Pn Tomographic Velocity and Anisotropy beneath the Iran Region
Pn Tomographic Velocity and Anisotropy beneath the Iran Region
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DOI：
10.1785/0120100141
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发表时间：
2012-02
影响因子：
3
通讯作者：
Yan Lu;B. Liu;S. Pei;Youshun Sun;M. N. Toksoez;Xiangfang Zeng
中科院分区：
文献类型：
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作者：
Yan Lu;B. Liu;S. Pei;Youshun Sun;M. N. Toksoez;Xiangfang Zeng
我们给出了伊朗地区下方上地幔的层析成像速度和各向异性模型。研究中使用了来自伊朗台网86个台站以及国际地震中心133个台站的共计74375个Pn震相读数。该研究采用了赫恩（1996）提出的Pn走时层析成像方法。层析成像结果显示出一些有趣的异常。伊朗地区下方的平均Pn速度约为8.0千米/秒，最大速度扰动约为3% - 4%。扎格罗斯山脉和里海下方的高Pn速度可能是由于洋壳/岩石圈物质的存在。在厄尔布尔士和高加索地区下方发现低Pn速度，这可能是由于这些地区的火山以及新生代中期火山/深成岩导致的较高温度或部分熔融。反演速度支持这样一种观点，即阿拉伯板块俯冲到伊朗高原下方的地幔中可能导致了热物质的上涌。对于射线覆盖良好的区域，同时获得了分辨率良好的Pn各向异性模型和速度模型。在板块碰撞区域（扎格罗斯和厄尔布尔士），快速Pn各向异性方向与碰撞弧以及由于跨断层挤压和沿断层伸展产生的纯剪切变形形成的大型逆断层平行。在高加索地区下方，Pn各向异性结果表明橄榄石晶体的优势排列方向与板块运动方向平行；然而，地表断层走向与地壳运动方向和各向异性呈近45°角。这些不同的变形表明地壳和上地幔之间可能存在解耦。由于火山作用，在高温下地壳和上地幔之间可能存在的解耦和不同变形很容易增强，并且高加索下方的低速度也支持这一点。我们通过对视Pn速度进行方位平均验证了这些区域下方Pn各向异性的存在。
We present tomographic velocity and anisotropy models of the uppermost mantle beneath the Iran region. A total of 74,375 Pn phase readings from 86 stations of the Iranian network and 133 stations of the International Seismological Centre are used in the investigation. The study uses the Pn travel‐time tomography method proposed by Hearn (1996). The tomography results show some interesting anomalies. The average Pn velocity under the Iran region is approximately 8.0 km/s, and the maximum velocity perturbations are approximately 3%–4%. High Pn velocities under the Zagros Mountains and the Caspian Sea may be due to the presence of oceanic crust/lithosphere material. Low Pn velocities were found under the Alborz and Caucasus regions and may be due to higher temperatures or partial melting resulting from volcanoes and mid‐Cenozoic volcanic/plutonic rocks in these regions. The inversion velocities support the idea that the subduction of the Arabian plate into the mantle beneath the Iranian plateau may have resulted in the upwelling of hot material. The well‐resolved Pn anisotropy model is jointly obtained with a velocity model for the areas with good ray‐path coverage. In the plate collision regions (Zagros and Alborz), the fast Pn anisotropy direction is oriented parallel to the collision arc and to large reverse faults due to pure shear deformation from cross‐fault compression and along‐fault extension. Under the Caucasus regions, the Pn anisotropy results indicate that the preferred alignment of olivine crystals is parallel to the plate movement direction; however, the surface fault strike is at an angle of nearly 45° with the crustal movement direction and anisotropy. These differing deformations suggest potential decoupling between the crust and upper mantle. The possible decoupling and differing deformation between the crust and upper mantle are easily enhanced under high temperatures due to volcanoes and supported by low velocities beneath the Caucasus. We validate the existence of Pn anisotropy under these regions by azimuthal averaging of the apparent Pn velocity.