Three-dimensional shear wave velocity models for the Atlantic upper mantle

Abstract We investigate the three-dimensional shear-velocity structure of the upper mantle beneath the Atlantic Ocean using two approaches. One, we invert travel time measurements that were first presented in James et al., (2014) for approximately 10,000 fundamental-mode Rayleigh waves traversing the Atlantic basin, in the period range 30–125 seconds, to derive a layered 3-D model. Two, we explore continuous depth-dependent variations in vertically-polarized shear velocity, VSV, with a splined model by inverting phase velocities from the 2-D maps in James et al. (2014). For the layered model, we assume that VSV is constant in a given layer, which allows us to capture and compare the velocity structure in shallow and deep layers. Varying the boundary depth between the shallow and deep layers shows that age dependence is strongly observed in the shallow layers but has a very weak or no signal in the deep layers. This is the case even when the layer-separation boundary depth is as shallow as 100 km. For the splined models, we allow VSV to vary continuously with depth to observe gradual VSV changes with depth. Shear-velocity depth-profile comparisons reveal a significant reduction in age-dependence around 200-km depth. The splined-model approach also allows us to make quantitative shear velocity comparisons across several seafloor age regions and with results from previous studies of the Pacific basin (e.g., Nishimura and Forsyth 1989; Gaherty et al. 1996). We find that in some cases, shear velocities are higher in the Atlantic basin when compared to shear velocities in the Pacific basin for the same seafloor ages.