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Union of Soviet Socialist Republics
Devonian of Tien Shan
The Devonian of Tien Shan occurs in three regions each having a distinct geologic development. These may be designated Northern, Middle and Southern Tien Shan.
Northern Tien Shan had been a geosyncline in Precambrian and Early Palaeozoic time. Acidic and intermediate effusive rocks and continental redbeds filling isolated superimposed troughs are assigned a Devonian age. Their thickness is about 3000 metres. The deposits rest on an eroded Precambrian and Lower Palaeozoic surface, and are overlain by Lower Carboniferous deposits in a similar facies. Devonian magmatic rocks have not been recognized to date.
The Devonian of Middle Tien Shan is for the most part characterized by a platform to geosyncline transition. The Middle Devonian and Frasnian continental redbeds are most widely distributed; the thickness is 100-2500 metres. The Devonian deposits unconformably overlie erosional remnants of Silurian beds plus older deposits and are conformably overlain by the Lower Carboniferous. Many polymetallic deposits are associated with the Devonian rocks in the area.
The Devonian of Southern Tien Shan was deposited in an actively developing geosyncline. Sharply distinctive tectonic movements resulted in the formation of structural-facies zones characterized by different thicknesses and facies of exclusively marine Devonian deposits. Devonian sections were established composed only of detrital, carbonate, or effusive rocks, or of combinations of these. Thickness of Devonian deposits in the different structural-facies zones ranges from a few metres up to 4000 metres. The Devonian usually occurs in a continuous sequence with the underlying Silurian and overlying Carboniferous. Some gaps in sedimentation have been observed, in some cases with angular unconformity, in the Middle Devonian in the southern part of the district, and in the Lower Devonian in the northern part. Devonian plutonism is characterized by ultra-basic and basic rocks. A variety of hydrothermal deposits developed.
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