Finite Plate Motions

The motions of the plates described in Section 5.3 are termed geologically instantaneous as they refer to movements averaged over a very short period of geologic time. Such rotations cannot, therefore, provide information on the paths followed by the plates in arriving at the point at which the instantaneous motion is measured. Although it is a basic tenet of plate tectonics that poles of rotation remain fixed for long periods of time, consideration of the relationships between plates forming an interlinked spherical shell reveals that this cannot be the case for all plates (McKenzie & Morgan, 1969).

Consider the three plates on a sphere A, B, and C shown in Fig. 5.15a. PBA, PBC, and PAC represent Euler poles for pairs of plates that describe their instantaneous angular rotation. Let plate A be fixed. Clearly the poles PBA and PAC can remain fixed with respect to the relevant pairs of plates. Thus, for example, any transform faults developing along common plate margins would follow small circles centered on the poles. Consider now the relative movements between plates B and C. It is apparent that if A, PBA, and PAC remained fixed, the rotation vector of C relative to B (Bff>C) acts through PBC and is given by the sum of the vectors Bff)A and AroC that act about PBA and PAC, respectively. Thus, PBC lies within the plane of PBA and PAC and is fixed relative to A. Such a point, however, does not remain stationary with respect to B and C. Consequently, relative motion between B and C must take place about a pole that constantly changes position relative to B and C (Fig. 5.15b). Transform faults developed on the B-C boundary will not then follow simple small circle routes.

Even when a moving pole is not a geometric necessity, it is not uncommon for Euler poles to jump to a new location (Cox & Hart, 1986). In Fig. 5.16 the pole of rotation of plates A and B was initially at P1, and gave rise to a transform fault with a small circle of radius 30°. The new pole location is P2, 60° to the north of P1, so that the transform fault is now 90° from P2, that is, on the equator of this pole. The occurrence of this pole jump is easily recognizable from the abrupt change in curvature of the transform fault.

Menard & Atwater (1968) have recognized five different phases of spreading in the northeastern Pacific. In Fig. 5.17 it is shown that the numerous large fracture zones of this region appear to lie on small circles centered on a pole at 79°N, 111°E. If the fracture zone patterns are analysed in more detail, however, it can be seen that the fracture zones in fact consist of five different segments with significantly different orientations that can be correlated between adjacent fracture zones. The apparent gross small circle form of the fractures only represents the third phase of movement.

Euler Pole Plate Motion
Figure 5.15 The three plate problem. PAC, PBC, and PBA refer to instantaneous Euler poles between plates A and C, B and C, B and A respectively, and Aw, Ba>C, and BwA to their relative rotation vectors. In (b) P'BC is the present location of PBC. See text for explanation.

Pi Pi

Figure 5.16 (a) Rotation of plates A and B about pole P, produces arcuate fracture zones with a radius of curvature of 30°; (b) a jump of the pole of rotation to P2 causes the fracture zones to assume a radius of curvature of 90°. P' , represents the positions of pole P, after rotation about pole P2 (after Cox & Hart, 1986, with permission from Blackwell Publishing).

Figure 5.16 (a) Rotation of plates A and B about pole P, produces arcuate fracture zones with a radius of curvature of 30°; (b) a jump of the pole of rotation to P2 causes the fracture zones to assume a radius of curvature of 90°. P' , represents the positions of pole P, after rotation about pole P2 (after Cox & Hart, 1986, with permission from Blackwell Publishing).

Figure 5.17 Fracture zones in the northeastern Pacific showing trends corresponding to five possible spreading episodes, each with a new pole of rotation (redrawn from Menard & Atwater, 1968, with permission from Nature 219, 463-7. Copyright © 1968 Macmillan Publishers Ltd).

Fig: 5.18 Earthquake epicenters superimposed on a reconstruction of Australia and Antarctica (redrawn from McKenzie & Sclater, 1971, with permission from Blackwell Publishing).

It is thus apparent that in the northeastern Pacific sea floor spreading has taken place about a pole of rotation that was continually changing position by small discrete jumps. This progression has been analyzed and illustrated in greater detail by Engebretson et al. (1985).

Changes in the direction of relative motions of plates do not cause large-scale deformation of the plate boundaries but rather result in geometric adjustments of transform faults and ocean ridge crests. This may be a consequence of the lithosphere being thin at accretive margins and consequently of smaller mechanical strength (Le Pichon et al., 1973). That the adjustments are only minor, however, is appreciated from continental reconstructions such as shown in Fig. 5.18, where the earthquake foci associated with present day activity are superimposed on the pre-drift reconstruction. The coincidence of shape of the initial rift and modern plate margins indicates that there has been little post-drift modification of the latter.

The past relative positions of plates can be determined by the fitting of lineaments that are known to have been juxtaposed originally. One approach is to fit former plate margins. Fossil accretive margins are usually readily identified from their symmetric magnetic lineations (Section 4.1.7), and fossil transform faults from the offsets they cause of the lineations. Ancient transform faults on continents are more difficult to identify, as their direction may be largely controlled by the pre-existing crustal geology. Their trace, however, normally approximately follows a small circle route, with any deviations from this marked by characteristic tectonic activity (Section 8.2). Ancient destructive margins can be recognized from their linear belts of calc-alkaline magmatism, granitic batholiths, paired metamorphic belts, and, possibly, ophiolite bodies (Sections 9.8, 9.9).

The features most commonly used for determining earlier continental configurations are continental margins and oceanic magnetic anomalies. The former are obviously used to study the form of pre-drift supercontinents (Section 3.2.2). Because magnetic anomalies can be reliably dated (Section 4.1.6), and individual anomalies identified on either side of their parental spreading ridge, the locus of any particular anomaly represents an isochron. Fitting together pairs of iso-chrons then allows reconstructions to be made of plates at any time during the history of their drift (Section 4.1.7). With the additional information provided by the orientation of fracture zones, instantaneous rates and poles of spreading can be determined for any time during the past 160 Ma or so; the period for which the necessary information, from oceanic magnetic anomalies and fracture zones, is available.

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How To Have A Perfect Boating Experience

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