Teleseismic tomography was done to investigate the lithosphere beneath northwestern India which was affected by several magmatic episodes since Neoproterozoic times. Broadband stations were deployed in phases at 29 locations in an area of 300 km × 300 km in northwestern India. About 8000 high-quality P phases corresponding to 934 events were used. Relative residuals estimated through adaptive stacking were inverted using the Fast Marching teleseismic tomography to obtain the 3D velocity perturbations. The tomograms revealed an anomalous ≈ 100 km wide, cylindrical zone of low velocities (−2% to −2.3%) extending from a depth of ≈ 60 km to ≈140 km beneath the Barmer rift. A broader zone of low velocities (−1%) encompasses the upper mantle down to a depth of at least 200 km. The anomaly lies beneath the localized, small-volume, eruptive centers of the earliest phase of Deccan volcanism within and on the flanks of the rift which falls on the projected extension of the Reunion hotspot track along the rift system of northwestern India. Explaining such large anomalies requires a significant increase in temperature, presence of melt and/or compositional changes. Geochemical studies indicate that the polychronous (early Cretaceous to Paleogene) alkaline complexes above the anomalous zone are derived from hydrated mantle peridotite and contain an abundance of phlogopites, suggesting a hydrated and metasomatized lithosphere beneath northwestern India. A thermal source impinging from below upon a metasomatized, weak, and rifted lithosphere may result in the observed signature akin to that of a plume conduit that overprints the older signatures of the Neoproterozoic Malani magmatic event.

中文翻译：

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印度西北部巴默裂谷下方交代化岩石圈中羽流通道的地震证据

远震断层扫描是为了调查印度西北部下方的岩石圈，该岩石圈自新元古代以来就受到几次岩浆活动的影响。在印度西北部300公里×300公里的区域内分阶段部署了29个宽带站点。使用了对应于 934 个事件的约 8000 个高质量 P 相。使用快速行进远震层析成像技术对通过自适应叠加估计的相对残差进行反演，以获得 3D 速度扰动。断层扫描显示，在巴默裂谷下方，有一个约 100 公里宽的异常圆柱形低速区域（−2% 至 -2.3%），从约 60 公里深度延伸至约 140 公里。更广泛的低速区域 (-1%) 涵盖了上地幔，深度至少为 200 公里。该异常现象位于裂谷内及其侧翼德干火山活动最早阶段的局部小体积喷发中心下方，该裂谷落在印度西北部裂谷系统留尼旺热点轨道的预计延伸范围内。解释如此大的异常需要显着升高温度、存在熔体和/或成分变化。地球化学研究表明，异常带上方的多时（早白垩世至古近纪）碱性杂岩源自水合地幔橄榄岩，并含有丰富的金云母，表明印度西北部下方存在水合和交代化的岩石圈。从下方撞击交代化、弱化和裂谷的岩石圈的热源可能会导致观察到的特征类似于羽流管道的特征，该特征覆盖了新元古代马拉尼岩浆事件的较旧特征。