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    Increasing evidence from seismic methods shows that anisotropy within subduction zones should consist of multiple layers. To test this, we calculate and model shear wave splitting across the Alaska-Aleutians Subduction Zone (AASZ), where previous studies have argued for separate layers of anisotropy in the subslab, slab and mantle wedge. We present an updated teleseismic splitting catalogue along the span of the AASZ, which has many broad-band seismometers recently upgraded to three components. Splitting observations are sparse in the Western Aleutians, and fast directions are oriented generally trench parallel. There are significantly more splitting measurements further east along the AASZ. We identify six regions in the Central and Eastern Aleutians, Alaskan Peninsula and Cook Inlet with a high density of splits suitable for multilayered anisotropy analyses. These regions were tested for multilayer anisotropy, and for five of the six regions we favour multiple layers over a single layer of anisotropy. We find that the optimal setup for our models is one with a dipping middle layer oriented parallel to palaeospreading. A prominent feature of our modelling is that fast directions above and below the dipping layer are generally oriented parallel to the strike of the slab. Additionally, we lay out a framework for robust and statistically reliable multilayer shear wave splitting modelling.

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    The Alaska–Aleutian subduction zone represents an ideal location to study dynamics within a mantle wedge. The subduction system spans several thousand kilometres, is characterized by a slab edge, and has ample seismicity. Additionally, the majority of islands along the arc house broad-band seismic instruments. We examine shear wave splitting of local-S phases originating along the length of the subduction zone. We have dense measurement spacing in two regions, the central Aleutians and beneath Alaska. Beneath Alaska, we observe a rotation in fast splitting directions near the edge of the subducting slab. Fast directions change from roughly trench perpendicular away from the slab edge to trench parallel near the boundary. This is indicative of toroidal flow around the edge of the subducting Alaska slab. In the central Aleutians, local-S splitting is primarily oriented parallel to, or oblique to, the strike of the trench. The local-S measurements, however, exhibit a depth dependence where deeper events show more consistently trench-parallel directions indicating prevalent trench-parallel mantle flow. Our local-S shear wave splitting results suggest trench-parallel orientation are likely present along much of the subduction zone excited by the slab edge, but that additional complexities exist along strike.

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  3. Abstract

    In the past decade, mounting evidence has pointed to complex, layered structure within and at the base of the mantle lithosphere of tectonically quiescent continental interiors. Sometimes referred to as negative velocity gradients or midlithospheric discontinuities (MLDs), the origin of intralithospheric layering has prompted considerable discussion, particularly as to how they may result from continent formation and/or evolution. Previous Sp receiver function analysis in Australia (Ford et al., 2010, found evidence for complex lithospheric layering beneath permanent stations located within the North, South, and West Australian Cratons and characterized these as MLDs. This study provides an update to the original study by Ford et al. (2010, Sp receiver function results are presented for 34 permanent, broadband stations. We observe the lithosphere–asthenosphere boundary (LAB) on the eastern margin of the continent, at depths of 75–85 km. The cratonic core of Australia has discontinuities within the lithosphere, with no observable LAB. On the western margin of the continent, we observe several stations with an ambiguous phase that may correspond to an MLD or the LAB. We also observe multiple negative phases at most stations, suggesting a complex and heterogeneous lithosphere. Australian MLDs are likely linked to the presence of hydrous minerals in the midlithosphere and may result from ancient processes such as subduction, plume interaction, or melt infiltration from the paleo‐LAB.

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