Segmentation of the Aleutian‐Alaska Subduction Zone Revealed by Full‐Wave Ambient Noise Tomography: Implications for the Along‐Strike Variation of Volcanism

Abstract

The along-strike variations of the velocity, thickness, and dip of subducting slabs and the volcano distribution have been observed globally. It is, however, unclear what controls the distribution of volcanoes and the associated magma generation. With the presence of nonuniform volcanism, the Aleutian-Alaska subduction zone (AASZ) is an ideal place to investigate subduction segmentation and its relationship with volcanism. Using full-wave ambient noise tomography, we present a high-resolution 3-D shear wave velocity model of the AASZ for the depths of 15-110 km. The velocity model reveals the distinct high-velocity Pacific slab, the thicker, flatter, and more heterogeneous Yakutat slab, and the northeasterly dipping Wrangell slab. We observe low velocities within the uppermost mantle (at depth <60 km) below the Aleutian arc volcanoes, representing partial melt accumulation. The large crustal low-velocity anomaly beneath the Wrangell volcanic field suggests a large magma reservoir, likely responsible for the clustering of volcanoes. The Denali volcanic gap is above an average-velocity crust but an extremely fast mantle wedge, suggesting the lack of subsurface melt. This is in contrast with the lower-velocity back-arc mantle beneath the adjacent Buzzard Creek-Jumbo Dome volcanoes to the east. The back-arc low velocities associated with the Pacific, the eastern Yakutat, and the Wrangell slabs may reflect subduction-driven mantle upwelling. The structural variation of the downgoing slabs and the overriding plate explains the change of volcanic activity along the AASZ. Our findings demonstrate the combined role of the subducting slab and the overriding plate in controlling the characteristics of arc magmatism. Plain Language Summary The subduction of oceanic plates underneath the continent plate is a complicated, three-dimensional process. The geometry of the downgoing plate and the associated volcanic activity vary along the subduction margin. It is not clearly understood what controls the distribution of arc volcanoes. We utilize an advanced seismic imaging technique to construct a detailed seismic velocity model of the Aleutian-Alaska margin, from crust to the uppermost mantle. The velocity model reveals multiple downgoing slabs, with various seismic velocities, thicknesses, and dip angles. The imaged Pacific slab, Yakutat slab, and Wrangell slab correlate spatially with the Aleutian arc volcanoes, the Buzzard Creek-Jumbo Dome volcanoes, and the Wrangell volcanoes, respectively. The mantle wedge, the wedge-shaped space below the overriding crust and above the downgoing slab, is characterized by different seismic velocities below these three volcanic areas. The Denali volcanic gap, a region with the absence of volcanic activity, is located above a seismically fast mantle wedge and an average crust. There is no melt below this volcanic gap based on our observations. Our findings explain the along-strike variation of volcanic activity along the Aleutian-Alaska subduction zone and demonstrate the combined role of the s...

Amphibious Shear Wave Structure Beneath the Alaska‐Aleutian Subduction Zone From Ambient Noise Tomography

“…Serpentinization of the Aleutian forearc has been reported by several existing seismic tomography studies (e.g., Martin‐Short et al., 2018; Yang & Gao, 2020). The vertical cross‐section B‐B' (Figure 8b) indicates reduction in shear wave velocity at the forearc near the Kodiak island and the 15 km Vs slice shows the region has the most significant Vs reduction than elsewhere, probably infers more fluid‐saturated material.…”

“…Indeed, thermal modeling studies (e.g., Abers et al., 2017) have suggested low temperature in the Aleutian mantle forearc, providing the environment for the serpentinization process. Previous tomographic studies also suggested that the Aleutian forearc is low in seismic velocities (e.g., Eberhart‐Philllips et al., 2006; Martin‐Short et al., 2018; Yang & Gao, 2020), which further increase the possibility of serpentinization of the Aleutian forearc. The imaged serpentinized region could be physically related to the seismicity gap (Figure 8b), as serpentine generally exhibits stable‐sliding aseismic behavior in the laboratory (e.g., Peacock & Hyndman, 1999).…”

“…As mentioned in the introduction, the Alaska‐Aleutian subduction zone exhibits remarkable along‐strike variations in historic seismic activity (Davies et al., 1981) and geophysicists have devoted great effort investigating the structure and physical process occurring at the subduction zone (e.g., Li & Freymueller, 2018; Shillington et al., 2015; Yang & Gao, 2020). The subduction zone, particularly the Semidi segment, could potentially trigger a devastating tsunami toward the western coast of the US mainland (von Huene et al., 2016).…”

Lithospheric Formation and Evolution of Eastern North American Continent

“…Our model reveals strong structural variations within eastern North America. It has been demonstrated that more data and/or a more accurate methodology result in stronger velocity contrasts (e.g., Hung et al., 2004; Becker, 2012; Gao & Shen, 2014; Gao, 2018; Gao et al., 2020; Savage et al., 2017; Yang & Gao, 2020). Furthermore, we ignore seismic anisotropy and attenuation in wave simulation and inversion.…”

“…Incorporation of P ‐wave velocities in kernel calculation and inversion provides additional constraints for the shallow crust, which significantly reduces the influence of P ‐wave velocity uncertainties on deeper structures. We invert for the shear velocity perturbations based on a damped least squares scheme (Montelli et al., 2004; Gao & Shen, 2014; Yang & Gao, 2020), with the damping and smoothing parameters selected in terms of the tradeoff between model norm and variance reduction (Figure ). The velocity model was progressively and iteratively updated for a total of six iterations (Figures and ).…”