Mantle transition zone shear velocity gradients beneath USArray

作者：Schmandt Brandon^*

来源：Earth and Planetary Science Letters, 2012, 355: 119-130.

DOI：10.1016/j.epsl.2012.08.031

• mantle transition zone
• 520 km discontinuity
• 410 km discontinuity
• 660 km discontinuity
• subduction

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 Vs 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+magnesiowiistite reaction is the dominant cause of the 660 in the region.

• 出版日期2012-11-15