Mantle transition zone shear velocity gradients beneath USArray
Broadband P-to-s scattering isolated by teleseismic receiver function analysis is used to investigate shear velocity (V_S) gradients in the mantle transition zone beneath USArray. Receiver functions from 2244 stations were filtered in multiple frequency bands and migrated to depth through P and S tomography models. The depth-migrated receiver functions were stacked along their local 410 and 660 km discontinuity depths to reduce stack incoherence and more accurately recover the frequency-dependent amplitudes of P410s and P660s. The stacked waveforms were inverted for one-dimensional V_S between 320 and 840 km depth. First, a gradient-based inversion was used to find a least-squares solution and a subsequent Monte Carlo search about that solution constrained the range of V_S profiles that provide an acceptable fit to the receiver function stacks. Relative to standard references models, all the acceptable models have diminished V_S gradients surrounding the 410, a local V_S gradient maximum at 490–500 km depth, and an enhanced V_S gradient above the 660. The total 410 V_S increase of 6.3% is greater than in reference models, and it occurs over a thickness of 20 km. However, 60% of this V_S increase occurs over only 6 km. The 20 km total thickness of the 410 and diminished V_S gradients surrounding the 410 are potential indications of high water content in the regional transition zone. An enhanced V_S gradient overlying the 660 likely results from remnants of subduction lingering at the base of the transition zone. Cool temperatures from slabs subducted since the late Cretaceous and longer-term accumulation of former ocean crust both may contribute to the high gradient above the 660. The shallow depth of the 520 km discontinuity, 490–500 km, implies that the regional mean temperature in the transition zone is 110–160 K cooler than the global mean. A concentrated V_s gradient maximum centered near 660 km depth and a low V_S gradient below 675 km confirms that the ringwoodite to perovskite+magnesiowustite reaction is the dominant cause of the 660 in the region.