New paleomagnetic data from the Huanan Block, China, and Cretaceous tectonics in Eastern China

Abstract

A total of 72 grey-green fine-grained sandstone and siltstone core samples (from 10 sites) were collected from the Early Cretaceous Jieshan and Hekou Formations at Yongan, in the Huanan Block of southeastern China (Fujian Province). After removal of an erratic soft magnetic component in fields of 10–20 mT, stable magnetizations were retrieved from both rock units which are carried mainly by single-domain/pseudo-single-domain (SD/PSD) magnetite or titanomagnetite, as suggested by isothermal remanent magnetization, coercivity measurements and microscopic observations. The Hekou Formation yields a tilt-corrected pole at 66.9°N, 221.4°E ( n=5 sites, K=203.3, A 95=5.4°). A fold test on this result suggests that the magnetization was acquired before tilting which occurred shortly after deposition (between Early and Late Cretaceous). This magnetization is related to a primary sedimentary fabric, as indicated by the low-field anisotropy of magnetic susceptibility (AMS) measurements. The positive fold test and occurrence of a primary sedimentary fabric suggest a depositional origin for the stable magnetization. The Jieshan Formation gives a pole at 70.8°N, 228.7°E ( n=5 sites, K=47.7, A 95=11.2°) in geographic coordinates, which is statistically indistinguishable from the tilted pole for the Hekou Formation. The associated stable magnetization corresponds to a secondary fabric. These features indicate that the Jieshan Formation has been remagnetized during Cretaceous time. The predominance of SD/PSD magnetite or titanomagnetite and igneous activities in eastern Fujian during the Cretaceous suggest a viscous thermal remanent magnetization origin for the secondary magnetization. The Early Cretaceous pole from the Hekou Formation is similar to those for the Yangzi Block, suggesting that the Huanan Block has been accreted to the Yangzi Block at least since Early Cretaceous time. A high consistency of Cretaceous poles from the Sino-Korean, Yangzi and Huanan Blocks as well as Hainan Island indicates that eastern China has been a coherent tectonic unit since Cretaceous time, although there might have been a small amount of northward displacement of the sampling area with respect to the Yangzi Block during the Early-Late Cretaceous period as a result of the overthrust of the Huanan Block onto the Yangzi carbonate platform. Comparison of Cretaceous poles for eastern China with the Eurasian apparent polar wander path (APWP) suggests that no paleomagnetically detectable relative motion has taken place since the Cretaceous. The Cretaceous poles of eastern China, however, display a paleomagnetically significant westward motion (about 9° in angular distance), which is not observed in the Eurasian APWP. This difference may be indicative of a younger age corresponding to the Cretaceous-Tertiary boundary for all Late Cretaceous poles from eastern China, or more probably may imply a minor error in the definition of the Eurasian APWP.