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Upper Mantle Anisotropy in North America and the Pacific From Global Adjoint Tomography

Description: 

The Earth's upper mantle exhibits significant anisotropy due to its composition and deformation throughout its evolution and dynamics. Despite its importance, there is no consensus on current anisotropic tomographic models. Additionally, discrepancies persist between results derived from different data sets, such as surface waves and shear-wave splitting observations. Using the transversely isotropic global adjoint model GLAD-M25 as the starting model, we developed GLAD-M28-AZI—a global azimuthally & radially anisotropic model constructed through 3D spectral-element simulations. GLAD-M28-AZI incorporates the full complexity of wave propagation and data sensitivities within an iterative adjoint tomography framework. We used minor- and major-arc Rayleigh and Love waves from 300 earthquakes during iterations. Despite our limited data, GLAD-M28-AZI achieves a continental-scale resolution in areas with sufficient data coverage. The model reveals azimuthal anisotropy in subduction zones perpendicular to the trench at depths of 50–100 km, which transitions to a trench-parallel orientation at 200–300 km in regions such as South America, New Zealand, and the Western Pacific where deeper anisotropy patterns are consistent with SKS splitting observations. As an example of continental scale observations, in North America, in addition to common features with continental-scale tomographic models, we observe distinct trench-perpendicular azimuthal anisotropy in Alaska where the north-south fast direction on the east of Alaska is interrupted by the Rockies in the south and the cratonic border along the McKenzie River at around 200 km. Azimuthal anisotropy depicts the east-west extension in the Basin and Range and consistency with plate motions, showing depth-dependent correlation with SKS splitting measurements. We will discuss the implications of our observations on GLAD-M28-AZI compared to other models and data sets for understanding continental to global scale mantle dynamics.

Session: Earth’s Structure from the Crust to the Core - III

Type: Oral

Date: 4/16/2025

Presentation Time: 02:30 PM (local time)

Presenting Author: Ebru

Student Presenter: No

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