Forearc Ages Reveal Extensive Short-Lived and Rapid Seafloor Spreading Following Subduction Initiation

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The Izu–Bonin–Mariana (IBM) forearc contains a record of a widespread and short-lived subduction initiation event in the early Eocene. We present new high-precision ages to determine the rates and length scales of ocean crust production following subduction initiation along the IBM convergent margin. We also explore the implications of these ages for potential causes of subduction initiation and its far-field effects. Zircons extracted from gabbros collected by Shinkai 6500 diving within the ophiolitic sequence exposed in the outer Bonin and Mariana forearcs give crystallization ages (±2σ) of 51.94 ± 0.13 Ma and 51.81 ± 0.03 Ma, respectively, consistent with a near-synchronous seafloor spreading after initiation of subduction along the entire length of the IBM system. 40Ar/39Ar ages of forearc basalts (51.34 ± 0.78 Ma) and boninites (51.27 ± 0.09 Ma to 50.33 ± 0.55 Ma) cored upslope of the Bonin sample gabbro during IODP Expedition 352 further reveal that the outer forearc comprises oceanic lithosphere with MORB to boninite compositions created by seafloor spreading during a remarkably short time period of 0.6–2 Myr at a full-rate of c. 70 mm/yr. This time period is consistent with those of some well-dated ophiolites. Thus, near-trench seafloor spreading to form basalt then boninite lithosphere is an extensive, rapid, but short-lived process after subduction initiation.