The Green Dog That Did Not Bark: The Curious Incidence of Seismic Anisotropy Beneath Mid-Ocean Islands
Abstract
Active-source seismic refraction and teleseismic receiver functions (RFs) suggest that the oceanic crust beneath hotspot islands is often underlain by a layer with seismic velocities intermediate to crust and mantle. Extensive magmatic underplating beneath the oceanic Moho has been proposed to feed a smaller volume of basaltic melt that erupts at the seafloor and (eventually) atop a subaerial volcano. However, a lateral gap in the underplated layer beneath the Hawaii swell, and tilted-axis anisotropy there and beneath other islands, argue against the lateral flow of a gabbroic or ultramafic sill. Upward migration of basaltic magma along tilted fractures to intercalate with mantle peridotite could explain the seismic observations, but this model requires a lens of partial melt to supply the magma across the entire hotspot swell. An alternate hypothesis is "metasomatic underplating" whereby crustal fractures develop during magma ascent and allow seawater to infiltrate and to serpentinize the sub-Moho mantle. Metasomatic underplating would lower seismic wavespeeds, promote buoyancy of the hotspot swell, and induce textural anisotropy as metamorphic expansion of olivine-rich peridotite promotes cracks that extend serpentinization. The differential expansion of mantle peridotite and crustal gabbro promotes new fractures in the crust along which seawater descends to the Moho, allowing metasomatic underplating to spread far from centers of active volcanism. Rare serpentinized mantle xenoliths suggest that crack textures develop during serpentinization at depth and could contribute anisotropy to the underplated layer. The discovery of iron-oxidizing microbial mats on the seafloor flank of the Loihi volcano, and many locations of diffuse low-temperature fluid venting worldwide, is consistent with the circulation of metasomatic fluids with reducing chemistry, sourced from serpentinization at depth. Metasomatism from fluid infiltration is slow, subject to cycles of permeability blockage and fresh fracturing. Post-eruptive uplift of Santa Maria Island (Azores) and the asymmetry of the Hawaiian swell, relative to its active volcanism, suggest that underplating requires 2-4 Myr to complete. Sub-Moho volume expansion helps explain stress geometry of lithospheric seismicity beneath Hawaii.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.T33B..08P
- Keywords:
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- 7218 Lithosphere;
- SEISMOLOGYDE: 7220 Oceanic crust;
- SEISMOLOGYDE: 8120 Dynamics of lithosphere and mantle: general;
- TECTONOPHYSICSDE: 8416 Mid-oceanic ridge processes;
- VOLCANOLOGY