Implications of earthquake focal mechanism data for the active tectonics of the south Caspian Basin and surrounding regions
Implications of earthquake focal mechanism data for the active tectonics of the south Caspian Basin and surrounding regions
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DOI：
10.1111/j.1365-246x.1994.tb04679.x
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发表时间：
1994-07
影响因子：
2.8
通讯作者：
K. Priestley;C. Baker;J. Jackson
中科院分区：
文献类型：
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作者：
K. Priestley;C. Baker;J. Jackson
南里海盆地是阿尔卑斯-喜马拉雅带内一个相对平静的地块，但周围是高地震活动区。我们使用的震源机制的16个地震的震源参数，我们确定从反演的体波和15个其他地震的机制，以确定在南里海盆地周围的地震带的断层的样式。伊朗西北部塔莱什山脉下和里海西南部近海的地震具有浅层逆冲机制，表明伊朗西北部的大陆地壳正在推覆南里海盆地的“海洋状”地壳。伊朗北方里海以南的地震显示出震源机制的混合。高阿尔布尔茨山既有高角度逆断层作用，也有左旋走滑断层作用。再往南，在伊朗高原中部的边缘，观察到斜向左侧反转机制。看来，伊朗中部和里海盆地南部之间的NE方向的缩短被划分为纯粹的左旋走滑和逆冲在西北向高Alborz，但容纳在低海拔的斜向断层。里海东部Kopet Dag山脉的地震也显示了高角度反向和走滑断裂机制的混合，可能是斜滑划分为走滑和逆冲运动的另一个例子。正断层机制在质心深度为35-50公里占主导地位的地震活动带，横跨里海中部。正断层地震的意义是个谜。这些事件不太可能代表里海盆地南部和欧亚大陆之间的运动，因为它们意味着一种运动感，这种运动感与沉积物中观察到的地形和褶皱不相容。两个浅源地震在约12公里深的这个带，一个小的事件和其他一个大的第二次子事件的多地震，有逆冲机制，表明缩短发生的北方里海的大陆地壳是推力在'海洋般的'地壳的南部里海盆地。缩短也建议在里海中部地震带南部的沉积盖层褶皱的方向。我们认为，这种缩短确实代表了里海相对于欧亚大陆的NNE运动，但运动是缓慢的，并没有产生许多地震。较深的正断层事件可能与初始俯冲板片的弯曲或下倾延伸有关。如果里海相对于欧亚大陆的运动确实是缓慢的，那么伊朗中部和里海盆地南部之间的阿尔博尔兹河的运动将与伊朗和欧亚大陆之间的运动几乎相同，正如以前所假设的那样。南部的塔列什-阿尔博尔兹山脉和北部的北方里海大陆地壳对南里海盆地的逆冲推覆作用的综合效应将导致南里海盆地的最终破坏，并可能在大陆内部形成一个中等深度的倾斜地震带，类似于目前在兴都库什观察到的地震带。
SUMMARY The south Caspian Basin is a relatively aseismic block within the Alpine-Himalayan Belt, but is surrounded by zones of high seismicity. We used the focal mechanisms of 16 earthquakes whose source parameters we determined from inversion of body waves and the mechanisms of 15 other earthquakes to determine the style of faulting in the seismic belts surrounding the south Caspian Basin. Earthquakes beneath the Talesh Mountains of north-west Iran and immediately off-shore in the south-west Caspian Sea have shallow thrust mechanisms, showing that the continental crust of north-west Iran is overthrusting the ‘oceanic-like’ crust of the south Caspian Basin. Earthquakes south of the Caspian Sea in northern Iran show a mixture of focal mechanisms. Both high-angle reverse faulting and left-lateral strike-slip faulting mechanisms are observed in the high Alborz Mountains. Farther south, on the edge of the central Iran plateau, oblique left-lateral reverse mechanisms are observed. It appears that the NE direction of shortening between central Iran and the south Caspian Basin is partitioned into pure left-lateral strike-slip and thrusting in the WNW-trending high Alborz, but is accommodated by oblique faulting in lower elevations. Earthquakes in the Kopet Dag Mountains east of the Caspian Sea also show a mixture of high-angle reverse and strike-slip faulting mechanisms and may be another example of the partitioning of oblique slip into strike slip and thrust motion. Normal faulting mechanisms at centroid depths of 35–50 km dominate in the belt of seismicity that extends across the central Caspian Sea. The significance of the normal faulting earthquakes is enigmatic. It is improbable that these events represent the motion between the southern Caspian Basin and Eurasia for they imply a sense of motion that is incompatible with the observed topography and folding in the sediments. Two shallow earthquakes at about 12 km depth in this belt, one a small event and the other a large second subevent of a multiple earthquake, have thrusting mechanisms suggesting that shortening occurs as the continental crust of the northern Caspian Sea is thrust over the ‘oceanic-like’ crust of the southern Caspian Basin. Shortening is also suggested by the orientations of folds in the sedimentary cover south of the central Caspian Sea seismic belt. We suggest that this shortening does indeed represent a NNE motion of the Caspian Sea relative to Eurasia, but that the motion is slow and has not produced many earthquakes. The deeper, normal-faulting events may be related to bending or down-dip extension of the incipient subducted slab. If the motion of the Caspian Sea relative to Eurasia is indeed slow, then the motion in the Alborz between central Iran and the south Caspian Basin will be almost the same as that between Iran and Eurasia, as has previously been assumed. The combined effect of the overthrusting of the south Caspian Basin by the Talesh-Alborz Mountains in the south, and by the continental crust of the northern Caspian Sea in the north will lead to the eventual destruction of the south Caspian Basin and the possible formation of an intermediate-depth, dipping seismic zone within the continental interior, similar to that presently observed in the Hindu Kush.