利用P波多重路徑法研究伊朗高原中部與鄰近區域之地函過渡帶

伊朗高原為阿拉伯板塊與歐亞大陸板塊斜向碰撞所形成，過去研究大多著重於地殼至上部地函岩石圈，對於深部之速度構造了解有限，僅有全球尺度的走時層析成像研究在伊朗高原下方的地函深處發現高速異常，並將其解釋為兩大陸在閉合前新特提斯海洋板塊隱沒下去之殘餘物。本研究使用高解析度P波多重路徑資料分析大陸碰撞帶（主要針對伊朗高原中部）與鄰近相對穩定區域下之地函過渡帶。當地震波經過深度410及660公里不連續面時，會因為速度不連續而形成多重路徑，其相對振幅及相對到時對於地函過渡帶之一維速度構造非常敏感，可以透過波形模擬來良好約束一維模型。本研究結合數個來自高加索及東土耳其寬頻地震網，在研究區域建立了十組長約800至1300公里之P波多重路徑剖面。研究結果顯示，地函過渡帶的速度梯度在北邊穩定之圖蘭平台及南邊之伊朗高原皆略比全球平均一維速度模型iasp91高，使得660公里不連續面之速度增幅均明顯小於iasp91之5.6%，其值分別為4.5%及4.1%。然而，伊朗在地函過渡帶之速度只比周遭的圖蘭平台快不到0.6%，與走時層析成像發現之高異常比1至3%有出入。我們推測可能隱沒板塊尚未延伸至過渡帶，或是板塊殘片太小且分散而造成本研究低估異常之強度。另外我們也發現圖蘭平台和其他歐亞大陸及印度穩定地塊下之660公里不連續面速度增幅均介於4%左右，可能為歐亞區域的地函代表特徵。

關鍵字

伊朗高原；地震波多重路徑；上部地函；大陸碰撞

並列摘要

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.

並列關鍵字

Iranian Plateau ； Triplicate waveform ； upper mantle ； continental collision

參考文獻

Adamia, S.A., Chkhotua, T., Kekelia, M., Lordkipanidze, M., et al. (1981), Tectonics of the Caucasus and adjoining regions: implications for the evolution of theTethys ocean. J. of Struct.Geol. 3, 437–447.

Agard, P., J. Omrani, L. Jolivet, and F. Mouthereau (2005), Convergence history across Zagros (Iran): constraints from collisional and earlier deformation, Int. J. Earth Sci., 94(3), 401-419.

Agard, P., P. Monie, W. Gerber, J. Omrani, M. Molinaro, B, Meyer, L. Labrousse, B. Vrielynck, L. Jolivet, and P.Yamato (2006), Transient, synobduction exhumation of Zagros blueschists inferred from P-T, deformation, time, and kinematic constraints: Implications for Neotethyan wedge dynamics, J. Geophys. Res., 111(B11), doi:10.1029/2005JB004103.

Alavi, M. (1994), Tectonics of the Zagros orogenic belt of Iran: new data and interpretations, Tectonophysics, 229, 221-238.

Alinaghi, A., Koulakov, I. and Thybo, H. (2007), Seismic tomographic imaging of P- and S-waves velocity perturbations in the upper mantle beneath Iran, Geophys. J. Int., 169: 1089–1102. doi: 10.1111/j.1365-246X.2007.03317.x.

國際替代計量

利用P波多重路徑法研究伊朗高原中部與鄰近區域之地函過渡帶

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