Plates Group of rocks all moving in the same direction - PowerPoint Presentation

1. PlatesGroup of rocks all moving in the same directionCan have both oceanic and continental crust or just one kind.

2. Three Types of Plate BoundariesTransformDivergentConvergent

3. Divergent Margin BasinA distinctive suite of basin develops where plates rift and separate as a result of sea floor spreading. Continental margins under these conditions are referred to as “divergent” or passive margin

4. DOMINGHigh heat flow, thermal expansion of crust - few km high / 1000’s km across.High, dry climateRIFTING Normal faults accommodate stretchNO ocean crustPossible formation of triple junctionHigh, dry climate: non-marine sandstones, conglomerates in basinEast African Rift ZoneDRIFTINGCrust thins and initial intrusion of basalts as new seafloorShallow ocean; high evaporation Later “normal” marine sedimentDrainage reverses in to the sea.Gulf of California (5 Ma) Red Sea (10 Ma)How Do Divergent Margins Develop?

5. Rifted SettingRifted basin are essentially non-orogenic, located either on craton margin (pericratonic) or within craton (intracratonic). Such basins develop in divergent setting mainly by extensional faulting. The pericartonic rift basins develop during continental rifting by subsidence of cratonic margin. Such basins are “Atlantic type” which are located in passive margins of continents.

6. 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).

7. The intracratonic basin developed by continental mass. They are generally located in crustal weak zones along primordial fault /tectonic trends e.g. ancient suture zone.The rifting is associated with crustal thinning and high heat flow during initial stages of basin development. Block faulting and igneous activity as a result of crustal stretching followed by subsidence during thermal cooling are common processes.

8. A great variety of sedimentary environments can exist within rift, ranging from non marine (fluvial, lacustrine, to eolian) to marginal marine (delta, esturine, tidal flat) and marine (shelf and submarine fan). The deposits of rift basins include conglomerate, sandstones, shales, turbidites,coal, evaporite and carbonates.

9. Divergent Plate Boundary

10. Divergent Plate BoundaryUsually start within continents—grows to become ocean basinFig. 2.6

11. Continental Rifts• East Africa, Rio Grande rift• Beginning of ocean formation although it may not get that far• Rifting often begins at a triple junction (two spreading centers get together to form ocean basin, one left behind).

12. Divergent Plate Boundaries

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14. Aulacogen and failed riftThe aulacogen is a special type of rift formed by failed arms of triple junction during continental separation.One arm of ridge-ridge-ridge (RRR) junction commonly “fails” after a few million years. The spreading is aborted shortly before or after the development of true oceanic crust, while complete continental separation occurs on the other two arms. The failed arm form deep trough extending at high angle away from the margin.

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16. The continental margin may later collide with another continental plate forming a suture.This event may reactivate structures in failed rift producing a mildly deformed trough trending obliquely from suture. These types of rift are called aulacogens, a term coined by Russian geologist N. Shatski long before the development of plate tectonic.Deposition of thick sequence takes place in aulacogens over long periods of time. These deposits include nonmarine (e.g. alluvial fan) sediments, marine shelf deposits and deeper water facies such as turbidites. E.g. Reelfoot rift in which Mississipi river flow and Amazon rift in which amzon river flow.

17. Driving Mechanism of Plate TectonicsThought to be convection of the mantle.Friction at base of the lithosphere transfers energy from the asthenosphere to the lithosphere.Convection may have overturned asthenosphere 4–6 times.

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19. Cratonic basin Thermal fractionation of the mantle to produce continental crust took place relatively rapidly during Archaean because of greater production of radioactive heat in the early phases of earth evolution. The result was that by the end of the Archaean 65-70 percent of the present continental mass had been generated (Windley, 1977). This Archaean crust now forms the core of a series of shield complexes, around which Proterozoic and Phanerozoic crust has been accreted. Subduction and suturing generate extensive plutonism and regional metamorphism which result in gradual thickening and stabilization or cratonization of the crust.

20. Intracranic BasinThese basins occur in the interior of continental masses away from margin. They are normally circular, oval or elongate in shape. The sediments of these basins are generally found free from severe deformation and metamorphism due to their sheltered location within stable cratons. The sediments are characterised by mature clastics and carbonates. Sheet sands and carbonate platform are common.

21. The following Proterozoic basin are classified as interior basin.VindhyanChattishgarh-BasterCuddapahKaladgiBhima basin

22. Vindhyan basin Vindhyan basin is a typical example of interior basin. The basin shows very little deformation except near the margins which are affected by two major faults e.g. GBF in west and Son-Narmada fault in south. The sedimentary fill comprises alternating siliciclastics, shales and carbonates. The other basins are assigned to this class based on their geometry, sediments characteristics and structural styles which compare well with those of model interior basin.

23. Intracratonic rift basinThis type of basin occurs on continental crust, either in the interior of the present plate or at crustal margins of old continental plates. They are caused by divergence and tension within the continental block and by subsidence along reactivated primordial crust. Horst and graben are dominant features. Differential subsidence along margin faults make the basin asymmetrical.

24. Intracratonic grabens are developed within the Indian shield during Late Paleozoic along paleosuture or along majaor primordial faults. Their formation marks crustal stretching and aborted rifting during a earlier rift phase preceding break-up of Gondwanaland.The typical features of Intracratonic grabens are as follows:-Linear basin geometry with high length to breadth ratio.-One or both basin boundaries bounded by basin margin fault.

25. -Structural consists of host graben complexes.-magmatic activity common as dykes and sills.