Izu-Bonin forearc tectonics subsequent to Pacific Plate subduction initiation: resolving a 15 Ma hiatus enigma
Abstract
International Ocean Discovery Program Expedition 352 sites U1439 - U1442 obtained sedimentary-volcanoclastic sequences that allow the reconstruction of the Izu-Bonin forearc tectonic evolution. The oldest recorded biostratigraphic ages of the sediment cover (Late Eocene to Early Oligocene) imply a hiatus of up to 15 million years between the formation of the igneous basement around 52 to 47 Ma and the onset of pelagic sedimentation.
At the forearc upslope sites (U1439, U1442) extension resulted in the formation of asymmetric, fault-bounded basins. Displacement along confining normal faults is in the range of 310 meters, resulting in stretching by ~16%-19% at strain rates in the range of 1.58 x 10-16 to 4.62 x 10-16. Downslope site U1441 is characterized by symmetric graben structures bound by conjugate normal faults. Displacement along these faults is ~ 660 m to 770 m, resulting in extension in the range of ~4 at strain rates of 9.66 x 10-15 to 3.31 x 10-15. Incipient roll-back of the Pacific Plate ~15 Ma after igneous basement formation triggered upper plate extension that was accommodated by normal faulting with initial syn-tectonic sedimentation from Late Eocene to Early Oligocene times. This was followed by waning displacement with slow-rift to post-rift sediment deposition during the transition from rifting along the margins to spreading at the center of the Shikoku-Parece-Vela basin system. Although the extensional strain rates appear to be low, these rates just represent initial upper crust rifting within the IBM forearc. Towards the arc and the backarc, rifting was assisted by magmatism probably related to the Oki Daito Plume, so that effective stresses drop and the lithosphere weakened locally. The 'boninite sites' (U1439 and U1442) are therefore situated between the more intensely stretched trench-near downslope sites, and the magmatically weakened arc / rear-arc / backarc sites. Accordingly, the boninite sites form a less stretched, neutral domain between the retreating trench and the spreading arc and backarc. A major detachment potentially commenced along the upwelling mantle and related plutons beneath the IBM arc and rear arc. The dismembered parts, now exposed along the Kyushu-Palau Ridge, therefore represent an arc-derived allochthon along the western margin of the Shikoku - Parece Vela Basin system.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V51G0200K
- Keywords:
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- 3613 Subduction zone processes;
- MINERALOGY AND PETROLOGY;
- 8170 Subduction zone processes;
- TECTONOPHYSICS;
- 8185 Volcanic arcs;
- TECTONOPHYSICS;
- 8413 Subduction zone processes;
- VOLCANOLOGY