1. Faults III
Lecture 29
Gly 326

2. Compression and deformation
In areas of compression, we have several options for deformation
Dependent on lithology, rock strength, confinement, fluid occurrence, etc.

3. Reverse/Thrust faults
Reverse fault recap:
Hanging wall moves up relative to the footwall
If shallow angle (<30°), we use the term thrust fault
Characteristic of regional compression  leads to shortening
Often with associated drag folds and more
Vocab: Detachment/Décollement: A subhorizontal gliding plane
Base of a listric fault
Flat of a thrust fault

4. Thrust faults: Horses and Wedges
A thrust fault is a reverse fault in which the dip of the fault plane is very low… (<30o)

5. Boyar and Elliot, 1982

6. Thrust systems Imbricate fans
Repeated series of thrusts sharing a dip direction
Faults flatten out into the basal thrust (décollement)
Fault dip depends on erosion level
Blind thrust: No surface expression
Normal sequence of imbrication
thrusts: younger in transport direction

7. Thrust systems Duplexes
Form when sub- horizontal thrust propagation “sticks”
Bound by floor and roof thrusts
May contain many individual horses (recall: Horse = fault bounded rock unit)

8. Duplexes Hinterland dipping duplex (Normal duplex)
Normal thrust progression
Horse bounding ramps have small overall displacements (slice length > fault slip)
New horse formation tilts older horses back (hinterland-ward)
Most common duplex structure

9. Duplexes Antiformal duplex
Greater displacement of thrust ramps (slice length = fault slip)
Horses piled one upon another
overall anticlinal geometry from fault rollover folding

10. Duplexes Foreland dipping duplex (Forward dipping duplex)
Out of sequence thrust formation
Ramp displacement > length of horse (older horses travelled farther than younger)

11. Thrust systems
Stacked geometry: Floor of an upper level duplex or imbricate sequence may be the roof of a lower duplex
Back thrusts: Antithetic faults in the thrust system. Associated with pop-up or triangle zone
Blind thrust: Tip point does not break the surface or merge w/ another fault

12. Thrust faults – associated folds
Fault bend fold
“Drag” folds in a fault zone
Fault propagation and drape fold
Detachment folds

13. Thrust faults – associated folds
Fault-bend fold:
Hanging wall sheet moves over a ramp
Thrusting begins, beds flex into kink-like folds as they move from flat to ramp, & from ramp to flat
Anticline grows in height & width, width of kink-like fold increases

14. Thrust faults – associated folds
Multiple Ramps  Multiple Fault Bend Folds

15. Thrust faults - associated folds
Fault propagation folds
Hanging wall folded during faulting
Fault and fold form at same time
As thrusting takes place, the fault tip migrates steadily upwards. Beyond the tip the fault is replaced by an overturned syncline.
Above the fault tip in the hanging wall there is a complementary fault propagation fold – an overturned anticline whose axial plane dips more steeply than the thrust.
Footwall relatively undeformed

16. Thrust faults - associated folds
Fault propagation fold

17. Thrust faults - associated folds
Fault propagation folds (several faults)

18. Thrust faults - associated folds
Fault propagation folds (several faults)

19. Thrust faults - associated folds
Detachment folds
Occurs above flats in bedding parallel to the flat
Folds expressed in competent (mechanically strong) layers
Incompetent layers flow to fill in the gaps beneath mechanically strong layers, changing unit thicknesses

20. Thrust faults - associated folds
Detachment fold

21. Thrust faults – associated folds
Continued detachment folding may result in fore and back thrusts as compression continues Pop-up geometry: Faulted top of a detachment fold

22. Thrust systems
Flats and ramps in 3D!

23. Lateral ramps act like transform faults  also called tear faults
Thrust systems
Lateral ramps act like transform faults  also called tear faults

24. Thrust faults – large scale geometry
Klippe: Erosional remnant of a thrust nappe. Sometimes the only indication of minimum displacement distance on a nappe
Fenster/Window: Erosional exposure of the footwall to a nappe

25. Keystone Thrust, Nevada
Paleozoic
Jurassic
(View to northwest)

26. HIMALAYAN FRONTAL THRUST
Himalayan Frontal thrust- active thrust slicing up foreland basin
Flat on ramp geometry here in poorly consolidated sediments

27. The McConnell thrust, Alberta.
View to North
The McConnell thrust, Alberta.
ramp

28. Example: Lewis Thrust Fault

29. Example: Lewis Thrust Fault
fenster
klippe

30. Lewis Thrust Fault (cont'd)
Source: Breck P. Kent
pre-Cambrian Limestone over
Cretaceous Shales!!!

31. Thrust faults – large scale geometry
Horse
Horse
Horse