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Transform fault

One of the three fundamental types of boundaries between the mobile lithospheric plates that cover the surface of the Earth. Whereas spreading centers mark sites where crust is created between diverging plates, and subduction zones are where crust is destroyed between convergent plates, transform faults separate plates that are sliding past each other with neither creation nor destruction of crust. The primary tectonic feature of all transform faults is a strike-slip fault zone, a generally vertical fracture parallel to the relative motion between the two plates that it separates. Strike-slip fault zones are described as right-lateral if the far side is moving right relative to the near side (for example, the Queen Charlotte zone; Fig. 1), left-lateral if it is moving to the left (for example, the North Caribbean zone; Fig. 1). Not all such fault zones are plate-bounding transform faults. Small-scale strike-slip faulting is a common secondary feature of many subduction zones, especially where plate convergence is oblique, and of some spreading centers, especially those with propagating rifts; it also occurs locally deep in plate interiors. The distinguishing characteristic of a transform fault is that both ends extend to a junction with another type of plate boundary. At these junctions the divergent or convergent motion along the other boundaries is transformed into purely lateral slip. See also: Earth crust; Plate tectonics; Subduction zones

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Types

Transform faults are most readily classified by the types of plate boundary intersected at their ends, the variety of lithosphere (oceanic or continental) they separate, and by whether they are isolated or are part of a multifault system. The common oceanic type is the ridge-ridge transform, linking two literally offset axes of a spreading center (for example, Clipperton and Siqueiros transforms; Fig. 1). Also common are transform faults that link the end of a spreading center to a triple junction, the meeting place of three plates and three plate boundaries. For example, the Panama transform links a spreading axis on the mid-oceanic ridge between the diverging Cocos and Nazca plates to a Cocos-Nazca-Caribbean plate triple junction at the continental margin of Central America. See also: Lithosphere; Mid-Oceanic Ridge

Fig. 2  The effect of changing plate motion on transform faults and their fracture zones. (a) East-west plate motion; offset spreading axes with a left-lateral North Transform and a right-lateral South Transform. (b) Change in motion; a small rotation of the direction of plate motion adds components of plate convergence to North Transform and divergence to South Transform. (c) Adjustment to change; fault is segmented into two fault zones aligned parallel to the new plate motion, which are linked by a short new spreading axis.

Other types are long trench-trench transforms at the northern and southern margins of the Caribbean plate, and the combined San Andreas/Gulf of California transform, which separates the North American and Pacific plates for 1500 mi (2400 km) between triple junctions at Cape Mendocino (California) and the mouth of the Gulf of California (Fig. 1). Strike-slip faulting in the Gulf of California (and on the northern Caribbean plate boundary) occurs along several parallel (en echelon) zones linked by short spreading centers, and the overall structure is more properly called a transform fault system; similar fault patterns are found at many ridge-ridge transforms. The San Andreas part of this plate boundary exhibits another type of transform fault system, one with several simultaneously active zones that overlap, rather than replacing each other in stepwise fashion; this pattern may be characteristic of wholly continental transforms.

Geology of oceanic transforms

The structure of ridge-ridge transforms on mid-oceanic ridges varies to some extent with their length, ranging 6–600 mi (10–1000 km), and with the rate of slip of their strike-slip faults 0.8–8 in. (20–200 mm) per year, but the structure depends mainly on the geologic history of changing plate motions. In the absence of such changes, where transform faults separate plates that have maintained the same motion for millions of years, the characteristic structure is a transform valley parallel to the direction of relative plate motion, and thereby transverse to the mid-oceanic ridge. The valley floor is occupied by a strike-slip fault zone, a band of shattered rock only 300–600 ft (100–200 m) wide, that is often marked by a groove or rift in the sea floor. There is some correlation of valley depth and transform length, and most ridge-ridge transforms longer than 60 mi (100 km) have valleys deeper than the 3300–6600-ft-deep (1000–2000-m) axial rift valleys typical of the crests of slow-spreading ridges. Slow-slipping transform valleys are generally deepest at their ends, at their orthogonal intersections with axial rift valleys, whereas the deepest parts of fast-slipping valleys are usually in their midsections, their ends being partly filled with lava that spills over from the intersecting spreading axes. See also: Lava

Transform valleys are structural troughs opened by a small component of extension across ridge-ridge transform faults. Where the faults are strictly parallel to relative plate motion, the origin of this extensional stress is probably thermal contraction of the young lithosphere accreted at the intersecting spreading axes. Larger components of extension, creating deeper transform valleys, can result from small angular changes in the direction of plate motion; conversely, opposite changes can add a component of valley-closing compression. The motions of most oceanic plates are changing continuously, albeit slowly, affecting all the transform faults along their boundaries. A small rotation in the direction of plate motion has an opposite effect on the stresses at adjacent left-lateral and right-lateral transforms (Fig. 2). Such a motion has affected adjacent transform faults on part of the East Pacific Rise, where relative plate motion (spreading direction) has rotated anticlockwise by about 5° in the past 5 million years. The resulting convergence across the left-lateral Clipperton transform has closed its transform valley and thrust up a median ridge 2000 ft (600 m) high, with the strike-slip fault zone along its crest. The same rotation has caused extension at the right-lateral Siqueiros transform (Fig. 1). Some transforms react to a change of this sort by opening wider and deeper transform valleys, often accompanied by uplift of the valley margins by 0.6–1.8 mi (1–3 km) to form structures known as transverse ridges. In some cases, opening transform faults by adding a component of extension allows seawater to penetrate deep into the lithosphere, chemically altering the upper mantle to a low-density rock (serpentinite) that rises along the fault zone, forming median ridges similar in shape but quite different in origin to those at compressed transform faults. In a few examples, crustal divergence across the transform fault allows magma to leak out, building yet another type of median ridge. The Siqueiros transform shows a more common response to the change in plate motion. Instead of maintaining the same strike-slip fault zone, and adding an extensional component to its lateral motion, it developed a set of new fault zones, each parallel to the new plate motion, and slightly oblique to the overall trend of the transform fault system. Magma does leak out within the fault system, but only at the short new spreading axes which link the parallel fault zones. See also: Serpentinite

The geology of ridge-ridge transform faults is sensitive to the history of recent changes in the direction of plate motion, and the pattern of the fracture zones that they leave on the flanks of the mid-oceanic ridges provides a record of these changes. Fracture zones are bands of rough topography that extend down ridge flanks from the ends of transform faults. Their name is inherited from an early false interpretation that they are belts of strike-slip faulting across the flanks of mid-oceanic ridges. A foundation of the theories of sea-floor spreading and plate tectonics was the recognition that fracture zones are not active fault zones, merely the seams between crust that differs in age becuase it has spread different distances from laterally offset spreading axes. The lateral offsets occur at transform faults, so fracture zones are their inactive continuations. The azimuth of a fracture zone is parallel to the direction of plate motion at the time that the crust on its younger side spread off the risecrest, with fracture zone bends and kinks marking changes in direction. Mapping the trends of fracture zones is a principal method of investigating the past movements of lithospheric plates. See also: Marine geology

Geology of continental transforms

Transform faults within the continental lithosphere fracture crust that is much thicker and less homogeneous than oceanic crust. Perhaps as a result, the fault zones tend to be less straight, with many local deviations from azimuths parallel to relative plate motion. Bends in the fault zones add components of extension or compression to the dominantly strike-slip motion, resulting in along-strike alternations of collapsed extensional basins and uplifted compressional ridges. A well-known compressional bend is the Big Bend of the San Andreas fault zone north of Los Angeles (Fig. 3), where oblique convergence of the Pacific and North American plates is raising the San Bernandino and San Gabriel mountains. Some of the sediment-filled basins formed along continental transform faults are important petroleum reservoirs, and secondary deformation on the margins of the fault zone commonly folds the sediment layers to form trapping structures for oil fields. See also: Basin; Petroleum geology

Fig. 3  Map showing parts of the San Andreas and Gulf of California fault systems. Many minor but still earthquake-generating strike-slip fault zones have been omitted. The abandoned offshore fault zones, relics of a time when the plate boundary was closer to the continental margin, still have a low level of residual earthquake activity. The individual fault zones are San Andreas (SA), Hosgri (H), South San Andreas (SSA), San Jacinto (SJ), Elsinore (E), San Clemente (SC), San Benito (SB), and Guaymas (G).

Most continental transform fault systems have several belts of faulting, with complex spatial patterns of overlapping and splaying fault zones, and complex geologic histories, involving constant shifting of the share of the total interplate displacement among several zones, accompanied by the birth of new fault zones and the abandonment of others. In southern California, for example, motion on the San Andreas transform fault system is now concentrated on three narrow fault zones (Fig. 3) which differ in age and are accompanied by a multitude of less active subparallel zones, some of which may become dominant traces in the near geologic future. Very detailed geologic studies are needed to unravel the histories of continental transform fault systems, which do not leave fracture zone traces like their oceanic counterparts. The total lateral displacement between the two sides of a fault zone, commonly amounting to tens or hundreds of miles, can be estimated by recognizing the two displaced halves of preexisting geologic features that were split and separated by fault motion. On a much shorter time scale, recent displacements can be monitored by offsets in human-made features such as fence lines.

Some of the less active fault zones in the region of the San Andreas and Gulf of California systems are senescent rather than nascent transform faults. Until 6 million years ago, transform faulting was centered west of Baja California and west of the southern California coastline, and the inland shift of the Pacific–North America plate boundary has caused the almost complete cessation of faulting on the offshore San Benito and San Clemente fault zones.

Shearing continental margins

The now-inactive San Benito fault zone was representative of an important class of transform faults that extend along continental margins, at or near the boundary between oceanic and continental lithosphere. A still-active example is the Queen Charlotte fault zone off the British Columbia coast (Fig. 1). Continental margins shaped by lateral shearing (transform faulting) have very steep continental slopes, but often with steps on the slope called marginal plateaus, crustal blocks that have subsided between the shifting fault zones of a transform fault system. The shifting is commonly away from the oceanic/continental boundary into adjacent weaker continental lithosphere; indeed, the San Andreas fault system can be considered a marginal shear zone that has shifted unusually far inland.

Most shearing margins of western North America were formerly, with an earlier arrangement of a lithospheric plates, convergent (subduction zone) margins. Shearing margins with active transform faults also play a role during the birth of ocean basins by continental rifting. Initial rifting, as in the split of North America from Africa about 200 million years ago, is commonly on laterally displaced fractures that develop into spreading axes linked by transform faults. These transform faults become part of the oceanic/continental boundary once continental separation has proceeded far enough for sea-floor spreading to occur, and eventually become ridge-ridge transforms once a risecrest develops in the new ocean basin. Many of the ridge-ridge transform faults on the Mid-Atlantic Ridge are inherited from fault zones that once formed shearing parts of the continental margin. Shearing margins occur on the boundaries of the very small, young ocean basins that have opened by the splitting of Baja California from mainland Mexico (for example, Guaymas Basin, with the Guaymas transform fault on its northeast side; Fig. 3). See also: Continental margin

Earthquakes

Along a few strike-slip fault zones, lithospheric plates slide quietly and almost continuously past each other by the process called aseismic creep. Much more often, frictional resistance to the sliding in the brittle crust causes the accumulation of shear stresses that are episodically or periodically relieved by sudden shifts of crustal blocks, creating earthquakes. The largest lateral shifts (slips) of the ground surface along major continental transform faults have been associated with some of the largest earthquakes on record; in 1906 the Pacific plate alongside 270 mi (450 km) of the San Andreas Fault suddenly moved an average of 15 ft (4.5 m) northwest relative to the North American plate on the other side, and the resulting magnitude-8.2 earthquake destroyed much of San Francisco. The average slip in this single event was equivalent to about 150 - 250 years of Pacific–North American plate motion.

The maximum size of earthquake that a transform fault can generate is limited by the length of the fault, though generally, even in large earthquakes like the one in San Francisco in 1906, a fault does not fail along its entire length. The frequency of earthquakes is controlled by the average speed of relative plate motion across a transform fault plate boundary and, for fault systems with multiple overlapping fault zones, by the share of this motion that is carried by any individual fault. However, many local geologic and tectonic factors intrude to complicate estimates of how frequently a particular transform fault will produce earthquakes of any specified size or destructive power, and it is still more difficult to predict the exact timing of such an event. Extrapolation of the past record is probably the best method of estimating future magnitudes and frequencies, becuase many transform faults do seem to have a characteristic size of large earthquakes, and a consistent ratio of small to large events. In most cases, however, especially for remote oceanic transform faults, the record is too short and too incomplete to be of much practical use. See also: Earthquake; Fault and fault structures; Seismology

Peter Lonsdale

Bibliography

• W. G. Ernst (ed.), The Geotectonic Development of California, 1981
• P. J. Fox and D. G. Gallo, The geology of North Atlantic transform plate boundaries and their aseismic extensions, The Geology of North America, vol. M, pp. 157–172, 1986
• P. J. Fox and D. G. Gallo, Transforms of the eastern central Pacific, The Geology of North America, vol. N, pp. 111–124, 1989
• J. T. Wilson, A new class of faults and their bearing on continental drift, Nature, 207:343–347, 1965
• Alifazeli=egeology.blogfa.com