The seismic structure of island arc crust and reflection imaging of ultra-deep roots beneath the eastern Aleutian island arc (Invited)
Abstract
Seismic velocity models derived from refraction surveys are commonly used to determine the crustal thickness and to constrain the composition of oceanic island arcs. Crustal thicknesses vary from as little as 10 km in the Bonin arc to 35 km in the Aleutian and northern Izu arcs. Although globally island arcs appear to have a mafic composition, intermediate composition crust is inferred in parts of the Izu arc. Seismic velocity models are commonly poorly constrained at depths of 20-50 km beneath the arc due to the difficulty in recording first arrivals at the long offsets required to directly infer the velocity; however, the absence of a sharp velocity contrast at the Moho appears to be a first order characteristic of island arc crust, and indicates the existence of a broad crust-mantle transition zone. Multichannel seismic reflection surveys complement refraction surveys by revealing structures associated with variations of density and seismic velocity at the scale of a few hundred meters or less to depths of 60 km or more. An integrated interpretation of coincident reprocessed seismic reflection data and velocity models from the Aleutian arc shows that the eastern arc is characterized by reflectors that extend continuously from the lower arc crust to >50 km depth, which is considerably deeper than the crustal thickness of 27-35 km previously inferred from coincident wide-angle seismic surveys. Since relative to the overlying crust the upper mantle is commonly homogeneous and hence non-reflective, we interpret these reflectors to be gabbro, garnet gabbro, and pyroxenite intrusions within two 50 km-wide roots, which represent a >25 km thick heterogeneous transition from mafic lower crustal rocks to ultramafic mantle rocks. We suggest that the reflectivity is linked to repeated differentiation and intrusion of mantle-derived melts into the sub-arc lithosphere, and that the depth of these roots shows that fractionation of arc crust can extend well below the seismically determined Moho. Since these deep roots are not evident beneath the central Aleutian arc, either the roots form sporadically, perhaps as a consequence of an elevated magmatic supply, or such roots ultimately founder into the underlying mantle due to their relatively high mass density.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.V14B..02C
- Keywords:
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- 8104 TECTONOPHYSICS Continental margins: convergent;
- 8185 TECTONOPHYSICS Volcanic arcs;
- 3060 MARINE GEOLOGY AND GEOPHYSICS Subduction zone processes