利用遠震接收函數探究小高加索與鄰近地區的地殼速度構造

Abstract

阿拉伯和歐亞大陸板塊自漸新世（約三千兩百萬年前）以來的碰撞形成了遍布第四紀火山的大高加索、小高加索與東北安那托利亞高原，為地質複雜的構造區塊。前人研究已經發現地殼厚度從安納托利亞往東北方向逐漸增加，在南喬治亞（小高加索和大高加索之間）達到最厚的52公里左右。然而，受限於稀疏的測站分布，只能對高加索地區的莫荷面空間變化粗略地解析，在高加索與鄰近地區，至今尚缺乏詳細的地殼構造研究。 本論文蒐集並分析2012年1月至2016年6月期間的遠震資料，主要來自中研院地球所與臺大地質系分別在喬治亞與亞美尼亞新建置的寬頻地震網，觀測網涵蓋大部分的小高加索和第四紀火山，將小高加索區域的平均站間距加密至約30至40公里，另加入其他當地的寬頻地震網，以提升測站包覆性。本研究總共使用42個測站的高品質地震資料，利用時間域迭代法計算每個測站的P波接收函數推求研究區域各測站下方的地殼構造。 使用H-κ疊加法所得的整體結果顯示，地殼厚度從東北安納托利亞高原的40公里朝東北方向增加，地殼最厚在東北小高加索與東大高加索下方，可達50至52公里，而莫荷面深度最淺的地方則在西喬治亞靠近黑海沿岸處（25至35公里）。此外，本研究在中部亞美尼亞高原的阿拉加茨火山和格加馬火山周圍觀測到區域性相對較淺的莫荷面（~32公里）與較高的地殼平均Vp/Vs（1.80至2.07），從逆推求得的一維速度模型也顯示該區底下有較厚的下部地殼低速構造，這些現象指示地殼的基性組成很高，或是下部地殼有部分熔融存在。在亞美尼亞莫荷面之下的上部地函也發現低速構造，此結果和前人的Pn波速度研究一致，而莫荷面附近的低速可能都與上湧地函的熱異常有關。本研究發現喬治亞南部地區的小高加索莫荷面呈深度較兩側淺的拱形（最淺處的莫荷面約39公里深），而且其中一測站在大約5公里深處有一明顯的低速層，需要進一步證實是否為鄰近扎瓦赫季高地底下的岩漿庫或熱液作用所致。

Geologically complex zones of the Greater Caucasus, Lesser Caucasus and NE Anatolian Plateau, where Quaternary volcanoes are abundant, have been formed by the continental collision between Arabia and Eurasia since Oligocene (~32 Ma). Previous studies have indicated that the crustal thickness increases from the NE Anatolia toward the northeast direction with the deepest Moho of ~52 km under the southern Georgia between the Lesser and Greater Caucasus. However, the spatial variation of Moho in the Caucasus can only be roughly constrained due to sparse seismic arrays. No estimates on detailed crustal structures have been made by far for the Caucasus region. In this thesis, the data are collected and analyzed for tele-seismic events between January 2012 and June 2016 from a new broadband seismic network in Georgia and Armenia, which were supported by Academia Sinica and National Taiwan University. The average station interval in the Lesser Caucasus is improved to roughly 30-40 km and other broadband stations are also included for better coverage. A total of 42 high-quality stations are used in this study. P-wave receiver functions are calculated using time-domain iterative method for constraining the crustal structures beneath each station in the study area. The results of H-κ sacking show that overall the crustal thickness increases toward NE from 40 km under the NE Anatolian Plateau to the maximum of ~50-52 km beneath the northeastern Lesser Caucasus and the eastern flank of Greater Caucasus. The shallowest Moho is located under western Lesser Caucasus near the cost of Black Sea (25-35 km). Moreover, locally thin crust (~32 km) and anomalously high Vp/Vs (1.80-2.07) that accompany a thick low-velocity layer in the lower crust from the 1-D velocity model inverted are found around the Volcano Aragats and Ghegam of central Armenia plateau, suggesting an overall high mafic content of crust or the presence of partial melt in the lower crust. Low velocity structures are also found under the Moho in Aremenia, which are consistent with previous Pn studies. Relatively low speeds around Moho are both probably associated with the thermal anomaly caused by upwelling of upper mantle. In southern Georgia of the Lesser Caucasus, an arched structure of Moho is observed, with shallowest Moho depth of ~39 km at the center and deepening toward east and west. There is apparent low-velocity layer at the depth of ~5 km, which require more evidences to confirm whether it is related to magma chamber or hydrothermal process under the Javakheti highland.