Material on this page is copyright © 2018 John W.F. Waldron
Present-day examples include the
Ancient examples include:
Numerous examples in the Rockies and Appalachians
Numerous examples in East African Rift.
Note that some transfer faults get preserved in passive continental margins, especilly near large offsets in the margin (e.g. south edge of Grand Banks).
Many diagrams for strike-slip show the orientation of the instantaneous (infinitesimal) strains in strike-slip zones, assuming simple shear.
If strains are small, it is assumed that the principal stresses are parallel to the infinitesimal strain axes, so these diagrams have been usefully used to predict and interpret the orientations in which structures form.
Note that ideal simple shear is a plane strain, and can be treated in 2D, though the 'plane' is horizontal in this case. Hence a map view is more informative than a cross-section. Cross-sections may show either shortening or extension, depending on the orientation of the line of section and the sense of shear.
Structures may include:
With finite strain everything rotates, so structures may not end up in the orientation in which they formed.
It's possible to predict the rotation of structures with simple shear. The diagram shows the eventual rotation of structures formed in response to the instantaneous strain.
Note that the instantaneous strain axes remain at 45° to the shear zone boundary throughout deformation in ideal strike-slip.
However, the finite strain axes become progressively rotated (clockwise in the above dextral example).
Individual structures may rotate from the field of extension into the field of shortening. As a result an initial extensional structure (e.g. a normal fault) may become reactivated as an oblique or strike-slip fault and then inverted as a shortening structure such as a reverse fault).
What we have called 'ideal strike-slip' is a plane strain, with the 'plane' horizontal. There is neither extension nor shortening in the vertical direction.
In long straight thrust belts, or in long straight rifts, bulk plane strain is a reasonable assumption; departures from plain strain would produce space problems at the end of the belt.
In a belt of strike slip, plane strain can never be assumed. Extension in the vertical direction can easily occur, leading to the formation of a topographic ridge; material can be removed by erosion. Shortening in the vertical direction can also easily occur, leading to the formation of a sedimentary basin, which may be filled by deposition.
In ideal strike-slip, particle paths are parallel to the shear-zone boundary. In transpression and transtension the paths of particles diverge or converge with the shear zone boundary. The angle between the particle paths and the overall shear zone is known as alpha.
Note that transpression and transtension sound like dynamic terms. However, they are not, they are kinematic terms. For this reason it's incorrect to refer to "transtensional stress" or "transpressional stress". (The technical reason for this is that stress is described by a symmetric tensor that has no rotation component; it's the boundary conditions, not the stress, that cause rotation in a strike-slip zone. In contrast, deformation is described by an asymmetric tensor that captures both distortion and rotation.)
Transpression describes a state of strain that combines strike-slip motion, horizontal shortening and vertical extension.
The relative amounts of shortening and strike-slip can be expressed in the convergence angle alpha which ranges from zero (ideal strike-slip) to 90 degrees (ideal convergence)
We can make some generalizations about the strain. If transpression proceeds without volume loss, then some material needs to be extruded from a transpressional zone. That material typically goes upwards - the easier escape route.
Transpressional strain is most clearly developed at restraining bends and stepovers on strike-slip faults - a right-hand bend on a sinstral fault or a left-hand bend on a dextral fault.
A typical product of extrusion from a transpressional zone is a positive flower structure
Transpressional zones are typically characterized by oblate (flattening) strains.
At alpha angles greater than 20°, shortening predominates over strike-slip shear, the vertical finite strain axis becomes the longest (S1). When this happens, stretching lineations will switch from horizontal to vertical. This phenomenon is seen in the history of many shear zones and is an indication of pure-shear-dominated transpression.
Folds in transpressional zones may nucleate early in the strain history, and rotate progressively towards the shear zone boundary. Fabrics also rotate, but if the fabric follows the finite S1 S2 plane it will not rotate as fast as material lines. Hence fabrics in transpression zones are often not perfectly axial planar to folds, but instead transect the folds. Dextral transpressional zones have counterclockwise transpression; sinistral transpression produces clockwise transpression.
Transpression results in
Transtension is a combination of horizontal extension, vertical shortening, and strike slip. The relative importance of extension vs strike slip can be represented by the divergence angle alpha, which ranges from zero (ideal strike-slip) to 90 degrees (ideal extension).
The most common transtensional zones are at releasing bends on strike-slip faults, and in oblique rifts. Transtensional zones are characterized by lithospheric thinning and are sometimes the sites of deep sedimentary basins (pull-apart basins).
Ductile fabrics are scarce in most transtensional zones in outcrop. However, if a crustal extension is superimposed on ideal strike-slip kinematics, the resulting strain ellipsoid has its vertical S2 shortened relative to the ideal strike-slip case, producing a prolate strain ellipsoid. At depth we might expect such zones to show a weak subhorizontal foliation and strong lineations oblique to the SZB.
The shortening direction in transtension may be vertical or horizontal. If pure-shear predominates, alpha is greater than 20° and a vertical S3 axis is predicted. Alternatively, in simple-shear dominated transtension, the shortening axis S3 is horizontal.
Transtension is the style typical of pull-apart basins, but occurs in other environments such as oblique rifts.
The most characteristic structure in cross-sections of pull-apart basins is a downward-converging configuration of faults with combined normal and strike-slip offsets - a negative flower structure
Transtension results in
Transtensional environments produce lithospheric thinning and are therefore tend to produce sedimentary basins that may be sites of hydrocarbon accumulations
Transtensional basins tend to be characterized by
Examples