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Sedimentary Basins related to Volcanic Arcs

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strike-slip. Back-arc basins. lie behind the magmatic arc ... Cratonic 'sag' Basins: E.g. Chad Basin, Africa. Abyssal Plains. ... – PowerPoint PPT presentation

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Title: Sedimentary Basins related to Volcanic Arcs


1
Sedimentary Basins related to Volcanic Arcs
  • M08353 Basin Analysis

2
Reading - start with
  • Reading, H.G. Sedimentary Environments
  • 2nd edition. Tectonics Sedimentation chapter by
    Mitchell Reading
  • 3rd edition. Volcaniclastics chapter by Orton, p.
    549-

3
Volcanic arcs may develop...
  • within oceanic lithosphere, where ocean floor
    subducts beneath ocean floor, and an island arc
    results, e.g. Lesser Antilles arc
  • or at the edge of a continent, where oceanic
    lithosphere subducts beneath continental
    lithosphere, and a continental margin magmatic
    arc forms, e.g. Andes

4
Basins related to volcanic arcs
  • fore-arc
  • back-arc
  • intra-arc
  • All may be either submarine or subaerial, or may
    have marine subaerial parts
  • Much sediment is supplied from active arc.

5
Fore-arc basins
  • Lie in the arc-trench gap, between volcanic arc
    and submarine trench
  • range from small basins on trench slope to large
    basins (50 to 100 km wide, and gt 500 km long)
    with thick fills (several km)
  • Basins tend to become wider and shallower with
    time, partly because of accretion at trenches

6
fore-arc basins
  • Sediment sources
  • volcanic arc
  • outer arc
  • longitudinally froma continent
  • Tectonic style varies
  • compressional
  • extensional
  • strike-slip

7
Back-arc basins
  • lie behind the magmatic arc
  • often the site of extension thinning of crust
  • may overlie either ocean or continental crust
  • oceanic back-arc basins are eventually subducted
    and destroyed, or preserved in thrust complexes
    related to ocean closure.
  • back-arc basins on continental crust - more
    varied facies, because of terrigenous input
    higher preservation potential.

8
Intra-arc basins
  • Sedimentary basins within magmatic arcs, between
    volcanoes, or between older and younger belts of
    the arc
  • Some are fault-bounded and subside rapidly.
    Faulting due to extension within arc, or flexure
    of lithosphere due to weight of volcano.
  • With time, position of the arc migrates, and
    basins may change between intra-arc, back-arc and
    fore-arc.

9
Sediment supply and transport
  • Sediment supply varies according to volcano
    behaviour, governed by magma viscosity and gas
    content.
  • In deep water, explosive activity is suppressed
    by hydrostatic pressure.
  • More silicic magmas in more evolved arcs -
    therefore greater explosive activity, more supply
    of pyroclastic sediment.

10
Sediment transport and deposition is controlled
by
  • topography - both subaerial and submarine
  • volcanic processes, especially eruption column
    height, direction of pyroclastic flows
  • sediment transport systems - e.g. rivers,
    prevailing winds

11
Subduction Zones
12
Subduction zones
  • also termed convergent or consuming plate margins
  • occur where adjacent plates move toward each
    other and relative motion is accommodated by one
    plate over-riding the other.
  • These zones are classified as either oceanic or
    subcontinental, depending on the overriding
    plate.
  • If the "subducting" plate is continental,
    subduction will cease and a mountain belt will
    form within a collision zone.

13
Where do subduction zones occur?
  • along the "Ring of Fire" around the Pacific
    Ocean.
  • Two short subduction zones occur at the Lesser
    Antilles, at the eastern side of the Carribean
    plate and the South Sandwich Island plate.
  • Three short segments of the Alpine Himalayan
    system involve subduction of oceanic lithosphere.
  • the Calabrian and Aegean boundaries in the
    Mediterranean Sea
  • Makran boundary along the SW boundary of the Iran
    plate.

14
Physiography
  • Outer Swell
  • Outer Trench Wall
  • Trench
  • Forearc (Arc-Trench Gap)
  • Volcanic Arc
  • Back-Arc

15
Physiography 2
  • Outer swell
  • Low topographic bulge (a few hundred meters of
    relief)
  • develops just outboard of where subducting plate
    bends down into the mantle.
  • Outer Trench Wall
  • Slope on ocean floor between the outer swell and
    the trench floor.
  • Slope dip is typically -5 degrees

16
  • Trench
  • Deep valley that develops at the plate boundary.
  • Continuous for 1000s of km
  • typically 10 - 15 km deep (5 - 10 km below
    surrounding ocean floor.)

17
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18
Forearc (Arc-Trench Gap)
  • Consists of region between trench and the arc.
  • steep inner trench wall (lower trench slope)
  • dips of - 10 deg
  • flattens into a gentle slope termed the forearc
    basin (upper trench slope).
  • The inner trench wall is usually separated from
    the forearc by the outer ridge.
  • The accretionary prism underlies the inner trench
    wall, the outer ridge and part of the forearc
    basin.

19
Volcanic Arc
  • Active arc built on a topographically high region
    of older rocks, the arc basement
  • may be a shallow marine platform or an emergent
    region of older rocks.
  • In continental arcs, the basement is continental
    crust standing a few kms above sea level.
  • Volcanoes in island arcs are usually 1 - 2 km
    above sea level. Volcano elevation in continental
    arcs is strongly influenced by continental crust
    thickness.

20
Back-Arc
  • Area behind the volcanic arc.
  • In island arcs this region consists of basins
    with oceanic crustal structure and abyssal water
    depths.
  • Sometimes remnant arcs are preserved behind the
    island arcs.
  • On continents this region is the continental
    platform which may be subaerially exposed, or the
    site of a shallow marine basin.

21
Gravity
  • Typically, similar free-air gravity profiles
  • 50 mGal gravity high associated with the outer
    bulge
  • 200 mGal low associated with the trench and
    accretionary prism
  • 200 mGal high associated with the arc.
  • Isostatic anomalies have the same polarity as the
    free-air gravity
  • Suggests that the gravity anomalies are caused by
    the dynamic equilibrium imposed by the system by
    compression.
  • Compressional forces cause the trench to be
    deeper and the arc to have less of a root than
    they would be if only isostatic forces were at
    work.

22
Structure from Earthquakes
  • Subduction zones are characterized by dipping
    seismic zones termed Benioff zones or
    Wadati-Benioff zones
  • Result from deformation of the downgoing
    lithospheric slab. The zones have dips ranging
    from 40 to 60 deg

23
  • Because, the slab is colder and more dense than
    surrounding asthenosphere, it's position can be
    mapped seismically as high velocity anomalies and
    as high "Q" (little attenuation of seismic waves)
    zones in the mantle. High Q, and high velocity
    are thought to correspond to relatively high
    density, cold material

24
earthquake hypocenters related to their position
within the slab
  • Shallow depths
  • predominantly thrust faults within the upper part
    of the downgoing plate or in the adjacent
    overriding plate.
  • Down to depths of 400 km, down-dip extension.
  • In some slabs, down-dip extension is found in the
    upper part of the slab, accompanied by down-dip
    compression at the base of the slab. The
    extension probably results from the lithosphere
    being pulled into the mantle by the weight of the
    downgoing portion.

25
  • Deep slabs usually show down-dip compression
  • may result from increased viscous resistance at
    depth.
  • deeper part of the slab will feel a push from the
    weight of the shallower portion of the slab.
  • Between 70 - 300 km, faulting may occur due to
    dehydration of serpentinite.
  • From 300 - 700 krn may also be due to the sudden
    phase change of olivine to spinel which may be
    accommodated by rapid shearing of the crystal
    lattice along planes on which minute spinel
    crystals have grown.

26
Structural Geology- Trenches
  • Trenches normally contain flat-lying turbidites
    deposited by currents flowing down into the
    trench from the overriding plate or along the
    axis of the trench. The outer swell is probably
    caused by elastic bending of the subducting
    plate.

27
Forearc
  • may be underlain either by the accretionary
    prism or arc basement rocks covered by a thin
    veneer of sediments or both.
  • Where there is little sediment accumulation on
    the subducting plate, island arc or continental
    basement may extend all the way to the lower
    trench slope and little or no accretionary prism
    may occur.
  • Forearc basement may draped by a thin veneer of
    sediment, and is commonly cut by normal faults
    toward the trench.

28
Accretionary Prism
  • wedge of deformed sedimentary rocks
  • the main locus of crustal deformation
  • Rocks are typically cut by numerous imbricate
    thrust faults that dip in the same direction as
    the subduction zone.
  • As more material is added to the toe of the
    wedge, the thrusts are moved upwards and rotate
    arcwards.
  • Rocks within the accretionary prism are derived
    from the downgoing and/or overriding plates.

29
Accretionary Prism
  • At the toe of the wedge, sediments are added thru
    offscraping
  • propagation of the basal thrust into undeformed
    sediments on the subducting plate.
  • This process results in progressive widening of
    the wedge, and eventually a decrease in dip on
    the subduction zone.

30
Accretionary Prism
  • When sediments on the downgoing plate are
    subducted without being disturbed they can still
    be added to the prism thru underplating
  • propagation of the basal thrust into the
    downgoing undeformed sediments to form a duplex
    beneath the main part of the prism.

31
Subduction Erosion
  • erosion and subsequent subduction of rocks from
    the toe of the prism.
  • Sediment on the subducting plate that is not
    added to the overriding plate thru these
    processes may descend into the mantle and
    contribute to the generation of arc magmas.

32
Forearc Basin
  • Wide sedimentary basin
  • develops above irregular basement on the upper
    part of the arc-trench gap.
  • Sediments from the active arc or arc basement
    rocks
  • deposited by turbidity currents traveling along
    the basin axis or perpendicular to the arc.
  • asymmetric basin
  • inner part of the upper slope basin subsides
  • outer edges rises due to accretion at the toe of
    the wedge.
  • high-P, low-T metamorphism
  • increases in grade toward the inner forearc
    region
  • in the direction of subduction

33
Arc
  • Arc basement
  • older more deformed and metamorphosed rocks in
    platform on which the modem arc is built.
  • oceanic rocks
  • On the continents, complex continental basement.
  • Volcanic arc
  • a chain of largely andesitic stratovolcanoes
    spaced at fairly regular intervals of 70 km.
  • The structural environment of these arcs is
    commonly extensional (numerous normal faults)
  • volcanoes in grabens termed volcanic depressions.
  • underlain by large plutonic bodies (e. g. the
    Sierra Nevada).

34
Arcs
  • Metamorphism
  • common and suggest a high geothermal gradient.
  • Much of the lower crust may be at the melting
    temperature of granite.
  • Sediments
  • debris from active volcanoes.
  • deposited as turbidites.
  • In tropics, settings these volcanogenic sediments
    may interfinger with carbonate reefs.
  • In continental arcs, sediments are often
    deposited subaerially.

35
Back-arc
  • extensional tectonics and subsidence.
  • In oceans arc-derived sediments are deposited in
    an ocean basin behind the arc termed the back-arc
    basin.
  • In continents, sediments are deposited into
    basins on the continental platform and are termed
    foreland basins or retro-arc basins.

36
Foreland Fold and Thrust Belts
  • Relation between foreland fold and thrust belts
    and subduction not understood
  • not all continental arcs display these features.
  • Possible explanations if there is a relation
  • Thrust belt caused by compression at margin of
    overriding plate due to subduction of hot,
    buoyant lithosphere.
  • Thrust belt associated with shallow dip of a
    downgoing slab.
  • Thrust belt associated with subduction of an
    aseismic ridge.

37
Models of thermal processes in subduction zones
  • Rate of Subduction
  • The faster the descent of the slab, the less time
    it has to absorb heat from the mantle.
  • Slab Thickness
  • The thicker the descending slab, the more time it
    takes to come into equilibrium with the
    surrounding lithosphere.
  • Frictional Heating
  • occurs at top of slab due to friction as slab
    descends and is resisted by the lithosphere.

38
  • Conduction
  • heat into slab from the asthenosphere
  • Adiabatic Heating
  • associated with compression of slab with
    increased pressure at depth.
  • Heat of Radioactive Decay
  • decay of radioactive minerals in the oceanic
    crust (minor)
  • Latent Heat of Mineral Phase Transitions
  • olivine-spinel transition at 400 km is
    exothermic. Spinel-oxide transition at 670 km
    could be either exothermic or endothermic.

39
  • All thermal models show that the downgoing slab
    maintains its thermal identity to great depths
    (e. g. contrasts of 700 deg C can still exist at
    700 krn depth).

40
If the slab is so cold, how do we get enough
heating to cause arc magmatism?
  • Melting of Slab in Presence of Water
  • Partial melting may take place at lower
    temperatures due to presence of water as slab
    dehydrates. Water is released by transition of
    amphibolite to ecologite, and dehydration of
    serpentinite at depths of - 100 km.
  • Corner Flow and Melting of Mantle
  • Downgoing slab may cause flow of hot mantle into
    the comer of the overriding mantle where it
    impinges on the downgoing slab. This may provide
    enough heat to cause melting.

41
Origins of back-arc basins
  • Entrapment of previous oceanic crust
  • Change of plate motion may lead to abandonment of
    a fragment of oceanic crust behind the arc.
    (e.g., Aleutian Basin and West Philippines Basin
    )
  • Formation of new crust - behind the arc. 3 models
  • Spreading caused by forceable injection of a
    diapir rising from the downgoing slab.
  • Spreading induced in the overriding plate by the
    viscous drag in the mantle wedge caused by the
    motion of the downgoing plate (comer flow).
  • Spreading induced by the relative drift of the
    overriding plate away from the downgoing slab
    (slab fixed with respect to mantle). This is also
    termed roll-back.

42
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45
Ocean-ocean plate convergent boundary.
46
Structure of a continent-ocean convergent
boundary.
47
Continent-continent collision.
48
Mid-ocean ridge divergent boundary showing
transform faults.
49
Two stages of an opening rift.
50
slab-pull due to its greater density
51
Reconstruction _at_ 250 million years ago
52
Concept of a Basin
  • Three dimensional architecture of basin fill.
  • Affected by spatial and temporal pattern of
    tectonic subsidence
  • Lithospheric deformation process.
  • Three basic causes of subsidence
  • Loading and flexure (like an elastic plate).
  • Thermal and density changes - isostasy.
  • Faulting - isostasy.
  • Sea level changes.
  • Sediment supply rates and source position
    (drainage basin outlets).

53
Basin Classification
  • Extensional
  • Half graben/graben - Rift Basin, e.g. Basin and
    Range, U.S.A.
  • Mature oceanic speading - e.g. Atlantic margin
    (Passive Margin).
  • Syn-rift, post-rift megasequences.
  • Compressional
  • Foreland basin - flexural loading of the Earth's
    lithosphere.
  • Two types
  • Collisional, e.g. Himalaya.
  • Back arc, e.g. Andes/Precordillera.
  • Piggy-back basin (thrust sheet top basin).
  • Strike-Slip Basins
  • E.g. Dead Sea, Israel.

54
Basin Classification Cont.
  • Passive Margins
  • E.g. Atlantic Margin.
  • Subduction Related
  • Oceanic trench, e.g. Marianas Trench.
  • Fore-arc basin, e.g Taiwan or Median Valley in
    Scotland.
  • Back-arc basin, e.g. Sea of Japan.
  • Cratonic "sag" Basins
  • E.g. Chad Basin, Africa.
  • Abyssal Plains.
  • Predictive models of facies distributionsuseful
    for subsurface exploration of oil or
    understanding dispersal of pollutants.

55
Benioff Zones
  • Earthquakes occur at shallow, intermediate and
    deep levels beneath subduction zones
  • The earthquakes define a plane which begins at
    the trench and dips at about 45 beneath the arc
  • This dipping plane of earthquake foci is called
    the Benioff Zone
  • The Benioff Zone follows the upper part of the
    descending oceanic plate
  • Shallow earthquakes also occur through the arc

56
Island Arcs
  • Island arcs are of chains of volcanically active
    islands arranged in a curved arc
  • An ocean trench occurs on the oceanwards side
  • Island arcs first develop on oceanic crust
  • The crustal thickness in an arc is intermediate
    between oceanic and continental
  • Volcanic activity begins abruptly at a Volcanic
    Front about 200 - 300 km in from the trench
  • The volcanic front and trench are separated by an
    Arc-Trench gap with no volcanism

57
Island Arc Volcanism
  • Volcanic rocks in island arcs are mostly of
    andesitic composition
  • The magmas originate mostly by partial melting of
    subducted oceanic crust and overlying mantle
  • Melting begins when the slab descends to about
    100 km depth, forming the volcanic front Partial
    melting of basaltic ocean crust
  • Rising magmas Volcanic eruptions

58
Chemical Differentiation
  • Mid-Ocean Ridge
  • Partial melting of Mantle basalt magma
  • Subduction Zone
  • Partial melting of Basalt crust andesite magma
  • Mature Arcs
  • Partial melting of Andesite crust rhyolite
    magma
  • All of this is an irreversible chemical
    differentiation of the mantle in several stages
  • Continental crust grows by accumulation of
    increasingly silica-rich rocks

59
Ocean trench Sedimentation
  • Unconsolidated sediment from the ocean floor is
    scraped off the descending plate at the trench
  • Slices of the oceanic crust may be included as
    ophiolite belts
  • These rocks form a complex rock mass called an
    Accretionary Wedge
  • The Accretionary Wedge is buckled upwards as new
    material is pushed beneath its base
  • The chaotic jumble of rocks in the Accretionary
    wedge is called a Tectonic Mélange Accretionary
    Wedge

60
Metamorphic Rocks and Subduction
  • High Temp - Low Pressure
  • Metamorphism
  • Occurs in the core of volcanic arcs
  • Abnormal heating of the crust
  • thermal effects of subduction-related magmatism
  • High Pressure-Low Temp
  • Metamorphism
  • occurs in the accretionary wedge
  • cold rocks are dragged to great depths and then
    upthrust again Granites

61
Basin Analysis
  • Basins
  • Topographically low places where sedimentary
  • materials accumulate.
  • Basins are characterized by accomodation space.
  • Accomodation is created by
  • Eustatic sea-level rise
  • Subsidence

62
Basin Analysis
  • Basin analysis is the detailed investigation of
    the processes that
  • Form basins
  • Fill basins
  • Alter basins
  • Uplift (invert) basins
  • Destroy basins
  • Requires sedimentology, stratigraphy,
    hydrogeology, petroleum geology, seismology,
    geophysics, geochemistry, paleontology, etc.

63
Types of Basins
  • Intracratonic
  • Rift related
  • Strike-Slip related
  • Collision / Subduction related

64
  • _Intracratonic Basins that form within
    continental crust _Intracratonic Basins that
    form within continental crust
  • Rifts Divergent plate boundaries that
    eventually develop into spreading centers.

65
  • Rifts Divergent plate boundaries that
    eventually develop into spreading centers.
  • _Initial rift sediments are arkosic sandstones
    interbedded with basalts.
  • _Rifts then flood and deposit a thick sequence of
    evaporites.
  • _Then marine sedimentationtakes over.

66
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67
The Tonga Arc - Lau Back-arc Basin System
  • CLSC central Lau spreading center
  • ELSC east Lau spreading center
  • VFR Valu Fa ridge (ELSC????????)

(ETZ extensional transform zone)
68
ELSC Zone 3
Decreased Magma Supply
  • ELSC
  • ??????????? 60-110 km
  • ???? 60-95 mm/yr,?? ?2,000-3,000 m
  • aMBA?????????????????and/or????(?????????)
  • ????????????????????
  • ?????????????????????????,??????VFR????????
  • Zone 3
  • ????? Zone2??2-3.5 km??
  • ??zone2??1 km??
  • aMBA??????
  • Zone2???????????????Abyssal hills

69
Character and Position of Spreading Axes
  • ????,??????????(?????????????)???? VFR???
  • ???????????????????????
  • ???????????????????????????(?????)
  • ?????????????? ELSC???
  • ??????????????
  • ???????,????????????????????????????????,?????????
    ?????(????????aMBA?????)
  • ?????????????? CLSC???
  • ???????????????????????????????,?????????
  • ????????

ELSC????????????????,CLSC?????????(?????)??????
70
Basins basin types
  • Basin a region of depressed crust, typically
    with greater thicknesses of sediment accumulation
    than surrounding regions.
  • Basins form by tectonic processes that cause the
    crust to subside, and so to create large amounts
    of accommodation space. Rapid A-space growth
    translates into abundant and well-preserved
    organic matter within the sediment.
  • The abundance of organic matter combined with the
    low grade burial metamorphism that sediments in
    thick accumulation experience translate into
    major oil and gas accumulations.
  • Basins provide the most complete successions of
    the geological record.

71
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.

72
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).

73
Rift basins.
  • Smallish volumes of sediment (100s to a few 1000
    m of accommodation space) in localized,
    fault-bounded basins.
  • Dominantly terrestrial (subaerial and lacustrine)
    and arid as result of uplifted margins that cast
    rain-shadows into the basins as well as their
    small drainage capture area.
  • Early rift sediments typically heterogeneous with
    basement clasts and volcanics dominant. Grains
    with low physical and chemical maturity. Late
    rift sediments often include extensive evaporites
    from episodic oceanic invasions (reflects
    variable rates of subsidence and sea level rise
    and fall). Remobilized salt becomes important in
    passive margin history later (see below).

74
Passive margin basins.
  • Lots of accommodation space (10,000 to 15,000 m
    or more) from combination of thermal and sediment
    load-driven subsidence.
  • As oceans widen, marine conditions come to
    dominate. Early phases often still arid and
    dominated by carbonate deposition and fringing
    reefs along the shelf edge (at low latitude
    sites)
  • With continued subsidence the fringing mountains
    diminish, drainage basin size grows, and clastic
    sediment supply rates increase. In humid regions
    or times of low sea level, deposition switches to
    the clastic off-lap suite with formation of the
    familiar shelf environments typical association
    of deltas, beaches, and sub-tidal clastic shelf
    deposits grading outward to deep sea fans and
    contourites in continental rise settings.
    Prograding sediments build a wide continental
    shelf upon a voluminous accumulation of clastic
    or carbonate (or some mix of clastic and
    carbonate).
  • The clastic off-lap suite is well developed along
    the present Atlantic coast of North America,
    South America, Africa, and Europe. The southern
    passive margin of Laurentia (what is now the
    eastern margin of North America) was dominated in
    the Cambrian and Ordovician entirely by
    carbonates. Both are major source of petroleum.

75
Destructive (convergent) margin basins
  • Foredeep Basins
  • Foreland Basins.
  • Facies patterns.

76
Foredeep Basins
  • Basin forming mechanisms dominantly a
    combination of load-related and density-related
    subsidence. Loading leads to flexure of the
    crust. Subsidence near the load is matched by a
    compensating (but smaller amplitude) up-bend at
    the periphery of the down-bend.
  • At destructive plate boundaries, dense, old
    oceanic crust subducts beneath continental crust
    or younger, less-dense oceanic crust, forming an
    elongate, deep oceanic trench.
  • Interaction between the down-going plate and the
    overriding plate generates thrust slices (slabs
    of rock and deformed sediment) sheared off of one
    or the other plate, which are thrust ocean-ward,
    overtop of the down going plate.
  • The subduction zone may also include abundant
    sediment supplied to the trench (especially in
    humid climates ex. central Andean margin of
    South America).
  • Thrust and sediment load causes subsidence of the
    crust, deepening the basin and creating large
    amounts of additional accommodation space. This
    is a foredeep basin a basin that form from a
    combination of trench (density driven) and
    load-driven subsidence in the down-going (lower)
    plate. It is located ocean-ward of the volcanic
    arc and is rimmed distally by an uplifted
    forebulge. Ex north Australian shelf-Banda Arc
    collision.

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Foreland Basins.
  • Thickened crust of an island arc complex or
    continental crust at the margin of the upper
    plate leads to intense interaction between the
    down-going plate and the upper plate. This
    generates considerable additional thickening and
    deformation of the upper plate via thrust slices
    that stack up to form (together with the
    volcanoes) a tall, complex mountain system and
    its isostatic roots. Ex Andes Mountains.
  • Thrusts migrate away from the collision
    (subduction) zone toward the "foreland." Thrust
    and sediment loads causes subsidence of the crust
    forming a deep flexural basin (8-10 km
    accommodation space) followed by an uplifted
    forebulge and a much smaller second order basin
    (a few 100s of meters A-space at most). Ex
    Amazon Foreland Basin.

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Facies patterns.
  • Load-driven flexure is rapidly accommodated by
    subsidence/uplift.
  • Rapid growth of accommodation space in the
    foreland or foredeep basin leads to trapping of
    sediments in the source area and strong
    transgression and sediment starvation as relative
    water depth increases over a period of 104 - 105
    years.
  • Rapid uplift on the foreland or peripheral bulge
    results in loss of accommodation space, but
    facies response varies greatly with local
    conditions the state of eustatic sea level.
    Possibilities range from subaerial exposure to
    prograding carbonate or siliciclastic facies.
    Unlikely to exhibit transgression.
  • Basin fill shows strong progradation of sediments
    filling deep basin. Sediments grade upward and
    proximally from black shales to flysch (mixed
    shale and turbiditic litharenites), to near shore
    clastics (deltaic and shelf-like sediments) to
    alluvial plane deposits (fans rivers, etc.).
    Early basin fill with abundant, well preserved
    organic material.
  • As the system ages, the mountains erode and the
    sediments become less volcanic, more plutonic and
    metamorphic (granitic) source-rock dominated, and
    become more mature both physically and
    chemically. These late phase deposits are
    sometimes referred to as mollasse.

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