小高加索與亞美尼亞高地震波非均向性之區域變化

Abstract

高加索是兩千五百萬年前阿拉伯板塊開始與歐亞板塊碰撞所形成的大陸造山帶，陸—陸板塊的碰撞通常伴隨著火成活動與岩石圈構造變化，前人研究發現碰撞後的火成活動自11 Ma時由東安納托利亞高原遷移至伊朗西北部，而亞美尼亞高地擁有大範圍且年輕（<2 Ma）的火成活動。此外，東安納托利亞高原至小高加索與亞美尼亞高地一帶，皆被認為下方的岩石圈地函已不存在。然而，受限於儀器佈放範圍與地球物理觀測所解析的深度，至今高加索地區的地函動力學仍尚未釐清。為了瞭解此處岩石圈的形變和軟流圈流動，本研究利用喬治亞及亞美尼亞的區域地震網紀錄之10年資料，分析SK(K)S波相的剪力波分離（shear-wave splitting, SWS）以獲得震波非均向性的側向變化。為求得可靠的剪力波分離參數——快方向與延遲時間，我們藉由主成份分析量化質點運動的線性程度，並且比對交互相關法與切向分量最小能量法得到的測量結果。 本研究得到46個測站紀錄的1191個高品質SWS量測結果，其中有167筆品質優良且分離的剪力波，其平均快方向為45°，延遲時間為0.92秒，與岩石圈厚度已減為40-50 km的東安納托利亞SWS結果（快方向約45°及延遲時間1.3秒）一致。大範圍呈現東北—西南的快方向雖然與小高加索山脈走向垂直，但與絕對板塊運動（65°）大致平行，表示岩石圈與其底下的軟流圈的耦合良好。然而，平均快方向與絕對板塊運動方向仍相差約20°，此偏差暗示著有另一非均向性介質存在，利用順推模擬證明雙層地函流的累加才符合平均快方向，兩層地函流分別是淺部的板塊耦合地函流與深部的大尺度地函流。本研究最後所推得的深部地函流呈現南—北走向，與深度100至200 km 的表面波全球地函非均向性結果一致，或許可與前人提出的特提斯對流胞作鏈結。此外，亞美尼亞高地正下方為未拆沉的岩石圈西緣，相較上述的大尺度非均向性特徵，亞美尼亞高地的快方向順時鐘增加17°、延遲時間減少40%。此現象可與因應厚度變化而改變的地函流有關，強烈的地函溫度側向差異促進瑞利—泰勒不穩定性（Rayleigh-Taylor instability），使鉛直方向的地函流加劇。

The Caucasus in west Asia is a natural laboratory to study dynamics of continental collision between the Arabian and Eurasian Plates that initiated ~25 Ma. Lithospheric delamination beneath the Anatolian-Caucasus region was proposed by previous studies to explain the migration of post-collisional volcanisms from eastern Anatolian Plateau to NW Iran starting 11 Ma. In particular, the lithospheric mantle is absent in the eastern Anatolian Plateau, Lesser Caucasus and Armenian Highland where volcanic activities are pervasive within 2 Ma. Due to limited station coverage and the depth constraints from previous geophysical observations, the upper mantle dynamics beneath the Caucasus region is still poorly understood. In this study, we clarify the deformation of lithosphere and flow of asthenosphere using shear-wave splitting (SWS) of SK(K)S phases from regional array in Georgia and Armenia, which has been operated for nearly 10 years. To reliably determine the fast-direction and delay time of splitted SK(K)S phases, we carefully compared results from both rotation correlation and transverse minimization methods, with the linearity of particle motion quantified by principal components analysis. In total, we obtain 167 good- and fair-quality SWS parameters from 1191 analyzed SK(K)S phases for 46 stations. The average fast-direction and delay time are 45° and 0.92 s, respectively. Both splitting parameters are greatly consistent with the previous results in the eastern Anatolia (~45° and 1.3 s), where lithosphere is as thin as 40-50 km due to delamination. The NE-SW direction is almost perpendicular to the trend of Lesser Caucasus Mountain belt but is sub-parallel to the absolute plate motion (APM) at 65° from north, suggesting mantle flow influenced strongly by the lithosphere-asthenosphere coupling. However, the difference of 20° between the average fast-direction and APM requires an additional anisotropic layer. As the result of forward modeling, the average fast-direction can be expressed by a two-layer mantle flow model, namely the plate-coupled and deep mantle flow. The lower layer anisotropy exhibits sub-N-S fast-direction which agrees well with the global surface-wave at the depth of ~150 km or deeper, likely associated with the previously proposed Tethyan convection cell. Given the similar azimuth in such vast area, we presume the average anisotropy correspond to a long-term & large-scale asthenospheric flow. In addition, the detailed pattern of SWS reveals that the fast-direction deviates from the main average by 17° clockwise and the corresponding delay time decreases by 40% right beneath central Armenia near the western edge of undelaminated lithosphere. Such a correlation may relate to the modified flow at the thickness transition where lateral thermal gradient promotes Rayleigh-Taylor instabilities and the vertical-oriented mantle flow nearby is enhanced.