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Subsidence

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Breve explicación de los diferentes tipos de subsidencia

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Subsidence

  1. 1. SUBSIDENCE July 2008 Sedimentary Basin Analysis
  2. 2. Sumary <ul><li>Subsidence </li></ul><ul><li>Gravity Loading Hypothesis </li></ul><ul><li>Thermal Hypothesis * </li></ul><ul><li>Crustal Thinning hypothesis </li></ul><ul><li>Normal-fault based mechanisms </li></ul><ul><li>Conclusions </li></ul><ul><li>References </li></ul>
  3. 3. Subsidence Passive continental margins are those associated with continental rifting and the subsequent formation of ocean basins. They differ from active continental margins which are associated with subduction. Arapahoe Basin, Colorado, USA Subsidence is a cooling of the surface of the earth. Causes include sediment loading, tectonic activity, and thermal contraction during cooling of the crust.
  4. 4. Gravity Loading Hypothesis <ul><li>This attributes subsidence to sediment load (effectively replacing seawater with denser sediment). </li></ul><ul><li>The amount of subsidence depends on </li></ul><ul><li>- Relative densities of seawater </li></ul><ul><li>- Sediment </li></ul><ul><li>- The underlying mantle </li></ul>
  5. 5. Gravity Loading Hypothesis <ul><li>Gravity loading Hypothesis </li></ul><ul><li>( Bott 1978) </li></ul><ul><li>This depends on replacing low density water by higher density sediment . </li></ul>
  6. 6. Thermal Hypothesis <ul><li>When a basin is formed, high angle faults force sedimentation in the basins, but that is not enough to form the actual basin. Deep lithospheric cooling, which results in thermal contraction in order to regain isostatic equilibrium, specifically causes thermal subsidence. Thermal Subsidence usually takes over as the main basin forming component on after tectonic subsidence has ceased. </li></ul>
  7. 7. Thermal Hypothesis Thermal Hypothesis of Sleep 1971 (After Bott, 1978) Thermal hypothesis of Sleep. This was the first to recognise that heating up the mantle (by a plume or whatever) could produce substantial crustal uplift (and erosion), followed by thermal subsidence. 1. 2. 3.
  8. 8. Thermal Hypothesis <ul><li>Thermal hypothesis modification according to Falvey (1974), who argues that heating will cause dense granulite to form . </li></ul>
  9. 9. Crustal Thinning hypothesis <ul><li>- The continental crust and the lithosphere have an upper brittle zone, 20 km thick, overlying a much weaker layer which deforms by ductile flow. </li></ul><ul><li>- Thus crust may thin by progressive creep of middle and lower crustal material towards the sub-oceanic upper mantle. </li></ul><ul><li>- It is argued that this may give rise to jerky subsidence. </li></ul>
  10. 10. Crustal Thinning hypothesis <ul><li>After the initial rifting the lower crust deforms by plastic flow. Could the lower continental crust flow UNDER oceanic crust in the manner shown? </li></ul>
  11. 11. Normal-fault based mechanisms <ul><li>Early hypotheses assumed that graben formation required a wedge of crust about 60 km wide to sink isostatically between inward-dipping normal faults. As the upper crust forms graben by wedge subsidence the ductile lower crust compensates by plastic flow. </li></ul>Graben Formation by wedge subsidence
  12. 12. Conclusions <ul><li>Passive margin formation is important to understanding thermal subsidence, because the thermally subsided basin in extensional zones occurs between the ridge and the passive margin. </li></ul><ul><li>Basins that are formed by rifting and thermal subsidence don’t necessarily stay basins. They may suffer from uplift and erosion . </li></ul><ul><li>En las márgenes pasivas, la subsidencia se forma como resultado del proceso de rifting. </li></ul>
  13. 13. References <ul><li>- http://www.le.ac.uk/ </li></ul><ul><li>http://www.geosci.usyd.edu.au/ </li></ul><ul><li>BOTT, M.H.P. 1979. Subsidence mechanisms at passive continental margins. American Association of Petroleum Geologists Memoir 29, 8-19? </li></ul><ul><li>BOTT, M.H.P. 1982. The mechanism of continental splitting. Tectonophysics 81, 301-309. </li></ul><ul><li>KUSZNIR, N.J. & ZIEGLER, P.A. 1992. The mechanics of continental extension and sedimentary basin formation: a simple-shear/pure-shear flexural cantilever model. Tectonophysics 215, 117-131. </li></ul><ul><li>WHITE, R.S., SPENCE, G.D., FOWLER, S.R., McKENZIE, D.P., WESTBROOK, G.K. & BOWEN, A.N. 1987. Magmatism at rifted continental margins. Nature 330, 439-444. </li></ul><ul><li>- WHITE, R.S. & McKENZIE, D.P. 1989. Magmatism at rift zones: the generation of volcanic continental margins and flood basalts. Journal of Geophysical Research 94, 7685-7729. </li></ul>
  14. 14. THANKS

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