Deep seismic structure of the Indian shield, western Himalaya, Ladakh and Tibet
Deep seismic structure of the Indian shield, western Himalaya, Ladakh and Tibet
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DOI：
10.1016/j.epsl.2011.05.016
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发表时间：
2011-07
影响因子：
5.3
通讯作者：
S. Oreshin;L. Vinnik;S. Kiselev;S. S. Rai-S.;K. Prakasam;A. Treussov
中科院分区：
文献类型：
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作者：
S. Oreshin;L. Vinnik;S. Kiselev;S. S. Rai-S.;K. Prakasam;A. Treussov
印度地盾、喜马拉雅西部、拉达克和西藏地震台站的 P 和 S 接收器函数采用一种方法进行处理，该方法提供 P 和 S 速度及其比率与深度的函数估计。印度地盾北部和小喜马拉雅地区的 P660 和 P410 相位之间的时间差比正常的 24 秒大 1.0-1.5 秒。这是低温的影响，意味着印度盾的消耗材料已到达过渡区。印度地盾某些台站的 P410s 和 S410p 相位波形表明，在 410 公里不连续面上方有一层薄薄（几十公里）的低 S 速度层，这通常与地幔上涌有关。印度地盾下深度小于 180 公里的地幔 S 速度为 4.4–4.5 公里/秒，远低于前寒武纪地盾的典型速度 4.7 公里/秒。我们主要通过最近（第三纪？）高速地幔龙骨的交代改变来解释这种低S速度。在喜马拉雅山西部、拉达克和西藏西部（但不包括西藏东部）以下，深度小于 100-150 公里的地幔 S 速度约为 4.7 公里/秒，Vp/Vs 异常低，我们认为这个高速层是大印度北部地幔岩石圈的残余。在印度地盾的大多数位置，高 S 速度（3.5 km/s 或更高）在中地壳和下地壳中占主导地位，并且升高的 S 速度伴随着 Vp/Vs 比率的增加（1.8-2.1 与标准的 1.73）。在喜马拉雅山麓，地壳厚 50-55 公里，几乎完全由高 S 速度（3.7 公里/秒及以上）岩石组成，中部具有增加的 Vp/Vs 比，下地壳具有标准的 Vp/Vs 比。这一观测结果表明，印度板块的上地壳在碰撞带被刮掉，而下地壳则与地幔岩石圈一起高速俯冲。相对于拉达克，高速是导致 P 波远震走时异常约 1 秒的原因。在喜马拉雅山下，地壳中的 Vp/Vs 比率是正常的，这表明相对于印度地盾地壳的成分发生了变化。在拉达克和西藏地区，地壳中再次观察到异常高的 Vp/Vs 比。我们的分析揭示了西藏下方 20 公里至 45 公里深度之间存在部分熔融的低速地壳带。此前，45公里的不连续性被解释为榴辉岩化的影响。
P and S receiver functions from seismograph stations in the Indian shield, Western Himalaya, Ladakh and Tibet are processed with a method which provides estimates of the P and S velocities and their ratio as a function of depth. The time difference between the P660s and P410s phases in the north of the Indian shield and the Lesser Himalaya is 1.0–1.5 s larger than the normal 24 s. This is an effect of a low temperature with implication that the consumed material of the Indian shield has reached the transition zone. The waveforms of the P410s and S410p phases at some stations in the Indian shield are indicative of a thin (a few tens of kilometers) low S velocity layer atop the 410-km discontinuity, which is usually related to mantle upwelling. The mantle S velocity under the Indian shield at depths less than 180 km is 4.4–4.5 km/s, much lower than the 4.7 km/s, typical for Precambrian shields. We explain this low S velocity mainly by a recent (Tertiary?) metasomatic alteration of the high-velocity mantle keel. Beneath the western Himalaya, Ladakh and western Tibet (but not eastern Tibet) the S velocity in the mantle at depths less than 100–150 km is around 4.7 km/s, Vp/Vs is anomalously low, and we argue that this high-velocity layer is a remnant of the mantle lithosphere of the northern Greater India. At most locations in the Indian shield high S velocities (3.5 km/s and more) are dominant in the middle and lower crusts, and the elevated S velocity is accompanied by an increased Vp/Vs ratio (1.8–2.1 versus the standard 1.73). In the foothills of the Himalaya, the crust is 50–55 km thick and consists almost entirely of a high-S-velocity (3.7 km/s and more) rock with the increased Vp/Vs ratio in the middle and the standard Vp/Vs ratio in the lower crust. This observation suggests that the upper crust of the Indian plate is scraped off in the collision zone, whereas the high-velocity lower crust is subducted jointly with the mantle lithosphere. The high velocities are responsible for the P-wave teleseismic travel time anomaly of ~ 1 s relative to Ladakh. Under the Himalaya the Vp/Vs ratio in the crust is normal, which suggests a change in composition relative to the crust of the Indian shield. Under Ladakh and Tibet the anomalously high Vp/Vs ratio in the crust is observed again. Beneath Tibet our analysis reveals a low-velocity crustal zone of partial melt between the 20-km and 45-km depths. Previously, the 45-km discontinuity was interpreted as the effect of eclogitization.