非洲東部阿法爾裂谷岩石圈地震移位成像

Abstract

板塊間的相對運動產生不同形式的板塊邊界，裂谷及中洋脊等地貌在板塊相互拉張的地區因應而生。這種板塊邊界一般認為可能與岩石圈受到熱及拉張作用有關，但大陸是如何開始張裂(continental breakup)以至張裂後轉變為海底擴張(seafloor spreading)的過程仍不完全清楚。非洲阿法爾裂谷位於南紅海張裂(Southern Red Sea Rift)、亞丁灣張裂(Gulf of Aden Rift)以及衣索比亞張裂(Main Ethiopian Rift)的交界處，正處於大陸張裂環境中，並向南紅海轉變為海底擴張，因此是了解大陸張裂初期過程的最佳地點。過去一般認為阿法爾的大陸張裂可能起源於地幔熱柱影響使得岩石圈減薄，但其受影響的位置、程度、和演化過程仍不清楚，而掌握岩石圈的構造是進一步了解阿法爾演化機制的關鍵。本研究取用阿法爾地區的地震陣列資料，利用地震移位成像技術(seismic migration)萃取並疊加散射波相以求取陣列下方的散射源(即可能的不連續面)分佈位置。本論文呈現了阿法爾區域第一個高解析度的散射波移位成像結果。我們發現在二維成像剖面的北段(阿法爾西北緣與衣索比亞高原交界)與南段(阿法爾南緣與衣索比亞張裂帶交界)各有一明顯的不連續面存在。跨過南段位於75公里深的不連續面波速由快變慢，可能代表岩石圈－軟流圈邊界層(lithosphere-asthenosphere boundary)，或可用熱物質上湧造成波速變化加劇來解釋。至於此熱作用來自地幔熱柱或是因張裂導致的上地幔局部被動流場，則需要進一步探討。另一方面，不同於南段，跨過剖面北段的不連續面波速由慢轉快，並由70公里往南漸深至90公里，其代表的構造意義尚待釐清。

Different plate boundaries are characterized by the way the plates move relative to each other. One kind of plate boundaries is divergent boundary, which occurs where two plates slide apart. At zones of continent-to-continent rifting, divergent boundaries will be where the new ocean basin forms as the continents split, accompanied by volcanic activities and earthquakes. The origin of divergent boundaries are thought to be associated with heating and stretching in the lithosphere. The Afar Rift system in east Africa is an ideal natural laboratory for investigating the process of divergent boundary and incipient continental rifting. It is located at the triple junction of three rifts, Southern Red Sea Rift (RSR), GOA (Gulf of Aden Rift) and MER (Main Ethiopian Rift), and the ongoing continental breakup at Afar transitions to seafloor spreading toward the southern Red Sea. Previous studies favor the explanation that Afar is influenced by a mantle plume that weakens the lithosphere through thermal erosion and magma infiltration. However, the location and degree of plume influence are debated. In this study, recordings of seismic array at Afar are used to produce the first high-resolution seismic images from scattered wavefield of Afar lithosphere, to detect upper-mantle discontinuities. We reveal two seismic discontinuities on the north and south section of our 2-D profile, centered between the north-western Afar and Ethiopia Plateau and between the south-western Afar and Main Ethiopian Rift. The discontinuity in the south is identified at ~75 km depth characterized by a pronounced velocity reduction, which may be interpreted as the lithosphere-asthenosphere boundary (LAB) or as velocity contrast enhanced by hot mantle upwelling. The other discontinuity to the north dips southward from 70 to 90 km depth, characterized by a velocity increase with depth, in contrast with the observation in the south. Our study shows evidence that the Afar lithosphere is likely modified by thermal effect in the uppermost mantle, where the presence of mantle plume cannot be excluded.