Receiver function approaches have proven to be valuable for the investigation of crustal and upper mantle discontinuities whose sharp changes in seismic velocities cause wave conversions. While the crustal and mantle transition zone discontinuities are largely understood, the X-discontinuity at 250-350 km depth is still an object of controversial debate. The origin and global distribution of this structure with a velocity jump of 1.5-4.8 per cent for compressional and shear waves is still unexplained. Although the crustal and mantle transition zone discontinuities beneath SW Morocco and surroundings have been investigated, only a few studies observed the X-discontinuity and place the depth at 260-370 km beneath the region of western Morocco. In order to better locate and characterize the X-discontinuity beneath southwest Morocco, we create P-wave receiver functions using data recorded by the Morocco-Münster array and detect the X-discontinuity at apparent depths of 285- 350 km. In the western part of our study region we find apparent depths of ∼ 310-340 km. The eastern part of the study area appears more complex: we locate two velocity jumps at apparent depths of around 285- 295 km and 330-350 km in the northeast, and in the southeast we find a discontinuity at apparent depths of 340-350 km. Due to the large depth range and the twofold appearance of the X-discontinuity, we suggest that two different phase transitions cause the X-discontinuity beneath SW Morocco. The velocity contrasts at larger depths likely point to the coesite-stishovite phase transition occurring in deep eclogitic pools. The shallower depths can be explained by the transition from orthoenstatite to high-pressure clinoenstatite which requires the reaction between eclogite and peridotite to form orthopyroxene-rich peridotite. This reaction is likely related to previously proposed small-scale mantle upwellings beneath SW Morocco. Since both phase transitions require eclogite occurrence, the location of the X-discontinuity in this region can be used to indicate the location of recycled oceanic crust.

中文翻译：

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使用P波接收器功能，在摩洛哥西南部和相邻大陆架下方的X不连续性的位置和特征

事实证明，接收器函数方法对于研究地壳和上地幔的不连续性是有价值的，因为地壳和上地幔的不连续性会引起地震速度的急剧变化而引起波转换。尽管人们对地壳和地幔过渡带的不连续性已广为人知，但在250-350 km深度处的X不连续性仍然是有争议的话题。对于压缩波和剪切波，这种结构的速度跳跃为1.5-4.8％，其起源和整体分布尚无法解释。尽管已经调查了西南摩洛哥及其周围地区的地壳和地幔过渡带不连续性，但只有少数研究观察到X断续，并将深度置于摩洛哥西部地区下方260-370 km。为了更好地定位和表征摩洛哥西南部下方的X不连续性，我们使用摩洛哥-明斯特（Morgan）阵列记录的数据创建P波接收器功能，并在285- 350 km的表观深度检测X不连续性。在研究区域的西部，我们发现约310-340 km的表观深度。研究区域的东部显得更为复杂：我们在东北的285-295 km和330-350 km的表观深度处发现了两个速度跃变，而在东南的340-350 km的表观深度处发现了一个不连续性。由于较大的深度范围和X轴不连续性的双重外观，我们建议两个不同的相变引起西南摩洛哥下方的X轴不连续性。在较大深度处的速度对比可能指向在深层弧辉岩池中发生的堇青石-水滑石相变。较浅的深度可以通过从钙长辉石到高压斜变辉石的转变来解释，这需要榴辉岩和橄榄岩之间的反应以形成富含邻苯二茂的橄榄岩。这种反应可能与先前在摩洛哥西南部提出的小规模地幔上升有关。由于两个相变都需要榴辉岩的发生，因此该区域中X断续的位置可以用来指示再生洋壳的位置。