The Upper‐Mantle Structure Beneath Alaska Imaged by Teleseismic S‐Wave Reverberations

• Published in: Journal of geophysical research. Solid earth 2023-06, Vol.128 (6), p.n/a
• Main Authors: Hao, Shangqin, Shearer, Peter, Liu, Tianze
• Format: Article
• Language: English
• Subjects: Accretion; Alaska; Arrays; Continental crust; Crustal deformation; Crustal thickness; Deformation; Density; Deposition; Depth; Discontinuity; Earth mantle; Earth sciences; Elevation; Geophysics; Gravity anomalies; Isostasy; Magma; mantle transition zone; Moho; Mountains; Ocean circulation; S waves; seismic discontinuity; Subduction; Subduction (geology); S‐wave reverberation; Tectonics; Thickness; Tomography; Transition zone; Upper mantle; Upwelling
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2023JB026667

Alaska is a tectonically active region with a long history of subduction and terrane accretion, but knowledge of its deep seismic structure is limited by a relatively sparse station distribution. By combining data from the EarthScope Transportable Array and other regional seismic networks, we obtain a high‐resolution state‐wide map of the Moho and upper‐mantle discontinuities beneath Alaska using teleseismic SH‐wave reverberations. Crustal thickness is generally correlated with elevation and the deepest Moho is in the region with basal accretion of the subducted Yakutat plate, consistent with its higher density due to a more mafic composition. The crustal thickness in the Brooks Range agrees with the prediction based on Airy isostasy and the weak free‐air gravity anomaly, suggesting that this region probably does not have significant density anomalies. We also resolve the 410, 520, and 660 discontinuities in most regions, with a thickened mantle transition zone (MTZ) and a normal depth difference between the 520 and 660 discontinuities (d660‐d520) under central Alaska, indicating the presence of the subducted Pacific slab in the upper MTZ. A near‐normal MTZ and a significantly smaller d660‐d520 are resolved under southeastern Alaska, suggesting potential mantle upwelling in the lower MTZ. Beneath the Alaska Peninsula, the thinned MTZ implies that the Pacific slab may not have reached the MTZ in this region, which is also consistent with recent tomography models. Overall, the results demonstrate a bent or segmented Pacific slab with varying depths under central Alaska and the Alaska Peninsula. Plain Language Summary Alaska attracts attention from the Earth science community due to its complicated tectonic setting, including active subduction, crustal deformation, and extensive magmatism, but its deep structure remains enigmatic due to limited observations. With newly deployed dense Transportable‐Array seismic stations across Alaska, we study Alaska's deep structure using a new imaging method. We find that the crust is thick beneath the northern and southern mountains and thin in the central flatlands, with a correlation between crustal thickness and elevation. The thickest crust lies in the region with basal accretion of the subducted Yakutat slab, an oceanic plateau with higher density than the normal continental crust. We also image the 410, 520, and 660 discontinuities beneath Alaska. Combining prior results with our findings, the subducted slab may have