Magnetic inclination shallowing problem and the issue of Eurasia’s rigidity: insights following a palaeomagnetic study of Upper Cretaceous basalts and redbeds from SE China

Abstract

Redbeds are an important source of palaeomagnetic data, but they often record inclinations shallower than that of the ancient local geomagnetic field. Discrepancy of palaeopoles from Cretaceous redbeds in South China Block (SCB) and the coeval Eurasia reference pole is commonly attributed to inclination shallowing. However, redbed-derived palaeomagnetic data from the block have rarely been critically assessed with data from coeval volcanic rocks that should be unaffected by the problem. Here, we address the issue using high-quality palaeomagnetic data from Upper Cretaceous (∼95 Ma) amygdaloidal basalts and coeval redbeds from Jiangshan, Zhejiang Province and Guangfeng, Jiangxi Province. Stepwise thermal and alternating field demagnetizations isolated stable components that in the basalts are carried by a mixture of magnetite and titanomagntite and in the sedimentary units by haematite. The stable components are regarded as primary based on positive intraformational conglomerate tests and a regional tilt test. The redbeds yield a tilt corrected mean direction of D = 20.9°, I = 35.8°, α95 = 8.7°, N = 6, which is statistically indistinguishable from the mean direction of the basalts (D = 17.6°, I = 38.1°, α95 = 8.6°, N = 11), suggesting that the former do not suffer from the problem. In addition, analysis of the other Late Cretaceous SCB palaeopoles reveals two groups with one at relatively high (‘H’, ∼80°N) latitudes and the other at relatively low (‘L’, ∼70°N) latitudes. Importantly, each comprises palaeopoles from both redbeds and volcanic rocks, and reasonable consistency exists within each group, further attesting that SCB redbeds do not suffer significant inclination shallowing. Comparison of the SCB palaeopoles with a newly defined coeval reference pole for Europe indicates an ∼11° separation. Since inclination shallowing, over 1000 km tectonic shortening, and apparent polar wander appear unlikely, the ∼11° discrepancy may provide evidence for the non-rigidity of the Eurasia plate. Consequently, a new Late Cretaceous reference pole for the stable SCB is defined at 72.3°N, 235.2°E, where A95 = 3.2° and N = 6.