Terrestrial Palaeoecology and Global Change

164 Valentin A. Krassilov. Terrestrial Palaeoecology
relative stability of the fault network over times rules out a possibility of large-scale
polar wander.
Rotational forcing is reflected by the series of transcurrent offsets of meridional
lineaments, such as mid-ocean ridges, with the amplitudes increasing at critical latitudes.
The maximum shear zone is the equatorial.
The faulted lithosphere adjusts to rotational acceleration by the lateral, as well as
radial, shifts of its constituent blocks mediated by their marginal shear belts. These develop
as global fold belts over the density boundaries of continental and oceanic lithosphere.
Not only the Pacific and Tethys belts, but also the presently “passive” Atlantic and
Arctic margins were active in the Cretaceous, displaying a typical mosaic structure of
stable blocks fringed by ophiolitic thrust zones. Tectonomagmatic events are synchronous
over the fold belts attesting to their developmental integrrity.
The marginal magnetic anomalies are tracks of the leaky shear faults that decouple
continents from the surrounding oceanic crust. Their impregnating magmatic wedges
are periodically exhumed from under the volcanic – sedimentary cover that is thrust,
with high-pressure metamorphism, over the continental margin. The exhumed ultramafic
– mafic material is involved in ophiolitic nappes that impose the high-temperature metamorphism,
granitization and comagmatic volcanism. The nappe belts, cut off by marginal
shear and pulled apart by transcurrent faulting, develop as island-arc systems, with extensional
backarc basins and trenches.
Asymmetry of the western/eastern marginal structures, as in the present-day Pacific
or the Cretaceous Atlantic (with the Greater Antilles opposing to the Canary – Cape
Verde arc) betrays rotational forcing, as does the westward drift of volcanic activity (the
Coriolis vector) and the equatorward propagation of volcanic bulges (the Eötvös vector).
Mid-ocean rises are S-cracks generated by the marginal couple of forces. Impregnation
of magmatic material in the crest zone and flanking faults generates a series of magnetic
anomalies, with an age progression reflecting a sequential cooling and subsidence.
Chronology of tectonmagmatic events over the fold belts and cratonic areas reveals
a global cyclicity of tectonic processes. Thus, the Cretaceous Period is bracketed by the
major intracratonal rifting events over the lower boundary, the Late Jurassic/Neocomian,
giving rise to the basaltic provinces of eastern Brazil, Namibia, Barents Shelf, Mongolia
and elsewhere, and over the upper boundary, the Cretaceous/Tertiary, with the
even more prominent trap magmatism about 65 Ma terminating the 180 m.y. Mesozoic
megacycle. A mid-Cretaceous Austro-Alpine thrusting phase of the western Tethys,
circum-Pacific belt and elsewhere, accompanied by basaltic volcanism of oceanic ridges,
divides the subordinate Early/Late Cretaceous cycles about 30 m.y. each.
These periodicities are typical of the Phanerozoic time scale, which is based on biotic
events causally linked to tectonomagmatic evolution (their relations are discussed in
IX.3-4). The first order cycles of about 180 m.y. (two such cycles in the Palaeozoic)
roughly correspond to revolution of the Solar system around the center of the Galaxy