Mantle discontinuities and reflectors beneath the Arctic Ocean and Aleutian-Alaska subduction zone: Evidence for MORB crust at the top of the lower mantle

We investigate the properties of mantle discontinuities beneath the Arctic Ocean and the Aleutian-Alaska subduction zone with underside reflections of PP and SS waves. The depth distribution of the 410-km, 520-km and 660-km discontinuities suggest a relatively normal mantle transition zone beneath the Arctic Ocean. In contrast, beneath the Aleutian-Alaska subduction, there exists a cold mantle transition zone, indicating its link with the subducted Pacific plate. Additionally, the detection of deep reflectors (720∼798 km) beneath the eastern Aleutians, identified by precursors with opposite polarity to the main phase, introduces complexity in explaining the slab geometry. Based on the mineralogical modeling for superhydrous phase B, mid-ocean ridge basalt (MORB) and a mechanical mixture of MORB and harzburgite along different geotherms, these deep reflectors are interpreted as cold MORB crust at 722–747 km depths and the post-garnet phase transition at 763–798 km depths, both associated with the Pacific slab. This slab may deform or buckle at the bottom of the mantle transition zone beneath the eastern Aleutians, facilitating transporting the MORB component below the slab. On the other hand, an uplifted 660-km discontinuity observed in the adjacent Alaskan region suggests the possibility of akimotoite–related transitions linked to subduction, or a variation in subduction depth where the slab has not yet reached the 660-km discontinuity. Our observations likely depict a complex slab geometry along the Aleutian-Alaska trench, that is, the slab appears to reach the top of lower mantle beneath the eastern Aleutians, while residing within the mantle transition zone beneath central Alaska.