Mantle discontinuities beneath Arctic ocean and Aleutian-Alaska subduction zone

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 distributions of the 410-km and 520-km discontinuities suggest a relatively normal mantle transition zone beneath the Arctic ocean and a cold mantle transition zone with the subducted Pacific plate beneath Aleutian-Alaska subduction. The depth of the 660 km discontinuity shows normal behavior beneath the Arctic Ocean. However, the detection of deep reflectors with opposite polarities in depth range of 720~770 km beneath the eastern Aleutians introduces additional complexity for explaining the slab morphology.  We test several plausible compositions using mineralogical modeling along a subduction geotherm. The deep reflectors are interpreted as mid-ocean ridge basalt (MORB) crust associated with the Pacific slab that may deform or buckle at the bottom of the mantle transition zone beneath the eastern Aleutians.  Meanwhile, an uplifted 660-km discontinuity observed in the adjacent Alaska region suggests a different subduction depth, where the slab may penetrate the 410-km discontinuity but does not reach the 660-km discontinuity,  consistent with previous regional studies. Our observations thus depict a complex slab geometry along the Aleutian-Alaska trench, that is, the slab  may reach the top of the lower mantle beneath eastern Aleutian but remains at the base of the transition zone underneath central Alaska.