| Summary: | 碩士 === 國立臺灣大學 === 地質科學研究所 === 103 === Iranian plateau is formed by the continental collision between Arabia and Eurasia. Recent studies of regional seismic tomography show P- and S-wave anomalies in the uppermost mantle beneath Iranian Plateau, but the structure of mantle transition zone (TZ) remains inconclusive. The high-velocity anomalies in the tomographic images are typically interpreted as the remnants of Neo-Tethys slab in the uppermantle that was subducted beneath Iran prior to the collision.
In this study, we utilize triplicate waveforms of P-waves to investigate the structure of TZ beneath deformed Iranian Plateau and the surroundings that are tectonically stable. The rays passing through the 410- and 660-km seismic discontinuities form multiple arrivals, whose relative amplitude and timing are most sensitive to the velocity contrast across the TZ boundaries. By combining several broadband arrays in eastern Anatolia and the Caucasus, we construct 10 seismic profiles, each about 800 to 1300 km long, to constrain the vecloties under the study area based on modeling of waveforms.
Our results show that the velocity gradient in the TZ under the Turan platform and Iranian plateau are both higher than that of the global average model iasp91. This also makes the contrasts of the two models at the 660 km discontinuity relatively small (about 4.5% for Turan and 4.1% for Iran) comparing to the 5.6% in iasp91. Although the P-wave speed in the base of TZ under Iranian plateau is faster than Turan, the amount of anomaly is inappreciably small (only ~0.6%). This is inconsistent with the tomography result in the previous study which indicates 1-3% of variations in the TZ. Our results infer that the subducted/break-off slab had not reached to the deeper TZ. Alternative interpretation is that, the seismic anomaly is underestimated in our study if the fragments of the slab are small in size and separated apart in the sampled area. Finally, the contrasts at the 660 km discontinuity under Turan is comparable to other tectonically stable areas in Euraisa and India. Perhaps 4% contrast is a representative background value for continental Asia.
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