Transcript Fore-arc basin

Sedimentary Basins
Related to
Volcanic Arcs
Types of Basins
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Rift related
Collision / Subduction related
Intracratonic
Strike-Slip related
Two stages of an opening rift.
Oceanic Crust
Ocean-ocean plate convergent
boundary.
Structure of a continent-ocean
convergent boundary.
Continent-continent collision.
Mid-ocean ridge divergent
boundary showing transform
faults.
Basin Concept
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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.
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Sea level changes.
Sediment supply rates and source position (drainage basin
outlets).
Basins related to volcanic arcs
» Fore-arc
» Back-arc
» Intra-arc
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
 All may be either submarine or subaerial, or may have marine &
subaerial parts
 Much sediment is supplied from active arc.
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Basin Classification
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Passive Margins: e.g. Atlantic Margin.
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Active Margins:
– Oceanic trench (Marianas Trench).
– Fore-arc basin (Taiwan, Peru, Sumatra).
– Back-arc basin, e.g. Sea of Japan.
Cratonic “Sag" Basins: e.g. Chad Basin, Africa.
Abyssal Plains.
Predictive models of facies distributions:
useful for subsurface exploration of oil or understanding dispersal
of pollutants.
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Sediment and deposition
Sediment Source
– Sediment supply varies according to volcanic behaviour, governed by
magma viscosity and gas content.
– More silicic magmas in more evolved arcs - therefore greater explosive
activity, more supply of clastic sediment.
Sediment and deposition are controlled by:
– topography - both subaerial and submarine
– volcanic processes, especially eruption column height, direction of flows
– sediment transport systems - e.g. rivers, prevailing winds
Subsidence
Subduction Zones
Subduction zones
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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.
Slab Density
Slab Density
Island Arcs
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Island arcs are of chains of volcanically active islands arranged in a
curved arc
An ocean trench occurs on the ocean-wards 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
Ocean trench Sedimentation
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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
Fore-Arc Key words
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Outer Swell
Outer Trench Wall
Trench
Accretionary Wedge
Volcanic Arc
Benioff Zone
Coupling
Slab dip
Sediments
Hydrocarbon
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 > 500 km long) with thick
fills (several km)
 Basins tend to become wider and
shallower with time, partly because of
accretion at trenches
Fore-arc Basin
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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.
Fore-arc basement may draped by a thin veneer of sediment, and is
commonly cut by normal faults toward the trench.
Fore-arc (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 fore-arc basin (upper trench
slope).
 The inner trench wall is usually separated from the fore-arc by the
outer ridge.
 The accretionary prism underlies the inner trench wall, the outer
ridge and part of the fore-arc basin.
Volcanic Arc
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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.
Gravity
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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.
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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.
Gravity
Structure from Earthquakes
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Subduction zones are characterized by
dipping seismic zones termed Benioff zones
or Wadati-Benioff zones
Result from deformation of the down going
lithospheric slab. The zones have dips
ranging from 40 to 60 deg
Various types of stress states within the
subducting slab (Compression and
Extensional stress)
Stress on Slab
Benioff Zones
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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
Earthquake within the slab
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Shallow depths
predominantly thrust faults within the upper part of
the down-going plate or in the adjacent overriding
plate.
Down to depths of 400 km, down-dip extension.
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.
Slab Earthquake
Accretionary Prism
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At the toe of the wedge, sediments are added thru off
scraping
propagation of the basal thrust into under-formed
sediments on the subducting plate.
This process results in progressive widening of the wedge,
and eventually a decrease in dip on the subduction zone.
When sediments on the downgoing plate are subducted
without being disturbed they can still be added to the prism
thru under-playing
propagation of the basal thrust into the downgoing underformed sediments to form a duplex beneath the main part
of the prism.
Fore-arc Basin
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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.
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asymmetric basin
– inner part of the upper slope basin subsides
– outer edges rises due to accretion at the toe of the wedge.
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high-P, low-T metamorphism
– increases in grade toward the inner fore arc region
– in the direction of subduction
Volcanic-Arc
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Metamorphism
– common and suggest a high geothermal gradient.
– Much of the lower crust may be at the melting temperature of granite.
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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.
Sediments & H C
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Thin sedimentary section (1-2 km) due to the small amount
of hinterland available for sediment source.
If the hinterland is large, more sediments can be deposited
but the fine sediment will plug the reservoir and will reduce
the permeability (rich in feldspars)
Due to the thin sediments (low thermal) and the low
permeability, Fore-arc basins have produced little quantity of
oil.
Examples: All basins along continental margins (e.g.
Sacramento, San Joaquin, Barbados, Peru, Java, Sumatra,
Makran, Guatemala, Alaska, etc.)
Fore-Arc
Basin
Fore-arc Evolution
Fore-Arc Basin