Page 71 - Global Tectonics

58 CHAPTER 3

Figure 3.4 Fit of the southern continents and India (redrawn from Smith & Hallam, 1970, with permission from Nature
225, 139–44. Copyright 1970 Macmillan Publishers Ltd).

Gondwana. The first such reconstruction was per- not always occur as the location of the rift is often
formed by Smith & Hallam (1970) and is illustrated in controlled by the geology of the supercontinent, and
Fig. 3.4. The shapes of the continental edges of the east takes place along lines of weakness that may run paral-
coast of Africa, Madagascar, India, Australia, and Ant- lel to the geologic grain. However, there remain many

arctica are not quite so well suited to fitting as the geologic features that can be correlated across juxta-
circum-Atlantic continents. However this reconstruc- posed continental margins, some of which are listed

tion has been confirmed by subsequent analysis of the below.
record of magnetic lineations in the Indian Ocean
1 Fold belts. The continuity of the Appalachian
(Section 4.1.7).
fold belt of eastern North America with the
Caledonian fold belt of northern Europe,
illustrated in Fig. 3.5, is a particularly well-
3.3 GEOLOGIC studied example (Dewey, 1969). Within the
sedimentary deposits associated with fold belts
there is often further evidence for continental
EVIDENCE FOR drift. The grain size, composition, and age
distribution of detrital zircon minerals in the
CONTINENTAL DRIFT sediments can be used to determine the nature
and direction of their source. The source of
sediments in the Caledonides of northern
The continental reconstructions discussed in Sections Europe lies to the west in a location now
3.2.3 and 3.2.4 are based solely on the geometric fi t of occupied by the Atlantic, indicating that, in the
continental shelf edges. If they represent the true past, this location must have been occupied by
ancient configurations of continents it should be pos- continental crust (Rainbird et al., 2001; Cawood

sible to trace continuous geologic features from one et al., 2003).
continent to another across the fits. The matching of 2 Age provinces. The correlation of the patterns of

features requires the rifting of the supercontinent ages across the southern Atlantic is shown in
across the general trend of geologic features. This does Fig. 3.6, which illustrates the matching of both