Chapter 7 Deformation and Metamorphism

1. Chapter 7Deformation and Metamorphism • Three parts • Metamorphic settings and processes • Deformation of rock units • Metamorphic rocks and facies

2. 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

3. 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)

4. 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

6. What type of stress is involved in stretching? 30 • Compressional • Tensional • Shearing 0 0

7. Direction of Stress Horizontal - Buckle Vertical - Bend

10. Sheep Mountain AnticlineBighorn Basin, Wyoming Beds dip away form core Oldest beds in core

12. Large Structures Black Hills, S.D Michigan Basin

13. 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

21. Velocity of plates San Andreas Fault BEMT SBCC

23. Velocity of plates • SBCC • 26.8 N • -26.8 E • -3.9 V = ~38mm/yr to the northwest • BEMT • 5.0 N • -5.4 E • -2.1 V = ~7.3 mm/yr to the northwest RelativeStrike Slip Motion BEMT SBCC SBCC is moving ~5 times more quickly than BEMT

24. Pressure reduces strength

26. Metamorphism

27. Rock Cycle

28. Meta  1.    Changed in position or form     2.    After      3.    About

29. 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

30. Agents of Metamorphism • Heat • Pressure • Chemically Active Fluids

31. 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

32. Powder River Basin

34. Wyodak CoalPowder River Basin, Wyoming

35. Wyodak Mine, Gillete, Wyoming

36. 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

37. Pressure • Confining pressure applies forces equally in all directions • Rocks may also be subjected to differential stress which is unequal in different directions

39. Agents of Metamorphism • Chemically active fluids • Mainly water with other volatile components • Enhances migration of ions • Aids in recrystalizationof existing minerals

40. Agents of Metamorphism • Chemically Active Fluids • Water • CO2

41. 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)

42. 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

45. Metamorphism and Metamorphic Rocks

46. 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

47. Quartzite

48. Rochford, S.D.

49. 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