Text reading: Chapt. 7 of Keary & Vine.

Some terminology:

• strike-slip fault: strike-slip component, but dip unspecified.
• wrench fault: strike-slip component and subvertical orientation.
• transcurrent fault: major wrench fault.
• transform fault: plate boundary fault system linking other boundary types.
• boundary parallel movement: just as it sounds.
• transpression: combined major strike-slip and minor convergent component across boundary/mobile zone.
• oblique convergence: combined major convergent and minor strike-slip component across mobile zone.
• transtension: combined major strike-slip and minor divergent motion component across boundary/mobile zone.

Examples of plate boundaries with major strike-slip components or of major continental transcurrent faults.

• San Andreas fault system.
• Alpine fault, New Zealand.
• Dead Sea Transform.
• Red River fault, S central China.
• De Geer fault zone, Svalbard and NE Greenland.
• Anatalya complex, Turkey.
• Nares Strait (?), between Greenland and Canadian Arctic.
• Great Glen fault, Scotland.

What are methods of determining offset history (sense of motion, amount, rates).

• offset of distinctive geologic features:
  • sedimentary assemblages with unique point sources (e.g. unique channels and fans), e.g. San Andreas.
  • igneous bodies, e.g. San Andreas.
  • oblique older geologic belts, e.g. Alpine fault.
• offset of geomorphic features (gives more recent history/rates). Neotectonics is a key word for this approach.
• geodetically (gives modern pattern). PFO observatory. GPS networks becoming more common.
• inferred from plate motions, e.g. Nares Strait.
• strain analysis of ductile fault zone (gives a minimum).

On a plate scale what geometry should a transcurrent boundary have if it is to have pure strike-slip motion along its length?

Why do continental transforms not show such a geometry?

Locking and releasing bends.

Negative and positive flower structure models.

• classic expression is of a deep seated, subvertical fault zone that branches out with ascent. The faults are oblique slip; i.e. they have both dip and strike-slip movement patterns.
• positive flower structure has a hanging wall up component, i.e. a contractional component with uplift.
• a negative flower structure has a hanging wall down component, i.e. an extensional component.
• seen seismically. Can be reproduced in clay models.
• if this pattern characterizes the length of the zone, an important question is how is the extensional or contractional component accomodated at depth?

Evolution of a coupled, en echelon, system.

In class exercise: modeling of evolution of strike-slip features in homogenous material. Using wet sand or clay as a deforming medium, we will simply watch the evolution of layers of original intact material overlying a subvertical discontinuity moving in a strike-slip fashion. One of the interesting lessons here is the complexity of the pattern that develops in such a simple set-up. As the experimental set-up deforms answer the following questions. What is the first set of structures to develop? What orientation do they have? What happens to them as deformation continues? What structures develop subsequently? How do structures link? In general, how would you describe the resulting feature after movement is complete?

Characteristic features of a coupled en echelon system include:

• oblique slip and en echelon faults.
• for transpression, curved upthrust geometries.
• rotation of structures with time.
• very complicated histories.

Decoupled transpression, an alternate mode.

• type of strain partitioning where orogen parallel shear and orogen perpendicular contraction are accomodated on different sets of structures.
• shear often concentrated in narrow zones where as contraction by necessity must be more widely distributed.
• Spitsbergen's fold-thrust belt is a good example.
• already been addressed in discussion of Fitch faults, Sunda style tectonism and oblique subduction.
• transpression can be (and usually is) partially decoupled.

Block rotation in transcurrent zones.

• determination from paleomag studies.
• roller bearing tectonics.

Sedimentation in strike-slip basins:

• moving sources.
• very deep basins.
• fault controlled sedimentation.

References:

Atwater, T., 1970, "Implications of plate tectonics for the Cenozoic evolution of western North America," Bull. Geol. Soc. Amer., vol. 81, no. 12, p. 3513-3536. This is a classic, early paper on the evolution of the San Andreas system with time. This paper was one of the early ones that showed the incredible explanatory power of plate tectonic paradigms in explaining 'continental' geology.

Ben-Avraham, Z. & Zoback, M., 1992, Transform-normal extension and asymmetric basins: an alternative to pull-apart models; Geology, v. 20, p. 423-426.

Crowell, J.C., 1979, "The San Andreas fault system through time," Jour. Geol. Soc. (London), vl. 136, p. 293-302. A good, solid, descriptive paper by a geologist who has devoted much of his career to studying and understanding the San Andreas. This is a good first paper to read to get introduced to California geology.

Howell, V.C., 1975, "Hypothesis suggesting 700km of right slip in California along northwest-oriented faults," GEOLOGY, vol. 3, no. 2, p. 81-83. This short paper introduces one to some of the types of arguments used in trying to document the movement history and magnitude for transcurrent boundaries; a problem made more difficult by the observation that historically the movement may be concentrated along different faults at different times.

Keller, E. A. & Pinter, N., 1996, Active Tectonics; Prentice Hall, Saddle River, New Jersey, 339 p. this is a good primer at the undergraduate level on neotectonics and approaches taken to describe and measures such movements.

Lowell, J.D., 1972, Spitsbergen Tertiary Orogenic belt and the Spitsbergen fracture zone: Geol. Soc. America Bull., vol. 83, p. 3091-3102. I couldn't help throwing this one in. I disagree with many specific aspects of Lowell's model, but it is an important and useful starting point.

Mount, V. S. & Suppe, J., 1992, Present-day stress orientations adjacent to active strike-slip faults: California and Sumatra: Journal of Geophysical Research, v. 97, p. 11,995-12,013.

Scholz, C.H., 1977, "Transform fault systems of California and New Zealand; similarities in their tectonic and seismic styles," JGS, (London), vol. 133, pt. 3, p. 215-229. A useful comparison between the two best known transcurrent faults in the world; this paper considers the type of earthquake patterns and behavior in transcurrent faults.

Sylvester, A.G., (compiler), 1984, Wrench Fault Tectonics, AAPG Reprint Series, #28, 374 p. A collection of classic papers including some of the above. It is divided into 3 sections: Observations, Experiment, and Theoritical Studies. The only weakness in this collection is that it heavily emphasizes the San Andreas. A few more articles on the Alpine fault wouldhave been useful.

Wilcox, R.H. and others, 1973, "Basic Wrench Tectonics," Bull. AAPG. Vol. 57, no. 1, 74-96. A classic paper, that is a must for anyone working with transcurrent tectonics; this paper presents a powerful predictive model for the myriad of structures that can form in a wrench fault setting.