Is the Qinling Orogen an Indosinian Collisional Orogenic Belt?

Since the late 1980s, the Qinling orogen has been considered an Indosinian collisional orogenic belt by some Chinese and foreign geologists. However, no trace of a Triassic oceanic basin or a Paleozoic oceanic basin that continued to Triassic has been found as yet in the Qinling area. It has been suggested that the Devonian-Triassic of the Qinling area is characterized by coastal to shallow marine sediments, and there are no pelagic sediments or ophiolites. There is a clear regional angular unconformity between the Devonian and its underlying rocks. The Shangdan fault is not an Indosinian but a Caledonian suture zone, both sides of which (i.e. the southern margin of the Sino-Korean plate and the northern margin of the Yangtze plate) have a clear record of the Caledonian orogeny. The Pb isotopic compositions of the Liuling Group of the Middle-Upper Devonian, deposited on the northern margin of the Yangtze plate, are similar to those of the North Qinling belt. The detrital zircon age spectrum also suggests that the sediments came from the North Qinling orogenic belt. Mafic rocks from the Mianlue and Sanligang ophiolitic melanges within the Mianlue Indosinian suture zone show a Proterozoic age, and the Sanligang ophiolitic melange is overlain by the sediments of the Nanhuan-Sinian. The so-called Mianlue suture zone is actually a huge regional fracture zone, to the north of which was the passive margin of the Yangtze plate in Early Paleozoic and to the south lay the Yangtze paraplatform (small craton), the core of the Yangtze plate. Therefore, the Indosinian orogen of the Qinling should be attributed to the thrusting and overlapping of the continental crust rather than the continent-continent collision after the disappearance of the oceanic basin. The ultrahigh pressure (UHP) metamorphic belt of the Dabieshan, the eastern extension of the Qinling orogen, is considered an important piece of evidence suggesting that the Qinling was an Indosinian collisional orogen. However, the Dabieshan UHP metamorphic rocks were formed under dynamic UHP conditions during the orogenic process. Therefore, it is inappropriate to convert the estimated thermodynamic pressure to depth by assuming that the pressure was static simply due to burial. Accumulating data from field geological observations, structural geology, metamorphic petrology, isotope geology, geochemistry, geophysics, and physical experiments all indicate that the UHP metamorphism did not occur in the upper mantle but in the earth crust. Structurally, the upper crust of the South Qinling-Dabie region can be subdivided, from top to bottom, into four layers:unmetamorphosed sedimentary rocks, epidote-glaucophane schists, high pressure metamorphic rocks, and UHP metamorphic rocks. The lower crust is represented by the upper amphibolite-granulite facies rocks which were not affected by the UHP metamorphism. The coesite-bearing metamorphic rocks are merely sheeted slices of about 10-12 km thick in the lower part of the upper crust. Based on these observations, our preliminary conclusion is that the low-high-ultrahigh pressure tectonic system of the upper crust in the South Qinling-Dabie region was the result of multilayered decollement above the lower crust, acting as a main shear slide plane during the Indosinian orogeny. The UHP metamorphic rocks of the lower part of the upper crust were formed in response to repeated transient UHP events in the focal area, where frequent earthquakes were likely caused by severe shearing.