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Published June 1, 2017 | Supplemental Material
Journal Article Open

Seismic evidence for the depression of the D″ discontinuity beneath the Caribbean: Implication for slab heating from the Earth's core

Abstract

The lowermost 100–300 km of the Earth's mantle commonly regarded as the thermal boundary layer (TBL) of mantle circulation is characterized by its complex physical properties. Beneath the Caribbean this so-called D″ layer features relatively high velocities and abrupt impedance increase at the top (designated as the D″ discontinuity). These seismic characteristics have been attributed to the accumulation of ancient subducted slab material and the phase transition in the major lower mantle mineral of pervoskite. Geodynamic models predict that the blanketing cold slabs may trap enough heat from core to be buoyantly destabilized, and eventually broken apart and entrained into the bottom of the convection cell. Here we explore the D″ structure with unprecedented resolution through modeling traveltimes, amplitudes, and waveform shapes from the USArray. We find an east-to-west asymmetrical undulation of the D″ discontinuity with a V-shaped depression of ∼70–160 km over a lateral distance of 600 km beneath northern South America. The shear velocity perturbations vary in the same trend showing the most pronounced reduction of ∼3–4% below the thinnest D″ layer in close proximity to an intermittently undetected discontinuity. The strong correlation between the D″ topography and velocity variations indicates the phase transition boundary has been perturbed or even disrupted by the large lateral temperature gradient of slab material which has been reheated from the core over extended periods of time.

Additional Information

© 2017 Elsevier B.V. Received 25 December 2016, Revised 24 March 2017, Accepted 26 March 2017, Available online 7 April 2017. We thank Eh Tan and Daoyuan Sun for fruitful discussions on the model interpretation. IRIS DMC provides earthquake waveform data used in our study. Editor Peter Shearer and two anonymous reviewers provide suggestions and comments that help greatly improve our manuscript. This work was supported by Ministry of Science and Technology of Taiwan grants, 101-2116-M-002-026 and 102-2116-M-002-025.

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