Anna Mahr Courtier

A water-rich transition zone beneath the eastern United States and Gulf of Mexico from multiple ScS reverberations

We examine mantle discontinuities beneath the United States and Gulf of Mexico using multiple ScS reverberations from earthquakes in Central and South America captured by 65 broadband and long-period seismometers across the United States. The depths of discontinuities and the impedance contrasts across them were estimated using a hierarchical waveform inversion and stacking method. The path-averaged depth of the 410-km discontinuity varies moderately across the study area and is particularly shallow (∼395 km) beneath the eastern United States. Topography on the 660-km discontinuity is more subdued and is close to the global mean depth. The 520-km discontinuity is seen consistently across the study area, though both the depth and the impedance contrast of the discontinuity vary significantly. Corridors in the eastern United States and Gulf of Mexico have extremely strong 520-km discontinuities relative to the corresponding 410-km and 660-km discontinuities. We attribute the shallow 410-km and strong 520-km discontinuities beneath the eastern United States and Gulf of Mexico to a locally water-rich transition zone.

Seismic anisotropy associated with continental lithosphere accretion beneath the CANOE array, northwestern Canada

We examine upper-mantle seismic anisotropy beneath the cordillera and craton of northwestern Canada using the multi-event station averages of shear-wave splitting of SKS, SKKS, and sSKS phases recorded during the Canadian Northwest Experiment (CANOE). Splitting times derived from multi-event averaging at each station range from 0 to 1.4 s, with an array average of ∼0.65 s. Over broad portions of the array, fast directions are coherent and roughly consistent with the direction of absolute plate motion in a hotspot reference frame, suggesting that coherent asthenospheric fabric underlies the plate in much of the region. Within this broad framework, fast directions and splitting times show marked variability over length scales of 50–200km, and are generally correlated with surface and/or crustal tectonics. Anomalous splitting is observed across an ancient suture within the cratonic lithosphere, apparently associated with complex dipping fabric produced during continental assembly. The deformation front of the Canadian Rockies correlates with a significant change in splitting behavior, consistent with the front range demarking the craton-cordillera transition within the mantle. Splitting times are small across much of the cordillera, indicating that lithospheric and/or asthenospheric fabric is weaker or less coherent than beneath the craton. In the western cordillera, fast directions rotate abruptly to parallel the plate boundary, implying that fabric associated with plate-boundary deformation extends ∼200km into the North American continent.

The influence of temperature, bulk composition, and melting on the seismic signature of the low-velocity layer above the transition zone

We report a new technique to describe seismic velocity and impedance anomalies atop a seismic low-velocity layer (LVL) at 350 km depth. We model shear wave speed reductions detected with Ps conversions beneath the Hawaiian Islands and negative impedance contrasts detected with ScS reverberations beneath the Coral Sea in the South Pacific, by varying the bulk solid composition, reference potential temperature, dihedral angle of melt, and melt composition. For a given bulk solid composition, the effects of elevated temperature and melt volume fraction on the seismic properties trade off with one another. At a given temperature, the calculated melt volume fraction is nearly insensitive to variations in the bulk solid composition. A low volume fraction of low dihedral angle melts mimics the seismic signature of a higher volume fraction of high dihedral angle melts. Despite stronger lateral variations in the LVL structure beneath Hawaii compared to the Coral Sea, both regional averages are similar. For a basalt volume fraction of 0.2 and a dihedral angle of 10°, we estimate regional averages of 1.1 ± 0.8 vol % melt at a depth of 350 km beneath the Hawaiian Islands for a reference potential temperature of 1800 K and 1.2 ± 0.005 vol % melt at a depth of 350 km beneath the Coral Sea region for a reference potential temperature of 1500 K. Our model of the seismic signal is unable to distinguish between melt compositions of mid-ocean ridge basalt and carbonated peridotite melts at such small melt volume fractions.