Deep Challenges in the Southern Marianas
Abstract
The southern Mariana convergent margin (SMCM) attracts global attention because of the Challenger Deep, the deepest point in the ocean. The region is perhaps more interesting because it is so different from the rest of the Izu-Bonin-Mariana (IBM) convergent margin. There are three main differences: the SMCM forearc is much narrower; arc volcanoes are missing; and the backarc basin is much closer to the trench. These differences reflect the fact that the SMCM upper plate has been widening for several millions of years while the Pacific plate subducts beneath it; today it is widening at a rate of at least 45 mm/y (GPS at Guam) while the Pacific plate subducts beneath it at 30 mm/y. As the forearc widens, new parts of the Pacific plate must begin subducting beneath it. Westward propagating subduction initiation is suggested by the subducting slab reaching deeper in the east (~240 km deep slab) than 350 km to the west (~120 km deep slab). Forearc crust in the west is 12-18 km thick but it is unclear what this is composed of and how it formed. Forearc crust for the rest of IBM formed during subduction initiation in Eocene time but southern Mariana forearc crust must have a different origin. It is unclear how widening of the SMCM is accomplished, but volcanism must be important. Some remnants of older crust have been found in the forearc so mechanical extension also occurs but we cannot yet estimate what is proportion of magmatic vs. mechanical extension. Crustal seismicity is today concentrated in the west, so extension may mostly occur there now. The Moho is exposed in the inner trench wall, and several kilometers of mantle peridotite are exposed below this, providing unparalleled opportunities to study the upper mantle in situ. One seafloor seep field with a chemosynthetic community has been discovered just below the Moho; many more such vents likely exist in the SMCM forearc but the low energy output of these vents will make finding them challenging. We haven't begun to study the crust of the downing Pacific plate here, but swath mapping reveals major normal faults that likely exposed crustal sections and may host hydrothermal vents associated with serpentinization of the downing plate. Opportunities for international co-operation to study this challenging region exist and future studies of this region are eagerly anticipated.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V54B..06S
- 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