UC Berkeley

It is generally accepted that mantle plumes are responsible for hotspot chains, and as such provide insight to mantle convection processes. Among all the hotspots, the Hawaii chain is a characteristic example that has been extensively explored. But many questions remain including, what is the shape, size and orientation of the plume conduit? To what extent can the seismic structure of the plume be mapped? Can we see a continuous plume conduit extending from the lower to the upper mantle? At what depth do melting processes occur? In the first part of this thesis, we are trying to answer these questions from a seismic imaging perspective. We combine constraints from three data sets (body waves, ballistic surface waves and ambient noise) to create 3D images of the velocity structure beneath the Hawaiian island chain from a depth of ~800 km to the surface. Our multiphase 3D model results indicate there is a large deep-rooted low velocity anomaly rising from the lower mantle. At transition zone depth the conduit is located to the southeast of Hawaii. A 2% S-wave anomaly is observed in the core of the plume conduit around 700 km depth which, once corrected for damping effects, suggests a 200-250C temperature anomaly assuming a thermal plume. In the upper mantle, there is a horizontal plume “pancake” at shallow depths beneath the oceanic lithosphere, and there is also a second horizontal low-velocity layer in the 250 to 410 km depth range beneath the island chain. We suggest this feature is a deep eclogite pool (DEP), an interpretation consistent with geodynamic modeling.

The second major part of this thesis is imaging the Cascadia subduction plate. First, we present a novel 3-D pre-stack Kirchhoff depth migration (PKDM) method for teleseismic receiver functions. The proposed algorithm considers the effects of diffraction, scattering and traveltime alteration caused by 3-D volumetric heterogeneities. It is therefore particularly useful for imaging complex 3-D structures such as dipping discontinuities, which is hard to accomplish with traditional methods. Next, we present a 3D model of upper mantle seismic discontinuity structure below Cascadia using this migration method. In this model, multiple and primary signals are separated by our analysis. The 410km discontinuity is observed across the entire image together with lithosphere-asthenosphere boundary (LAB). A fine analysis of the primary and multiple reverberated phases allows imaging of the Juan de Fuca plate dipping below the North American continent. At two frequency bands (5 s and 10 s period), the main seismic discontinuities in the plate are limited to a downward increase of shear-wave velocities. Our model shows along-strike variations in the visibility of this discontinuity. To the southern and northern ends of the subduction system, the discontinuity is clearly observed down to the transition zone. In the center under central Oregon, this structure is however missing, which leaves a seismic discontinuity gap within the subducted oceanic plate at the depth between ~150 km and ~300 km. We attribute the observed heterogeneity of the discontinuity structure inside oceanic plate to different hydration and plume-slab processes prior to and during subduction.