Beneath the continents and island arcs, a seismic discontinuity is often detected around 300 km depth, referred to as the X-discontinuity. Various mineralogical and petrological mechanisms have been put forth to explain its occurrence. Because of the large depth variability, it is challenging to explain its origin invoking a single mineralogical phase transition. In order to investigate this discontinuity beneath India, we analyzed 10,216 P wave receiver functions at seismological stations deployed on the Indian shield and the Himalayas. We detect the X-discontinuity as a sporadic and thin feature, in the depth range of 246–335 km, with a sharp shear velocity jump of 2.5%–3.6%. It neither bears a clear tectonic affinity nor has any correlation with the transition zone discontinuities. Interpreting its origin due to a single mineralogical change warrants a large spatial variation in the mantle temperature. Therefore, we suggest that the observed widespread X-discontinuity beneath the Indian shield owes its origin to two mechanisms, that is, Orthoenstatite to high pressure Clinoenstatite transformation which shifts to lower pressures (∼2 GPa) due to the presence of water (0.13 wt% H₂O) in MgSiO₃ and coesite-stishovite transition occurring at 8–11 GPa due to excess silica in an eclogitic component derived from the Tethys oceanic lithosphere subducted during lower Eocene. The identification of such a discontinuity could allow tracking of subducted material within the upper mantle providing a measure of mantle geochemical heterogeneity.

中文翻译：

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印度地盾下的 X 不连续性——地幔中残余特提斯大洋岩石圈的证据

在大陆和岛弧下方，通常在 300 公里深度附近检测到地震不连续性，称为 X 不连续性。已经提出了各种矿物学和岩石学机制来解释它的发生。由于深度变化很大，因此很难通过单一矿物相变来解释其起源。为了调查印度下方的这种不连续性，我们分析了 10,216 P波接收器在部署在印度地盾和喜马拉雅山脉的地震台上发挥作用。我们在 246-335 公里的深度范围内将 X 不连续性检测为一个零星的薄特征，剪切速度急剧增加 2.5%-3.6%。它既没有明显的构造亲和力，也与过渡带的不连续性没有任何关联。由于单一矿物学变化而解释其起源需要地幔温度的大空间变化。因此，我们认为在印度地盾下方观察到的广泛的 X 不连续性归因于两种机制，即正方沸石到高压斜方辉石转变，由于水的存在（0.13 wt % H ₂ O) 在 MgSiO _{3 中}由于来自特提斯大洋岩石圈的榴辉岩组分中过量的二氧化硅在始新世下俯冲，柯石英-stishovite 转变发生在 8-11 GPa。这种不连续性的识别可以跟踪上地幔内的俯冲物质，从而提供地幔地球化学异质性的测量。