Page 246 - Global Tectonics

232 CHAPTER 8

evidence of folding and faulting associated with dextral margin is then in contact with cooling oceanic litho-
motion within a 10- to 20-km-wide zone along the Côte sphere and its subsidence evolves in a manner similar to
d’Ivoire–Ghana marginal ridge (Edwards et al., 1997; other rifted passive margins (Section 7.7.3).
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 epi- 8.5 CONTINUOUS
sodes of strike-slip deformation (Attoh et al., 2004). The

first involved a combination of strike-slip motion and VERSUS
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 DISCONTINUOUS
result of a change in the direction of motion in the
transform. DEFORMATION
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 8.5.1 Introduction
illustrated diagrammatically in Fig. 8.17c–f. In phase 1
(Fig. 8.17c) there is contact between two continents. The distributed nature of deformation on the conti-
Strike-slip motion results in brittle deformation of the nents compared to most oceanic regions has led to the
upper crust and ductile deformation at depth (Section invention of a unique framework for describing conti-
2.10), giving rise to pull-apart basins and rotated crustal nental deformation (Sections 2.10.5, 5.3). One of the
blocks (Section 8.5). In phase 2 (Fig. 8.17d), as rifting most important aspects of developing this framework
and crustal thinning accompany the formation of a involves determining whether the motion is accommo-
divergent margin, the contact is between normal thick- dated by the movement of many coherent blocks sepa-
ness continental lithosphere and thinner, stretched rated by discrete zones of deformation or by a more
continental lithosphere. The newly created rift basin spatially continuous process. In some areas, the pres-
experiences rapid sedimentation from the adjacent con- ence of large aseismic regions such as the Great Valley
tinent and subsidence associated with the crustal thin- and Sierra Nevada in the southwestern USA (Figs 7.8,
ning (Section 7.7.3). The sediments are folded and 7.10) imply that part of the continental lithosphere
faulted by the transform motion and blocks of material behaves rigidly. However, in other areas, such as the
are uplifted (Basile & Allemand, 2002), forming scarps Walker Lane and Eastern California Shear Zone, seis-
and marginal ridges (see also Section 6.2). This tecto- micity reveals the presence of diffuse zones of deforma-
nism is recorded in unconformities in the sedimentary tion that are better approximated by a regional velocity

sequence and other structures imaged in seismic refl ec- field rather than by the relative motions of rigid blocks.
tion profi les (Attoh et al., 2004). In phase 3 (Fig. 8.17e) To distinguish between the possibilities, geoscien-
new oceanic lithosphere emerges along a spreading tists use combinations of geologic, geodetic, and seis-
center to establish an active ocean–continent transform. mologic data to determine the degree to which
At this stage there is contact between the faulted conti- deformation is continuous or discontinuous across a
nental margin and oceanic crust. The faulted margin region (Thatcher, 2003; McCaffrey, 2005). Determining
passes adjacent to the hot oceanic crust of the spreading the characteristics of these regional velocity fi elds is
center and the thermal exchange it experiences results important for developing accurate kinematic and rheo-
in heating and differential uplift within the faulted logical models of deforming continental lithosphere
margin, especially near the continent–ocean boundary. (Section 8.6), and for estimating where strain is accumu-
Seismic data suggest magmatic underplating in the deep lating most rapidly and, thus, where earthquakes are
portions of the continental crust, where the magmatic most likely to occur.
features align with the transform faults (Mohriak & In models involving continuous velocity fi elds, even
Rosendahl, 2003). In phase 4 (Fig. 8.17f) the transform though the upper brittle crust is broken into faults, the
is only active between blocks of oceanic crust and thus faults are predicted to be relatively closely spaced, have
appears as a fracture zone (Section 6.12). The faulted small slip rates, and extend only through the elastic part