SEDIMENTARY GEOLOGY AND DYNAMIC SEDIMENTOLOGY OF THE DEVONIAN STRATA IN THE WEST QINLING OROGENIC BELT: DEVONIAN PALAEOGEOGRAPHY,STRATIGRAPHIC SEQUENCES AND TECTONIC EVOLUTION

The Qinling orogenic belt is a composite continental orogenic belt with a long evolutionary history.In the belt,the Devionian strata crop out broadly with obvious facies variation.The emphasis in this paper is placed on the Devonian palaeogeography,stratigraphic sequences and tectonic evolution in this region.
Based on the major factors controlling sequence development,the Devonian stratigraphic sequences in the Qinling orogenic belt may be classified into three types;(l)the sea-level change-controlled sequences (SC type),which nay be subdivided into fourteen third-order sequences developed in the small cratonic basins in the Middle Qinling microplate;(2)the tectonic subsidence-controlled sequences (TC type) in the synorogenic basins and adjacent forebulge zone,and (3)the coupling sequences controlled by the combination of the two factors outlined above(STC type) in response to the overthrusting events during orogenesis.
The collision-orogenesis in the late Caledonian and early Hercynian show a very complex history in the Qinling orogenic belt.The North Qinling orogenic belt,an immature and inhonogeneous orogenic zone,resulted from the oblique and irregular border collision between the North China plate and Middle Qinling microplate.The orogenesis was migrated from west to east,and conspicuous in western Qinling.The closing of South Qinling Ocean and collision between the Middle Qinling microplate and South China plate are also oblique,and migrated from east to west.The orogenic process is characteristic of "closing but no collision" and "collision but no orogeny".The regenerated rift troughs were developed in central Qinling in the late Hercynian and Indosinian,and closed giving rise to extensive mountain building.