Plate boundaries with major strike-slip components.



A strike-slip plate boundary with a simple and 'pure' geometry could be fairly boring. Luckily, most transcurrent boundaries show a complexity of geometries, and take advantage of a host of crustal irregularities so that they are usually very interesting, showing a rich array of patterns and behavior.

Some terminology for strike slip faults:

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

As you might guess there are multiple approaches.

Transpression and transtension

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 tend to show such a geometry?

In this schematic example of the boundary between two plates (A and B) on an equal angle stereonet plotted so that the pole of rotation and associated small and large circle paths for movement between the two plates are shown, one can see how bends in the orientation of a plate boundary can produce transpression and transtension. Remember that the small circle paths are the ones that points on a rigid plate interior will follow. From the geometry it then follows that: section ED of the plate boundary will be pure convergence, EF will be dextral transpression, FG will be pure dextral transcurrence, GH will be dextral transtension, HI will be pure dirvergence, IJ will be sinistral transcurrence (and has a transform fault geometry), JK will be pure divergence, and KL will be dextral transcurrence (and also with a transform fault geometry). Underlying stereonet image created with stereonet program - infor at .

The simple approach is that locking bends produce transpression and releasing bends produce transtension. The terminology may be thought of reflecting how crust is stronger in compression than in extension, and so bends with a convergent component may lock up and produce larger earthquakes.

The image to the right is an example of a locking bend geometry on a small scale for a sinistral fault. Image source: .

A standard model for the architecture of zones of transpression and transtenion are positive and negative flower structures (respectively). This architecture can be thought of as coupled transpression and transtension, because both the strike-slip and contractional components are taken up on the same set of faults, which should be characterized by oblique slip.

Block diagrams of positive and negative flower structures from USGS report: .

Evolution of a coupled, en echelon, system - linkage of earlier formed structures.

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?

YouTube video of similar experiment done in gypsum by Marc Holland - . Note the changes along strike in the features developed.

YouTube video of sand box experiment with releasing and locking bends from "The Geo Models" effort - .

Coupled versus decoupled transpression and trantension

Characteristic features of a coupled en echelon system include:

Decoupled transpression, an alternate mode:

Block rotation in transcurrent zones

Diagram from USGS publication showing the rotation of Puerto Rico in a complex sinistral deformation zone. From: D.G. Masson and K.M. Scanlon, 1991, The neotectonic setting of Puerto Rico - , and modified from modified from GSA Bulletin, 1991. v. 103, no. 1, p. 144-154.

Sedimentation in strike-slip basins

These are usually associated with transtension:



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.

Cunningham, W. D. & Mann, P. (eds) Tectonics of Strike-Slip Restraining and Releasing Bends. Geological Society, London, Special Publications, 290, 169 – 188.

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 would have been useful.

ten Brink, U. S. & Ben-Avraham, 1989, The Anatomy of a pull-apart basin: seismic reflection observations of the Dead Sea Basin; Tectonics, 8, 333-350.

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.

Course materials for Plate Tectonics, GEOL 3700, University of Nebraska at Omaha. Instructor: H. D. Maher Jr., copyright. This material may be used for non-profit educational purposes with appropriate attribution of authorship. Otherwise please contact author.