Terrestrial Palaeoecology and Global Change

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Chapter 5. Tectonic factors of global changes 141 Fig. 61. A schematic origin of the circum-Pacific ophiolite nappes: I. Subcrustal material is thrusted from beneath the continent over the oceanic crust that is isostatically elevated. II. The bulging nappe zone is faulted, with subcrustal material penetrating the fault zone; hanging nappes are back-thrusted upon the continental margin; oceanic crust exhumed from under the nappes sinks as a back-arc basin. (2) The volcanic–sedimentary cover is disentangled and napped upon adjacent oceanic crust to be re-sheared toward the continent in the next phase, (3) The ultramafic bodies of Benioff zone, exhumed from under the nappes, form a median cordillera of the shear belt, shedding ophiolitic olistolites to the residual flysch troughs. The associated metamorphism evolves from the low pressure/high temperature type produced by friction heating to the high pressure/low temperature type beneath the nappe piles, while the lavas show a concomitant progression from the predominantly toleiitic in the lower horizons to andesitic and trachitic above. This model is further discussed in respect to Cretaceous examples. V.6.3. Cretaceous circum-Pacific structures The circum-Pacific fold belt is formed by consolidation of numerous crustal blocks. The mosaic structure, comprising, from north to south, the well-defined Chukotskian, Kolymian, Omolonian, Okhotskian, Bureyan, Khankian and a number of smaller blocks has been first recognised on the western Pacific margin between Siberian craton and the coastal ranges (Fig. 62). The blocks are cratonic, surrounded by thrust belts emplaced upon their margins. Anticlockwise rotation by a system of sinistral shear is well documented for the Omolonian Massif in the Russian Northeast (Bondarenko, 1996). Such rotating blocks are considered below as the millstones of global fold belts. The Kolymian, Omolonian and Chukotskian blocks are surrounded by complete, though asymmetrical, annular shear zones with Palaeozoic ophiolitic nappes re-sheared during the Late Jurassic/Early Cretaceous phase. Consolidation of the blocks and their accretion to the Siberian craton have been accomplished with the granitoid intrusions (the Kolymian batholites and coeval granitoids elsewhere), after which the tectonic activity has shifted to the east (Archipov, 1984; Natapov & Stavsky, 1985; Alexeev, 1987).
142 Valentin A. Krassilov. Terrestrial Palaeoecology Fig. 62. Structural zones and the major crustal blocks of northwestern Pacific margin. Blocks: (Bu) Bureyan, (Ch) Chukotskian, (Kh) Khankian, (Ko) Kolymian, (Om) Omolonian, (Wo) West-Okhotoian, (Ya) Yamato. (1) Terrestrial volcanic belt, (2) Foredeep, (3) Cretaceous island arcs, (4) Areas of back-arc extension.
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Terrestrial Palaeoecology and Global Change

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