Determination and analysis of long-wavelength transition zone structure usingSSprecursors

Abstract

S U M M A R YGlobal mapping of 410 and 660 km discontinuity topography and transition zone thickness has proven to be a powerful tool for constraining mantle chemistry, dynamics and mineralogy. Numerous seismic and mineral physics studies suggest that the 410 km discontinuity results from the phase change of olivine to wadsleyite and the 660 km discontinuity results from the phase change of ringwoodite to perovskite and magnesiowustite. Underside reflections of the 410 and 660 km discontinuities arrive as precursors to SS. With the recent development of a semi-automated method of determining SS arrivals, we have more than tripled the Flanagan and Shearer (1998a) data set of handpicked SS waveforms. We are able to increase resolution by stacking waveforms in 5 • rather than 10 • radius bins as well as increasing data coverage significantly in the southern hemisphere. The resulting SS-S410S and SS-S660S times are heavily influenced by upper-mantle velocity structure. We perform a joint inversion for discontinuity topography and velocity heterogeneity as well as performing a simple velocity correction to the precursor differential times and find little difference between the two methods. The 660 km discontinuity topography and transition zone thickness are correlated with velocities in the transition zone whereas the 410 km discontinuity topography is not. In addition, the 410 km discontinuity topography is not correlated with the 660 km discontinuity topography, rather anticorrelated, as expected due to the opposite signs of the Clapeyron slopes of their respective phase changes. These results suggest that, whereas the topography of 660 km discontinuity could be dominated by thermal effects, the topography of the 410 km discontinuity is likely dominated by compositional effects. In addition, unlike previous studies which find less topography on the 410 km discontinuity than on the 660 km discontinuity, our 410 and 660 km topography have similar amplitudes.

Imaging Mantle Heterogeneity with Upper Mantle Seismic Discontinuities

“…There exist a variety of approaches for studying discontinuity structure using the SS precursors (Deuss and Woodhouse 2002;An et al 2007;Houser et al 2008;Lawrence and Shearer 2008), including sensitivity tests and synthetic modeling of precursor behavior in response to 3-D heterogeneities (Chaljub and Tarantola 1997;Zhao and Chevrot 2003;Bai and Ritsema 2013). Here, we implement the slowness stacking and distance exclusion procedures of Schmerr and Garnero (2006), Schmerr and Thomas (2011), which is similar to the methodology of Flanagan and Shearer (1998b), except that epicentral distances with interfering seismic waves are excluded from the stack.…”

Sharp mantle transition from cratons to Cordillera in southwestern Canada

“…From the regional solution we obtain average MTZ discontinuity depths of 419.0 km and 667.0 km for the 410 and 660, both are deeper than the respective global averages of 409-410 km and 650-660 km [Gu et al, 2003;Deuss and Woodhouse, 2002;Houser et al, 2008;Houser and Williams, 2010]. The resulting MTZ thickness is 248 km, which is 6 km thicker than the global average [Flanagan and Shearer, 1998;Gu et al, 2003;Lawrence and Shearer, 2006;Tauzin et al, 2008].…”

The latest geodynamics in Central Asia: primary and secondary mantle melting anomalies in the context of orogenesis, rifting, and lithospheric plate motions and interactions

“…Утонение переходного слоя в целом подтверждается антикорреляцией его верхней и нижней границы [Shen et al, 1998;Li et al, 2000Li et al, , 2003Owens et al, 2000;Hooft et al, 2003], хотя в некоторых регионах (например, в западной части Северной Америки) границы 410 и 660 км антикоррелиру-ются друг с другом не всегда [Houser et al, 2008].…”