Tertiary paleomagnetism of North and South China and a reappraisal of late mesozoic paleomagnetic data from Eurasia: Implications for the cenozoic tectonic history of Asia

Abstract

To contribute to the understanding of the Cenozoic tectonic history of China, and eastern Eurasia in general, we collected paleomagnetic samples from Tertiary rocks in both North and South China. Our Miocene paleomagnetic data for northern China are from four widely separated basalt sections in the Inner Mongolia Autonomous Region and Shandong Province. Both normal and reversed polarities of the characteristic magnetization are present in the stratigraphie sequence, and the mean normal and reversed directions are closely antiparallel to each other. The fact that the same characteristic direction was carried by both magnetite and hematite suggests that the hematite was probably formed by auto-oxidation processes during primary cooling of the basalts. The paleomagnetic data from these four areas also pass a regional generalized fold test. The corresponding mean Miocene paleomagnetic pole for northern China is longitude 261.6°E, latitude 88.3°N (A 95 = 6.4°). Our Early Tertiary paleomagnetic results for south China are from two sandstone sections around the city of Nanning (22.7°N, 108.3°E) in Guangxi Zhuang Autonomous Region. High-temperature characteristic directions isolated in 31 samples from five sites pass both tilt and reversal tests. The paleomagnetic pole position determined from the five sites lies at longitude 228.0°E, Latitude 83.6°N (A 95 = 8.5°). The Tertiary paleomagnetic poles for North and South China are statistically indistinguishable from each other and thus confirm that North and South China were fully assembled at least by the Tertiary. Our new Tertiary paleomagnetic data are consistent with the hypothesis that the major continental blocks of North and South China were in the same position as today during the Tertiary. A critical evaluation and selection of paleomagnetic results from the Late Jurassic onward for Siberia, Europe and North and South China further corroborate this hypothesis. Most previously existing Jurassic and Cretaceous paleomagnetic poles from Europe/Siberia are of uncertain reliability and should not be used for tectonic reconstruction. Using poles derived from transferring reliable data from other continents, we show that the previous paleomagnetic inferences of large eastward displacements between these Chinese blocks during the Cenozoic are no longer warranted. The available paleomagnetic data that meet our acceptance criteria are also of interest concerning the boundary conditions of the propagating extrusion model for Asian tectonics. Additional Mesozoic and Cenozoic paleomagnetic data from Siberia and North and South China are still needed to resolve the smaller, second-order displacements that appear to characterize the Cenozoic tectonic history of this part of Asia.