利用雷利波相速度圖探討安納托利亞岩石圈構造

Abstract

安納托利亞位在地中海的最東側，其北部屬於阿爾卑斯-喜馬拉雅造山帶的一支。安納托利亞為高度地表變形的區域，其構造由阿拉伯板塊與歐亞板塊碰撞於安納托利亞東北邊以及希臘隱沒帶回退於安納托利亞西南邊主導。安納托利亞的地殼構造已經透過接收函數、數個噪訊分析以及地震層析成像有很好的了解。然而，安納托利亞的岩石圈構造還需要進一步的研究。另外，向西移動的安納托利亞到底是受「阿拉伯板塊與歐亞板塊碰撞」的推力還是受「希臘隱沒帶回退」的拉力作用的結果，是目前仍需釐清的問題。 本研究中，我們透過安納托利亞地區寬頻地震測站紀錄到的垂直分量雷利波來構建有很好約束的高解析度均向性與非均向性相速度圖。首先，利用52個近乎均勻分布的地震測站接收到的1868個來自全球的地震以雙站法量測沿280個地震測站對的雷利波頻散曲線(週期範圍20-300秒)，然後我們將這些頻散曲線逆推得到高解析度的均向性與非均向性相速度圖。 我們的結果顯示在東安納托利亞高原有相對低速異常，與前人研究一致，這說明軟流圈上湧導致東安納托利亞由較熱的地幔支撐。前人提出希臘隱沒板塊持續往地幔延伸而整體隱沒系統往西南方回退。我們的雷利波非均向性量測結果也顯示安納托利亞東部的快方向為南北向而安納托利亞西部的快方向為東北西南向。我們的非均向性排列方式與GPS的量測很一致。阿拉伯板塊向北碰撞歐亞板塊導致安納托利亞板塊向西擠出，安納托利亞受到西邊的希臘隱沒帶回退帶動往西南方向移動。因此我們提出安納托利亞地體構造形變受到其下方地幔流動的牽引以及周圍區域特構造特性影響，如阿拉伯板塊與歐亞板塊沿著縫合帶(Bitlis-Zagros Suture Zone)碰撞與希臘隱沒系統回退。

Anatolia is a highly deformed region. Its tectonics is dominated by the Arabia－Eurasia collision in the northeast and the Hellenic subduction rollback in the southwest. The Anatolian Plate is located in the easternmost Mediterranean and its northern part belongs to the Alpine-Himalayan orogenic belt. The crustal structure of the Anatolian region is already well known, with a number of receiver function analyses and several ambient noise as well as travel-time tomographic studies. However, the lithospheric structure of the Anatolian region has not been well resolved. An open question is whether the western extrusion of Anatolia is caused by the “push” resulting from the Arabia-Eurasia collision or by the “pull” from the rollback of the Hellenic subduction system. In this study, we use vertical-component Rayleigh-waves recorded at broadband seismic stations in the Anatolia region to obtain well-constrained high‐resolution isotropic and azimuthally anisotropic phase-velocity maps. We investigate the variation of Rayleigh-wave phase velocities beneath Anatolia in a broad period range (20-300s) by inverting the fundamental-mode Rayleigh-wave dispersion curves measured by the two-station technique. Using 52 stations nearly evenly distributed in the region and 1868 global earthquakes, we measured Rayleigh-wave dispersion curves along 280 interstation paths in the Anatolia region. These dispersion curves are then inverted for high-resolution isotropic and azimuthally anisotropic phase-velocity maps at periods between 20 and 300 seconds. Our models show relatively slow velocity anomalies in the crust and upper mantle beneath the East Anatolian Plateau (EAP), consistent with previous researches, indicating that the EAP is supported by hot mantle due to the asthenosphere upwelling. Other Studies suggested that the downgoing Hellenic slab continues and the rollback of the subduction system is oriented in the southwestern direction. This is also in good agreement with our Rayleigh-wave anisotropy model with a N-S fast direction in eastern Anatolia and a NE-SW fast direction in western Anatolia. Theses anisotropic patterns are consistent with the GPS-derived velocity field showing a northward collision of Arabia with Eurasia, a westward extrusion of the Anatolian Plate, and a NE-SW trench-ward motion caused by the rollback of the Hellenic subduction system. Therefore, we suggest that the engine of Anatolian deformation is driven by the mantle flow underneath as well as tectonic effects from neighboring regions, such as the collision of Arabia with Eurasia along the Bitlis-Zagros Suture Zone and the rollback of the Hellenic subduction system.