SH Velocity Structures in the Transition Zone Beneath South America and Northeast China

The stable phase assemblages in the transition zone region are sensitive to mantle composition, temperature and chemical interactions between the olivine- and pyroxene-normative components. With the accumulation of in-situ measurements of elastic properties and accurate determination of phase equilibria data, we can now explore various chemical interactions and quantitatively calculate seismic velocity profiles for various mineralogical models. Mantle composition and thermal models, and their regional variations, can thus be quantitatively constrained by jointly modeling mineral physics data and seismic observations sampling different regions. In this study, we constrain fine seismic structures in the transition zone beneath South America and northeast China, and explore compositional and thermal models in these two regions based on mineral physics modeling. We use the triplicated phases recorded in the epicentral distance range of 10^o~- 30^o~ for a deep event occurring in South America subduction zone and two deep events occurring in northeast Asia. The triplication phases near a discontinuity place tight constraints on the velocity gradients above and below the discontinuity and the velocity jump across the discontinuity. The termination distance of the AB phases (the branch turning above the discontinuity) is sensitive to the velocity gradient above the discontinuity and the move-out of the CD phases (the branch traveling below the discontinuity) is controlled by the velocity gradient below the discontinuity. Seismic observations indicate that the SH velocity gradients above the 660 km discontinuity are larger than the Preliminary Reference Earth Model (PREM) beneath both South America and northeast China, But model differences exist for the two regions. The gradient above the 660 km discontinuity is larger for South America than for northeast China; while the model for South American has a shear velocity jump across the discontinuity and a velocity gradient below this discontinuity similar to PREM, the model for northeast China has a velocity jump larger than PREM and a velocity gradient smaller than PREM below the discontinuity. The observed triplications near the 410-km discontinuity beneath northeast China also indicate that the region has, compared to PREM, a larger velocity gradient above the 410 km discontinuity, a larger velocity jump across the 410 km discontinuity and a smaller velocity gradient below the discontinuity. In joint modeling of the mineral physics and seismic data, we calculate seismic velocity profiles for a variety of compositional and thermal models using the Tr660 program developed by Weidner and Wang (1998). We then obtain the most appropriate compositional and thermal models based on the criterion that their predicted seismic profiles best match those inferred from the seismic data and the synthetics of those predicted seismic profiles best fit the seismic data.