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首页> 外文期刊>Journal of Asian earth sciences >Group velocity dispersion characteristics and one-dimensional regional shear velocity structure of the eastern Indian craton
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Group velocity dispersion characteristics and one-dimensional regional shear velocity structure of the eastern Indian craton

机译：东印度克拉通的群速度扩散特征和一维区域剪切速度结构

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In the past three years, a semi-permanent network of fifteen 3-component broadband seismographs has become operational in the eastern Indian shield region occupying the Archean (similar to 2.5-3.6 Ga) Singhbhum-Odisha craton (SOC) and the Proterozoic (similar to 1.0-2.5 Ga) Chotanagpur Granitic Gneissic terrane (CGGT). The reliable and accurate broadband data for the recent 2015 Nepal earthquake sequence from 10 broadband stations of this network enabled us to estimate the group velocity dispersion characteristics and one-dimensional regional shear velocity structure of the region. First, we measure fundamental mode Rayleigh- and Love-wave group velocity dispersion curves in the period range of 7-70 s and then invert these curves to estimate the crustal and upper mantle structure below the eastern Indian craton (EIC). We observe that group velocities of Rayleigh and Love waves in SOC are relatively high in cdmparison to those of CGGT. This could be attributed to a relatively mafic-rich crust-mantle structure in SOC resulting from two episodes of magmatism associated with the 1.6 Ga Dalma and similar to 117 Ma Rajmahal volcanisms. The best model for the EIC from the present study is found to be a two-layered crust, with a 14-km thick upper-crust (UC) of average shear velocity (V-s) of 3.0 km/s and a 26-km thick lower-crust (LC) of average V-s of 3.6 km/s. The present study detects a sharp drop in V-s (similar to-2 to 3%) at 120-260 km depths, underlying the EIC, representing the probable seismic lithosphere-asthenosphere boundary (LAB) at 120 km depth. Such sharp fall in Vs below the LAB indicates a partially molten layer. Further, a geothermal gradient extrapolated from the surface heat flow shows that such a gradient would intercept the wet basalt solidus at 88-103 km depths, suggesting a 88-103 km thick thermal lithosphere below the EIC. This could also signal the presence of small amounts of partial melts. Thus, this 2-3% drop in V-s could be attributed to the presence of partial melts in the upper mantle related to the earlier volcanic episodes viz. back-arc volcanism associated with the Archean/Proterozoic subduction, 1.6 Ga Dalma volcanism, and similar to 117 Ma Rajmahal volcanism. The main result of our modeling provides evidences for the absence of Keel or thick lithosphere below the EIC. (C) 2016 Elsevier Ltd. All rights reserved.
机译：在过去的三年中，由15个3分量宽带地震仪组成的半永久性网络已在印度东部盾构地区投入使用，该地区占据了太古宙（类似于2.5-3.6 Ga），辛格哈姆-奥迪萨克拉通（SOC）和元古代（类似至1.0-2.5 Ga）乔塔纳格布尔花岗岩片麻岩地层（CGGT）。来自该网络10个宽带站的2015年尼泊尔最近地震序列的可靠，准确的宽带数据，使我们能够估算该地区的群速度散布特征和一维区域剪切速度结构。首先，我们测量7-70 s周期范围内的基本模态瑞利波和洛夫波群速度弥散曲线，然后将这些曲线求反以估计东印度克拉通（EIC）之下的地壳和上地幔结构。我们观察到，在SOC中，瑞利波和洛夫波的群速度与CGGT相比具有较高的一致性。这可能归因于SOC中相对镁铁质的壳幔结构，这是由与1.6 Ga Dalma相关的两次岩浆作用引起的，类似于117 Ma Rajmahal火山作用。从本研究中得出的EIC的最佳模型是两层地壳，其平均剪切速度（Vs）为3.0 km / s的14 km厚的上地壳（UC）和26 km的厚处平均Vs的下地壳（LC）为3.6 km / s。本研究检测到EIC下方120-260 km深度处的V-s急剧下降（约2-3％），代表了120 km深度处可能的岩石圈-软流圈边界（LAB）。 Vs低于LAB的急剧下降表明部分熔融。此外，从地表热流推断出的地热梯度表明，该梯度将拦截深度为88-103 km的湿玄武岩固相线，表明EIC下方的热岩石圈厚度为88-103 km。这也可能表示存在少量的部分熔体。因此，Vs下降2-3％可能是由于上地幔中存在与早期火山爆发有关的部分熔体。与太古代/元古代俯冲有关的后弧火山活动，1.6 Ga Dalma火山活动，类似于117 Ma Rajmahal火山活动。我们建模的主要结果为EIC下方没有龙骨或厚岩石圈提供了证据。 （C）2016 Elsevier Ltd.保留所有权利。

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