Page 70 - Global Tectonics

CONTINENTAL DRIFT 57

in the Bay of Biscay which is closed by this rotation. pattern of magnetic lineations. Geologic and geometric
Geologic evidence (Section 3.3) and information pro- considerations suggest that the Paleozoic crustal blocks
vided by magnetic lineations in the Atlantic (Section which underlie Central America were originally situ-
4.1.7) indicate that the reconstruction represents the ated within the region now occupied by the Gulf of

continental configuration during late Triassic/early México, an area existing within the reconstruction (Fig.
Jurassic times approximately 200 Ma ago. 3.3). The North Atlantic started to open about 180 Ma
Examination of Fig. 3.2 reveals a number of overlaps ago, and the South Atlantic somewhat later, about

of geologic significance, some of which may be related 130 Ma ago. The poles of rotation of the North and

to the process of stretching and thinning during the South Atlantic were sufficiently different that the
formation of rifted continental margins (Section 7.7). opening created the space between North and South
Iceland is absent because it is of Cenozoic age and its America now occupied by the Caribbean. This also
construction during the opening of the Atlantic post- allowed a clockwise rotation of the Central American
dates the reconstruction. The Bahama Platform appears blocks out of the Gulf of México to their present loca-
to overlap the African continental margin and main- tions. About 80 Ma ago the poles of rotation of the
land. It is probable, however, that the platform repre- North and South Atlantic changed to an almost identi-
sents an accumulation of sediment capped by coral on cal location in the region of the present north pole so
oceanic crust that formed after the Americas separated that from this time the whole Atlantic Ocean effectively
(Dietz & Holden, 1970). Similarly, the Niger Delta opened as a single unit.
of Africa appears to form an overlap when in fact it
also developed in part on oceanic crust formed after
rifting. 3.2.4 The reconstruction
A major criticism of the reconstruction is the overlap
of Central America on to South America and absence of Gondwana
of the Caribbean Sea. This must be viewed, however,
in the light of our knowledge of the history of the Geometric evidence alone has also been used in the
opening of the Atlantic based, for the most part, on its reconstruction of the southern continents that make up

110˚ 100˚ 90˚ 80˚

30˚ Yucatan block 30˚
C

20˚ 20˚
Chortis block

0 400 km 100˚ 90˚ 80˚

Figure 3.3 Reconstruction of the Central American region within the Bullard et al. fit of the continents around the

Atlantic (Fig. 3.2). C, location of pre-Mesozoic portions of Cuba (redrawn from White, 1980, with permission from Nature
283, 823–6. Copyright 1980 Macmillan Publishers Ltd).