Sedimentary Basins Related to Volcanic Arcs

1
Sedimentary Basins Related to Volcanic Arcs
2
Types of Basins

• Rift related
• Collision / Subduction related
• Intracratonic
• Strike-Slip related

3
Two stages of an opening rift.
4
Oceanic Crust
5
Ocean-ocean plate convergent boundary.
6
Structure of a continent-ocean convergent
boundary.
7
Continent-continent collision.
8
Mid-ocean ridge divergent boundary showing
transform faults.
9
Basin Concept

• 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).

10
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.

11
Basin Classification

• Passive Margins e.g. Atlantic Margin.
• 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 distributionsuseful
  for subsurface exploration of oil or
  understanding dispersal of pollutants.

12
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

13
Subsidence
14
Subduction Zones
15
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.

16
Slab Density
17
Slab Density
18
Island Arcs

• 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

19
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

20
Fore-Arc Key words

• Outer Swell
• Outer Trench Wall
• Trench
• Accretionary Wedge
• Volcanic Arc
• Benioff Zone
• Coupling
• Slab dip
• Sediments
• Hydrocarbon

21
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

22
Fore-arc Basin

• 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.

23
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.

24
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.

25
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.

26
Gravity
27
Structure from Earthquakes

• 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)

28
Stress on Slab
29
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

30
Earthquake within the slab

• 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.

31
Slab Earthquake
32
Accretionary Prism

• 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 under-formed sediments to form a duplex
  beneath the main part of the prism.

33
Fore-arc 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 fore arc
  region
• in the direction of subduction

34
Volcanic-Arc

• 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
Sediments H C

• 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.)

36
Fore-Arc Basin
37
Fore-arc Evolution
38
Fore-Arc Basin