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Upper mantle seismic anisotropy beneath the Kachchh rift zone, Gujarat, India, from shear wave splitting analysis
Journal of Earth System Science ( IF 1.9 ) Pub Date : 2020-04-20 , DOI: 10.1007/s12040-020-1373-5
Bhoopendra Singh , Prantik Mandal

Unravelling the anisotropic behaviour of the upper mantle helps to shed light on its present and past deformation processes. In this study, we attempt to explore the seismic anisotropy prevailing within the upper mantle beneath the Kachchh rift zone through shear wave splitting analysis. We have measured the splitting parameters (e.g., fast axis orientation (\( \varPhi \)) and delay time (\( \delta t \))) using SKS/SKKS core refracted phases from 112 teleseismic events recorded at NGRI network in the Kachchh region, during 2006–2009 and 2013–2016. The ‘\( \varPhi \)’ and ‘\( \delta t \)’ estimates vary from N34° to N73°E and 0.80 to 1.5 s, respectively. The average vector means of ‘\( \varPhi \)’ and ‘\( \delta t \)’ for all the stations are found to be \( {\text{N}}\left( {58 \pm 10} \right) \)°E and (0.99 ± 0.19) s, respectively. Measurements of 59 good SKS/SKKS splitting parameters from 112 earthquakes reveal that the upper mantle is highly anisotropic beneath the Kachchh rift zone with an average fast axis orientation of \( {\text{N}}\left( {58 \pm 10} \right) \)°E, which is deviated nearly \( (\sim{\text{N}}18 \)°E) from the absolute plate motion (APM) direction (~N40°E) of the Indian plate in a no-net-rotation reference frame. This deviation of fast axis orientation from APM direction may be attributed to the effect of Kachchh rift zone as well as the presence of structural imprints of the 65 Ma Deccan mantle plume in the study region. And the average delay time of (0.99 ± 0.19) s is consistent with the global average (1 s) for continents. Furthermore, the modelled seismic layer thicknesses reveal that anisotropic sources beneath study region are associated with both the lithospheric deformation processes (e.g., 184 Ma African rifting, 88 Ma Madagascar rifting, 65 Ma Deccan mantle plume) as well as asthenospheric flows.



中文翻译:

基于剪切波分裂分析的印度古吉拉特邦Kachchh裂谷带下的上地幔地震各向异性

揭示上地幔的各向异性行为有助于揭示其当前和过去的变形过程。在这项研究中,我们试图通过剪切波分裂分析来探讨卡赫奇裂谷带下方上地幔中普遍存在的地震各向异性。我们使用来自NGRI网络记录的112个远震事件的SKS / SKKS岩心折射相位测量了分裂参数(例如,快速轴方向(\(\ varPhi \))和延迟时间(\(\ delta t \))。卡奇地区,2006-2009年和2013-2016年。“ \(\ varPhi \) ”和“ \(\ delta t \) ”的估计值分别在N34°至N73°E和0.80至1.5 s之间变化。' \(\ varPhi \) '和'的平均向量均值所有站的\(\ delta t \) '分别为\({\ text {N}} \ left({58 \ pm 10} \ right)\) °E和(0.99±0.19)s 。对112次地震中59个良好的SKS / SKKS分裂参数的测量表明,上地幔在Kachchh裂陷带下方高度各向异性,平均快轴方向为\({\ text {N}} \ left({58 \ pm 10} \ right)\) °E,几乎偏离\((\ sim {\ text {N}} 18 \)°E)从无网旋转参考系中印版的绝对板运动(APM)方向(〜N40°E)开始。快速轴方向与APM方向的这种偏离可能归因于Kachchh裂谷带的影响以及研究区域内65 Ma Deccan地幔柱的结构烙印。平均延迟时间(0.99±0.19)s与各大洲的全球平均时间(1 s)一致。此外,模型化的地震层厚度表明,研究区域下方的各向异性源与岩石圈变形过程(例如184 Ma非洲裂谷,88 Ma马达加斯加裂谷,65 Ma Deccan地幔羽)和软流圈流动有关。

更新日期:2020-04-20
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