Lg-wave attenuation tomography in the Alaskan mainland: Implications for the formation of volcanic gap and clustered volcanism

The Alaskan mainland overlies the subducting Pacific plate and Yakutat microplate as they subduct beneath the southern margin of the North American plate. South-central Alaska features massive volcanoes of different types, including intraplate volcanoes, Aleutian arc volcanoes, and a group of densely clustered volcanoes called the Wrangell volcanic field (WVF). How the Denali volcanic gap (DVG) formed and why the Wrangell volcanoes are clustered remain vigorously debated. Investigating the crustal thermal structure can be crucial for understanding subsurface magmatic activity. Seismic attenuation, or the quality factor Q, usually provides good constraints on the viscoelastic structure and is sensitive to thermodynamic processes in the lithosphere, such as partial melting and high-temperature magmatism. Regional Lg-waves propagating in the continental crust waveguide are an ideal phase for investigating crustal attenuation. In this study, based on vertical-component waveform data recorded by 20 permanent and temporary seismic networks in Alaska, we established a high-resolution broadband crustal Lg-wave attenuation model for Alaska and nearby regions. Strong Lg attenuation is observed beneath the volcanoes in south-central Alaska, indicating thermal anomalies and possible melting in the crust. In contrast, the central Yakutat terrane and DVG are characterized by weak Lg attenuation, suggesting the existence of a cool crust that prevents hot mantle materials from invading the crust. This cool crust is likely the reason for the DVG. Quarter-toroidal crustal melting with strong attenuation is revealed around the Yakutat terrane. This curved zone of crustal melting, possibly driven by toroidal mantle flow, weakly connects the Wrangell and Buzzard Creek-Jumbo Dome magmatic chambers.

This research was supported by the National Natural Science Foundation of China (42430306 and 42404067), the China Postdoctoral Science Foundation (2024M751295) and the Postdoctoral Fellowship Program of CPSF (GZC20240638).