Palaeomagnetic investigation on Early–Middle Triassic sediments of the North China block: a new Early Triassic palaeopole and its tectonic implications

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SUMMARY

Palaeomagnetic and rock magnetic study is reported of Early–Middle Triassic terrestrial sediments from the Jiaocheng and Yushe areas, Shanxi province. Thermomagnetic analysis indicates that magnetite and haematite are the main magnetic carriers. Thermal demagnetization shows that the majority of Early Triassic specimens from the Jiaocheng area contain an intermediate-temperature component (ITC) between 500 –250 °C and a high-temperature component (HTC) between 500–680 °C. The ITC has a negative fold test and might be a Cretaceous or recent overprint. The HTC contains both normal and reversed polarities with positive fold and C-level reversal tests. We interpret the HTC as primary magnetization. Unfortunately, we failed to isolate a meaningful mean direction from Middle Triassic rocks of the Yushe area. New Early Triassic palaeopole from the Jiaocheng area confirms an ∼ 10° counter-clockwise displacement of the Taihang Early Triassic palaeopoles relative to their Ordos counterparts. This study thus suggests that the North China block (NCB) has generally behaved as rigid block since the Early Triassic, but there might be an ∼9° counter-clockwise rotation of the Taihang terrane with respect to stable NCB in the Late Triassic. The slight rotation of the Taihang terrane does not bear significantly on what has been previously concluded concerning the Mesozoic history of the NCB—South China block (SCB) collision. However, Triassic palaeomagnetic data from stable NCB indicate that the east end of the NCB and SCB did not undergo large convergence or significant opposite latitudinal displacement after the initial contact at the end of the Late Permian.

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