Lower crustal metastable gabbro and volcanism in warm subduction systems
Abstract
Along-strike variation of seismicity and volcanism is an important feature of the Cascadia Subduction Zone. To the north, intraslab activity is dense, and the 127 quaternary vents are tightly clustered around the five volcanoes. To the south, seismic activity is sparse and the more than 2000 quaternary vents form a near continuous array from Bumping Lake to the southeast of Mt. Rainier to very close to the California border. While most of the volcanism in the Cascades lies along the quaternary axis, there are four features (in the central region) that lie within the forearc: the Indian Head and Portland vent fields, Mt. St. Helens, and Mt. Rainier. Mt. St. Helens has been frequently active since 1980, and Mt. Rainier is one of the most potentially dangerous volcanoes in the United States. We present models derived from a 2-D Generalized Radon Transform seismic study across central Washington State near Mt. Rainier and show how they are distinctly different from models obtained in similar studies conducted to the north (across Vancouver Island and into British Columbia) and south (in Oregon near Mt. Jefferson). Specifically, the persistence of a low velocity feature coincident with the subducting lower oceanic crust and uppermost mantle is present here to depths of approximately 100 km. We argue that this feature is best interpreted as a metastable gabbro lower crust underlain by a moderately hydrated uppermost mantle, most likely rich in chlorite. This scenario requires a fairly narrow set of environments for slab hydration offshore, and suggests that important along-strike variation in the volcanic arc may be driven by differences in slab hydration.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.T31F2991M
- Keywords:
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- 1031 Subduction zone processes;
- GEOCHEMISTRYDE: 7240 Subduction zones;
- SEISMOLOGYDE: 8104 Continental margins: convergent;
- TECTONOPHYSICSDE: 8413 Subduction zone processes;
- VOLCANOLOGY