GEOLOGICAL CHARACTERISTICS OF THE ANCIENT QINLING RIFT ZONE AND ITS SIGNIFICANCE IN TECTONICS

Prof. Zhang Wenyou regarded a graben system as a rift system, He pointed out that"a graben is equivalent to an embryonic form of a geosynclinal systemi;a eugeosyncline is equivalent to an oceanic crustal graben, while a miogeosyncline is equivalent to a continental crustal graben".The authors have studied .the strata, contact relationships, fractures, sedimentary formations, magmatic activity, metamorphism, isotope geology and geophysical features of the "Northern Qinling Caledonian fold belt of the Qinling fold system" that was originally assigned to the "Qinling axis". According to the results of the study, the, authors consider the fold belt to be a consumed oceanic rift zone （fault-fold zone）. It is different from the Eastern Qinling fault-warp （miogeosyncline）.Westwards it is in the main equivalent to the Northern Qilian graben （so called by Zhang Wenyou） . From view-point of plate tectonics, it is an eastwest-trending suture line of fau blocks. There are five near-east-west-trending deep faults （foundational ai lithospheric faults） , between which occur ophiolites and mid oceanic ridge tholeiites. The geophysical features are manifested by the presence of a zone of sudden change in crustal thickness and a zone of gravity anomaly gradients. The ancie（?）t Qilian-Northern Qinling rift zone, greater than 2000 km in length, is bounded on the north by the Tarim-North China （Ordos and Taihang） fault block and on the south by the Altay-Qingkang-Eastern Qinling fault-warp,i.e. there had been a near-east-west-trending Qinling-Qilian old ocean between China’s northern and southern old continental crusts. Its role in the geological history is equivalent to that of the modern mid-oceanic ridges （oceanic rifts）. It is both the starting point of oceanization and a growth point of ocean ic crust, and constitutes an important link for continentization of oceanic crust. It was just due to reciprocal motion and compression of the fault blocks at both sides of the old ocean that a stratigraphic and paleontologic demarcation line was formed between North China and South China with Qinling as the boundary.