Week 10

Faults III. Read pages 340-371 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.

Normal faulting p.340-357

• Regional Tectonic Environments
• Ah, So Simple
• Discovery of Low-Angle Normal Faulting/Detachment Faulting
• Profett's Discovery of Low-Angle Normal Faults at Yerington
• Current Cross-Sectional Pictures of Normal Faulting
• Clay Models of Normal Faulting
• McClay Models of Normal Faulting
• Inversion Tectonics p.357

Strike-Slip Faulting p.357-371

• Regional Tectonic Settings
• Strike-Slip Faulting in Guatemala
• The Alpine Fault in New Zealand
• The San Andreas Fault
• "Ernst Cloos" Clay-Cake Modeling of Strike-Slip Faulting
• Modeling of Strike-Slip Faulting by Wilcox, Harding, and Seely
• Bends and Stepovers Along Strike-Slip Faults
• Strike-Slip Duplexes p.370
• Back to the San Andreas p.370-371

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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. How does the absolute magnitude of extension in major normal fault systems compare with the magnitude of shortening across compressional orogens?
2. Where would normal and thrust faults be developed in a strike-slip fault system?
3. Draw diagrams to illustrate the principal features associated with a normal fault system.
4. Describe the structures that would be found along a continental right-slip strike-slip fault. Include diagrams illustrating a map view of the evolving fault system.
5. Define the following terms: escape tectonics, negative flower structure, restraining bend, growth fault, antithetic fault, rollover anticline.

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Faults III

Normal faults

Regional Tectonic Environments

Areas of continental extension

• localized as rift valleys (Red Sea ~160% extension) associated with spreading centers preserved on continental margins (Gettysburg basin) (overhead)
• failed rifts (aulacogens) in continental interior (S. Oklahoma, Reelfoot, CNARS)
• widespread over broad areas (Basin & Range) - extension upto 250% (overhead)

Non-regional patterns

• radial normal fault patterns associated with domal structures (e.g. Gulf Coast salt domes)
• associated with folds, allow extension of fold arc

Normal fault in Cambrian clastic rocks in Wind River canyon, Owl Creek Mountains, Wyoming (left) and in Triassic Chugwater Formation, Seminoe Mountains, central Wyoming (right, Note lens cap for scale).

Fault geometry - the old picture

• classic horst and graben geometry with 60^o faults
• synthetic and antithetic splay faults (overhead)
• modified by rotation of fault blocks on listric normal faults

Fault geometry - the new picture

• low angle normal faults (detachment faults) discovered in Basin and Range province
• example from Yerington district, Nevada
• possible mechanisms to generate low angle faults
• rotation of initially high-angle faults - domino style faulting
• core-complex development with isostatic uplift (overhead)
• ramp/flat fault geometries (overhead)

Strike-slip faults

Regional Tectonic Environments

Areas of continental strike-slip faults

• continuations of transform fault systems marking plate boundaries (San Andreas, California; Alpine fault, New Zealand; Motagua fault, Guatemala (overhead)
• escape tectonics - associated with compressional orogens (Himalayas) (overhead)

Non-regional patterns

• tear faults in thrust fault systems (e.g. Pine Mountain thrust sheet, Appalachians)

Fault geometry

Mapping of strike-slip systems and modeling of such environments has recognized a family of structures. Movement along a strike-slip shear zone generates a local s₁ oriented at 45^o to the shear zone boundary.

_{Structures in map view}

• Riedel shears - conjugate set of faults oriented at ~15 (R shears) and 75^o (R' shears) to shear zone boundary (the R shears form preferentially) (overhead)
• P shears form at ~10^o to future fault zone, as near mirror image of R shears
• Through going, braided fault zone (Y shear) forms from combination of P, R, and R' shears
• Bends in faults within fault zone are areas of local extension (releasing bends) or shortening (restraining bends). Fault blocks enclosed by such bends create pull-apart basins and pop-up structures. (overhead)
• En echelon folds form, axes trend ~45^o from future fault zone (overhead)

Structures in cross section view

Depending upon how a cross section is constructed through a strike slip fault zone it may show extensional or compressional features.

• Cross section profiles tend to show a single fault at depth that diverges upwards into a flower or palm tree structure.

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