Palaeomagnetism of Middle Triassic red bedded cherts from southwest Japan: equatorial palaeolatitude of primary magnetization and widespread secondary magnetization

Abstract

SUMMARY Middle Triassic (Anisian) red bedded cherts were collected from the Ajiro area in Kyushu Island, southwest Japan, for a palaeomagnetic study. Thermal demagnetization showed five distinct remanent magnetization components from the cherts. The lowest temperature component below 200 °C (component A) is a present-day viscous overprint. The second- (up to 420 °C, component B) and third-demagnetized (up to 580 °C, component C) components are secondary magnetizations of thermoviscous and chemical origin, respectively, both of which reside in magnetite. The fourth-removed component (component D) was demagnetized at 630 °C and is a secondary magnetization carried by haematite. Components B and C are considered to be a result of widespread remagnetization in southwest Japan during the Cretaceous; the basis of this consideration is that red chert remagnetizations from an area about 600 km away from the present study area, are of the same origins with unblocking spectra similar to those observed in this study. The last-removed component during thermal demagnetization (up to 695 °C, component E) is a primary magnetization carried by haematite. The directions of this component, after correction for the inclination flattening, yielded a mean palaeolatitude of 2°S at the time of deposition in the Middle Triassic. The Ajiro cherts accreted as part of an accretionary complex to the eastern margin of the South China Block during the Middle Jurassic and were then translated northwards from the continental margin to their present position through a margin-parallel translation process owing to the oblique convergence of the oceanic plate.