Margins

Where a transform fault develops during continental rifting the continental margin is defined by the transform fault and is termed a transform continental margin. The history of such a margin, first considered by

Scrutton (1979), reflects its initial contact with its continental counterpart on the adjacent plate and subsequent contact with oceanic lithosphere and an ocean ridge as the separation proceeds. These margins differ from rifted or passive margins (Section 7.7) by a narrow (<30 km) continental shelf and a steep ocean-continent transition zone.

One of the best-studied transform margins is the Ivory Coast-Ghana margin in the north of the Gulf of Guinea. This margin formed during the Early Creta ceous opening of the South Atlantic, which was accompanied by transform motion within what is now the Romanche Fracture Zone (Fig. 8.17a) (Mascle & Blarez, 1987; Attoh et al., 2004). The margin has undergone little subsequent modification and so can be considered to represent a fossil transform margin.

The Ivory Coast-Ghana margin displays a triangular-shaped continental shelf, a steep (15°) continental slope, and a narrow (6-11 km) ocean-continent transition zone (Fig. 8.17b). Seismic reflection data provide

Ghana Margin Basins

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.

evidence of folding and faulting associated with dextral motion within a 10- to 20-km-wide zone along the Côte d'Ivoire-Ghana marginal ridge (Edwards et al., 1997; Attoh et al., 2004). The folds display northeast-trending axes that are compatible with dextral motion. The faults record both strike-slip and dip-slip (south-side down) displacements that appear to reflect at least two episodes of strike-slip deformation (Attoh et al., 2004). The first involved a combination of strike-slip motion and extension on northeast-trending faults, leading to the formation of pull-apart basins (Section 8.2). The second involved strike-slip motion and folding, possibly as a result of a change in the direction of motion in the transform.

On the basis of these and other observations, it has been possible to reconstruct the large-scale evolution of the Ivory Coast-Ghana margin. Four main phases are illustrated diagrammatically in Fig. 8.17c-f. In phase 1 (Fig. 8.17c) there is contact between two continents. Strike-slip motion results in brittle deformation of the upper crust and ductile deformation at depth (Section 2.10), giving rise to pull-apart basins and rotated crustal blocks (Section 8.5). In phase 2 (Fig. 8.17d), as rifting and crustal thinning accompany the formation of a divergent margin, the contact is between normal thickness continental lithosphere and thinner, stretched continental lithosphere. The newly created rift basin experiences rapid sedimentation from the adjacent continent and subsidence associated with the crustal thinning (Section 7.7.3). The sediments are folded and faulted by the transform motion and blocks of material are uplifted (Basile & Allemand, 2002), forming scarps and marginal ridges (see also Section 6.2). This tecto-nism is recorded in unconformities in the sedimentary sequence and other structures imaged in seismic reflection profiles (Attoh et al., 2004). In phase 3 (Fig. 8.17e) new oceanic lithosphere emerges along a spreading center to establish an active ocean-continent transform. At this stage there is contact between the faulted continental margin and oceanic crust. The faulted margin passes adjacent to the hot oceanic crust of the spreading center and the thermal exchange it experiences results in heating and differential uplift within the faulted margin, especially near the continent-ocean boundary. Seismic data suggest magmatic underplating in the deep portions of the continental crust, where the magmatic features align with the transform faults (Mohriak & Rosendahl, 2003). In phase 4 (Fig. 8.17f) the transform is only active between blocks of oceanic crust and thus appears as a fracture zone (Section 6.12). The faulted margin is then in contact with cooling oceanic lithosphere and its subsidence evolves in a manner similar to other rifted passive margins (Section 7.7.3).

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