Subparallel dipping heterogeneities in the mid-lower mantle

Data from western U.S. short-period seismic networks show anomalous later phases which we interpret as S-to-P converted waves from heterogeneities in the mid-lower mantle (depths of ∼1000-1850 km) northeast of the Mariana subduction zone. The geometry of the objects is mapped by a two-step approach: (1) computing the S-to-P scattering likelihood with the arrival time, slowness, and azimuth of the later arrivals measured by array techniques relative to direct P waves, and (2) by direct use of the waveforms using semblance. At least three subparallel objects are detected in the lower mantle study region, with the dips of 30° to 40° southward and with a lateral extent larger than 500 km by 300 km. The elastic properties associated with the heterogeneous objects appear to change within several kilometers, considering their high efficiency at converting short-period waves (wavelengths of 5-10 km). Previous seismic tomography studies have not predicted significant velocity anomalies around the heterogeneities. This is probably because they are below the resolution limit of present methods. The three major heterogeneities are separated from each other nearly 300 km vertically, and significant anomalies seem absent between each object. Nevertheless, semblance cross sections with a finer grid size (0.1° × 0.1° × 10 km) of the clearest intermediate object (at 1400-1600 km) show a hint that the object is not a single interface but is actually composed of double interfaces nearly 50 km apart. Purely thermal anomalies of kilometers scale must be transient features in the mantle and thus are highly unlikely to be the cause of the heterogeneities. They should rather represent either sharp chemical variations in major element composition or sharp changes in preferred orientations of anisotropic lattice of the lower mantle minerals. Thermal anomalies, however, may be superimposed on such variations because of heat generation either by concentrated radioactive nuclei or by viscous shear heating due to deformations. A plausible tectonic interpretation of these dipping objects is that they represent remnants of subducted lithospheric plates which preceded the current subduction of the Pacific plate. Although we cannot make conclusive arguments on what the mechanism of creating such heterogeneities from these observations alone, we suggest that they may represent stretched, thinned, and duplexed rocks within chaotic mantle flows.