| Abstrakt: |
We study the collision of the Ontong Java Plateau, reversal of the Solomon Islands arc, and subduction of the Woodlark Basin spreading ridge in the southwest Pacific. Double‐sided subduction and intra‐arc deformation, not just polarity reversal, characterize the rapid Australia‐Pacific plate convergence. The rifted margins of the 0–6.2‐Ma Woodlark Basin separate its unflexed young lithosphere without a trench from the older flexed lithosphere with >8‐km‐deep trenches subducting to either side. A gap evident in the seismicity and tomography between the NE‐subducted slabs of the Solomon Sea and Australia plates indicates that the Woodlark lithosphere likely melts and/or is quickly (<1 m.y.) resorbed into the mantle. A section of the Pacific Plate remains attached within the gap and provides the slab pull to form the Malaita accretionary prism. Quaternary arc‐composition volcanoes that occur on both sides of the Woodlark Basin subduction front are not sourced from a mantle wedge overlying a Wadati‐Benioff zone, but come from and through breaks in the young incoming and subducting lithosphere and the overturning mantle beneath. Though the basin is too weak to sustain plate flexure, arc volcanoes on the subducting crust result in locally strong coupling to the forearc, manifest by tsunamigenic earthquakes. At 150–400‐km depth, the subducted slabs under the Solomon Islands are near vertical. They are near‐horizontal between the 410 and 660‐km mantle discontinuities beneath the Woodlark Basin and further south, whereas beneath the Solomon Sea Basin they dip southwest and penetrate the 660‐km discontinuity, having been detached by the north‐dipping Solomon Sea slab. Plain Language Summary: The Pacific Plate continues to subduct at a reduced rate following the collision of the Ontong Java Plateau with the Solomon Islands. Opening the 0–6.2 Ma Woodlark Basin formed a window between the slabs then subducted from the south that allowed the opposing Pacific slab to remain attached there, which provided the slab pull to form the Malaita Accretionary Prism. It also allowed arc‐composition volcanoes to erupt on both sides of the southern subduction front, being sourced from and through breaks in the young incoming, melting and resorbing lithosphere and the overturning mantle beneath. Though the Basin is too weak to sustain plate flexure and form a bathymetric trench, arc volcanoes on the subducting crust result in locally strong coupling to the forearc. A seismicity gap beneath the Russel Islands has not ruptured in >120 years. The subducted slabs under the Solomon Islands are near vertical at 150–400 km. They are near‐horizontal between the 410 and 660 km mantle discontinuities beneath the Woodlark Basin and further south, being still attached to the Pacific Plate beneath Guadalcanal and Makira, whereas beneath the Solomon Sea Basin they dip southwest and penetrate the 660 km discontinuity, having been cross‐cut and detached by the north‐dipping Solomon Sea slab. Key Points: Tomography extends earthquake images of slabs subducted to great depths under the Solomon Islands from both sidesAbove a slab window, arc‐composition volcanoes are sourced from and through breaks in the young incoming and subducting lithosphereThe lithosphere is too weak to sustain plate flexure yet is strongly coupled to the forearc by arc volcanoes on the subducting crust [ABSTRACT FROM AUTHOR] |
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