PALAEOTECTONIC ACTIVITIES: AN INVESTIGATION OF DEVONIAN SEDIMENTARY FACIES AND PALAEOGEOGRAPHY IN THE QINLING—BASHAN AREA

This paper elucidates, on the basis of the reconstruction of sedimentary facies and palaeogecgraphy, the Devonian palaeoenvironmental vicissitudes in the Qinling—Bashan area, which were characterized by the gradual transgression from south to north and from west to east; the migration of depositional or subsidence centres from south to north; the northward migration of the volcanic zones and the variations from basic to intermediate-acidic compositions; the northward migration of the ancient islands; the variations in ancient submarine topography lower in the south and higher in the north in the early stage, or vice versa in the later stage, and the extensional faulting which led to the transgression all over the region. It is believed that all the regular changes mentioned above may be attributed to the external factors. And internally, the Qinling-Bashan area lay between the North China and Yangzi plates. The increasing suturing of the southern and northern plates during the Caledonian allowed the disappearance of the oceanic crust, the elevation of the crust and the retreat of sea water. Later in the Devonian, as a result of the northward shifting of the North China plate, the study area is thought to be in an extensional and faulted environment where the continental crust became thinner and elongated. The crust downwarped slowly and thus the transgression occurred, accompanied by faulting and weak volcanic activities. It follows that the Devonian time represents a transitional stage, i. e. a relatively stable stage between active Caledonian and Indo-Chinese epochs.