VI Strike-slip faults

Faults where a large component of the slip vector is horizontal, parallel to the strike of the fault.. Strike-slip faults that are plate boundaries are called transform faults. Other major strike slip faults within continental crust are called transcurrent faults, but the distincition is not sharp - some zones of continental deformation are divided into microplates by some authors and not by others.

Many major transcurrent faults are actually fault zones - multiple strands of faults separate blocks that (at least for periods of time) are relatively rigid. We infer that at depth these zones are underlain by ductile shear zones at mid-crustal level. Major strike slip shear zones are found in metamorphic rocks of deeply eroded orogens.

Characteristics of individual strike-slip faults

Orientation

Many but not all strike slip faults are steep. Close to the earth's surface, Anderson's theory of stress and faulting tells us that strike slip faults predominated when 2 is vertical. This situation leads to conjugate strike slip faults intersecting in a vertical line.

Typically rather straight in map view

Cobequid fault looking E

Cobequid fault looking W

Close-up of fault zone

Separation and slip

Definition of strike slip tells us that horizontal component of slip predominates. Under these circumstances, strike separation of a surface is always in the same sense (sinistral or dextral) as the slip but dip separation may vary depending on the dip of the surface.

Dip separation depends mainly on the orientation of the intersected surface and has little to do with the true slip.

Under circumstances where there is a small amount of dip slip, but the beds intersected are even more gently dipping, the possibility arises that the separation may have the reverse sense of the slip (ie. dextral separation but sinistral slip etc.)

If deformation extends into surrounding rocks it may produce a number of associated structures.

Structures in adjoining rocks

R and R' fractures, P shear fractures.

If surface of earth is allowed to rise or fall in response to stress in wall of strike-slip fault, then associated dip slip faults and or folds may form.

Twiss & Moores Fig 7.4 shows orientation

Note that if ductile shearing occurs adjacent to the fault, any or all of these features can be rotated. (In this case, a dextral fault all will rotate clockwise. Note that Twiss and Moores show this effect only in the case of folds in their fig 7.4).

Note that though many strike slip faults are approximately straight (or rather, arcs of small circles) most have irregularities. These can be characterized as left or right bends (Twiss & Moores fig 7.5)

On a left lateral fault, left bends are releasing, right bends are restraining

On a right lateral fault, right bands are releasing, left bands are restraining

Transpression & transtension

Ramsay & Huber fig 23.38

Where there is a component of shortening across a belt of significant strike slip, the environment is described as transpressional. Restraining bends are localized transpressional zones.

Where there is a component of extension across a belt of significant strike-slip, the environment is described as transtensional. Releasing bends involve localized transtension.

Ramsay & Huber fig 23.37 shows restraining & releasing bends

Terminations of strike slip faults

Slip can die out into distributed deformation, or fault can connect with another type of structure.

Twiss & Moores Fig 7.10 shows connection with normal faults and with thrusts.

Characteristics of strike-slip zones

Pure strike-slip

A straight purely strike-slip fault can move without causing any distributed deformation in the wall rocks, so much of the interest in strike-slip zones relates to features where there is a departure from pure strike slip: transpression and transtension at a large scale, releasing and restraining bends at small scale (really just small scale transpressional and transtensional structures respectively).

However, variations in rate of strike-slip within zone can be accommodated by cross-faults with extension or shortening, or by transverse folds.

Ramsay&Huber fig 23.45

Transpression

Transpression is associated with the formation of faults with a reverse sense of slip, or a component of reverse slip. When these root into a main fault they show a structure called a positive flower structure, consisting of 'horses' separated by thrust faults that steepen at depth (note: the reverse of the typical listric structure of thrust belts.

Ramsay & Huber fig 23.40

In transpressional zones, we predict that there will be bulk instantaneous shortening of a line perpendicular to the zone. Shortening structures (thrusts, folds) are particularly likely to be developed, and will be initiated in an orientation more nearly parallel to the zone than in the pure strike-slip case. Rocks within the zone, if there is ductile strain, will tend to be deformed by a flattening style of strain (ie initially spherical objects will be drawn into oblate cushion or pancake shaped ellipsoids.)

Transtension

Transtension is associated with formation of faults with normal sense of slip, or a component of normal sense. When these root into a main fault, then the structure is a negative flower structure. A sedimentary basin called a pull-apart basin may form above a negative flower structure. Note that in the case of large strains in a strike-slip belt, the normal faults may be rotated through perpendicular to the belt, into an orientation where there is instantaneous shortening across them. Hense, with large strains an originally extensional structure may become inverted in a strike-slip belt.

In transtensional zones, we predict that there will be bulk instantaneous stretching of a line perpendicular to the zone. Extensional structures (e.g. normal faults) are particularly likely to be developed, and will be initiated in an orientation more nearly parallel to the zone than in the pure strike-slip case. Strain ellipsoids will tend to be prolate (football or cigar shapes).

Combinations of transpression and transtension can result in significant rotations about subhorizontal axes

R&H 23.41

Kinematic analysis of strike-slip zones

Extensional and compressional zones typically have a linear dimension parallel to the belt length that stays roughly constant length; hence the plane strain assumption used in balancing is a good approximation for many such belt. In contrast, for most strike-slip deformation, there is no direction that is constrained to maintain constant length. The intermediate bulk strain axis is typically vertical, but can freely shorten or extend by changes in the elevation of the Earth's surface and/or the depth to the base of the lithosphere; hence plane strain assumptions are almost always invalid.

So, balancing and geometrical techniqes have progressed much less far in the case of strike slip belts as compared with thrust and rift zones.