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 arcforms, 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 distributions:useful 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 > 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