Sinistral to Normal Faulting along the Tan-Lu Fault Zone: Evidence for Geodynamic Switching of the East China Continental Margin

The Tan-Lu fault zone (TLFZ) along the East China continental margin (ECCM) experienced a change from sinistral to normal faulting in the late Mesozoic. Thirty-four laser ablation (LA)-ICPMS zircon U-Pb dates for plutons and volcanic rocks along the TLFZ indicate that extension-related magmatism started as early as 136 Ma. The development of pre-eruption rift basins along the TLFZ during the earliest Early Cretaceous further constrains the onset time of the Tan-Lu normal faulting to the beginning of Early Cretaceous (ca. 145 Ma). Association of extensive rifts, metamorphic core complexes, and magmatism along the margin with the Tan-Lu normal faulting suggests an Early Cretaceous extensional regime for the ECCM that also started at the beginning of the Early Cretaceous, about 145 Ma. An undeformed granite dike that intrudes the sinistral ductile shear zone yields an LA-ICPMS zircon U-Pb age of 122 Ma. Seven 40Ar/39Ar plateau ages of mica samples from mylonites in the Tan-Lu sinistral ductile shear zone range from to Ma, and these are 129.5 +/- 0.8 to 101.8 +/- 0.6 considered to represent cooling ages related to later normal faulting. A white mica 40Ar/39Ar plateau age of 149.8 +/- 0.9 Ma is interpreted as the cooling age of sinistral faulting. It is suggested that the sinistral faulting took place before 150 Ma (Late Jurassic), rather than in the Early Cretaceous, as previously proposed. The Tan-Lu sinistral faulting developed under a transpressive regime along the ECCM during the Late Jurassic. It is inferred that the switch from Late Jurassic transpression to Early Cretaceous extension is due to a shift from oblique, shallow subduction of the Izanagi Plate to orthogonal, steep subduction of the Pacific Plate.