1. Bryce Canyon National Park, UT

2. Bryce Canyon National Park, UT

3. Today’s topics
Intro to Fractures
Faults and Fault classification

4. Fractures
Fractures are the main structures associated with “Brittle” deformation. Found almost everywhere at solid Earth’s surface. Most characteristic of deformation in upper crust.
Important for reconstructing tectonic evolution of geologic areas, and impact fluid flow (e.g., water, oil, gas) in the subsurface  can act has permeable channels or impermeable barriers depending on nature of host rocks and fault rocks
Joints
Fault

5. Fractures What is a fracture?
Subplanar surface or discontinuity across which cohesion has been lost (although cohesion can be later healed)
Typically very localized (narrow) but they all have some finite thickness.
Two main families of fractures
Extensional fractures
joints, fissures, veins
displacement perpendicular to walls
Shear fractures
displacement parallel to walls
if displacement is large, we call these FAULTS

6. Fracture Types Extensional fractures joints, fissures, veins
displacement perpendicular to walls
Shear fractures
displacement parallel to walls
if displacement is large, we call these FAULTS

7. Fractures Fracture formation in response to principal stresses
We’ll spend a lot more time on stress later in the semester, but the image to the left depicts the main types of fractures and their most common geometric relationship to the 3 principal stresses:
s1 (maximum)
s2 (intermediate)
s3 (minimum)
Stresses are compressive when pointing inward
Note that this figure (from Ch. 7 in Fossen textbook) could also be useful when you do the analysis of 6-mile fold in labs 4, 5, and 6.

8. Conjugate Shear fractures/faults
© Cambridge University Press 2011

9. Areal Photograph of high angle joints in Permian Cedar Mesa Sandstone from Canyonlands National Park, UT

10. Joint planes in Esplanade Sandstone

11. Intro. to Faults

12. Fault classification Based on two basic observations:
Orientation of the fault surface
(strike/dip of the surface)
Nature of the relative displacement
across the fault (sense of shear)
(trend/plunge of slip or displacement vector)

13. Fault terms and classification
For non-vertical faults ,there is a “footwall” and a “hanging wall”
A brief note on stereonets. This is a graphical method for projecting 3-D orientation for planes and lines onto a 2-D plane (paper). You will much more with sterenets in lab, but you could get a head start by looking at Appendix B in your textbook or Fossen’s e-module on stereographic projection (
© Cambridge University Press 2011

14. Net slip of a fault Net slip (or displacement)
Hangingwall
Footwall
Net slip
(or displacement)
The line on the fault surface that connects two points that were together before deformation
It is a vector (direction and magnitude), and within the fault plane has strike-slip and dip-slip components

15. A and A’ were adjacent before faulting

16. Fault Types 2 other special classes: 2 endmember classes:
Dip-slip
Oblique-slip
Normal
Reverse
Strike-slip
Rotational
2 endmember classes:
Dip-slip (2 types)
Strike-slip (2 types)

17. Fault systems in the Western U.S.
Active thrust faults
Cenozoic & active
normal faults
overprinting Mesozoic
thrust faults
Cenozoic Thrust faults
Mesozoic Thrust faults
Cenozoic & Active Strike-Slip faults

18. Common map symbols for faults Normal faults -hatches or balls on
“downthrown side”
If dip unknown,
U-up, D-down
Reverse/Thrust faults
“teeth” on hanging wall
Strike-slip faults
-arrows indicate
Sense of slip
Common map symbols
for faults

20. Black Mountains, Death Valley
East
West

21. Black Mountains, Death Valley
East
West
Footwall
Hanging Wall

23. Chief Mountain, Glacier National Park
klippe
“allochthonous”
“autochthonous”

24. Relative motion (sense of slip) of
strike-slip faults

25. Right-Lateral (Dextral)

26. Left-Lateral (Sinistral)

27. Arrow Heads ( ) and Tails ( )
Block moving toward you like an arrowhead heading towards you
Block moving away from you like seeing the feathers of an arrow

28. Arrow Heads ( ) and Tails ( )
Right-lateral or left-lateral?

29. Arrow Heads ( ) and Tails ( )
Left-lateral

30. Arrow Heads ( ) and Tails ( )
Sinistral or Dextral?

31. Arrow Heads ( ) and Tails ( )
Dextral

32. Basic Fault Types Right-lateral (dextral) strike-slip fault
Izmit, Turkey
Left-lateral (sinistral) strike-slip fault
Guatemala

33. Oceanic Strike Slip fault: Transforms

34. Some other general characteristics of faults (of all types)