T24A-08
Water Tectonics: Evidence That Hydration Plays a Role in Tectonism

Tuesday, 15 December 2015: 17:45
306 (Moscone South)
Anthony R Lowry1, Derek Schutt2, Marta Perez-Gussinye3, Janine S Buehler4, Michael A Berry5, Xiaofei Ma1 and Dhananjay Ravat6, (1)Utah State University, Logan, UT, United States, (2)Colorado State University, Geosciences, Fort Collins, CO, United States, (3)Royal Holloway University of London, Egham, United Kingdom, (4)University of California San Diego, La Jolla, CA, United States, (5)Utah State University, Geology, Logan, UT, United States, (6)University of Kentucky, Lexington, KY, United States
Abstract:
Several new observations provide evidence that water may play a more important role in tectonism than previously realized. Among these,

• Thermodynamical modeling suggests that hydration promotes crustal mineral assemblages with lowered vP/vS and decreased density. This sheds new light on the significance of low crustal vP/vS measured in the western U.S. Cordillera from joint inversion of EarthScope USArray receiver functions and gravity. vP/vS, previously interpreted in terms of quartz abundance, is strikingly low throughout the highest-elevation regions of the western Cordillera.

• Rheological modeling of flexural rigidity measurements also can be used to map water variations, primarily in the uppermost mantle. Mantle hydration estimated from flexural rigidity exhibits very similar spatial distribution to crustal hydration inferred from vP/vS, with the notable exception that the Wyoming craton has dry mantle lithosphere but a hydrous crust.

• In hydrated lithosphere of the high-elevation western U.S. Cordillera, Moho temperatures estimated from Pn velocities are systematically colder than predictions by simple geothermal models of surface heat flow. These differences can only be reconciled by invoking a previously unrecognized advective term in the deep thermal transfer.

• New and improved estimates of magnetic bottom are much deeper than the depth of the magnetite Curie temperature in some apparently hydrous lithosphere (notably, east of the Siletzia accreted terrane). This may indicate exotic magnetic mineralogies thought to occur only under hydrous conditions.

Lithospheric hydration may be driven either by dehydration of subducted slab or by entrainment of water into upwellings passing through the mantle transition zone. Conceptualizing hydration as a large-scale process accompanied by increased buoyancy and decreased ductile strength, coupled with widespread changes in mineralogy, mass and energy transfer may help to illuminate many otherwise enigmatic processes of uplift and strain.

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