Lecture index: Wilcox et. al. (1973) predictive model. / Locking and restraining bends. / Transpression, transtension and flower structures. / Models of decoupling. / Examples of major transcurrent zones. /

Readings:

Chapt. 7 - Strike-Slip Faults in Twiss, R. J. & Moores, E. M., 1992, Structural Geology, Freeman & Company, p. 113-127.

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

• strike-slip, wrench, transcurrent faults; orogen parallel motions
• en echelon structures
• Wilcox et al. model of en echelon structures: card decks and clay models
• block and structure rotations
• restraining and releasing bends
• transpression and transtension
• positive and negative flower structures
• Dead Sea Rift
• San Andreas
• Alpine fault, S Island New Zealand
• Spitsbergen's Tertiary fold-thrust belt
• deformation partitioning / decoupling and weak surfaces

Some related terminology:

• strike-slip fault: technically only refers to orientation of net slip vector, hence the fault could be shallowly dipping.
• wrench fault - vertical strike-slip fault (what people usually are thinking about when they say strike-slip fault).
• transcurrent fault - big wrench fault.
• transform fault - lithospheric in dimension.
• en echelon - refers to a stepped pattern. Consistently oriented structures, within but consistently oblique to enveloping surfaces

This model enjoys experimental, theoretical. and some empirical success.

2 basic assumptions:

• 1) displacement between 2 semi-rigid blocks is accommodated by deformation within an intervening mobile zone.
• 2) pattern of history of strain ellipse in simple shear deformation path is predictive guide to variety of structures that form within the shear zone.

Basic question is how can elongation of major strain ellipse axis and shortening of minor axis be accommodated by distinct structures (inhomogenous deformation)? One can think of an underlying ductile shear zone with a major component of simple shear influencing the pattern of brittle structures in the overlying crust.

Possible to form the following sets of en echelon features in isolation or combination:

• folds and thrusts:
  • initially 45° to mobile zone boundaries.
  • with time can rotate towards parallelism.
  • sigma three locally vertical by implication.
• normal faults:
  • start out 45° to mobile zone boundaries.
  • develop pull apart basins.
  • consider sedimentary history of such a basin: with time rotate to a more oblique position (away from parallelism).
  • on small scale sigmoidal tension gashes good examples of such a history.
  • sigma one locally vertical.
  • Tension gashes, Baraboo quartzite, WI
• secondary wrench faults:
  • synthetic set:
    • initially 15° from mobile zone boundaries.
    • same sense as main zone.
    • rotates towards parallelism
  • antithetic set:
    • initially 75° from boundary.
    • opposite sense from main zone.
    • sigma 2 is locally vertical.

Take this basic scenario through an evolution and get significant complexity. Key aspect of this is rotation component. Again, faults can rotate out of an appropriate position for slip and new faults develop.

Some problems and shortcomings associated with this model:

• assumes starts out with homogeneous material. What happens if faults localizes along previously existing structure?
• doesn't address history of linkages that must develop with time. A corollary of this is that this model may explain the early formational history of a transcurrent zone well, but not the 90% or so of the activity afterwards.

Another example of a fault bend geometry that again causes fault block strain, but without the hanging vs. footwall distinction. Basically the block movement vector is at an angle to the slip plane/fault zone so that there is a material gap or overlap to be accommodated, translating to an extensional or contractional component to deformation.

This geometry can be seen at a variety of scales.

It may originate from:

• linkage of en echelon structures.
• previous anisotropy at an angle to the block movement vector.
• change in kinematics.

Ff large enough scale (lithospheric) then can a locking bend is transpressive and a releasing bend is transtensive.

Major strike-slip zones often characterized by significant geometric and kinematic change along strike - that very complexity can be a distinctive trait.

Defined in cross section perspective:

• faults steepen with depth.
• faults are predominantly strike-slip, but with a consistent dip-slip component.
• steep portion often not well imaged seismically.
• consideration of 3-d deformational balancing.

Flower structures as local shallow strike-slip duplexes.

The basic idea is that if there are transpressional or transtensional conditions, an alternate response is to have the strike-slip and dip-slip components accommodated on separate but parallel structures. Instead of a positive flower structure you could have a parallel set of thrusts and vertical strike-slip faults. This can be approached from an energy perspective.

Decoupling is promoted when a very weak slip surface is involved. This can be a previously existing surface or develop with deformation.

This behavior had been modeled in laboratory studies.

Oblique subduction zones often shows this behavior at a large scale.

New Zealand's Alpine fault

• N Island - volcanic zone, oblique subduction (joins Tonga Kermadec arc to N).
• S Island - no volcanic arc, predominantly transpressional regime.
• reason for such quick changes? near pole of rotation.
• main fault strand - Alpine fault.
• transpressional (plate motion vector oblique to fault trace).
• E side uplift 17,000' in 4 +/- 2 Ma (Sheppard et. al. 1975).
• 450 km of dextral offset: Dun mountain ultramafics provide displaced marker unit.

Displacement amounts:

• Molnar & others - 330 km in last 10 Ma on basis of geology.
• Wellman, 1973, - 300 km in past 10 Ma on basis of plate motions.
• recent movement rates .45 to 2.3 cm/year

There are en echelon folds developed and oblique motion on the main fault - can consider it a coupled or only partially decoupled system system.

Note northern faults forming - more pure transcurrent. Why? as thicken crust resistance increases, and more energy efficient to form new crust?

San Andreas fault system

Movement history?

• Mathews, 315 km offset of 23.5 Ma volcanic construct; 1.34 cm/yr.
• Anderson, 1.25 cm/yr over last 10 Ma.
• about 2.5 cm/yr for relative plate motion. Deficit - other faults.
• Crowell - total of 350 km on San Andreas fault, 1000 km on diffuse zone.

Along strike kinematics change (north to south):

• San Francisco locked area.
• middle creep section.
• Transverse ranges, Garlock fault.
• Salton trough, transtensional.

Cajun Pass drill hole and stresses along the San Andreas:

Anomalies associated with the San Andreas fault that need to be explained:

• sigma one perpendicular to fault (aftershocks, in-situ measurements).
• fold and thrust structures parallel to strike-slip fault (not oblique or en echelon).
• low heat flow.

Model of decoupling and very low strength faults (e.g. Mount & Suppe, 1987).

Dead Sea Transform

• overall sinistral transtensional fault basin.
• basins, rhombochasms, classic example of pull-apart basins?
• a distinct anomaly - basins decidedly asymmetric (see reading). Interpreted as decoupled transtension.

EPFZ system in the southern Appalachians.

Additional references:

McCaffrey, R. & 7 others, 2001, Strain Partitioning during oblique plate convergence in northern Sumatra: geodetic and seismologic constraints and numerical modeling; JGR, 105, p. 28,363-28,376.

Mount, V. S. & Suppe, J., 1987, State of stress near the San Andreas fault: Implications for wrench tectonics; Geology, 15, 1143-46

Sieh, K. & Natawidjaja, D., 2001, Neotectonics of the Sumatran fault, Indonesia, JGR, 105, 28,295-28,326. This is a beautifully done paper with abundant information and maps.

Zoback, M. D., & Beroza, G. C., 1993, Evidence for near-frictionless faulting in the 1989 Loma Prieta, California, earthquake and its aftershocks; Geology, 21, 181-185.

Copyright Harmon D. Maher Jr., This may be used for non-profit educational purposes as long as proper attribution is given. Otherwise, please contact me. Thank you.