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ECU GEOL 1500 - Structural Geology - Fall 2014

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OVERVIEWINTRODUCTION TO STRESS AND STRAINTYPES OF DEFORMATIONSUMMARY OF DEFORMATIONTYPES OF STRESSSTANDING STRESS OR PRESSURE3 TYPES OF DIRECTIONAL STRESSSTRIKE AND DIPFAULTS AND JOINTSJOINTSDIP SLIP FAULTS - NORMAL AND REVERSESTRIKE SLIP FAULTS - LEFT LATERAL AND RIGHT LATERALEVIDENCE OF FAULTINGSAN ANDREAS FAULTREVIEW OF PLATE BOUNDARIESUNCONFORMITIESAngular Unconformity - a series of sedimentary rocks form, next the rocks areDeformed by folding or tilting, next a nondeposition or massive erosion event occurs, then a series of younger horizontal sedimentary rocks are deposited.Nonconformity – Intrusive igneous or metamorphic rock form, next a massiveerosion event occurs, then a series of younger sedimentary rocks are deposited.STRUCTURAL GEOLOGY, CRUSTAL DEFORMATION,FAULTS, FOLDS, AND MOUNTAIN BUILDINGOVERVIEWHow do rocks deform? Deformation usually occurs in the form of folding and faulting, whichhappens concurrently with regional metamorphism, igneous activity, regional uplift, and mountain building. A period of deformation, regional metamorphism, igneous activity, and regional uplift and mountain building is called an Orogeny. The Himalayan Orogeny“Animation of the Himalaya Orogeny”<http://www.classzone.com/books/earth_science/terc/content/visualizations/es1105/es1105page01.cfm?chapter_no=visualization>The Annapurna HimalayaGeology Class in Argham Jhajhamare, NEPALThe uplift and subsequent erosion of regions allows us to see parts of the Earth in 3 dimensions and to better understand deep crustal processes and materials.In terms of times scales, faulting can be episodic or gradual; however, folding is generally gradual. As you have learned previously in this course, erosion that occurs as a result of uplift (from faulting, folding, and mountain building) can be both gradual or episodic.INTRODUCTION TO STRESS AND STRAINRocks become deformed when they undergo Strain, which is a change in shape and/or volume in response to stress or pressure. Stress is an applied force per unit area and is measured in the same units as pressure. Hence, Stress causes Strain. Stress or Pressure can be Directional (Compression, Tension, or Shear) or Standing, such as Confining Pressure, which is simply a stress caused by the load or weight of rocks overlying other rocks. At shallow depths where Confining Pressures and Temperatures are relatively low, rocks tend to deform brittlely (i.e., fracturing or faulting) or elastically (i.e., recoverable gentle folding). At greater depths where Confining Pressures and temperatures are relatively high, rocks tend to deform plastically (i.e., nonrecoverable permanent folding).TYPES OF DEFORMATIONDeformation of rocks can be classified into Three Types.The first type of deformation is Elastic Deformation, in which strain (the change in shape and/or volume) is proportional to applied stress, and the deformation is recoverable. Hence, as stress increases, so does the amount of strain, and if the stress is removed, the material returns to or recovers its original shape. This type of deformation usually occurs at shallow depths where temperatures and confining pressures are low. The maximum stress that a substance can withstand and still show elastic behavior is called its Elastic Limit or Yield Point. At shallow depths breakage or brittle deformation occurs when the Elastic Limit is exceeded. The type of deformation that occurs when strain in a particular material exceeds its elastic limit depends on the pressure and temperature to which that material is subjected, time (how long the material has been subjected to stress), the rate at which the material has been subjected to stress, and the physical properties of the material. At great depths in the Earth where temperatures and confining pressures are relatively high, rocks undergo Plastic Deformation. In Plastic Deformation strain is not proportional to applied stress, and the deformation is not recoverable. Hence, in Plastic Deformation the amount of strain in a material may continue to increase even if stress increases only slightly or not at all, and Plastic Deformation is permanent. At shallow depths in the Earth where temperature and confining pressure are relatively low, rocks, especially massive ones like sandstone, granite, and gneiss, undergo Brittle Deformation. Brittle Deformation or Fracturing and Faulting of rocks is also permanent. SUMMARY OF DEFORMATIONIn summary four factors determine how rocks deform: (1) Pressure-Temperature environment in which the rock resides, which is mostly a function of depth; (2) the physicalstrength of the rock type, which is a function of its mineral composition and atomic structure. Rocks such as shale, rock salt, and rock gypsum tend to flow plastically at very shallow depths whereas rocks, such as granite, gneiss, quartzite, and massive sandstones tend to fracture or behavebrittlely at shallow depths; (3) time, or how long a rock has been subjected to stress; and (4) the rate at which it has been subjected to stress.TYPES OF STRESSSTANDING STRESS OR PRESSURE Confining Stress/Pressure - Stress that pushes inward or downward uniformly from many directions. Confining stress causes a reduction in volume and an increase in density of the materials under stress. Confining stress occurs anywhere there is a substantial load or weighton Earth materials, such as at depth in the Earth’s crust and the mantle. It decreases the possibility of brittle deformation (fracturing and/or faulting) and increases the tendency for materials to deform plastically.In addition to Confining Stress/Pressure there are also Directional Stresses that cause rocks to undergo a change in shape. There are 3 types of directional stress that cause deformation in rocks, and these are closely associated with the 3 types of Plate Boundaries.3 TYPES OF DIRECTIONAL STRESSCompressional Stress - Stress that pushes together on a rock body or compresses it. This type of stress makes the rock shorter (i.e., a change in shape). Compressional stresses are associated with Convergent Plate Boundaries, and cause most permanent folding (plastic deformation) and Reverse or Thrust Faulting.Tensional Stress - Stress that pulls a rock apart or extends its. This type of deformation increases the rock’s length (i.e., a change in shape). Tensional stresses are associatedwith Divergent Plate Boundaries and cause Normal Faulting.Shear Stress - Stress that causes a rock body to break along an


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