DEFINITION OF TRANSFORM PLATE MARGIN PROCESSES

Processes that occur where two plates are sliding past each other along a transform

plate boundary, either in the oceans or on the continents.

Famous examples of transform plate boundaries on

land include the San Andreas fault in California, the Dead

Sea transform in the Middle East, the East Anatolian transform

in Turkey, and the Alpine fault in New Zealand. Transform

boundaries in the oceans are numerous, including the

many transform faults that separate segments of the midocean

ridge system. Some of the larger transform faults in the

oceans include the Romanche in the Atlantic, the Cayman

fault zone on the northern edge of the Caribbean plate, and

the Eltanin, Galapagos, Pioneer, and Mendocino fault zones

in the Pacific Ocean.

There are three main types of transform faults, including

those that connect segments of divergent boundaries

(ridge-ridge transforms), offsets in convergent boundaries,

and those that transform the motion between convergent

and divergent boundaries. Ridge-ridge transforms connect

spreading centers and develop because in this way, they minimize

the ridge segment lengths and minimize the dynamic

resistance to spreading. Ideal transforms have purely strikeslip

motions and maintain a constant distance from the pole

of rotation for the plate.

Transform segments in subduction boundaries are largely

inherited configurations formed in an earlier tectonic regime.

In collisional boundaries the inability of either plate to be subducted

yields a long-lived boundary instability, often formed

to compensate the relative motion of minor plates in complex

collisional zones, such as that between Africa and Eurasia.

The development of a divergent-convergent transform

boundary is best represented by the evolution of the San

Andreas–Fairweather fault system. When North America

overrode the East Pacific rise, the relative velocity structure

was such that a transform resulted, with a migrating triple

junction that lengthened the transform boundary.

Transform Boundaries in the Continents

Transform boundaries on the continents include the San

Andreas fault in California, the North Anatolian fault in

Turkey, the Alpine fault in New Zealand, and, by some definitions,

the Altyn Tagh and Red River faults in Asia. Transform

faults in continents show strike-slip offsets during

earthquakes and are high angle faults with dips greater than

70°. They never occur as a single fault but rather as a set of

subparallel faults. The faults are typically subparallel because

they form along theoretical slip lines (along small circles

about the pole of rotation), but the structural grain of the

rocks interferes with this prediction. The differences between

theoretical and actual fault orientations lead to the formation

of segments that have pure strike-slip motions and segments

with compressional and extensional components of motion.

Extensional segments of transform boundaries form at

left steps in left-slipping (left lateral) faults and at right steps

in right-slipping (right lateral) faults. Movement along fault

segments with extensional bends generates gaps where deep

basins known as pull-apart basins form. There are presently

about 60 active pull-apart basins on the planet, including

places like the Salton trough along the San Andreas fault,

and the Dead Sea along the Dead Sea transform. Pull-apart

basins tend form with an initially sigmoidal form, but as

movement on the fault continues, the basin becomes very

elongate parallel to the bounding faults. In some cases the

basin may extend so much that oceanic crust is generated in

the center of the pull-apart, such as along the Cayman fault

in the Caribbean. Pull-apart basins have stratigraphic and

sedimentologic characteristics similar to rifts, including rapid

lateral facies variations, basin-marginal fanglomerate and

conglomerate deposits, interior lake basins, and local

bimodal volcanic rocks. They are typically deformed soon

after they form, however, with folds and faults typical of

strike-slip regime deformation.

Compressional bends form at right bends in left lateral

faults and left steps in right lateral faults. These areas are

characterized by mountain ranges and thrust-faulted terrain

that uplift and aid erosion of the extra volume of crust compressed

into the bend in the fault. Examples of compressional

(or restraining) bends include the Transverse Ranges along

the San Andreas fault, and Mount McKinley along the Denali

fault in Alaska. Many of the faults that form along compressional

bends have low-angle dips away from the main strikeslip

fault but progressively steeper dips toward the center of

the main fault. This forms a distinctive geometry known as a

flower or palm tree structure, with a vertical strike-slip fault

in the center and branches of mixed thrust/strike-slip faults

branching off the main fault.

In a few places along compressional bends, two thrustfaulted

mountain ranges may converge, forming a rapidly subsiding

basin between the faults. These basins are known as

ramp valleys. Many ramp valleys started as pull-apart basins

and became ramp valleys when the fault geometries changed.

A distinctive suite of structures that form in predictable

orientations characterizes transform plate margins. Compressional

bends form at high angles to the principal compressive

stress, and at about 30°–45° from the main strike-slip zone.

These are often associated with flower structures, containing

a strike-slip fault at depth, and folds and thrusts near the surface.

Dilational bends often initiate with their long axes perpendicular

to the compressional bends, but large amounts of

extension may lead to the long axis being parallel to the main

fault zone. Folds, often arranged in en echelon or a stepped

manner, typically form at about 45° from the main fault

zone, with the fold axes developed perpendicular to the main

compressive stress. The sense of obliquity of many of these

structures can be used to infer the sense of shear along the

main transform faults.

Strike-slip faults along transform margins often develop

from a series of echelon fractures that initially develop in the

rock. As the strain builds up, the fractures are cut by new

sets of fractures known as Riedel fractures in new orientations.

Eventually after several sets of oblique fractures have

cut the rock, the main strike-slip fault finds the weakest part

of the newly fractured rock to propagate through, forming

the main fault.

Transform Boundaries in the Oceans

Transform plate boundaries in the oceans include the system

of ridge-ridge transform faults that are an integral part of the

mid-ocean ridge system. Magma upwells along the ridge segments,

cools, and crystallizes, becoming part of one of the

diverging plates. The two plates then slide past each other

along the transform fault between the two ridge segments,

until the plate on one side of the transform meets the ridge on

the other side of the transform. At this point, the transform

fault is typically intruded by mid-ocean ridge magma, and the

apparent extension of the transform, known as a fracture

zone, juxtaposes two segments of the same plate that move

together horizontally. Fracture zones are not extensions of

the transform faults and are no longer plate boundaries. After

the ridge/transform intersection is passed, the fracture zone

juxtaposes two segments of the same plate. There is typically

some vertical motion along this segment of the fracture zone,

since the two segments of the plate have different ages and

subside at different rates.

The transforms and ridge segments preserve an orthogonal

relationship in almost all cases, because this geometry creates

a least work configuration, creating the shortest length

of ridge possible on the spherical Earth.

Transform faults generate very complex geological relationships.

They juxtapose rocks from very different crustal

and even mantle horizons, show complex structures, alteration

by high-temperature metamorphism, and have numerous

igneous intrusions. Rock types along oceanic transforms

typically include suites of serpentinite, gabbro, pillow lavas,

lherzolites, harzburgites, amphibolite-tectonites, and even

mafic granulites.

Transform faults record a very complex history of

motion between the two oceanic plates. The relative motion

includes dip-slip (vertical) motions due to subsidence related

to the cooling of the oceanic crust. A component of dip-slip

motion occurs all along the transform, except at one critical

point, known as the crossover point, where the transform

juxtaposes oceanic lithosphere of the same age formed at the

two different ridge segments. This dip-slip motion occurs

along with the dominant strike-slip motion, recording the

sliding of one plate past the other.

Fracture zones are also called the non-transform extension

region. The motion along the fracture zone is purely dipslip,

due to the different ages of the crust with different

subsidence rates on either side of the fracture zone. The

amount of differential subsidence decreases with increasing

distance from the ridge, and the amount of dip-slip motion

decreases to near zero after about 60 million years. Subsidence

decreases according to the square root of age.

Transform faults in the ocean may juxtapose crust with

vastly different ages, thickness, temperature, and elevation.

These contrasts often lead to the development of a deep topographic

hole on the ridge axis at the intersection of the ridge

and transform. The cooling effects of the older plate against

the ridge of the opposing plate influence the axial rift topography

all along the whole ridge segment, with the highest

topographic point on the ridge being halfway between two

transform segments. Near transform zones, magma will not

reach its level of hydrostatic equilibrium because of the cooling

effects of the older cold plate adjacent to it. Therefore,

the types and amounts of magma erupted along the ridge are

influenced by the location of the transforms.

Transform faults are not typically vertical planes, nor are

they always straight lines connecting two ridge segments. The

fault planes typically curve toward the younger plate with

depth, since they tend to seek the shortest distance through

the lithosphere to the region of melt. This is a least energy

configuration, and it is easier to slide a plate along a vertically

short transform than along an unnecessarily thick fault.

This vertical curvature of the fault causes a slight change in

the position and orientation of the fault on the surface, causing

it to bend toward each ridge segment. These relationships

cause the depth of earthquakes to decrease away from the

crossover point, due to the different depth of transform fault

penetration. Motion on these curved faults also influences the

shape and depth of the transform-ridge intersection, enhancing

the topographic depression and in many causing the ridge

to curve slightly into the direction of the transform. Faults

and igneous dikes also curve away from the strike of the

ridge, toward the direction of the transform in the intersection

regions.

Many of the features of ridge-transform intersections are

observable in some ophiolite complexes (on-land fragments

of ancient oceanic lithosphere), including the Arakapas transform

in Troodos ophiolite in Cyprus, and the Coastal Complex

in the Bay of Islands ophiolite in Newfoundland.

See also OPHIOLITES; PLATE TECTONICS; STRIKE-SLIP FAULT.

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