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Tectonic Forces and Geologic Structures
What are geologic structures? Why do geologic structures form? How do rocks respond to tectonic forces? How do different geologic structures form? How do geologic structures relate to plate tectonic setting? Why care about geologic structures?
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What are Geologic Structures?
Shapes, arrangements and interrelationships of bedrock units. Manifestations of tectonic forces at work.
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Why do Geologic Structures Form?
Geologic structures represent a rock’s response to tectonic forces. Stress Force on an object over a specified area. Strain Change in volume or shape of an object in response to stress. Often referred to as deformation.
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Types of Stress Compressive Tensional Shear Hydrostatic pressure
Forces directed toward each other. Tensional Forces directed away from each other. Shear Forces acting parallel to a plane. Hydrostatic pressure Forces directed towards each other in all directions.
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Stresses in the Earth Hydrostatic Confining Pressure
Due to weight of overlying column of rock. Tectonic Compression Convergent plate margins Tectonic Extension Divergent plate margins Shear Transform plate margins
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Response of Rocks to Stress
Elastic Reversible, no permanent change in volume or shape. Ductile (Plastic) Permanent change in volume or shape. Brittle (fracture) Irreversible change involving rapid rupture.
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Rock’s Response to Stress
Determined by: Temperature Higher temperature promotes ductile behavior. Confining pressure Higher pressure promotes ductile behavior. Strain rate and time Faster strain rate promotes brittle behavior. Material properties (composition) Silicates more brittle than carbonates, sulfates, etc. Water Presence of water promotes ductile behavior.
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Geologic Structures Grain-scale structures Outcrop-scale structures
Pores Solution cavities Outcrop-scale structures Folds Fractures Joints Faults Unconformities
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Mapping Geologic Structures
Strike Compass direction of planar feature. Dip Angle of dip - angle between planar feature and the horizontal. Direction of dip - compass direction in which angle of dip is measured.
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Terminology of Folds Folds are bends or wavelike features in layered rocks. Folds are classified by their form: Anticline Convex upward arch, oldest rocks in center. Syncline Convex downward arch, youngest rocks in center. Plunging fold Fold axis inclined relative to horizontal.
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Interpreting Folds Folds are further classified with regard to the intensity and nature of stresses operative (‘bending vs buckling’). Open folds Isoclinal Overturned Recumbent
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Fractures Joints Faults
Fracture in rock along which there has been no offset. Faults Fracture along a planar surface across which rocks have moved relative to each other. Slip is the distance of offset across a fault.
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Types of Faults & Tectonic Setting
Normal (dip-slip) faults Form due to tension forces as in a divergent tectonic setting. Reverse or Thrust (dip-slip) faults Form due to compression forces as in a convergent tectonic setting Strike-slip faults Form due to shear forces as in a transform tectonic setting. Can be left or right-lateral in offset.
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Why Care about Geologic Structures?
Hydrocarbons [Energy] in Rocks Source rock geologic formation in which hydrocarbon originates. Hydrocarbon trap any rock barrier that accumulates hydrocarbons by preventing upward migration. Cap rock impermeable rock that prevents upward migration of hydrocarbons. Reservoir rock porous, permeable rock in which hydrocarbons accumulate.
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Why Care about Geologic Structures?
Structural hydrocarbon traps Anticlines/Domes Faults Unconformities Salt domes
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Review - Tectonic Forces and Geologic Structures
What are geologic structures? Types of stress and stresses in the Earth Response of rock to stress Role of temperature, pressure, strain rate, mineralogy and water on rock deformation Folds (terminology and interpretation) Faults (types and tectonic setting) Structure and Resources
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