Foreland basin - flexural loading of the Earth's Foreland basin - flexural loading of the Earth's

lithosphere. lithosphere. – Two types: Two types:

Collisional, e.g. Himalaya. Collisional, e.g. Himalaya. Back arc, e.g. Andes/Precordillera. Back arc, e.g. Andes/Precordillera.

– Piggy-back basin (thrust sheet top basin). Piggy-back basin (thrust sheet top basin). Strike-Slip Basins:Strike-Slip Basins:

– E.g. Dead Sea, Israel. E.g. Dead Sea, Israel.

Basin Classification Cont.

Passive Margins:Passive Margins: – E.g. Atlantic Margin. E.g. Atlantic Margin.

Subduction Related:Subduction Related: – Oceanic trench, e.g. Marianas Trench. Oceanic trench, e.g. Marianas Trench. – Fore-arc basin, e.g Taiwan or Median Valley in Scotland. Fore-arc basin, e.g Taiwan or Median Valley in Scotland. – Back-arc basin, e.g. Sea of Japan. Back-arc basin, e.g. Sea of Japan.

Cratonic "sag" Basins:Cratonic "sag" Basins: – E.g. Chad Basin, Africa. E.g. Chad Basin, Africa.

Abyssal Plains.Abyssal Plains. Predictive models of facies distributions:Predictive models of facies distributions:

useful for subsurface exploration of oil or useful for subsurface exploration of oil or understanding dispersal of pollutants. understanding dispersal of pollutants.

Benioff Zones

Earthquakes occur at shallow, intermediate Earthquakes occur at shallow, intermediate and deep levels beneath subduction zonesand deep levels beneath subduction zones

The earthquakes define a plane which The earthquakes define a plane which begins at the trench and dips at about 45° begins at the trench and dips at about 45° beneath the arcbeneath the arc

This dipping plane of earthquake foci is This dipping plane of earthquake foci is called the Benioff Zonecalled the Benioff Zone

The Benioff Zone follows the upper part of The Benioff Zone follows the upper part of the descending oceanic platethe descending oceanic plate

Shallow earthquakes also occur through Shallow earthquakes also occur through the arcthe arc

Island Arcs

Island arcs are of chains of volcanically active Island arcs are of chains of volcanically active islands arranged in a curved arcislands arranged in a curved arc

An ocean trench occurs on the oceanwards sideAn ocean trench occurs on the oceanwards side Island arcs first develop on oceanic crustIsland arcs first develop on oceanic crust The crustal thickness in an arc is intermediate The crustal thickness in an arc is intermediate

between oceanic and continentalbetween oceanic and continental Volcanic activity begins abruptly at a Volcanic Front Volcanic activity begins abruptly at a Volcanic Front

about 200 - 300 km in from the trenchabout 200 - 300 km in from the trench The volcanic front and trench are separated by an The volcanic front and trench are separated by an

Arc-Trench gap with no volcanismArc-Trench gap with no volcanism

Island Arc Volcanism

Volcanic rocks in island arcs are mostly of Volcanic rocks in island arcs are mostly of andesitic compositionandesitic composition

The magmas originate mostly by partial The magmas originate mostly by partial melting of subducted oceanic crust and melting of subducted oceanic crust and overlying mantleoverlying mantle

Melting begins when the slab descends to Melting begins when the slab descends to about 100 km depth, forming the volcanic about 100 km depth, forming the volcanic front Partial melting of basaltic ocean crustfront Partial melting of basaltic ocean crust

Rising magmas Volcanic eruptionsRising magmas Volcanic eruptions

Chemical Differentiation

Mid-Ocean Ridge: Mid-Ocean Ridge:

Partial melting of Mantle basalt magmaPartial melting of Mantle basalt magma Subduction Zone:Subduction Zone:

Partial melting of Basalt crust andesite Partial melting of Basalt crust andesite magmamagma

Mature Arcs:Mature Arcs:

Partial melting of Andesite crust rhyolite Partial melting of Andesite crust rhyolite magmamagma

All of this is an irreversible chemical differentiation All of this is an irreversible chemical differentiation of the mantle in several stagesof the mantle in several stages

Continental crust grows by accumulation of Continental crust grows by accumulation of increasingly silica-rich rocksincreasingly silica-rich rocks

Ocean trench Sedimentation

Unconsolidated sediment from the ocean floor is Unconsolidated sediment from the ocean floor is scraped off the descending plate at the trenchscraped off the descending plate at the trench

Slices of the oceanic crust may be included as Slices of the oceanic crust may be included as ophiolite beltsophiolite belts

These rocks form a complex rock mass called an These rocks form a complex rock mass called an Accretionary WedgeAccretionary Wedge

The Accretionary Wedge is buckled upwards as The Accretionary Wedge is buckled upwards as new material is pushed beneath its basenew material is pushed beneath its base

The chaotic jumble of rocks in the Accretionary The chaotic jumble of rocks in the Accretionary wedge is called a Tectonic Mélange Accretionary wedge is called a Tectonic Mélange Accretionary WedgeWedge

Metamorphic Rocks and Subduction

High Temp - Low PressureHigh Temp - Low Pressure MetamorphismMetamorphism Occurs in the core of volcanic arcsOccurs in the core of volcanic arcs

– Abnormal heating of the crustAbnormal heating of the crust– thermal effects of subduction-related magmatismthermal effects of subduction-related magmatism

High Pressure-Low TempHigh Pressure-Low Temp MetamorphismMetamorphism

– occurs in the accretionary wedgeoccurs in the accretionary wedge– cold rocks are dragged to great depths and then cold rocks are dragged to great depths and then

upthrust again Granitesupthrust again Granites

Basin Analysis

Basins Basins – Topographically low places where sedimentaryTopographically low places where sedimentary– materials accumulate.materials accumulate.– Basins are characterized by accomodation space.Basins are characterized by accomodation space.– Accomodation is created by…Accomodation is created by…– Eustatic sea-level riseEustatic sea-level rise– SubsidenceSubsidence

Basin Analysis

Basin analysis is the detailed investigation of Basin analysis is the detailed investigation of the processes thatthe processes that

– Form basinsForm basins– Fill basinsFill basins– Alter basinsAlter basins– Uplift (invert) basinsUplift (invert) basins– Destroy basinsDestroy basins– Requires sedimentology, stratigraphy, hydrogeology, Requires sedimentology, stratigraphy, hydrogeology,

petroleum geology, seismology, geophysics, petroleum geology, seismology, geophysics, geochemistry, paleontology, etc.geochemistry, paleontology, etc.

Types of Basins

IntracratonicIntracratonic Rift relatedRift related Strike-Slip relatedStrike-Slip related Collision / Subduction relatedCollision / Subduction related

__Intracratonic – Basins that form within Intracratonic – Basins that form within continental crust continental crust __Intracratonic – Basins Intracratonic – Basins that form within continental crustthat form within continental crust

Rifts – Divergent plate boundaries that Rifts – Divergent plate boundaries that eventually develop into spreading centers.eventually develop into spreading centers.

Rifts – Divergent plate boundaries that Rifts – Divergent plate boundaries that eventually develop into spreading centers.eventually develop into spreading centers.

__Initial rift sediments are arkosic Initial rift sediments are arkosic sandstones interbedded with basalts.sandstones interbedded with basalts.

__Rifts then flood and deposit a thick Rifts then flood and deposit a thick sequence of evaporites.sequence of evaporites.

__Then marine sedimentationtakes over.Then marine sedimentationtakes over.

The Tonga Arc - Lau Back-arc Basin SystemThe Tonga Arc - Lau Back-arc Basin System

CLSCCLSC: central Lau spreading center: central Lau spreading center ELSCELSC: east Lau spreading center: east Lau spreading center VFRVFR: Valu Fa ridge (ELSC: Valu Fa ridge (ELSC の南のセグメの南のセグメ

ントント ))

(ETZ: extensional transform zone)

ELSC & ELSC & Zone 3Zone 3

ELSC火山フロントからの距離 : 60-110 km拡大速度 : 60-95 mm/yr ，深さ : 〜 2,000-3,000

maMBA が北に向かって上昇→地殻が薄くなる and/

or 密度上昇（安山岩から玄武岩へ）拡大速度は速いのにマグマ溜まりの反射なし中央海嶺に比べて非常に微量元素に枯渇した組成で

あり，島弧の特徴は VFR ほど顕著ではない

Zone 3地殻の厚さ : Zone2 より 2-3.5 km 薄い水深 :zone2 より 1 km 深いaMBA の高いところZone2 とは対照的な凹凸の少ない線上の

Abyssal hills

Decreased Magma Supply

Character and Position of Spreading AxesCharacter and Position of Spreading Axes

拡大軸が，マントルウェッジの端（水に富ん拡大軸が，マントルウェッジの端（水に富んだ枯渇したマントル）にあると だ枯渇したマントル）にあると [VFR[VFR の場の場合合 ]]

– プレートの発散によってマントルが上昇し更に溶融プレートの発散によってマントルが上昇し更に溶融– 火山フロントの火成活動によってできたメルトを引き寄火山フロントの火成活動によってできたメルトを引き寄

せる（島弧の特徴）せる（島弧の特徴）

火山フロントとの距離が遠いと 火山フロントとの距離が遠いと [ELSC[ELSC の場の場合合 ]]

– 火山フロントの影響は減少する火山フロントの影響は減少する– 起源マントルは，過去の拡大と火山フロントの活動によ起源マントルは，過去の拡大と火山フロントの活動によ

り枯渇度がさらに進んでいるので，生成されうるメルトり枯渇度がさらに進んでいるので，生成されうるメルトの量が減少（深く薄い地殻・高いの量が減少（深く薄い地殻・高い aMBAaMBA ・高い枯渇度）・高い枯渇度）

充分に火山フロントから遠いと 充分に火山フロントから遠いと [CLSC[CLSC の場の場合合 ]]

– 太平洋プレートのロールバックを埋める形で肥沃なマン太平洋プレートのロールバックを埋める形で肥沃なマントルが流移し，起源マントルとなるトルが流移し，起源マントルとなる

– 典型的な高速拡大典型的な高速拡大

ELSC の下のマントルが溶けにくくなって， CLSC が現在の位置にある（移動した？）と推測される

Basins & basin types

BasinBasin: a region of depressed crust, typically with : a region of depressed crust, typically with greater thicknesses of sediment accumulation greater thicknesses of sediment accumulation than surrounding regions. than surrounding regions.

– Basins form by tectonic processes that cause Basins form by tectonic processes that cause the crust to subside, and so to create large the crust to subside, and so to create large amounts of accommodation space. Rapid A-amounts of accommodation space. Rapid A-space growth translates into abundant and well-space growth translates into abundant and well-preserved organic matter within the sediment. preserved organic matter within the sediment.

– The abundance of organic matter combined The abundance of organic matter combined with the low grade burial metamorphism that with the low grade burial metamorphism that sediments in thick accumulation experience sediments in thick accumulation experience translate into major oil and gas accumulations. translate into major oil and gas accumulations.

– Basins provide the most complete successions Basins provide the most complete successions of the geological record.of the geological record.

Tectonic setting for the majority of basins

Constructive (spreading) margins: This setting produces a genetic series of basins from localized rift-related basins through narrow oceans (Red Sea phase) to voluminous passive margin basins.

Destructive (convergent) margins: This setting produces basins associated with subduction zones (foredeep basins), intra-arc spreading basins, back-arc spreading basins (Sea of Japan, Great Basin), and back-arc thrust (foreland) basins.

Transform (strike-slip) margins: mostly localized pull-apart basins such as the Gulf of California and Dead Sea. Grades into rift-type and thrust-type tectonics where relative plate motion is not fully strike-slip (i.e., transtensional and transpressional regimes).

Intra-cratonic basins. Fairly mysterious, concentrically subsiding basins formed on continental crust well away from plate boundaries. The cause of subsidence is unknown, but is generally considered to involve some combination of density-driven stress and in-plane stress.

Rifting and passive margin

Continental rifting begins with dome-formation. Produced by hot-spot volcanism (bimodal: dominantly basaltic plus some rhyolite from partial melting of the crust by basaltic magma and decompression melting).

Much extension by normal-faulting. Individual domes link up to form a more or less continuous series of rift valleys (grabens, as in the East African Rift zone). Extension results in thinning of the upper crust. The lower crust thins by ductile flow.

High heat flow produces thermal uplift; uplift leads to further thinning of the upper crust by erosion.

Continued extension produces oceanic crust between the newly formed, thinned continental margins. Eventually (over 107 years), sea floor spreading and thermal subsidence yields a distinct mid ocean ridge, open communication with the oceans (Red Sea phase) to form a permanent seaway.

Continued seafloor spreading carries the new continental margins away from the active tectonic zone. The much-extended crust cools and subsides over the next 108 yrs. as a passive continental margin ("passive margin," for short).

Rift basins.

Smallish volumes of sediment (100’s to a few Smallish volumes of sediment (100’s to a few 1000 m of accommodation space) in 1000 m of accommodation space) in localized, fault-bounded basins. localized, fault-bounded basins.

Dominantly terrestrial (subaerial and Dominantly terrestrial (subaerial and lacustrine) and arid as result of uplifted lacustrine) and arid as result of uplifted margins that cast rain-shadows into the margins that cast rain-shadows into the basins as well as their small drainage basins as well as their small drainage capture area. capture area.

Early rift sediments typically heterogeneous Early rift sediments typically heterogeneous with basement clasts and volcanics with basement clasts and volcanics dominant. Grains with low physical and dominant. Grains with low physical and chemical maturity. Late rift sediments often chemical maturity. Late rift sediments often include extensive evaporites from episodic include extensive evaporites from episodic oceanic invasions (reflects variable rates of oceanic invasions (reflects variable rates of subsidence and sea level rise and fall). subsidence and sea level rise and fall). Remobilized salt becomes important in Remobilized salt becomes important in passive margin history later (see below). passive margin history later (see below).

Passive margin basins.

Lots of accommodation space (10,000 to 15,000 m or Lots of accommodation space (10,000 to 15,000 m or more) from combination of thermal and sediment load-more) from combination of thermal and sediment load-driven subsidence. driven subsidence.

As oceans widen, marine conditions come to dominate. As oceans widen, marine conditions come to dominate. Early phases often still arid and dominated by carbonate Early phases often still arid and dominated by carbonate deposition and fringing reefs along the shelf edge (at low deposition and fringing reefs along the shelf edge (at low latitude sites) latitude sites)

With continued subsidence the fringing mountains With continued subsidence the fringing mountains diminish, drainage basin size grows, and clastic sediment diminish, drainage basin size grows, and clastic sediment supply rates increase. In humid regions or times of low supply rates increase. In humid regions or times of low sea level, deposition switches to the clastic off-lap suite sea level, deposition switches to the clastic off-lap suite with formation of the familiar shelf environments: typical with formation of the familiar shelf environments: typical association of deltas, beaches, and sub-tidal clastic shelf association of deltas, beaches, and sub-tidal clastic shelf deposits grading outward to deep sea fans and deposits grading outward to deep sea fans and contourites in continental rise settings. Prograding contourites in continental rise settings. Prograding sediments build a wide continental shelf upon a sediments build a wide continental shelf upon a voluminous accumulation of clastic or carbonate (or voluminous accumulation of clastic or carbonate (or some mix of clastic and carbonate). some mix of clastic and carbonate).

The clastic off-lap suite is well developed along the The clastic off-lap suite is well developed along the present Atlantic coast of North America, South America, present Atlantic coast of North America, South America, Africa, and Europe. The southern passive margin of Africa, and Europe. The southern passive margin of Laurentia (what is now the eastern margin of North Laurentia (what is now the eastern margin of North America) was dominated in the Cambrian and Ordovician America) was dominated in the Cambrian and Ordovician entirely by carbonates. Both are major source of entirely by carbonates. Both are major source of petroleum. petroleum.

Destructive (convergent) margin basins

Foredeep BasinsForedeep Basins Foreland Basins. Foreland Basins. Facies patterns. Facies patterns.

Foredeep Basins

Basin forming mechanisms: dominantly a combination of Basin forming mechanisms: dominantly a combination of load-related and density-related subsidence. Loading load-related and density-related subsidence. Loading leads to flexure of the crust. Subsidence near the load is leads to flexure of the crust. Subsidence near the load is matched by a compensating (but smaller amplitude) up-matched by a compensating (but smaller amplitude) up-bend at the periphery of the down-bend. bend at the periphery of the down-bend.

At destructive plate boundaries, dense, old oceanic crust At destructive plate boundaries, dense, old oceanic crust subducts beneath continental crust or younger, less-subducts beneath continental crust or younger, less-dense oceanic crust, forming an elongate, deep oceanic dense oceanic crust, forming an elongate, deep oceanic trench. trench.

Interaction between the down-going plate and the Interaction between the down-going plate and the overriding plate generates thrust slices (slabs of rock and overriding plate generates thrust slices (slabs of rock and deformed sediment) sheared off of one or the other plate, deformed sediment) sheared off of one or the other plate, which are thrust ocean-ward, overtop of the down going which are thrust ocean-ward, overtop of the down going plate. plate.

The subduction zone may also include abundant The subduction zone may also include abundant sediment supplied to the trench (especially in humid sediment supplied to the trench (especially in humid climates; ex. central Andean margin of South America). climates; ex. central Andean margin of South America).

Thrust and sediment load causes subsidence of the crust, Thrust and sediment load causes subsidence of the crust, deepening the basin and creating large amounts of deepening the basin and creating large amounts of additional accommodation space. This is a foredeep additional accommodation space. This is a foredeep basin: a basin that form from a combination of trench basin: a basin that form from a combination of trench (density driven) and load-driven subsidence in the down-(density driven) and load-driven subsidence in the down-going (lower) plate. It is located ocean-ward of the going (lower) plate. It is located ocean-ward of the volcanic arc and is rimmed distally by an uplifted volcanic arc and is rimmed distally by an uplifted forebulge. Ex: north Australian shelf-Banda Arc collision. forebulge. Ex: north Australian shelf-Banda Arc collision.

Foreland Basins.

Thickened crust of an island arc complex or Thickened crust of an island arc complex or continental crust at the margin of the upper plate continental crust at the margin of the upper plate leads to intense interaction between the down-leads to intense interaction between the down-going plate and the upper plate. This generates going plate and the upper plate. This generates considerable additional thickening and considerable additional thickening and deformation of the upper plate via thrust slices deformation of the upper plate via thrust slices that stack up to form (together with the volcanoes) that stack up to form (together with the volcanoes) a tall, complex mountain system and its isostatic a tall, complex mountain system and its isostatic roots. Ex: Andes Mountains. roots. Ex: Andes Mountains.

Thrusts migrate away from the collision Thrusts migrate away from the collision (subduction) zone toward the "foreland." Thrust (subduction) zone toward the "foreland." Thrust and sediment loads causes subsidence of the and sediment loads causes subsidence of the crust forming a deep flexural basin (8-10 km crust forming a deep flexural basin (8-10 km accommodation space) followed by an uplifted accommodation space) followed by an uplifted forebulge and a much smaller second order basin forebulge and a much smaller second order basin (a few 100’s of meters A-space at most). Ex: (a few 100’s of meters A-space at most). Ex: Amazon Foreland Basin. Amazon Foreland Basin.

Facies patterns.

Load-driven flexure is rapidly accommodated by Load-driven flexure is rapidly accommodated by subsidence/uplift. subsidence/uplift.

Rapid growth of accommodation space in the foreland or Rapid growth of accommodation space in the foreland or foredeep basin leads to trapping of sediments in the foredeep basin leads to trapping of sediments in the source area and strong transgression and sediment source area and strong transgression and sediment starvation as relative water depth increases over a period starvation as relative water depth increases over a period of 104 - 105 years. of 104 - 105 years.

Rapid uplift on the foreland or peripheral bulge results in Rapid uplift on the foreland or peripheral bulge results in loss of accommodation space, but facies response varies loss of accommodation space, but facies response varies greatly with local conditions the state of eustatic sea greatly with local conditions the state of eustatic sea level. Possibilities range from subaerial exposure to level. Possibilities range from subaerial exposure to prograding carbonate or siliciclastic facies. Unlikely to prograding carbonate or siliciclastic facies. Unlikely to exhibit transgression. exhibit transgression.

Basin fill shows strong progradation of sediments filling Basin fill shows strong progradation of sediments filling deep basin. Sediments grade upward and proximally from deep basin. Sediments grade upward and proximally from black shales to black shales to flyschflysch (mixed shale and turbiditic (mixed shale and turbiditic litharenites), to near shore clastics (deltaic and shelf-like litharenites), to near shore clastics (deltaic and shelf-like sediments) to alluvial plane deposits (fans & rivers, etc.). sediments) to alluvial plane deposits (fans & rivers, etc.). Early basin fill with abundant, well preserved organic Early basin fill with abundant, well preserved organic material. material.

As the system ages, the mountains erode and the As the system ages, the mountains erode and the sediments become less volcanic, more plutonic and sediments become less volcanic, more plutonic and metamorphic (granitic) source-rock dominated, and metamorphic (granitic) source-rock dominated, and become more mature both physically and chemically. become more mature both physically and chemically. These late phase deposits are sometimes referred to as These late phase deposits are sometimes referred to as mollassemollasse. .