A revised paleomagnetic pole from the mid-Neoproterozoic Liantuo Formation in the Yangtze block and its paleogeographic implications

Abstract

The paleogeographic relationship between South China and Australia during the Neoproterozoic is still hotly debated. Although a series of propositions for the close proximity between South China and Australia are suggested, their relative positions are varied. To better constrain the paleoposition of South China, we have carried out a new paleomagnetic study from the Neoproterozoic Liantuo Formation in two localities, Yichang and Changyang, in Hubei province, China. Stepwise thermal demagnetization reveals that a late Cretaceous remagnetization (component ‘L’, 350–500°C) was removed from most of the samples. A medium component (‘M’) isolated mainly between 350°C and 600°C, but in some cases with an upper limit of 640°C, yields a mean direction of Dg=59.5°, Ig=64.6°, α95=3.7° in situ; and Ds=81.6°, Is=61.5°, α95=4.1° after tilt correction. A steeper medium-high temperature component, ‘B’, was separated from 71 specimens with almost the same temperature range of ‘M’ (Dg=341.3°, Ig=78°, α95=5.6° in situ; and Ds=78.3°, Is=83.8°, α95=6.9° after tilt correction) and is similar to the ‘B’ component obtained by Evans et al. (2000). In addition, a high temperature component ‘H’ (Ds=102.5°, Is=62.4°, α95=3.7° after tilt correction) with both reversed and normal polarity was revealed up to 690°C, yielding a pole at 12.7°N, 157.4°E, with dp/dm=4.5°/5.8°. A change of magnetic polarity, with 5 magnetozones defined, is revealed in our sampling section, in which a reversed-normal polarity succession in the upper part of the section may be correlative with that obtained in a section 6.5km away by Evans et al. (2000), demonstrating the primary nature of the ‘H’ component. Combining our results with those of Evans et al. (2000) yields a grand mean pole from the upper Liantuo Formation at 13.2°N and 155.2°E with A95=5.3°. Considering the distribution of our sampling sites, and together with the results of a detailed recent SIMS U–Pb zircon study on the Liantuo Formation in the Three Gorges area (Lan et al., 2015), an age of 720Ma was assigned to component ‘H’ and its corresponding pole. After carefully reevaluating the Neoproterozoic apparent polar wander paths (APWP) from Australia and South China, we suggest that the South China block was located to the northwest of Northern Australia during the Ediacaran period. Subsequently, the South China block moved to the northwest of Western Australia through counter-clockwise rotation of the Northern Australian craton at the later time of 550Ma, or in the middle Cambrian.