Multiple tectonic events in the Miocene Japan arc: The Heike microplate hypothesis.

Abstract

It was a time of unrest for the Japan arc about 15 Myr ago. Magmatism, large-scale topography, and stress regime changed with the rapid paleomagnetic rotation of SW Japan. Magmatic zone expanded oceanward, while the event in NE Japan was older by one million years than in SW Japan. Stress regime in SW Japan changed from tensional to compressional just after the magmatism began. Here, a plate kinematic model is presented to account for the events with special reference to the magmatism in SW Japan as a key phenomenon. The magmatism has been ascribed to the subduction of the young Philippine Sea plate. Instead, it is suggested here that the hot mantle plumes that rose under the drifting Japan arc were the cause of the magmatism. A hypothetical microplate, named the Heike, is introduced in the paper. It is assumed that the plate was detached from the Pacific plate at ∼19 Ma and began anticlockwise rotation. The rotation resulted in a leaky transform boundary between the Heike and Pacific plates, creating a slab window that activated fore-arc magmatism NE Japan. Namely, hot mantle materials were raised to compensate the retreat of the Heike slab, and supplied heat to the magmas. Hot and volatile-rich plume was raised from the Pacific slab via the other slab window under western SW Japan by the roll back of the slab, activating the widespread magmatism in SW Japan. The microplate was demised at 15 Ma and the young and buoyant Shikoku Basin begun subduction. The buoyant subduction uplifted SW Japan and switched stress state in the arc. Our plate model is consistent with the superfast Japan Sea opening inferred from paleomagnetic data.