Week 9

Faults II. Read pages 304-313, 315-317, and 319-339 in Chapter 6: Faults.

• readings
• terms
• questions
• classnotes

You are expected to read all the sections listed below. Information from the sections in italics will be discussed in class. You are expected to read the other sections and you may be called on in class to answer questions based on that material.

Dynamic Analysis of Faulting p.304-319

• Anderson's Theory of Faulting
• Experimental Deformation and Coulomb Failure
• M.King Hubbert's Sandbox Illustrations of Coulomb Theory
• Exceptions to the Law and Special Considerations
• The Problem of Reverse Faults

• Regional Characteristics
• Thin-Skinned Deformation and Decollement
• Rich's Model of Bedding-Step Thrusting
• Ramp-Flat Geometry and Kinematics
• Horses, Imbricate Fans, and Duplexes
• The Moine Thrust, Northern Scotland
• Dahlstrom's Guidelines on Thrust Kinematics
• Mechanical Paradox of Thrusting
• The Wedge Model of Overthrusting
• Wedge Apparatus Experiments

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You should become familiar with the following terms during this weeks lectures and readings:

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You should be able to answer the questions below following this week:

1. What was the direction of tectonic transport for the Canadian Rockies?
2. What was the order of formation for the following thrusts illustrated in Figure 6.83: Bourgeau thrust, Brazeau thrust, Chatter Creek fault, McConnell thrust?
3. Explain how Anderson's theory of faulting and Coulomb's law of failure can be combined to account for the orientations of strike-slip, thrust-slip and normal-slip faults. What are the exceptions to this rule?
4. How would the development of a critical taper in an active thrust belt be influenced by the rate of erosion?
5. The cross section below is drawn through part of the Valley and Ridge province. Answer the following questions on the diagram. Which side of the image is closer to the foreland? Locate a fault plane where a hanging wall flat overlies a footwall ramp. Locate a fault plane where a hanging wall flat overlies a footwall flat. Locate a fault plane where a hanging wall ramp overlies a footwall flat. Outline a horse on the cross section. Which fault formed first? What term can be used to describe the assemblage of faults illustrated?

6. Define what is meant by: window & klippe; fault-bend folds; bow and arrow rule; critical taper; imbricate thrust system; duplex.
7. Highlight the differences in the evolution of fault-bend folds and fault-propagation folds.
8. Discuss the role of elevated fluid pressures in the transportation of large thrust sheets. Include a description of how changing fluid pressures influences the Mohr-Coulomb failure equation.

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Faults II

 How to determine slip on faults

1. using slickenlines, striations give direction of slip

• match hanging wall and footwall cutoffs to determine magnitude and sense of slip
• cut-off - line formed by the intersection of a bed with the fault plane

2. using drag folds to determine slip

• distortion of bedding due to shearing along a fault
• folds are convex toward direction of slip
• one exception - rollover anticlines along listric normal faults

Strain significance of faults

• slip on normal faults extends and thins layering (assuming it was originally horizontal)
• thrust and reverse faults shorten and thicken layering

Dynamic analysis of faulting

We can combine our previous discussion about:

• the relationship between s₁ and fault orientation
• with observations about principal stress orientations at the earth’s surface;

to explain why normal faults typically dip at 60o, thrust faults have an average dip of 30o, and strike-slip faults are normally vertical.

1. Coulombs’s law predicts faults should be oriented at ~30^o to s₁, maximum principal stress.
2. Anderson (1951) pointed out that shear stress must be zero at the earth’s surface.
3. We know that shear stress is zero parallel to principal stress directions.
4. The earth’s surface must be a plane containing two of the three principal stress directions.
5. The third direction is perpendicular to the first two, therefore, must be vertical.

The above conditions give us three possible combinations of principal stresses:

• s₁ vertical, s₂, s₃ are horizontal - normal faults;
• s₂ is vertical, s₁, s₃ are horizontal - strike-slip faults;
• s₃ is vertical, s₁, s₂ are horizontal - thrust faults.

conjugate faults - pairs of faults ~60^o apart, bisected by s₁

Exceptions to the rule

• anisotropic rocks - rocks with foliations oriented between ~25-45^o to s₁ will fail along the foliation plane.
• rocks with pre-existing fractures - fractures oriented relatively close to the ideal 30^o angle will be reactivated rather than have new fractures form.
• influence of s₂ on slip on pre-existing fractures - if s₁ = s₂, slip on the existing fracture will be skewed towards the s₃ direction. If s₂ = s₃, slip will be along the true dip direction of the fault.
• reverse faults - reverse faults are oriented too steeply relative to a horizontal s₁ to explain their formation. This can be explained by the reactivation of normal faults or by complex stress fields

Thrust Faults

associated with compressional orogens, e.g. Appalachians, Andes, Himalayas

• hinterland vs. foreland
• Appalachian orogen: plateau - valley & ridge - blue ridge - piedmont - coastal plain
• thin-skinned vs. thick-skinned - thin-skinned does not involve basement rocks, thick skinned does involve basement
• detachment horizons - in weak rocks within sedimentary section
• thrust sheet - volume of rock bounded by a thrust
• ramps and flats: hanging wall ramp, hanging wall flat, footwall ramp, footwall flat – complete exercise on handout
• fault-bend folds - flat-ramp-flat fault geometry
• fault-propagation folds - flat-ramp fault geometry (blind thrust)
• detachment folds - flat fault geometry
• horse - fault block surrounded by fault surfaces
• multiple fault systems: imbricate faults or duplex – controlled by presence of an upper detachment horizon
• displacement transfer – multiple thrust faults transfer displacement along the length of a thrust belt
• klippe – an erosional remnant of thrust sheet (hanging wall) surrounded by footwall rocks
• window – an exposure of the footwall of a thrust viewed through the hanging wall
• outlier & inlier – erosional landforms where younger rocks are surrounded by older rocks and older rocks surrounded by younger rocks, respectively (not to be confused with klippe and window)
• determination of shortening across thrust fault systems

Mechanical Paradox of Overthrusting

 Hubbert and Rubey showed that thrust sheets are too big to be moved by simply pushing from the back of the sheet. Imagine thrust sheets as rectangular blocks; the force necessary to push the block forward must overcome the normal force acting on the base of the block.

• The above calculation assumes that the maximum width of a thrust sheet would be approximately 10 km, clearly less than the length of many thrust sheets in fold and thrust belts.
• Consequently, Hubbert and Rubey proposed that normal stress must be offset by pore pressure (see Week 7) otherwise, thrust sheets would not move, but simply fracture at the hinterland margin.

Critical Taper

• The shape of fold and thrust belts is best represented as a wedge.
• Recent analyses of deformation in thrust belts have assumed that wedges develop a critical taper (angle between top [land surface] and bottom [detachment] surfaces of wedge) that is dependent upon the strength of the materials and the characteristics of the detachment plane.
• The wedge deforms internally until it reaches the critical taper and then is translated as a whole on the detachment plane. Erosion or the addition of material at the toe of the wedge will alter the taper and require further internal deformation

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