Page 164 - Global Tectonics

150 CHAPTER 6

Fig. 6.24 Differential topography resulting from
transform faulting of a ridge axis.

Fig. 6.25 Different types of basement morphology
Fig. 6.26 Development of a leaky transform fault across fracture zones (redrawn from Bonatti, 1978, with
because of a change in the pole of rotation. permission from Elsevier).

seismic velocities, and layer 3 is absent. The crustal the fracture zone from the younger, higher crust to
thinning may extend several tens of kilometers from the lower, older crust (Menard & Atwater, 1969;
the fracture zone. Geologically, this structure may rep- DeLong et al., 1977) (Figs 6.24, 6.25b). The rate of
resent a thin, intensely fractured and hydrothermally subsidence of oceanic lithosphere is inversely depen-
altered basaltic layer underlain by serpentinized ultra- dent upon the square root of its age (DeLong et al.,

mafic rocks. The apparent thickness variations may 1977), so the higher, younger crust subsides more

reflect different extents of serpentinization. The thin rapidly than the lower, older side. The combination of
mafi c crust is thought to be a result of reduced magma contraction in the vertical plane and horizontally per-
supply at ridge offsets as noted in Section 6.7. pendicular to the direction of the ridge axis would
Ocean fracture zones must bring oceanic crust of result in a small component of dip-slip motion along
different ages into juxtaposition. The depth of the sea the fracture zone away from the active transform fault.
floor is dependent upon its age (Section 6.4), and so it DeLong et al. (1977) have suggested that this small

would be expected that a scarp would develop across amount of dip-slip motion could give rise to fracture