Figure 5. Temporal cross sections across the Southwest Indian Ridge rift
valley drawn parallel to the spreading direction (not across the fracture
zone, but parallel to it), showing the postulated tectonic evolution of the
transverse ridge and Hole 735B (Dick et al., 1991a). The sequential
sections are drawn at about 18 km from the transform fault. Crust
spreading to the right passes into the transverse ridge and spreads
parallel to the transform valley. Crust spreading to the left spreads into
the rift mountains of the Southwest Indian Ridge parallel to the inactive
extension of the Atlantis II Fracture Zone. Dense stipple = mantle, filled
diamonds = gabbro, inverted "v" = basalt. A. Initial symmetric spreading,
possibly at the end of a magmatic pulse. Late magmatic brittle-ductile
deformation occurs because of lithospheric necking above (and in the
vicinity of whatever passes for a magma chamber at these spreading
rates). Hydrothermal alteration at high temperatures accompanies necking
and ductile flow in subsolidus regions. B. At some point, the shallow crust
is welded to the old, cold lithosphere to which the ridge axis abuts,
causing formation of a detachment fault and nodal basin, initiation of
low-angle faulting, continued brittle-ductile faulting, and
amphibolite-facies alteration of rocks drilled at Hole 735B. C. and D.
Block uplift of the rift mountains at the ridge/transform corner forms a
transverse ridge enhanced by regional isostatic compensation of the local
negative mass anomaly at the nodal basin. Initiation of the block uplift
terminates the extension driving cracking, and drastically reduces
permeability in the Hole 735B rocks, effectively terminating most
circulation of seawater and alteration. Greenschist-facies retrograde
alteration continues along the faults on which the block is uplifted to
account for the greenschist-facies alteration that predominates in
dredged gabbros.
To 176 Figure 6
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