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LSU GEOL 2020 - Lithospheric Mechanics

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Basin Analysis-Ch. 2 Lithospheric Mechanics(2 lectures)Fundamental physical properties used in to explain basin subsidence and uplift (SUMMARY)Most of what we think we know about basin evolution has come from explaining the patterns found in fundamental descriptions of geological surfaces. In the oceans, these descriptions include such parameters as the depth of the sea floor or the temperaturestructure in shallow holes. Over continents today we are also gaining new insights also by mapping topography and landscape evolution. e.g. LiDAR dataFor those portions of the crust or mantle we can not sample, rock mechanics lab experimental results provide invaluable insights into the behavior of rock properties at high temperatures (> 100 degrees C) and depths (> 1 km).If we think of the earth as an engine, overall basin evolution can be viewed as the interaction between self gravitation and the movement of the pieces of the engine against gravity. But, what is the fuel that drives this engine? It is in part the residual heat from theearly formation of the earth (primordial heat) and the heat generated by radioactively decaying elements in the mantle.This simple analogy breaks down because whereas the engine remains rigid throughout its performance, in the earth engine only the outer parts of this engine are relatively cool and the interior portions undergo fundamental physical and phase changes under the influence of heat and pressure.Flexural rigidityWe measure these forces of gravity and reaction to gravity not in terms of Newtons by using the concept of stress (See structural geology). Lithostatic stress is responsible for the increase of pressure with overall depth in the earth but it is the differential stress that creates the faults and folds.The outer skin of the earth down to depths where the temperature is cool enough and rock properties permit the earth can be visualized to be effectively elastic over long periods of time, e.g. hundreds of millions of years. e.g. rubber ballA conclusion is that mountain belts will not sag over time but will maintain their mechanical strength indefinitely for practical purposes. A measure of the strength of the crust is how much it bends to a given load. This value is known as the flexural rigiditywhich relates the bending moment (Force. Distance) and the local radius of curvature (degree of bending)Thermal conductivityFor a given temperature gradient how conductive heat transfer moves heat from areasof higher temperature to areas of lower temperature. (Notice that weather patterns show that masses of high pressure air move toward masses of lower pressure air. Hence tornadoes. Notice too that fluid flow around salt domes or in oceans are also affected by salinity gradients.) The efficiency of that transfer is the thermal conductivity. So, for a given temperature gradient (continental or oceanic geotherms) the amount of heat being passed across anygiven portion of the earth’s surface (heat flux-q) per unit time will depend on the thermalconductivity.Fourier’s Law: dzdTKq We can measure thermal conductivity with respect to standards as you can see in this overhead of a thermal conductivity measurements on board Leg ODP 150 New Jersey Margin in the summer of 1993. People are (L toR) Bryce Hoppie and Craig Fulthorpe. These needles contain heaters and temperature sensors. These needles measure the speedat which the temperature changes over time to calculate the conductivity of the material into which they are inserted.Experimental and observational summary of the mechanisms by which rocks deform (ROCK RHEOLOGY 2.3)You already should know from structural geology that there are at least 6 different factorsthat can affect rock deformation. See if you can name them off by heart:Review of structural geology:At the microscopic level convection in the mantle can be seen a the movement of a wave of atomic imperfections (diffusion) from areas of high stress to low stress or linear mis-arrangements in the crystal lattice (dislocation creep) through the peridotite(mantle rock).Creep is the plastic deformation of rock without the generation of macroscopic faults (i.e.brittle behavior). This wave of atoms or dislocations are the micro-geochemical-imperfections and micro-faults that occur when the material deforms. However, these blemishes in the chemistry and crystal structure can travel through the material aided by heat. These imperfections may travel to the edges of the body, (changing its overall shape) but leave a pristine mineralogical lattice behind in the inner self. Greater rates of movement of this imperfections occurs at greater temperatures. Apparent material “flow” can also occur at lower temperatures through assistance in diffusion with fluids in a process called pressure solution creep by dissolution of minerals in regions of highpressure and precipitation in areas of lower pressure. At low pressures and temperatures pressure-solution is an important mechanism to produce apparent linear viscous behavior ( strain rate is linearly proportional to applied stress).Ductile behavior can also occur at low temperatures (bag example) without diffusion and is known as cataclastic behavior. E.g. bag of marbles Cataclastic behavior can provide macroscopically ductile deformation within the brittle regime.A special case: Heat flow and rock behavior in the mantleHeat transferThere are three ways in which heat can be transferred. One is by simple conduction. Another is by radiative transfer and a third is by convection.Increase temperature has a marked effect on density without any other changes. So, material can become less dense than surrounding material and can experience buoyancy and the need to rise away from the earth’s center of mass.During convection of the mantle at a rate of a few centimeters per year even if no heat is added or taken away from the ascending or descending material temperature and phase changes do take place.For example by compression we know that materials heat up (air compressor burns,Pumps) We also know that if we decompress materials they can change state (e.g. water will boil at lower pressures and hence good cups of tea can not be brewed at high elevations). Mantle will melt upon decompression. Mantle will heat up upon compression.Geotherms and what we can learn about mantle processesFrom the mantle geotherm alone and assuming only conduction as a mechanism of getting heat to the


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