An Ocean Plate's Demise
The Molucca Sea is the site of a unique and slow-motion tectonic crash. This is the only location on the planet featuring a "divergent double subduction," where a single oceanic plate is sinking into the Earth's mantle from two opposite sides simultaneously. The Molucca Sea Plate is being forced downwards to the west beneath the Sangihe Arc and to the east beneath the Halmahera Arc. This process has created an inverted U-shaped slab of rock descending deep into the mantle.
This complex collision zone involves three major tectonic players: the Eurasian, Pacific, and Australian plates. The Sangihe Arc is part of the Eurasian margin, while the Halmahera Arc is part of the Philippine Sea Plate. The two volcanic arcs, which are roughly 250 kilometers apart, are converging on each other as the oceanic plate between them vanishes. Seismic imaging shows the Molucca Sea Plate slab extends to a depth of about 400 kilometers under the Halmahera Arc and possibly as deep as 600 kilometers under the Sangihe Arc. The subduction process is asymmetrical; the plate descends at an angle of approximately 40° beneath the Sangihe Arc.
A Region Forged by Collision
The immense pressure of this arc-arc collision has buckled the seafloor, creating a chaotic jumble of rock known as the Molucca Sea Collision Complex. This complex is a thick wedge of scraped-off seafloor sediments and crust. Rising from this complex is the Talaud-Mayu Ridge, a submarine feature that occasionally breaks the surface to form islands. These islands, including Talaud and Mayu, contain ophiolites—sections of oceanic crust and the underlying upper mantle that have been thrust upwards and exposed.
The entire region is one of the most seismically active in the world. The dual subduction zones generate two distinct, oppositely dipping Wadati-Benioff zones, which are planes of deep earthquake activity within the descending slabs. Earthquakes here can occur at depths greater than 600 kilometers. The subduction also fuels the volcanism that formed the Sangihe and Halmahera island arcs. As the collision continues, the Sangihe Arc is slowly overriding the Halmahera Arc, a process that will eventually lead to the complete closure of the Molucca Sea and the accretion of the volcanic arcs onto the larger continental margin.