Page 125 - Global Tectonics

112 CHAPTER 5

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 Pacifi c sea faults from the offsets they cause of the lineations.

floor spreading has taken place about a pole of rota- Ancient transform faults on continents are more diffi -
tion that was continually changing position by small cult to identify, as their direction may be largely con-
discrete jumps. This progression has been analyzed trolled by the pre-existing crustal geology. Their trace,
and illustrated in greater detail by Engebretson et al. however, normally approximately follows a small circle
(1985). route, with any deviations from this marked by charac-
Changes in the direction of relative motions of teristic tectonic activity (Section 8.2). Ancient destruc-
plates do not cause large-scale deformation of the plate tive margins can be recognized from their linear belts
boundaries but rather result in geometric adjustments of calc-alkaline magmatism, granitic batholiths, paired
of transform faults and ocean ridge crests. This may be metamorphic belts, and, possibly, ophiolite bodies
a consequence of the lithosphere being thin at accretive (Sections 9.8, 9.9).
margins and consequently of smaller mechanical The features most commonly used for determining

strength (Le Pichon et al., 1973). That the adjustments earlier continental configurations are continental
are only minor, however, is appreciated from continen- margins and oceanic magnetic anomalies. The former
tal reconstructions such as shown in Fig. 5.18, where are obviously used to study the form of pre-drift super-
the earthquake foci associated with present day activity continents (Section 3.2.2). Because magnetic anomalies
are superimposed on the pre-drift reconstruction. The can be reliably dated (Section 4.1.6), and individual

coincidence of shape of the initial rift and modern plate anomalies identified on either side of their parental
margins indicates that there has been little post-drift spreading ridge, the locus of any particular anomaly

modification of the latter. represents an isochron. Fitting together pairs of iso-
The past relative positions of plates can be deter- chrons then allows reconstructions to be made of plates

mined by the fitting of lineaments that are known to at any time during the history of their drift (Section
have been juxtaposed originally. One approach is to fi t 4.1.7). With the additional information provided by the
former plate margins. Fossil accretive margins are orientation of fracture zones, instantaneous rates and
usually readily identified from their symmetric mag- poles of spreading can be determined for any time

netic lineations (Section 4.1.7), and fossil transform during the past 160 Ma or so; the period for which the