• Faults are typically always non-planar
• Scales of irregularities from cm to 100's km
• Non-planarity results in deformation of wall rocks and/or development of gaps in fault plane (e.g. fibrous veins filling outcrop-scale faults).
• Predictable deformation of wall rocks used to infer fault geometry and vice versa

Tectonic Environments

• Spreading Centers
• Rifted continental margins: Continental margins around Atlantic for example
• Continental Rifts: North Africa, Basin and Range of Western US
• Continental Margin Slope Failures, Gulf of Mexico, also landslides

Normal Fault Systems:

• 

  Fault-line scarp of normal fault in Andes. Gneissic mylonites sample from this exposure. Represents exposure of a type of core complex (photo by Glenn Wallace).

• Traditionally steeply dipping faults inclined at about 60 degrees.
• 
• Many steeply dipping faults appear to intersect downward into a shallowly dipping detachment, or "low angle normal fault". This contrasts with the simple view of steep, planar normal faults that disappear into the earth.
• Faults my be listric or change dip (from steep to shallow)with depth (this is also true of thrust faults).
• 
• Most faults commonly dip in same direction in normal fault systems or are synthetic. Antithetic faults dip in the opposite direction.
• Fault-bend folds develop in normal fault systems, most notably rollover anticlines. As with thrust faults folds generated by movement of layers over changes in fault geometry. Changes in sedimentary layering on top of fault bend folds show their development.

Seismic line showing curved or listric fault with continuing sedimentation on basin formed by faulting.(from Twiss and Moores, 1992)

• Non-planar fault surface has many effects.
• Extensional duplexes or fault slices bounded above and below by normal faults develop in normal fault systems. In general a duplex is a fault-bounded block.
• Strain Field associated with normal faults. Extension perpendicular to fault strike (more on stress and strain later under mechanics of faulting). Circles deformed to ellipses, strain ellipse, elongated in direction of maximum extension.

Low Angle Normal Faults & Core Complexes

Extreme extension moves high grade rocks from below detachment up under low grade rocks and exposes high grade rocks at surface as gneissic "core complexes" Core complexes are rocks from mid-crustal depths and are mylonitic near detachment fault. Core complexes and associated detachments (low angle normal faults) are very common in Basin and Range of Nevada, eastern CA and Arizona.

 

Exposed mylonites at surface are known as a "core complex"

Whipple mountain map courtesy of Rebecca Dorsey (http://www.uoregon.edu/~rdorsey/Whips.GIF)

Cross Section by Eric Frost, San Diego State University

Detachment surface in Whipple Mtns. Gneissic basement complex below. Young volcanic rocks and scraps of basement above.

Whipple detachment fault. Note ledge of breccia marked by red arrow. Layering in hanging wall block strongly discordant to detachment. (from Eric Frost)

 

Indurated Breccia ledge. (from Eric Frost)

Contact of detachment surface.

Indurated breccia on detachment surface. Note coarse lineation parallel to pen.

 

Transitions from steep normal faults to detachment results in geometric problems--shown as voids, usually accommodated by deformation of rigid hanging wall block.

Steep normal fault above detachment surface.

Detail of normal fault. Note brecciation of rocks above and below and development of narrow fault core of finer-grained gouge.

Slickenlines on hanging wall. Note brecciated textures above and below slickenlined gouge face.

 

Strike Slip Faults

Tectonic Environments

• Transform Faults (spreading centers but also continental transforms like San Andreas)
• As tear faults connecting offset dip-slip fault systems

Features of Strike-Slip Faults

• Restraining or compressional bends-- folds and thrusts.

• The LA region is a prime example compression related to a restraining bend in the San Andreas Fault
• Releasing or extensional bends--normal faults and grabens

  Releasing bend structure, which direction did the strike-slip fault slip?

• Both releasing and extensional bends produce "flower structures" in cross section. Positive flower structure in case of compressional bend and negative in case of releasing bend.
• Transpression or compression normal to the fault during strike-slip and Transtension or extension normal to the fault during strike-slip. Can be used to describe motion across entire strike-slip fault zone, or that with in bends. Produces similar structures to restraining or releasing bends in strike-slip faults.
• Distributed Deformation in shear zone around planar strike-slip fault produces:
  • thrusts and folds
  • normal faults
  • related strike slip faults
  • all of these features can be produced by "simple shear" of the material around the fault zone with no compression or extension. Visualize a circle being deformed into an ellipse during shear along a fault zone and imagine where zone of compression and extension would occur. b

  • Structures developed in a shear zone. Original circle is deformed into an ellipse, as shown, with development of shear fractures, folds, extension fractures and thrust faults. No compression occurs across fault, only shear parallel to main fault trace. (from van der Pluijm and Marshak, 1997)

    Which way did the rocks get sheared in this image?

• Block rotations in strike-slip fault zones.