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Home , Transform fault

Two classes of transform faults

SrM^r" } Department of , Rutgers >, Newark, New Jersey 07102

ABSTRACT The motion is strike slip, and the displacement termina11 s ab- ruptly at both ends. In this model, both transform faults are ridge- A theoretical model and experimental work with cooling wax, as to-trench types (Wilson, 1965). They represent initial cuts in the well as a consideration of some faults that were originally cited as tennis ball (the ) and thus constitute original or funda- transform faults, have suggested that two classes of transform mental plate boundaries. It is proposed that all transform faults faults exist. They are termed "boundary transform fault" and that represent original or fundamental plate boundaries be called "ridge transform fault." They differ in the degree and manner in "boundary transform faults." which they delineate plate boundaries and probably also in age rel- We can apply the concept of boundary transform fault to the ative to a given plate. South American plate (Fig. 1). The present east boundary of this plate is the Mid-Atlantic Ridge. Initially, however, the east bound- INTRODUCTION ary was an incipient break in the combined continent of and . At that time, the Mid-Atlantic Ridge and its Since Wilson's (1965) original definition of transform faults and offsets due to transform faults did not exist. The west boundary of Sykes's (1967) confirmation of the of movement along them, the South American plate is the Peru-Chile Trench bordered by the the term "transform fault" has come to be applied to such contrast- . At the inception of the drift of South America westward rel- ing features as the smallest fault offsetting a mid-oceanic ridge and ative to Africa, there must have existed a fundamental break be- the — perhaps the 's most controversial tween the South American and Antarctic plates. Wilson (1965) in- structure. Wilson (1965) also introduced another major concept — terpreted the faults at the ends of the southern Antilles — at the that is, that transform faults offsetting a mid-oceanic ridge corre- southern end of the South American plate — as paired dextral and spond to offsets of equal magnitude along the margins of rifted sinistral transform faults. One of these, although its nature is not continents. Thus, according to this concept, transform faults along entirely clear, separates the Scotia and South American plates (For- ridges are inherited features. Sykes (1967), however, was intuitively syth, 1975) and thus constitutes a boundary transform fault. At the and correctly skeptical of the inherited aspect. In view of the widespread adoption of the term "transform fault" for such contrasting features and a clearer understanding of the na- ture of plate mechanics, it now seems appropriate to describe two fundamentally different kinds of transform faults. Two classes are here proposed: (1) boundary transform faults and (2) ridge trans- form faults. Boundary transform faults are those that constitute an initial or fundamental plate boundary. They apparently may belong to any of the types originally described by Wilson (1965) but prob- ably are least likely to be ridge-ridge types. Ridge transform faults are those that are ridge-ridge types and occur exclusively along a mid-oceanic ridge. They apparently are part of, and result from, the spreading mechanism. In many instances they are episodic in de- velopment and therefore are not inherited, but rather represent a second generation of structures.

BOUNDARY TRANSFORM FAULTS

The essence of plate is the concept of rotating, inter- nally nearly rigid lithospheric plates bounded by seismically active trenches, ridges, and transform faults. These characteristics and the nature of boundary transform faults can easily be visualized by using a modification of Cox's (1973) tennis ball experiment (Fig. 1). In this model, plates are created by cutting two arc segments (T-F and T'-F') concentrically about a pole of rotation and then connecting the two segments with additional radial cuts (T—T' and F—F'). As the wedge-shaped smaller plate is rotated about the pole, a trench forms along the leading edge, a ridge forms in the opening Figure 1. Tennis ball experiment, modified from Cox (1973). Cuts are wedge behind the trailing plate edge, and transform faults (T — F made in a tennis ball to form a small plate (TFTT'), which is rotated about a pole. A trench (Tr) forms along the leading edge; a ridge (ruled area R) and T' — F') form along the original arc segments. Along these forms along the trailing edge; and boundary transform faults are rep- transform faults, relative motion of the plates is tangential to the resented along the concentric arcs (T-F and T'-F'). SAP = South Ameri- plate boundaries. The "lithosphere" along these transform faults is can plate; CP = , AfP = African plate, AnP = Antarctic conserved in the sense suggested by McKenzie and Parker (1967). plate, and NP = Nazca plate.

Geological Society of America Bulletin, v. 87, p. 1127-1130, 3 figs., August 1976, ic. no. 60806.

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northern end of South America, there must also have existed a mechanism at the inception of spreading that permitted differential relative motion of the South American and Caribbean plates. Wil- son (1965) suggested that there are transform faults at the ends of the West Indies arc; the southern one of the two faults separates the Caribbean and South American plates and apparently consists of several faults such as the El Pilar, Oca, and Bocono (Maresch, 1974; Silver and others, 1975), although Meyerhoff and Meyerhoff (1972) doubted their continuity. Nevertheless, this fault or zone of faults across northern South America would constitute a fundamen- tal plate boundary and thus be classified as a boundary transform fault. None of these plate-bounding faults are the ridge-ridge type. Initially, Wilson (1965) suggested that several large faults offset- ting ridges were transform faults; these are the San Andreas and DeGeer (Spitzbergen) faults and a offsetting the Carlsberg Ridge, presumably the Owen fault. All are ridge-ridge faults, but their sheer magnitude would seem to set them apart from the mul- titude of much smaller offsetting fractures along the mid-oceanic ridge system. Furthermore, in the case of the San Andreas fault, its real nature is not by any means certain (see Hill, 1971). The Owen fracture, in view of its obvious relation to similarly trending major fracture zones in the southwest Indian as well as movement along it far beyond the ridge (Sykes, 1968), is clearly more than a simple ridge-offsetting transform fault. All three fractures are clas- sified here as boundary transform faults. Some other major faults that have been identified as transform faults but which are classified here as boundary transform faults for similar reasons are the Alpine and Denali faults; the Median Tec- tonic fault line of Japan, the Philippines, and New Guinea faults Figure 2. Schematic representation of the wax experiment, modified (Wellman, 1971); the Ninetyeast Ridge; the Investigator and from O'Bryan and others (1975). Plates are created in a wax crust by mak- Chagos faults (Sclater and Fisher, 1974); and the Sumba fault ing a circular cut (see T'-F' in Fig. 1) and a radial cut (see F-F' in Fig. 1). (Audley-Charles, 1975). As the paddle is rotated about a pole of rotation, new thin crust ("new floor") forms in the opening wedge (see ruled area R in Fig. 1). Well after RIDGE TRANSFORM FAULTS commencement of spreading, ridge transform faults develop and offset the medially located spreading center. Ridge transform faults may be visualized using an adaptation of Cox's (1973) tennis ball model and Oldenburg and Brune's (1972) along ridges have been repeatedly emphasized (Vogt and others, wax model. Cox conceived that along the trailing edge of the rotat- 1969, 1971; Vogt and Avery, 1974) and described in many areas ing plate (Fig. 1) a ridge would form. Instead of a simple ridge, — for example, the South Pacific (Molnar and others, 1975), the O'Bryan and others (1975), in their adaptation of these two east-central Pacific (Herron, 1972), the Gulf of California models, found that new sea floor (Fig. 2) formed in the area com- (Bischoff and Henyey, 1974), the North Atlantic (Eldholm and parable to the ruled area of Figure 1, and that well after commence- Windsich, 1974; Saemundsson, 1974), and the Indian Ocean ment of spreading, a spreading center offset by arcuate ridge trans- (Sclater and Fisher, 1974). The shifting nature of spreading cen- form faults developed. ters, the episodic formation of transform faults along a mid- In O'Bryan and others' (1975) wax model, the radial cut (Fig. 2) oceanic ridge, the close spacing of transform faults (most are 50 to is comparable to the radial cut F-F' of Figure 1, and the circular 200 km apart; Turcotte, 1974), the experimental formation of cut (Fig. 2) is comparable to arc segment T'—F' of Figure 1. Motion them, and the theoretical analyses of transform faults as possible of the wax plates was tangential to the circular cut (boundary thermal contraction phenomena (Collette, 1974; Turcotte, 1974) transform fault) and strike slip. The solidified wax layer (litho- clearly seem to indicate that most ridge-ridge transform faults sphere) was conserved. result from the act of spreading and are not inherited from pre- As the wax plates (Fig. 2) rotated, wax solidified (new sea floor) existing lines of weakness, as originally proposed by Wilson (1965). in the wedge-shaped opening. In this model of new floor, a medial Subscribing to Wilson's inheritance concept, LePichon and Fox ridge or spreading center developed and was offset by arcuate (1971) and LePichon and Hayes (1971) used first-order basement transform faults concentric about the pole of rotation (Fig. 3). structures, fracture zones, and marginal offsets on both sides of the Along the faults, motion was strike slip and tangential, and the wax Atlantic to suggest the history of both the North Atlantic and South was conserved. The model faults were normal to the ridge and were Atlantic openings. Yet in the entire length of the Atlantic that they of the ridge-ridge type (Wilson, 1965). They formed well after considered, they found specific correspondence only between con- commencement of spreading, and, with changes in direction and tinental marginal features and several fracture zones on the Mid- rate of spreading, the formation of the transform faults was dis- Atlantic Ridge in the equatorial Atlantic, the area used initially by tinctly episodic. The ridge or spreading center constituted a funda- Wilson (1965) to illustrate the inheritance concept of transform mental plate boundary in the model, but its existence was pre- faults. More recently, Olivet and others (1974) apparently found determined by the radial cut. The arcuate transform faults that correspondence between the Charlie-Gibbs and cer- formed did constitute plate boundaries, but did so only between tain continental marginal offsets, and they presumably considered ridge segments. These ridge transform faults merely modified and the position of the fracture zone as inherited. In addition, L. R. in a sense were only incidental to the fundamental boundary. Sykes (1975, oral commun.) suggested that the Falkland Fracture Since 1968 (see Menard and Atwater, 1968), the shifting of Zone and its South African equivalent, the Agulhas Fracture Zone spreading centers and the episodic formation of transform faults (Dingle and Scrutton, 1974) may correspond to a major offset of

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fault pattern on the basis of the extremely low spreading rates across the East African . The lack of rectilinear -transform fault systems due to slow spreading rates was also observed ex- perimentally by O'Bryan and others (1975).

SUMMARY

The two proposed classes of transform faults fundamentally reflect contrasting genesis. Boundary transform faults occur as major boundaries of plates. They occur along fundamental breaks in the lithosphere and are necessary initially to permit differential movement between discrete plates. Ridge transform faults resu|t from the spreading mechanism and are confined to one of the three primary types of plate boundaries — the spreading center. They bound plates only between the offset ridge segments. Theoretically, spreading could occur without them and apparently does at below-minimal spreading rates.

ACKNOWLEDGMENTS

We thank H. F. Garner, T. J. Herrón, L. R. Sykes, and W. W. Wiles, all of whom critically read the manuscript and offered help- ful criticisms.

REFERENCES CITED

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