關於愛琴海岩石圈的異質性;On the heterogeneous nature of the Aegean Lithosphere

NCUIR > College of Earth Sciences > Taiwan International Graduate Program for Earth System Science (NCU-Academia Sinica) > Electronic Thesis & Dissertation > Item 987654321/86214

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Title:	關於愛琴海岩石圈的異質性;On the heterogeneous nature of the Aegean Lithosphere
Authors:	藍杰;Ranjan, Pratul
Contributors:	國際研究生博士學位學程
Keywords:	地震學;愛琴海岩石圈;地震衰減;地震層析成像;俯衝帶;seismology;The Aegean lithosphere;seismic attenuation;seismic tomography;subduction zone
Date:	2021-08-23
Issue Date:	2021-12-07 12:17:43 (UTC+8)
Publisher:	國立中央大學
Abstract:	愛琴海形成大陸岩石圈，向東地中海地區俯衝的努比亞岩石圈推進。由於努比亞板塊回滾引起的伸展使愛琴海岩石圈變形，並產生了幾個裂谷/斷層帶、複雜的地質結構以及流體運移通道，這些都是傳播地震波的異質介質。除了一些當地的研究外，尚無法對這些介質如何跨越愛琴海影響地震波傳播進行定量估計。在此論文中，我的目標是研究這些異質特徵並量化它們在愛琴海地區的特性，以了解產生它們的潛在過程。此研究使用的數據從2002 年秋季至 2020 年春季，在愛琴海運行的主要地震網絡的地震圖和地震相位走時，對非均勻結構進行了三階段建模/成像。首先，使用地震圖中峰值能量到達的延遲時間來模擬地殼和上地幔小尺度速度擾動的光譜特徵。接著，使用地震圖包絡估計並繪製不同頻段地殼結構產生的固有衰減和散射衰減。最後，對愛琴海地殼的 3D 速度結構進行走時層析成像。透過以上分析，我發現愛琴海殼幔邊界是一個過渡帶，而地殼由三個主要岩性單元組成：極低速沉積物、低速矽質單元和高速變質單元。產生地震包絡加寬以及散射和固有衰減的地殼不均勻性僅限於四個子區域：科林斯裂谷、基克拉澤斯、克里特島和戈科瓦灣。橫跨愛琴海的正常斷層作用和前弧中的逆衝斷層作用可能會在地殼中產生顯著的地震包絡加寬和散射衰減。另一方面，來自火山中心下方以及變質核複合體內的流體移動可能導致地殼中的高固有衰減和高 Vp/Vs比。上地幔不均勻性主要出現在弧後部，很有可能會發生地幔楔形熔化，而南部前弧中的不均勻性較小，與地殼底鍍的板片物質有關。使用斷層掃描模型對孔隙度的預測，火山中心下方有 4-9% 的熔體含量，其中聖托里尼島表現出最高的熔化程度。橫跨愛琴海的大地震發生在低 Vp 和高 Vp/Vs 比的區域，說明流體活動是其觸發機制。 ;The Aegean forms the continental lithosphere thrusting towards the subducting Nubian lithosphere in the eastern Mediterranean region. The extension induced due to the Nubian slab rollback has deformed the Aegean lithosphere and created several rift/fault zones, complex geological structures as well as pathways for fluid migration, which act as heterogeneous media for the propagating seismic waves. A quantitative estimate of how these features across the Aegean impact seismic wave propagation is not yet available, apart from some localized studies. In my thesis, I aim to identify these heterogeneous features and quantify their properties across the Aegean to understand better the underlying processes that generated them. I use the seismograms and the seismic phase travel-time data from major seismic networks that operated in the Aegean between autumn 2002 to spring 2020 for a three-stage modeling/imaging of the inhomogeneous structure. First, I model the spectral characteristics of the crustal and upper mantle small-scale velocity perturbations using delay times for peak energy arrival in seismograms. Then, I estimate and map the intrinsic and scattering attenuation produced by the crustal structure in different frequency bands using seismogram envelopes. Finally, I perform the travel-time tomography for the Aegean crust for the 3D velocity structure. Using the above analyses, I find that the Aegean crust-mantle boundary exists as a transition zone and the crust is composed of three main lithological units: extremely low velocity sediments, low velocity silicic units, and high velocity metamorphic units. The crustal inhomogeneities, which generate seismic envelope broadening as well as scattering and intrinsic attenuation, are restricted to four sub-regions: the Corinth rift, the Cyclades, Crete and the Gulf of G?kova. Normal faulting across the Aegean and thrust faulting in the fore-arc likely generates significant seismic envelope broadening and scattering attenuation in the crust. On the other hand, fluid migration from below the volcanic centers as well as within metamorphic core complexes possibly cause high intrinsic attenuation and high Vp/Vs in the crust. Upper mantle inhomogeneities are mainly prominent in the back-arc, where mantle-wedge melting is expected, with small inhomogeneities in the southern fore-arc linked with slab material underplating the crust. The estimates of porosity using the tomography model suggests 4-9 % melt-fraction beneath the volcanic centers with Santorini exhibiting the highest degree of melting. Large earthquakes across the Aegean occur in low Vp and high Vp/Vs zones, indicating fluid activity as their triggering mechanism.
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