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CONTINENTAL TRANSFORMS AND STRIKE-SLIP FAULTS 231

Scrutton (1979), reflects its initial contact with its con- ceous opening of the South Atlantic, which was accom-
tinental counterpart on the adjacent plate and subse- panied by transform motion within what is now the
quent contact with oceanic lithosphere and an ocean Romanche Fracture Zone (Fig. 8.17a) (Mascle & Blarez,
ridge as the separation proceeds. These margins differ 1987; Attoh et al., 2004). The margin has undergone
from rifted or passive margins (Section 7.7) by a narrow little subsequent modification and so can be considered

(<30 km) continental shelf and a steep ocean–continent to represent a fossil transform margin.
transition zone. The Ivory Coast–Ghana margin displays a triangu-
One of the best-studied transform margins is the lar-shaped continental shelf, a steep (15°) continental
Ivory Coast–Ghana margin in the north of the Gulf of slope, and a narrow (6–11 km) ocean–continent transi-
Guinea. This margin formed during the Early Creta- tion zone (Fig. 8.17b). Seismic reflection data provide

(a)
75 W 60 W 45 W 30 W 15 W 0 W 15 E (c) G
10 N 10 N
RFZ
0 N South Africa 0 N
America (d) G
10 S 10 S
75 W 60 W 45 W 30 W 15 W 0 W15 E

(b)
4 W 3 W 2 W 1 W 0 1 E (e) G
6 N 6 N
u
Cote d’Ivoire Ghana d
Cape u
Three d
5 N Points u 5 N
d
u d G
Cote d’Ivoire 1000 (f)
Basin 2000
4 N 3000 4 N
Cote d’Ivoire–Ghana
4000 Marginal Ridge
Romanche FZ 5000 Guinea Basin
3 N 3 N
Divergence
Transform motion between continental crust
Transform motion between oceanic crust
Thick continental crust
Thinned continental crust
Oceanic crust
Ocean ridge axis
Marginal ridge with thinned crustal blocks
and deformed sediment

Figure 8.17 (a) Tectonic map of the equatorial Atlantic ocean showing major fracture zones that offset the

Mid-Atlantic Ridge (triangles) and location of (b) the Ivory Coast–Ghana transform margin (modified from Edwards
et al., 1997, by permission of the American Geophysical Union. Copyright © 1997 American Geophysical Union). RFZ,
Romanche Fracture Zone. Faults and folds in (b) are modified from data presented by Attoh et al. (2004). u, up; d, down.

(c–f) Simplified model of the formation of a transform continental margin (after Mascle & Blarez, 1987, with permission
from Nature 326, 378–81. Copyright © 1987 Macmillan Publishers Ltd). G, position of the Ghana transform margin.