Page 103 - Global Tectonics

90 CHAPTER 4

crest to the south of the fracture zone about 40 Ma crest segments and transform faults is more likely to be

ago. The change in offset of anomalies of the same modified, less dramatically, by ridge propagation
age implies a “ridge jump” of approximately 500 km (Section 6.11) and changes in the spreading direction
to the east (Harrison & Sclater, 1972). Similar but better (Section 5.9).
documented ridge jumps, which also greatly reduce
the offset of crust of the same age on either side of a
fracture zone, occur in the extreme south of the Atlan-
tic Ocean (Barker, 1979). Here, to the south of the FURTHER READING
Falkland–Agulhas Fracture Zone, the ridge crest has
jumped westward on three occasions since the opening
of the South Atlantic, that is at 98, 63 and 59 Ma. In
Cox, A. & Hart, R.B. (1986) Plate Tectonics. How it works. Blackwell
so doing it has reduced the original offset of 1400 km
Scientific Publications, Oxford.

to approximately 200 km. Other ridge jumps, producing Jacobs, J.A. (1994) Reversals of the Earth’s Magnetic Field. Cambridge
major changes in ridge crest geometry within the past University Press, Cambridge.
10 Ma, have occurred to the north of Iceland (Vogt et Jones, E.J.W. (1999) Marine Geophysics. Wiley, Chichester,
al., 1970) and along the crest of the East Pacifi c Rise England.
Merrill, R.T., McElhinny, M.W. & McFadden, P.L. (1996) The Mag-
in the east central Pacifi c (Herron, 1972).
netic Field of the Earth: paleomagnetism, the core and the deep
In general, however, ridge jumps are relatively rare,
mantle. Academic Press, San Diego.
as evidenced by the median position of oceanic ridges Opdyke, N.D. & Channel, J.E.T. (1996) Magnetic Stratigraphy. Aca-
between separated continents. The geometry of ridge demic Press, San Diego.