chp 7 - Deformation Three components of deformation Rigid...

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Unformatted text preview: Deformation Three components of deformation Rigid body translation Rigid body rotation Distortion (strain) Deformation results from forces (stress) Relationship between force and deformation depends on mechanical properties of rock Mechanical properties of rock depend on intrinsic and extrinsic parameters Intrinsic and Extrinsic Parameters that Control Rock Behavior Intrinsic Parameters Mineralogy Texture of rock Flaws in rock (cracks, pores) Extrinsic Parameters Temperature Pressure Time Chemical Environment Tectonic Forces Surface forces and body forces Stress is force on a surface Force per unit area (same as pressure) Magnitude and direction Stress in a body consists of a non-directed part and a directed part Confining pressure (non-directed) Equal in all directions Exerted on a rock or sediment that is buried Compresses material but does not distort it Tectonic Forces, continued Directed Stress Compressional Stress Squeezes a rock together in one direction Tensional Stress Pulls rock apart Shear Stress Acts parallel to but in opposite directions along a failure plane Tectonic Strain The distortion or deformation that results from an applied stress Elastic Distortion Produces volume and shape changes When stress is removed, body returns to original shape and volume Nonelastic (Permanent) Distortion Plastic Behavior Failure at critical stress, fracture or flow Viscous Rheology Ductile deformation Flow rate proportional to stress Rock can be solid, yet still behave like a fluid, depending on the nature of the applied stress. Experimental Rock Deformation Brittle and ductile behavior Rock Response to Stress Time Small stresses applied over long periods of time promote slow viscous flow Rapidly applied, large stresses promote elastic deformation and/or brittle failure. Temperature Higher the temperature, greater the tendency for viscous flow versus brittle faulting Pressure Higher the pressure, greater the tendency for viscous flow versus brittle faulting Rock Type Different rocks have different strengths and ductility Rock Response to Stress, continued Crustal rock at a particular stress and applied at a particular rate: At the surface of the Earth Low temperature and low pressure conditions Deforms by faulting Brittle behavior Same rock buried 30 km Higher temperature and pressure conditions Deforms by viscous flow Ductile behavior Deformation Structures in Crust Faults (brittle) Fracture surface separating two blocks that have moved Classified on basis of orientation and movement sense Dip-slip faults, movement up and down Normal Reverse Strike-slip faults, movement sideways Left-lateral Right-lateral Folds (ductile behavior) Continuous distortion of rock layers Anticline and syncline Definition of Footwall and Hanging Wall Normal Fault Hanging wall moves down relative to footwall Extensional or tensile stress state Produces crustal thinning, extension Reverse Fault Hanging wall moves up relative to footwall Compressional stress state Produces crustal thickening, shortening Strike-slip Fault Rock on one side moves horizontally past rock on other side Slip direction is parallel to strike Transform fault is a special type of strike-slip fault Oblique-slip Fault Both dip-slip and strikeslip component of motion Common in smaller faults Folds Anticline Layers folded concave downward Map view Oldest rocks are in the center, youngest rocks are at the flanks Syncline Layers folded concave upward Map view Oldest rocks are on the flanks, youngest rocks are in the center Monocline Fold with only one limb Directed Stress and Associated Deformation Mountain Building and Evolution of Continental Crust Orogenesis - process of mountain building Igneous and tectonic processes that vary with plate tectonic setting Crustal thickening, convergence Linear Mountain chains reflect plate boundaries Origin and evolution of continental crust 40% formed 3.8 BYA, 50% by 2.5 BYA Formed rapidly early on Now formed primarily through differentiation at subduction zones Mountain Chains Relative to Pangaea Pangaea assembled 290 MYA Older, interior belts Younger, marginal belts Tectonic Features of North America Craton, Craton, Precambrian Paleozoic mountain belt, Appalachian Mesozoic mountain belt, Cordilleran Continental Collision Cordilleran System Several phases of mountain building Active today shown by topographic relief ...
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This note was uploaded on 03/25/2008 for the course GEOL 101 taught by Professor Olinsky during the Fall '08 term at Texas A&M.

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