Seismic studies of the mantle transition zone

Using high-resolution, triplicate body waveforms, I investigate P- and S-wave velocities (VP and VS) of the mantle transition zone beneath two convergent margins---the northern Philippine Sea for an oceanic subduction and the Tibetan plateau for a continent-continent collision.; For the subduction case, I find correspondingly high VS in the back-arc of Izu-Bonin where the Northern Philippine Sea Anomaly (NPSA) was previously delineated in the lower portion of the transition zone by an average VP that is 1.5+/-0.5% higher than the surrounding regions. High VS of the NPSA reduces the contrast in VS across the 660-km discontinuity (DeltaVS) to about 3.9%, consistent with the notion that NPSA reflects a sub-horizontal remnant of cold Pacific slab, resting immediately above the lower mantle.; For the continental collision zone, I show that due to high VP near the bottom of the mantle transition zone, the contrast in VP across the 660-km discontinuity beneath central Tibet is small (around 3.1%): only about 70% of that beneath the northern Indian shield. This sub-horizontal anomaly of high V P is most likely a remnant of detached mantle lithosphere that recently sank to depth, thus providing key evidence for a direct connection between continental collision near the surface and deep-seated dynamics in the mantle. However the corresponding high VS or equivalently small DeltaVS of the cold anomaly is missing under Tibet. The sizable DeltaVS (∼7.7%) is quite comparable to those under old continents, implying other counteracting effects exist. The detached lithosphere is probably not only cold but also hydrate---a condition naturally met by the lithospheric mantle under the Qiangtang terrane: Prior to thickening during continent-continent collision, it was part of a mantle wedge which would have been hydrated by major episodes of subduction. This interpretation also interconnects well with that of the NPSA, a remnant of subducted oceanic lithosphere: The oceanic lithosphere is largely anhydrous when created, and except for its topmost portion, is expected to remain so after subduction, thus manifested in corresponding anomalies of high VP and VS in the TZ.