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Chapter 9 – FOLDS, FAULTS & GEOLOGIC MAPS. Rock Deformation. Stress Force acting on surface, per unit area Pressure Confining (uniform) Stress is equal from all directions Differential stress Stress is stronger from some directions Strain
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Rock Deformation Stress Force acting on surface, per unit area Pressure Confining (uniform) Stress is equal from all directions Differential stress Stress is stronger from some directions Strain Change in rock shape or volume in response to stress
Tension Divergent boundaries Normal faults Compression Convergent boundaries Reverse faults Shear Transform boundaries Strike-slip faults Rock Deformation
Elastic deformation Temporary change; rebounds when stress is gone Elastic limit (AKA yield point) Ductile deformation Permanent, gradual change; folds or bends AKA Plastic deformation Brittle deformation Permanent change; breaks or cracks Rock Deformation
Factors affecting rock deformation Temperature Confining pressure Rate of deformation Composition Structural geology Study of stress & strain Strike Compass orientation of line of intersection between horizontal plane & planar feature Dip Angle between tilted surface & horizontal plane Rock Deformation & Structural Geology
Faults & fractures: Dip-slip faults Normal fault Hanging wall moves downward relative to footwall below Reverse fault Hanging wall moves up & over footwall below Thrust fault Reverse fault with shallow dip angle Structural Geology
Strike-slip fault Movement is horizontal & parallel to strike of fault Right-lateral Left-lateral San Andreas Fault Structural Geology
Folds Bend or warp in layered rock Monocline 1-sided fold Anticline Layers fold upward (arch) Older rocks in core Syncline Layers fold downward (trough) Younger rocks in core Dome Broad area of up-warping Basin Broad area of down-warping Structural Geology
Geologic Maps Geologic map Shows locations, kinds, & orientation of rock units Shows structural features Topographic maps Shows shape of ground surface, as well as location & elevation of surface features Relief Contour lines Geologic cross section Diagram of geologic features underground
What is Metamorphism? Metamorphism Mineralogical, textural, chemical & structural changes in rocks Rocks exposed to elevated temperatures &/or pressure Rocks do NOT melt
Factors that influence metamorphism Temperature 150°C = “official” boundary between diagenesis & metamorphism Pressure Composition Pore fluids Pores Open spaces in rocks Migmatite Transition between metamorphic & igneous rocks What is Metamorphism? stress stress
Factors that influence metamorphism (cont’d) Stress Foliation Planar arrangement of textural features in metamorphic rock Gives rock layered or banded appearance Duration Rate of metamorphism What is Metamorphism?
Grade Degree of temperature & pressure a rock is exposed to during metamorphism Low-grade Low temperature & pressure conditions; minimal rock changes High-grade High temperature & pressure conditions; significant rock changes What is Metamorphism?
Metamorphic Rocks • Texture: foliation • Composition: minerals present
Metamorphic Rocks Rocks with foliation Slate Very fine grained metamorphic rock; low grade Metamorphic product of shale Phyllite Fine grained metamorphic rock; intermediate grade Further metamorphism of slate
Metamorphic Rocks Rocks with foliation Schist Visible individual mineral grains; intermediate grade Further metamorphism of phyllite Gneiss Course grained, high grade, strongly foliated (light/dark bands) Further metamorphism of schist
Rocks without foliation Quartzite Metamorphosed sandstone Marble Metamorphosed limestone Metamorphic Rocks
Type of metamorphism Contact metamorphism Rocks heated & chemically changed adjacent to magma intrusion Aureole Burial metamorphism Occurs after diagenesis Due to burial of sediments in deep sedimentary basins Metamorphic Processes
Type of metamorphism Regional metamorphism Metamorphism of extensive area of crust Plate convergence, collision, subduction Other types of metamorphism Fault zone Shock Pyro-metamorphism Metamorphic Processes
Metamorphic Facies Index minerals Minerals that form at specific temperature & pressure conditions Isograds Lines of equal grade Used to map index minerals