Geoscience Reference
In-Depth Information
Figure 9.35.
(a) A sonar image of the overlapping spreading centres on the East
Pacific Rise at 9
◦
N. There is no transform fault between the two tips of the ridge
segments which extend past each other. The deeper region between the two
spreading centres is termed the overlap basin. Such overlapping spreading centres
are temporary phenomena caused by misalignment of tensional cracks in
neighbouring ridge segments. As the magma rises episodically, the tips of the ridge
segments move back, forth and laterally. (Reprinted with permission from
Nature
(Macdonald
et al
. 1988,
Nature
,
335
, 217-25). Copyright 1988, Macmillan Magazines
Ltd.)
(b) A three-dimensional seismic reflection survey across the eastern limb of the
overlapping spreading centre on the East Pacific Rise at 9
◦
03
N, with north to the top
and south to the bottom of the image. A rectangular box (orange frame) has been
cut away to reveal the reflectivity at depth. The top surface (shown red) is the
seafloor. Reflections from layer 2A are blue. Reflections from the magma chamber
are orange and blue-green. The image shows approximately 8 km × 14 km. The
two-way time from seafloor to base of image is ∼1.5 s. Bodies of melt underlie the
eastern limb of the ridge and apparently pond beneath the overlap basin. The melt
bodies may be only 100 m thick. Imaging highly heterogeneous structures is
extremely difficult and the fine detail of melt transport beneath the OCS is complex,
but there must be a robust vertical magma supply from the underlying mantle.
Colour version Plate 22. Reprinted with permission from
Nature
(Kent, G. M.
et al
.
2000,
Nature
,
406
, 614-18) Copyright (2000) Macmillan Magazines Ltd.)
(c) The hierarchy of the segmentation of mid-ocean ridges. Ridge segments from
first to fourth order are S1, S2, S3 and S4. Discontinuities from first to fourth order
are D1, D2, D3 and D4. (Reprinted with permission from Macdonald, K. C., Sheirer,
D. S. and Carbotte, S., Mid-ocean ridges: discontinuities, segments and giant cracks,
Science
,
253
, 986-94. Copyright 1991 AAAS.)
