LSU GEOL 2020 - Subsidence and Thermal History

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Slide 1Geohistory AnalysisSubsidence History and Thermal HistoryBackstrippingSlide 5Slide 6Slide 7Subsidence in the forward senseSlide 9Slide 10Slide 11Slide 12Slide 13Slide 14Subsidence in the reverse senseSlide 16Slide 17Slide 18Flexural backstrippingChapter 9: Subsidence and Thermal HistoryThis presentation contains illustrations from Allen and Allen (2005)Geohistory Analysis•Quantitative analysis that produces subsidence of various geological boundaries and rates of sediment accumulation through time.•Coined by Van Hinte (1973). •Backstripping involves additional incorporation of isostatic (local, flexural, 1-D, 2-Dor 3-D) effects of sediment unloading.Subsidence History and Thermal HistorytimedepthpastpresentbasementLayer 1Layer 2Layer 3“Tectonic” subsidence”Backstripping•(Watts and Ryan, 1976)•Tectonic subsidence is what remains after the effects of sea-level and sediments are removed. –…. It is the background subsidence caused by stretching and thermal cooling of the lithosphere.BackstrippingBackstrippingFor original matlab program from Allen’s book: see the following link link to a matlab programSubsidence History and Thermal HistorytimedepthpastpresentLayer 1Layer 2Layer 3“Tectonic” subsidence”Thermal windowSubsidence in the forward sense0tSea-level0tSubsidence in the forward sense0tSea-level1t0t0tSubsidence in the forward sense1t0t1tSea-level0t0t1t1tSubsidence in the forward sense0t0t0t1t1t2t1t1t0tSubsidence in the forward sense0t0t0t1t1t2t1t1t0t0t2tSubsidence in the forward sense0t0t0t1t1t2t1t1t0t0t2t0t3tSubsidence in the forward sense•We need to know:•Sea-level (assumed constant in the example)•Original sediment thickness and hence water-depth (known at all times)•Type of isostatic behavior does the crust exhibit when loaded (flexural, or local/Airy)Subsidence in the reverse sense•Sea-level (never constant in the past)•Original sediment thickness ( we don’t know??)– We DO know current sediment thickness•Water-depths ( never constant)Subsidence in the reverse sense0t2t0t3t0t3t0czef-=Subsidence in the reverse sense0czef-=Porosity: The “new”decompacted thickness must conform to a pre-defined porosity-depth relation. This calculation is achieved by trial and error.Subsidence in the reverse sensePaleo-water depth: From biostratigraphic assemblages Sea-level estimations: Long-term changes are accepted. Flexural Model: Various Te values can be used.Flexural


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