Geoscience Reference
In-Depth Information
FAST SPREADING (12 cm yr
--1
)
A
TRANSFORM
A
A
′
0.8 Ma
6 km
CRUST
CRUST
10 km
MANTLE
TRANSFORM
50 km
RELIC
SPREADING
CENTRE
Simple
CRUST
CRUST
TRANSVERSE RIDGE
MANTLE
FRACTURE-ZONE VALLEY
Complex
CRUST
CRUST
(e)
(f)
100 km
A
′
(g)
Figure 9.34.
(
cont.
)
zones, perhaps up to 50 km wide, and have a series of ridge-fault segments.
It should not be surprising that fast-slipping transforms operate somewhat dif-
ferently from slow-slipping transforms. There is a difference of up to a factor
of ten in their slip rates, which means that their thermal structures are very
different. Figure 9.34 shows the features which might reasonably be expected
to result at a large-offset, fast-slipping transform fault. It is clear that there is
a wide shear zone made up of a series of ridge-fracture segments. The East
Pacific Rise between the Nazca and Pacific plates, the fastest-spreading ridge
segment in the world, is offset by a number of long transform faults. Results
from various bathymetric and tectonic studies of these faults show that they
can be best described as 20-150-km-wide shear zones of complex topography.
Work on the East Pacific Rise has resulted in more new terminology. Although
the terms
transform fault
and
fracture zone
are used, respectively, for the active
and inactive portions of faults, the terms
deviations in axial linearity
(DEVAL)
