Page 165 - Global Tectonics

OCEAN RIDGES 151

zone seismicity and deformation of rocks within the Lowrie et al. (1986) have noted that, in some fracture

floor and walls. zones, the scarp height may be preserved even after
Transverse ridges are often found in association with 100 Ma. They accept that some parts of fracture
major fracture zones and can provide vertical relief of zones are weak, characterized by active volcanism, and
over 6 km. These run parallel to the fractures (Bonatti, maintain the theoretical depths predicted for cooling
1978) on one or both margins. They are frequently lithosphere. Other parts, however, appear to be welded
anomalous in that their elevation may be greater than together and lock in their initial differential bathymetry.
that of the crest of the spreading ridge (Fig. 6.25c,d). The differential cooling stresses would then cause

Consequently, the age–depth relationship of normal flexure of the lithosphere on both sides of the fracture
oceanic lithosphere (Section 6.4) does not apply and zone. Future work will reveal if there is any systematic
depths differ from “normal” crust of the same age. The pattern in the distribution of strong and weak portions
ridges do not originate from volcanic activity within the of fracture zones.
fracture zone, nor by hotspot activity (Section 5.5), but There are certain oceanic transform faults in which
appear to result from the tectonic uplift of blocks of the direction of the fault plane does not correspond
crust and upper mantle. Transverse ridges, therefore, exactly to the direction of spreading on either side so
cannot be explained by normal processes of lithosphere that there is a component of extension across the fault.
accretion. Bonatti (1978) considers that the most rea- When this occurs the fault may adjust its trajectory so
sonable mechanism for this uplift is compressional and as to become approximately parallel to the spreading
tensional horizontal stresses across the fracture zone direction by devolving into a series of fault segments
that originate from small changes in the direction of joined by small lengths of spreading center (Fig. 6.26).
spreading, so that transform movement is no longer A fault system in which new crust originates is termed
exactly orthogonal to the ridge. Several small changes a leaky transform fault (Thompson & Melson, 1972;
in spreading direction can give rise to episodic compres- Taylor et al., 1994). An alternative mechanism for leaky
sion and extension affecting different parts of the frac- transform fault development occurs when there is
ture zone. This has caused, for example, the emergence a small shift in the position of the pole of rotation
of parts of transverse ridges as islands, such as St Peter about which the fault describes a small circle. The fault
and St Paul rocks, and their subsequent subsidence would then adjust to the new small circle direction by
(Bonatti & Crane, 1984). becoming leaky.