Localized Seismic Scatterers Near the Core-Mantle Boundary beneath the Caribbean Sea: Evidence from PKP Precursors

Localized Seismic Scatterers Near the Core-Mantle Boundary beneath the Caribbean Sea: Evidence from PKP Precursors A) Map of the Caribbean region showing the calculated points of ray paths exiting the core and entering the mantle. Blue circles indicate the events with precursors, with a good (> 5) signal to noise ratio and red diamonds indicate the events with no evident precursor, which also have a good (> 5) signal to noise ratio. B-D) Tomographic images of the lower mantle (Grand, 2002) with isosurfaces of 1.6% shear velocity perturbation and greater. The exit points are also plotted with the images showing the localization of the scatterers and their relationship to the surrounding high velocities. The red lines (and ages) are the estimated Farallon plate boundaries from plate and kinematic reconstructions between 100 and 46 Ma (Pindell et al., 2005).
<p>
The nature of the core-mantle boundary (CMB) region, the
D”, has been a focus of research for decades because it is a crucial part in understanding the evolution of geodynamics and whole earth structure. The D” region is very heterogeneous and has complicated seismic structures that require involvements of both chemical and thermal processes. One representative and well studied area is Central America and Caribbean,where the D” region is featured by a well defined seismic dis-continuity underlain by anisotropic and higher velocity materials. Here we used seismic phase arrivals from earthquakes in the Western Pacific recorded by broadband seismic stations in northern South America and the Caribbean to investigate the core-mantle boundary and the D” region beneath the central
Caribbean plate. We identified precursors to the PKP phases
in 14 events, which can be explained by a region of heterogeneous scattering located above the CMB beneath the central Caribbean plate. The seismic scatterers are localized to the vicinity between approximately 5°N to 20°N and -80°W to-65°W. Other similar distance events that display no precursors, interpreted to have traversed non-scattering regions on the core-mantle boundary, lie to the south and southwest of the identified cluster of seismic scatterers. We found that the small-scale seismic scatterers are not uniformly distributed, are concentrated, and are surrounded by relatively fast shear velocity perturbations in the lower mantle imaged with global seismic tomography. The fast velocity perturbations may be residual slab from the subducted Farallon plate and the scatterers to the east-northeast of the high velocity material could be remnants of heterogeneous material, perhaps former basaltic crust, that has been transported ahead or on top of the subducted slab material as it impacts and bends at the core-mantle boundary.
</p><p>References
</p><p>Grand, S.P., 2002. Mantle shear-wave tomography and the fateof subducted slabs. Phil. Trans. R. Soc. Lond. A, 360(1800):2475-2491.
</p><p>
Pindell, J. et al., 2005. Plate-kinematics and crustal dynamics of circum-Caribbean arc-continent interactions: Tectonic controls on basin development in Proto-Caribbean margins. In: H.G. Lallemant and V.B. Sisson (Editors), Caribbean-South American Plate Interactions, Venezuela. Geological Society of America, Boulder CO, pp. 7-52.
</p><p>Acknowledgements: The BOLIVAR project is funded by the National Science Foundation EAR0003572 and EAR0607801 and MSM was funded by a NSERC postdoctoral fellowship.</p>

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