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Chapter 7 Deformation and Metamorphism • Three parts – Metamorphic settings and processes – Deformation of rock units – Metamorphic rocks and facies Deformation • Deformation is a general term that refers to all changes in the original form and/or size of a rock body » Deformation involves • Force – that which tends to put stationary objects in motion or changes the motions of moving objects • Most crustal deformation occurs along plate margins Deformation involves • Stress or force applied to a given area • Stress = Force/Area (psi, tsf, kg/m2) • Geologic Stress is in kbar (1,000 X surface pressure) Types of stress • Compressional stress – shortens a rock body • Tensional stress – tends to elongate or pull apart a rock unit • Lithostatic or hydrostatic stress is equal in all directions and produces a chance in size (dilation). • Differential stress is excess of lithostatic stress and produces a change in shape (distortion). • Shear stress – produces a motion similar to slippage that occurs between individual playing cards when the top of the stack is moved relative to the bottom What type of stress is involved in stretching? 30 1. Compressional 2. Tensional 3. Shearing 0 Direction of Stress Horizontal - Buckle Vertical - Bend Sheep Mountain Anticline Bighorn Basin, Wyoming Beds dip away form core Oldest beds in core Large Structures Black Hills, S.D Michigan Basin Faults are classified • Attitude of fault • Vertical-near vertical • High angle > 45o • Low angle < 45o generally less than 20 o • Relative motion along fault – Dip slip • Normal • Reverse (thrust if dip is less than 20o) – Strike slip • Right lateral • Left lateral Velocity of plates San Andreas Fault BEMT SBCC Velocity of plates • SBCC Relative Strike Slip Motion – 26.8 N – -26.8 E – -3.9 V = ~38 mm/yr to the northwest BEMT SBCC • BEMT – 5.0 N – -5.4 E – -2.1 V = ~7.3 SBCC is moving ~5 times more quickly than BEMT mm/yr to the northwest Pressure reduces strength Metamorphism Rock Cycle Meta 1. 2. 3. Changed in position or form After About Metamorphism • The transition of one rock into another by temperatures and/or pressures unlike those in which it formed • Metamorphic rocks are produced from • Igneous rocks • Sedimentary rocks • Other metamorphic rocks Agents of Metamorphism • Heat • Pressure • Chemically Active Fluids Agents of Metamorphism • Heat • Most important agent • Recrystallization results in new, stable minerals • Two major sources of heat – Contact metamorphism – heat from magma – An increase in temperature with depth due to the geothermal gradient • Minor heat source – local burning Powder River Basin Wyodak Coal Powder River Basin, Wyoming Wyodak Mine, Gillete, Wyoming Agents of Metamorphism • Pressure and differential stress • Increases with depth • Confining pressure applies forces equally in all directions • Rocks may also be subjected to differential stress which is unequal in different directions Pressure • Confining pressure applies forces equally in all directions • Rocks may also be subjected to differential stress which is unequal in different directions Agents of Metamorphism • Chemically active fluids • Mainly water with other volatile components • Enhances migration of ions • Aids in recrystalization of existing minerals Agents of Metamorphism • Chemically Active Fluids – Water – CO2 Normal Water • Solid resists shape change, resists volume change very low capacity for dissolved chemicals can penetrate only relatively large openings (immobile in the ground) • Liquid easy shape change, resists volume change, surface tension very high capacity for dissolved chemicals can penetrate relatively small openings (mobile in the ground) • Gas easy shape change, easy volume change, no surface tension low capacity to carry dissolved chemicals easily penetrate the smallest openings (very mobile in the ground) Critical Water • T>273oC and 10 bars pressure • No surface tension, can be driven through the most minute openings, can transport lots of dissolved chemicals through relatively hot rocks • Critical water transports ions to reshape minerals or create new minerals with elongation perpendicular to maximum compressive stress Metamorphism and Metamorphic Rocks Protolith • The original rock type • Sets rock chemistry – Quartz sandstone – Quartzite – Muds – Al, Ca, Fe, Mg, – Carbonates – Ca, CO3, Mg, Fe • Influences fabric – Basalts – Muds Quartzite Rochford, S.D. • Metamorphism progresses incrementally from low-grade to high-grade • During metamorphism the rock must remain essentially solid • Metamorphic settings • Contact or thermal metamorphism – driven by a rise in temperature within the host rock Barrovian Sequence (muds) (George Barrows Metamorphic Mineral Sequence) Foliated Rocks • Slate • Phylite • Schist • Gneiss Nonfoliated Rocks • • • • Quartzite Marble Anthracite Metabasalt For Wednesday For Wednesday Earthquakes Chapter 13