Ultralow velocity zone and deep mantle flow beneath the Himalayas linked to subducted slab

Abstract

The origins of ultralow velocity zones, small-scale structures with extremely low seismic velocities found near the core–mantle boundary, remain poorly understood. One hypothesis is that they are mobile features that actively participate in mantle convection, but mantle flow adjacent to ultralow velocity zones is poorly understood and difficult to infer. Although deep mantle anisotropy observations can be used to infer mantle flow patterns, ultralow velocity zone structures are often not examined jointly with these observations. Here we present evidence from seismic waves that sample the lowermost mantle beneath the Himalayas for both an ultralow velocity zone and an adjacent region of seismic anisotropy associated with mantle flow. By modelling realistic mineral physics scenarios using global wavefield simulations, we show that the identified seismic anisotropy is consistent with horizontal shearing orientated northeast–southwest. Based on tomographic data of the surrounding mantle structure, we suggest that this southwestward flow is potentially linked to the remnants of the subducted slab impinging on the core–mantle boundary. The detected ultralow velocity zone is located at the southwestern edge of this anisotropic region, and therefore potentially affected by strong mantle deformation in the surrounding area. The presence of an ultralow velocity zone and seismic anisotropy in the lowermost mantle beneath the Himalayas is linked to subducted slab remnants and southwest mantle flow, according to analyses of seismic waves and mantle anisotropy measurements.