Abstract
The Lau back–arc basin and spreading centers exhibit significant along-strike variations in plate motion rates and distance between the spreading centers and the Tonga trench. The mantle processes are expected to be complex in this region and seismic anisotropy offers a key constraint. Linked geodynamic and mineral alignment models of the mantle wedge predict the scale of anisotropic variation and underlying assumptions can be tested by comparison to observed anisotropy. Three cross-basin transects allow us to isolate the effects of trench-spreading center distance as well as rates of convergence and spreading. Lattice preferred orientations (LPO) corresponding to each transect’s 2-D flow field are calculated for peridotite polycrystals. Predicted P-wave anisotropy of several percent and shear-wave split times up to 3.5s are determined, with measurable differences predicted between transects. Variation in the 2-D estimates of flow driven by each transect’s plate kinematics will induce along-strike flow in the mantle wedge. We find that the rate of such flow would likely not alter the subduction/spreading induced LPO sufficiently to explain the reported trench/arc parallel fast seismic directions in the Lau Basin; additional geodynamic factors must play a role.
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