Abstract:

Correlation between the Qinling and Douling Groups on the two sides of the Shangnan-Danfeng fault zone in the eastern Qinling Orogen has always been controversial. In this paper, we study the relationship between the Qinling and Douling Groups on both sides of the Shangdan fault, based on the 1∶50k aeroradioactivity and aeromagnetic data in the eastern Qinling Orogen. The Qinling Group is dominated by a more subdue negative magnetic field background, while the Douling Group is characterized by a stronger positive magnetic background. The stratigraphic range of the Qinling Group corresponds to the higher field in the general channel and thorium channel, while that of the Douling Group corresponds to the lower field in the main channel and thorium channel. Based on the reinterpretation of the seismic velocity and apparent resistivity profile in the eastern Qinling region, it is found that there are significant differences between them. Based on the aerogeophysical data and previous studies in the region, it is concluded that the primary rock types of the Qinling Group are carbonate and argillaceous rocks deposited in a stable continental margin, while those of Douling Group are mafic rocks formed in an active continental margin. Many previous workers considered that the North China and Yangtze plates were amalgamated in the Archean, and that the Qinling and Douling Groups were formed before 2000 Ma, yet the timing of the breakup for this amalgamated block was unknown. In this paper, it is inferred that the North China and Yangtze plates were separated at least from the early Proterozoic, and that an ocean may have opened between them.

Key words: Qinling Group, Douling Group, airborne geophysical prospecting, seismic sounding velocity, apparent resistivity