'Cracked crust' and 'pristine crust' — JAMSTEC reveals two-stage forearc formation at subduction initiation

Two layers of crust from different eras lie stacked beneath the seafloor. A JAMSTEC research team analysed drill cores from the Izu-Bonin Arc and, for the first time, revealed the crust formation process at subduction initiation using both rock physics and seismic data.

Forearc crust built by two volcanic stages

The Izu-Ogasawara (Bonin) Arc formed where the Pacific Plate subducts beneath the Philippine Sea Plate. A team led by JAMSTEC researcher Yuya Akamatsu analysed rock cores collected during IODP Expedition 352.

They found the first evidence that forearc crust formed through two distinct stages of volcanic activity immediately after subduction began.

'Cracked crust' and 'pristine crust'

The team focused on how fast seismic P-waves travel through the rocks. Rocks formed during initial seafloor spreading contain many internal fractures and show slower P-wave velocities. Rocks formed by later volcanism after spreading ceased have fewer fractures and faster P-wave velocities.

In other words, 'cracked crust' and 'pristine crust' are layered on top of each other beneath the seafloor.

Bridging rock physics and seismology

The breakthrough of this study lies in directly linking physical property data from drill cores with seismic velocity structure beneath the seafloor. Previously treated separately, geochemistry and geophysics are now integrated to explain the formation history of forearc crust as a unified story.

Future prospects

The approach established in this study will enable re-analysis of existing drill core samples and seismic data to reveal subduction zone evolution in greater detail. A new window has been opened into the internal structure and formation processes of crust that is difficult to observe directly.

For more on seafloor geology, see 'A time capsule at 3,000 m — first megalodon tooth found in situ.'

A note from the author: The contrast between 'cracked crust' and 'pristine crust' is fascinating. Two layers of crust from different eras lie stacked beneath the same seafloor.

Incidentally, the IODP Expedition 352 drilling sites are near Chichijima and Hahajima in the Ogasawara Islands. Quite a geological saga hidden beneath those waters.