| Seismic constraints on the transfer of mantle materials and magma from hotspots to mid-ocean ridges are the focus of this dissertation. I developed a three-dimensional P-wave velocity model of the Icelandic crust and uppermost mantle from tomographic inversion of over 3500 first-arrivals from local earthquakes in Iceland. With seismic rays traversing below the crust, the inversion reveals a pronounced P-wave velocity reduction (about 5%) in the uppermost mantle beneath central Iceland. The large velocity reduction requires an excess temperature of up to 500 degrees or, more likely, a combination of excess temperature and partial melt. The localized nature of the low-velocity anomaly beneath central Iceland and the lack of comparable velocity reduction along the spreading centers suggest a relatively focused melt supply of the Iceland hotspot. With a more realistic representation of the propagation of seismic waves, the Finite Frequency Seismic Tomography (FFST) can image the seismic structure with significantly higher resolution than ray-based tomography. In the second part of this dissertation, FFST was used to construct the P-wave velocity model of the mantle structure beneath the Azores hotspot. The inversion revealed a low velocity anomaly in the shallow mantle along the Azores islands and beneath the center of the Azores plateau. From ∼200 km to at least 400 km depth, the low velocity anomaly forms a cylindrical structure beneath northeast of Terceira, bending toward south to southwest in the shallow mantle. These results are consistent with other geophysical and geochemical observations and provide seismic evidence for the proposed mantle plume-ridge interaction model, in which the plume conduit supplies hot mantle materials and melts preferentially southwestward along a lateral sublithospheric channel. The crustal effects on travel times measured by waveform cross-correlation are frequency dependent. Through a series of synthesis experiments, I demonstrate that the frequency dependent crustal correction for FFST can be approximated, to first order, by cross-correlating the impulse responses of a crust model filtered in a specified narrow frequency band. |
