New Mesozoic apparent polar wander path for south China: Tectonic consequences

Abstract

A new paleomagnetic study in the South Sichuan basin (South China Block) has been carried out on sediments of Early Triassic to Middle Jurassic age. Detailed stepwise thermal and alternating field demagnetizations allowed us to isolate two to three components. A low‐temperature component (component A) falls close to the local present Earth field direction. A middle‐temperature component (component B) is found both in limestone and red beds of Early Triassic age, in limestones of Middle Triassic age, and finally in red beds of Jurassic age. The component B directions failed the fold test but show a significant increase of the ks/kg ratio at 60% when a progressive stepwise unfolding is applied. The directions obtained at 60% unfolding are grouped best and yield a pole position at 76.5°N, 174.1°E, A95 = 3.7° that is close to the Late Cretaceous pole of the South China Block (SCB) or the Early Tertiary pole of the Eurasian plate. Characteristic directions are finally obtained (component C) for Early Triassic and Early to Middle Jurassic sediments only, both with reversal and fold tests. They yield pole positions at 44.1°N and 217.2°E (A95 = 4.9°) and 79°N and 227°E (A95 = 4.5°), respectively. The Early Triassic result well fits other data of the same age from the SCB. The revised Late Permian to Cretaceous paleopole data provide a new Mesozoic apparent polar wander path for south China. The successive pole positions from Late Permian to Early Triassic indicate motion of the South China Block as a whole rather than local rotations linked to important strain within the block. The Early to Middle Jurassic pole is statistically indistinguishable from the compiled North China Block (NCB) poles and indicates that final suturation between the two blocks occurred by the end of the Triassic, which is coherent with the age of ultra high‐pressure metamorphism in the Dabie‐Sulu areas of eastern China. The Asian plate including the Mongol‐Central Asian Block, NCB, and SCB, however, was probably separated from Siberia by the Mongol‐Okhotsk ocean during Jurassic time.