Transient Acceleration and Termination of Japan Sea Opening Controlled by Oblique-Subduction-Induced Slab Window

Back-arc extension is commonly assumed to be a steady, long-term process. However, geological records from the Japan Sea reveal a short-lived phase of rapid opening during the early–middle Miocene, followed by abrupt cessation. The geodynamic origin of this transient behaviour remains debated. This study employs three-dimensional geodynamic numerical models to investigate how oblique subduction geometry influences slab dynamics, mantle flow, and back-arc extension.

Our results show that a sufficiently high trench obliquity promotes slab breakoff at shallow upper mantle depths, forming a slab window and triggering a short-lived episode of strong lateral (toroidal) mantle flow. This flow dramatically accelerates back-arc extension and generates pronounced along-strike variations in spreading rates. As the slab window sinks into the mantle transition zone, the associated lateral mantle flow rapidly weakens, leading to a sharp decrease and eventual termination of back-arc spreading.

In contrast, models with lower subduction obliquity exhibit no slab breakoff, lack significant lateral mantle flow, and fail to reproduce rapid back-arc extension. These results suggest that slab breakoff induced by oblique subduction provides an efficient yet transient mechanism for focusing back-arc extension. Our findings offer a unified geodynamic explanation for the timing, spatial pattern, and abrupt end of Japan Sea opening. The proposed mechanism may also help explain slab window formation and episodic back-arc activity in other obliquely convergent margins.