Terrestrial Palaeoecology and Global Change

Chapter 5. Tectonic factors of global changes
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V.5.1. Mid-ocean rises and transcurrent share
As noted above, the mid-ocean rises are S-shaped fault zones consisting of D-elements
over mid- to high latitudes and of O-elements over low latitudes. The Atlantic and
Indian mid-ocean rises are roughly parallel to each other and to the East-African transcontinental
rift system. The latter is flanked by swarms of minor faults and the same
must be true for the mid-ocean ridges which arise as leaky median cracks of thin oceanic
crust split by a pair of forces generated by marginal shear at the contacts with the
much thicker continental crust (Krassilov, 1985).
The plate tectonic concept of mid-ocean rises as sea-floor spreading features is based
primarily on interpretation of the linear magnetic anomalies as sea-floor increments. An
alternative interpretation of these lineaments as fault-bound crustal blocks (Krassilov,
1985) relates the age progression of remnant magnetization and sedimentary cover to a
sequential cooling and coeval subsidence of block-faulted crust away from the heated
ridge crest. This mechanism explains a correlation of successive magnetization events
with deposition rates, yet not precluding sediments older than remnant magnetization of
sea-floor basalts. In fact, although pre-Tertiary sediments are mostly assimilated by the
ridge volcanics (Saito et al., 1966), they are locally preserved in nodal basins at intersections
with transcurrent faults (Bonatti & Crane, 1982). Even Cretaceous coaly shales
and breccia with shallow-water fossils were occasionally found in the axial zone of Mid-
Atlantic Rise (Rezanov, 1993).
Heat flow over axial zone is evidence of friction heating rather than of extension and
passive intrusion of basaltic magma. Actually, a left-lateral shear over the ridge crest is
confirmed by geodetic work in Iceland (Vadon & Sigmundsson, 1997). The volcanics of
the rises are heterogeneous, varying from tholeiitic to calk-alkaline to rhyolitic, as might
be expected of a combined shear-extensionl kinematics, and even on the ridge crest the
tholeiites constitute no more than 50% of lavas (predominantly rhyolitic on Hekla in
Iceland: Thorarinsson, 1967). An extensional component indicated by tholeiitic petrochemistry
is imposed by a pull apart of ridge borders by trancurrent shear.
The mid-ocean rises are cut by a series of transcurrent shear faults that displace the
ridge crest segments giving them a broken-stick outline. At least some of transcurrent
faults extend into the adjacent continental crust (which does not comply with their plate
tectonic interpretation as transform faults). Thus, the Romansh Fault is continued, with
marginal offsets, to the intracontinental Amazon Rift System (Grabert, 1983) and, to the
east, to the inland Cameroon line marked by alkaline volcanics (Machens, 1973). The
fault-bound England Seamounts, Walvis Ridge and probably other transcurrent ridges
extend over continental margins (Shafter, 1984; Mayhew, 1986; Hurtubise et al., 1987).
In the North Atlantic, a transcurrent fault zone defines the southern margin of Biscay
and projects as the Main Pyrenean Fault (Vielzeuf & Kornprobst, 1984). In the Pacific,
the Tuamotu – Line island chain marks a fossil mid-ocean rise, with the left-lateral Papua
– Solomon Island shear zones as its trancrurrent fault. The Fossa Magna of Japan