Geodynamic modeling of subduction initiation and back-arc basin spreading at the IBM subduction zone
Abstract
The Izu-Bonin-Mariana (IBM) subduction zone at the Pacific Ocean is a nature laboratory for studying the spontaneous subduction initiation (SI). The recent results of IODP 351 expedition in the back-arc of the proto-Izu-Bonin subduction zone revealed that the geochemical compositions evolved from forearc basalt (FAB), boninitie, and tholeiitic to calc-alkaline. A relative vertical force is required for the older plate to sink into mantle and the hot asthenosphere to upwell, creating a lithospheric gap to form the FAB. As subduction continues, the melting of the subducting oceanic crust could occur, leading to the release of fluids and formation of boninites. A mature arc would be formed (e.g., IBM) where this gap is closed, and then followed by back-arc basin spreading. The relationship between the SI and back-arc basin extension is still a subject of debate. We conducted a series of 2D numerical models to investigate the SI processes and the formation of back-arc spreading. The SI is numerically simulated to occur at a transform fault. The results indicate that for cases of no convergent velocity or relatively low velocity (<1 cm/yr) between the two approaching plates, there will be sufficient asthenosphere upwelling to cause melting of the subducting oceanic crust and the formation of the FAB and boninites during the early stage of SI. However, without sufficient convergent velocity, a back-arc seafloor spreading would have been predicted to form immediately after the SI, which is inconsistent with the observations at the IBM system where the back-arc spreading occurred more than 20 Ma after the SI. In contrast, with a relatively fast convergent velocity (>7 cm/yr) between the approaching plates, the SI could occur without significant asthenosphere upwelling and thus no FAB is predicted to form, which is much more consistent with the observations of the IBM system. Furthermore, we found that a back-arc basin could jump to a new location towards the trench near the axis of the forearc, when the slab retreat is sufficiently fast and when the lithospheric strength at a volcanically active forearc is sufficiently weak. This mechanism could explained the episodic jumps of back-arc basins and the formation of two backarc basins (including the Mariana Trough) in the IBM system.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMV045...05C
- Keywords:
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- 8140 Ophiolites;
- TECTONOPHYSICS;
- 8147 Planetary interiors;
- TECTONOPHYSICS;
- 8410 Geochemical modeling;
- VOLCANOLOGY;
- 8413 Subduction zone processes;
- VOLCANOLOGY