Crustal Deformation. Earth Systems 3209

Transcription

1 Crustal Deformation Earth Systems 3209

2 Crustal Deformation pg. 415 Refers to all changes in the original form and/or size of a rock body. May also produce changes in the location and orientation of rocks. Most deformation occurs along tectonic plate boundaries. Sedimentary rock types deform more easily because they are softer

3

4 Force pg. 415 What tends to put stationary objects in motion or change the motion of moving bodies.

5 Stress Term used by structural geologists to describe force. Is the amount of force applied over a given area. Is a measure of how concentrated force is. May be applied uniformly in all directions (uniform stress) or non-uniform (differential stress) Is the action that strains rocks

6

7 Types of Stress a. Compressional - is differential stress that shortens and thickens a body of rock by folding, flowing and faulting - associated with convergent plate collisions

8 b. Tensional Stress - is differential stress that tends to lengthen or pull apart a rock unit - Associated with divergent plate boundaries where plates are rifted apart. - Causes displacement of rocks along faults

9 c. Shear force - Is differential stress that causes two adjacent rock units to slide past each other - Occurs along weaknesses such as bedding planes, foliation and fault lines - Produces large scale offsets along transform plate boundaries

10

11 Types of Deformation a. Elastic Deformation - where rocks return nearly its original size and shape when the stress is removed. - Occurs if stress is applied gradually over long periods of time

12 Elastic Deformation

13 b. Brittle Deformation - applies to a rock body where the elastic limit (strength) is surpassed and fractures - Occurs on the surface where there is no pressure from above and is rapid - Changes in rock are permanent

14 Brittle Deformation

15

16 c. Ductile Deformation - elastic limit of rock is surpassed causing the rock body to flow. - Changes occur in size and shape without fracturing - Changes in rock are permanent and occur slowly over time

17 Ductile Deformation

18

19 So What causes the different types of deformation?

20 Factors That Affect Deformation 1. Temperature - if temperatures are low then rock behaves like a brittle solid and fractures - as temperature increases rock becomes more elastic or ductile

21 2. Confining Pressure - is low at the surface and produces brittle deformation - Increases with depth and causes ductile deformation

22 3. Rock Type - sedimentary rocks are more ductile because because of weakness along bedding planes and in the cementation of sediments - Metamorphic rocks are more ductile due to weakness along lines of foliation - Rock salt, gypsum and shale are weak/ductile - Limestone, schist and marble intermediate

23 Cont d Igneous plutonic rocks have strong chemical bonding in their crystal structure and will exhibit brittle fracture.

24 4. Time Small stresses applied to rock over long periods of geological time may cause rocks to become elastic or ductile Rapid application of force causes brittle deformation (faulting)

25 Measurements of Strike and Dip pg Strike (trend) - is the compass direction of the line produced by the inclined rock layer/fault with a horizontal Plane - is a compass bearing

26

27 2. Dip (inclination) Is the angle of inclination to the surface of a rock/fault measured from a horizontal plane Includes the angle of inclination and a direction Is always at a 90 angle from the strike.

28

29 Types of Faults pg Dip-Slip Faults a. Normal b. Reverse c. Thrust 2. Strike-Slip Faults a. Left lateral b. Right lateral 3. Transform Faults

30 Fault pg. 426 Is a break in a rock mass along which movement has occurred. Sudden movements along faults are the cause of most earthquakes

31 Parts of a Fault

32

33 1. Dip-Slip Faults pg. 427 Movement is parallel to the dip (inclination) of the fault surface Movement is vertical

34 a. Normal Fault pg Hanging wall moves down relative to footwall Caused by tension Range from 1.0 m to several km to form fault block mountains Common along divergent plate boundaries

35

36

37

38 b. Reverse Faulting pg. 428 Hanging wall moves up relative to the footwall Dip > 45 Caused by compression Common in convergent plate boundaries and subduction zones

39

40

41

42

43

44 c. Thrust Fault Is a reverse fault that dips < 45 Hanging wall moves up and over footwall Hanging wall can move from mm to hundreds of km Common in convergent plate boundaries where extreme compression exists: Alps, Himilayas, Appalachians, Andes, Rockies

45

46

47

48

49 Dover Fault NL

50 Horst and Graben

51

52 Horst and Graben Horst is an up thrust fault block or a footwall surrounded by two hanging walls Graben is a down dropped fault block surrounded by two foot walls Occur together along divergent plate boundaries and produce rift valleys and fault block mountain ranges.

53

54

55 2. Strike-Slip Faults pg. 430 Blocks of rock move horizontal and parallel to the fault surface Are long and large Caused by a shear force Associated with transform plate boundaries Movement ranges from meters to hundreds of km

56 a. Left lateral Rock is displaced to the left as you face the fault line

57

58

59

60 b. Right lateral Rock is displaced to the right as you face the fault line Ex. San Andreas

61

62

63

64 3. Transform Faults Are very long strike-slip faults that displace mid oceanic ridges Mark the edge of a plate boundary where two plates slide past each other without creating or destroying crust. Connect divergent boundaries to convergent boundaries ex. San Andreas fault

65

66 Folding Is the bending of rock layers Caused by slow continual compressional forces at depth which results in ductile deformation Common in sedimentary and volcanic rocks Can be symmetrical, asymmetrical or overturned

67 Types of Folds 1. Anticline - upfolding or arching of rock layers - Looks like an A 2. Syncline - downfolds or troughs

68

69

70