DEFINITION OF STRIKE-SLIP FAULT

A fault in which movement is subhorizontal,

parallel to the strike of the fault. Intense seismic activity

and deformation characterize strike-slip faults, considerable

differential movements between blocks, weak metamorphism,

and extremely high rates of and variation in sedimentation. A

strike-slip fault may be a transform (plate boundary) fault

only if it penetrates the thickness of the lithosphere and ends

in a subduction zone or terminates at a spreading ridge.

Many strike-slip faults occur in wide zones that are characterized

by many branches of the fault that split, diverge,

and rejoin in an anastomosing style. Regions where faults

converge are typically associated with uplift, whereas regions

where fault strands diverge are characterized by subsidence.

Individual strands of strike-slip faults may end, with the displacement

being taken up by another nearby fault strand.

The region between faults that are linked in this way may

host an accommodation zone, where minor faults, folds, and

other structures may help transfer the displacement from one

fault strand to another.

Strike-slip faults are rarely straight but instead are characterized

by bends or steps. These steps may show extensional

or contractional deformation, depending on the

orientation of the step, with respect to the sense of motion

along the fault. Right steps in right-slipping (also known as

right lateral) faults, and left steps in left-slipping (or left lateral)

faults form extensional bends. Steps in the opposite

direction place the bend in compression, forming folds,

thrust faults, and other contractional structures. Sedimentary

basins known as pull-apart basins form at extensional bends,

whereas fold-thrust belts and rapid uplift characterize contractional

bends.

Many strike-slip systems are not purely strike-slip but

have a component of compression or extension on a regional

scale, known as transpression or transtension. Transtensional

regimes are associated with normal faulting, drape

folding, and volcanic activity. In contrast, thrust faulting,

high-angle reverse faulting, folding, uplift, and the formation

of flower structures characterize transpressive regimes.

Relative plate convergence directions can change and thus

change the strike-slip regime from transpressive to transtension,

or vice versa.

Different types of sedimentary basins are associated with

transpression and transtension. In transpression, folding,

thrusting, and vertical uplift of mountains forms small foredeep

basins, cannibalistic nappes, flower structures, and shallow

sedimentary basins. Most of the eroded sediments from

transpression are carried and transported elsewhere, outside

the mountain range, and deposited far away in cratonic or

oceanic basins.

In contrast, in zones of transtension, basins associated

with normal faulting tend to be deep and associated with

very rapid sedimentation, with very little material transported

outside the orogen. Pull-apart basins are often associated

with strike-slip systems. These form very quickly, may be

tens of kilometers deep, and have normal faults on two sides

and strike-slip faults on the other two sides. These basins

may contain virtually every type of sedimentary environment.

Individual facies have a small lateral extent, and conglomerates

and breccias may be locally derived. Lacustrine

sediments are hallmark deposits of pull-apart basins, with

the lakes being long and narrow, with thick accumulations

of sediments. These are typically bounded by alluvial fans

fed from one or both ends of the basin. Continual subsidence

leads to facies remaining relatively stationary. After

lakes are filled, they become sites of braided streams, shallow

alluvial plains, or other, relatively quiet, sedimentary

environments.

It is often a challenge to recognize ancient strike-slip

systems in the geologic record. Different segments of a

strike-slip fault system may show pure strike-slip relationships,

extensional belts, or contractional belts. Also, strikeslip

faults often form along boundaries between different

rock terranes, making it difficult to know if there has been

lateral displacement between the different belts. Thus, to recognize

strike-slip orogens it is necessary to make a regional

assessment of the tectonics and not examine only a small or

local area.

Some characteristics of strike-slip systems are diagnostic

and recognizable in old orogenic belts. In some locations it

may be possible to match up geologic units, such as plutons

or distinctive belts of rocks that have been laterally displaced

by the strike-slip fault. In other cases, alluvial fans may be

displaced from their sources, perhaps with distinctive clasts

derived from one valley recognizable in a displaced fan.

Strike-slip systems are characterized by the development of

numerous angular unconformities in one location (such as at

a restraining bend), at the same time that rapid and continuous

sedimentation may be occurring in a nearby location

(such as in a pull-apart basin).

The regional structural pattern may be used to determine

the orientation of regional stresses, and their relationship to

the main faults in the system. Predictable patterns of folds,

strike-slip, normal, and thrust faults are associated with

strike-slip systems, and these differ from thrust and normal

fault-dominated orogens.

The types of basin filling may aid in the identification of

ancient strike-slip orogens. Conglomerates, basin-interior

lake sediments, and local volcanic rocks dominate early fill of

pull-aparts, whereas later stages and transpressional phases

may be dominated by fluvial sedimentation. All of these features

must be assessed together, as a single basin could be

interpreted in many different ways.

See also STRUCTURAL GEOLOGY; PLATE TECTONICS;

TRANSFORM PLATE MARGIN PROCESSES.

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