接收函數層析臺灣區域底下深部隱沒的歐亞板塊

Receiver Function Imaging of Deep Penetration of the Eurasian Slab in the Mantle Transition Zone beneath Taiwan

劉昀姍(Yun-Shan Liu)

指導教授 ： 洪淑蕙

國立臺灣大學/理學院/地質科學研究所/碩士(2010年)

https://doi.org/10.6342/NTU.2010.00722

摘要

臺灣位在歐亞大陸板塊邊緣，菲律賓海板塊朝西北斜向碰撞歐亞大陸板塊，使臺灣處在兩隱沒板塊系統相接處：一為東北部花蓮外海菲律賓海板塊往北隱沒至歐亞板塊下方；另一為東南部以花東縱谷為界，歐亞大陸板塊往東隱沒至菲律賓海板塊下方。關於歐亞板塊隱沒系統是否往北延伸至臺灣中部底下，以及岩石圈是否隱沒至深部上地幔與臺灣地區的地體構造演化成因有密切關係，然而因缺乏 400 公里以下的深部地震和高解析度的三維地幔速度層析成像模型，至今對於解答該問題仍多所爭論。
根據礦物物理實驗研究以及地震波分析，410 和 660 公里處所出現的震波速度和密度不連續變化與上部地幔的主要組成礦物 olivine 的相變有關，兩不連續面之間稱為地幔過渡帶的厚度會受到溫度改變而影響相變發生的壓力或深度。當板塊隱沒至過渡帶時，因溫度較周圍地幔冷，使 410 和660 公里處的相變深度會分別變淺和加深，造成地幔過渡帶變厚。另外，若地幔過渡帶含水量高亦會造成 410 公里不連續面變淺、相變帶變寬的現象。因此透過接收函數解析地幔過渡帶厚度的側向變化將有助於了解臺灣底下冷隱沒板塊在地幔過渡帶的分布情形。本研究利用 1996 至 2008 年間臺灣寬頻地震網所紀錄到的遠震 P 波的尾波訊號計算得到接收函數，再利用共同點疊加法加強於 410 和 660 公里處由 P 波轉換成 S 波的轉換波 Pds 訊號，根據量測到的轉換波相對 P 的到時，得到 410 和 660 公里不連續面深度以及地幔過渡帶厚度側向起伏變化，進而根據實驗所得到的相變溫度和壓力(即深度)關係，推論臺灣底下地幔過渡帶的溫度異常，得知歐亞板塊在臺灣地區下方延伸和分布情形。
研究結果顯示，臺灣下方過渡帶厚度大於全球平均厚度 250 公里，溫度低於全球平均值。而臺灣南部的歐亞板塊隱沒系統可往北延伸至北緯 23.5° 的位置，且隱沒深度達到 410 公里不連續面以下，但並無往下延伸至上部地幔底部。南中國海海洋岩石圈往東沿著馬尼拉海溝隱沒至菲律賓海板塊下方時，將板塊內部含水礦物帶到 410 公里不連續面附近的深度，同時因脫水作用使隱沒板塊上覆地幔楔含水量升高，造成臺灣南部22°N 以南地區 410-不連續面有顯著抬升及相變帶寬度增加的情形，同時 22.5°N 附近區域的 660 公里不連續面有往下變深的趨勢，証實板塊隱沒深度達 660 公里。除此之外，南中國海海洋岩石圈堆積停留在地幔過渡帶底部的岩石圈地塊，可能造成臺灣及鄰近臺灣海峽下方660 公里不連續面溫度偏低。

關鍵字

接收函數 ； 轉換波 Pds ； 410公里不連續面 ； 660公里不連續面 ； 地幔過渡帶 ； 南中國海海洋岩石圈 ； 歐亞板塊 ； 菲律賓海板塊

並列摘要

Taiwan is located in the convergent plate boundary zone where the Philippine Sea plate has obliquely collided on the Asian continental margin. Two subduction zone systems of opposite polarity are juxtaposed under the central Taiwan: to the northeast the Philippine Sea plate (PSP) is obliquely underthrusting beneath the Eurasian plate (EP), and to the south the EP is subducting eastward beneath the PSP. To the further south of 22oN, it transforms to an ocean-ocean subduction zone where the South China Sea (SCS) oceanic lithosphere, part of the EP, is subducting eastward along the Manila trench beneath the PSP. One of the key questions closely related to the dynamic evolution of the Taiwan subduction-collision zone is whether the subduction of the aseismic Eurasian lithosphere takes place beneath the central Taiwan. If it does, to what depth has the subducted slab penetrated? Numerous tectonic models have been proposed to either support or go against the existence of the Eurasian lithosphere beneath central Taiwan. Because of the lack of deep earthquakes and high-resolution tomographic images of deep structure beneath Taiwan, the debate still remains unresolved.
It has been known that a sequence of phase transformations of olivine, the major mineral constituent in the upper mantle, primarily cause seismic velocity and density jumps at depths near 410 and 660 km. A decrease in temperature induced by a subducted slab would shoal and depress the boundaries of the phase transformations at the 410- and 660-km seismic discontinuity, respectively; thus increasing the thickness of the mantle transition zone (MTZ). Likewise, the water would raise and broaden the phase transition zone near 410 km depth.
In order to decipher the long-term debate on the depth extent of slab penetration beneath Taiwan, we conduct a receiver function analysis to map the lateral variation of topography on the 410- and 660-km discontinuities and further investigate the cold temperature and enriched water linked to the distribution of the subducted EP and SCS slab in the MTZ. The data are obtained from teleseismic P wave coda recorded by the broadband stations across Taiwan and adjacent offshore islands during 1996-2008. A common conversion point (CCP) stacking technique is employed to enhance the signals of Pds, a P-to-S conversion at the depth d. The differential (P410s-P) and (P660s-P) travel time residuals provide a means to delineate the phase boundaries associated with the 410- and 660-km discontinuities.
Overall the MTZ beneath Taiwan region is thicker than the global average of 250 km, so as the temperature is lower. In the south of Taiwan, the eastwardly-subducted EP is found to extend northward to 23.5°N, where the slab penetrates at least to 410 km depth but does not reach the base of the upper mantle. In the south of 22oN, the stacked receiver functions to the east of 121oE reveal the earlier P410s arrivals with broad and multi-peak pulses, whereas the P660s phase to the west of 121oE clearly arrives relatively late. While the water retained atop the 410-km discontinuity could result in the much elevated and broadening phase transition zone, analogous to those observed in the receiver functions, the subducted SCS slab may have carried hydrous minerals to great depths and released water into the overlying mantle wedge through the breakdown of these minerals. In addition, the remnants of cold subducted materials accumulated at the bottom of the MTZ may lower the temperature which causes the depressed 660-km discontinuity beneath the Taiwan and Taiwan Strait region.

並列關鍵字

receiver function ； Pds conversion ； 410-km discontinuity ； 660-km discontinuity ； mantle transition zone ； South China Sea oceanic lithosphere ； Eurasian plate ； Philippine Sea plate

參考文獻

Ai, Y., T. Zheng, W. Xu, Y. He, and D. Dong (2003), A complex 660 km discontinuity beneath northeast China, Earth Planet. Sci. Lett., 212, 63– 71

Ammon, C. J. (1991), The isolation of receiver function from teleseismic P waveforms, Bull. Seism. Soc. Am., 81, 2504-2510.

Bijwaard, H., W. Spakman, and R. Engdahl (1998), Closing the gap between regional and global travel time tomography, J. Geophys. Res., 103, 30055-30078.

Bina, C. R., and G. Helffrich (1994), Phase transition Clapeyron slopes and transition zone seismic discontinuity topography, J.Geophys. Res., 99, 15,853-15,860.

Bose, K., J. Ganguly (1995), Experimental and theoretical studies of the stabilities of talc, antigorite and phase A at high pressures with applications to subduction processes, Earth Planet. Sci. Lett., 136, 109-121.