Subducted slabs have been detected in the lower mantle for almost 30 years, yet the presence of foundered cratonic segments in the lower mantle is still unclear and inadequately investigated. We present the first P-wave radial anisotropy tomography of southern Africa (our model SA-RAnis2024), which reveals a contrasting feature of preserved northwest and modified southeast Kalahari cratonic root. Segments from the modified cratonic lithosphere are inferred to have dropped into the shallow lower mantle where seismic evidence of isolated high-velocity anomalies are observed. We detect such a high-velocity anomaly under the southwest margin of the Kalahari craton, which possibly detached from the southeast Zimbabwe craton at ca. 60 Ma based on plate reconstructions. Foundered segments can be partially brought back up to shallow depths, and contribute to the geochemical heterogeneity of younger lithosphere, through large-scale mantle convection.Cratons are the stable and rigid geological units comprising over 60% of continental landmass, and their modification or destruction have been revealed to play a significant role in continental topographical expression and in the locations of mineral deposits like diamond, gold, and platinum (Hu et al., 2018). High-seismic-velocity anomalies have been found in the upper mantle and mantle transition zone (MTZ) beneath modified cratonic belts and their surrounding oceanic areas (e.g., Shen et al., 2018). These have been interpreted as fragments of detached lithosphere, but whether such blobs can sink into the high-viscosity lower mantle is not well understood because of the relatively low resolution of seismic imaging in the deep mantle. Extensive studies have been conducted within cratonic areas with a history of subduction such as the Wyoming craton (USA) and the North China craton (e.g., Bedle et al., 2021), but studies under cratons with no post-Paleozoic subduction history, especially high-resolution lower mantle tomography studies, are rare.The absence of subduction beneath southern Africa over the past 500 m.y. (Kröner and Stern, 2004) makes it a natural laboratory to conduct such studies. Multi-episode large igneous intrusion events (Fig. 1) in southern Africa (Zhang et al., 2008; Celli et al., 2020) indicate that cratons present there (i.e., the Kalahari and Tanzanian cratons) have been modified to various degrees. Consistent with geochemical evidence, low seismic velocities reveal areas that are significantly affected by metasomatism. A recent continental-scale Vs model of the African lithosphere (Fig. 2C) reveals the occurrence of widespread lithospheric erosion and thinning under most of the African cratons over the past 200 m.y. (Celli et al., 2020). In contrast, a significant portion of the Kalahari craton is interpreted to retain a depleted and relatively thick lithosphere, as suggested by the latest high-resolution thermochemical model (Afonso et al., 2022). Such a difference is possibly attributed to different seismic data and methods employed. We used all available broadband seismic data to create the first P-wave radial anisotropic tomography model (SA-RAnis2024; Xi, 2024) of the mantle beneath southern Africa (south of 5°S) and further mapped preserved versus eroded lithosphere beneath the Kalahari craton (see the Supplemental Material1). The resulting radial anisotropy in our model serves as an efficient tool to determine the strength (amplitude) and direction (horizontal or vertical) of mantle flow or tectonic deformation.Depleted mantle in cold, thick lithosphere results in high-velocity anomalies (HVAs) (Fig. 2B; Figs. S1 and S2 in the Supplemental Material). Diamondiferous kimberlites have been used extensively as proxies for detecting thick cratonic lithosphere at the time of their eruption (Celli et al., 2020), as their diamond load indicates a kimberlite origin in the diamond stability field. The stability of the cratonic lithosphere may be enhanced by the attachment of depleted oceanic lithospheric mantle (carrying sublithospheric diamonds) to the continental keel during supercontinent assembly (Timmerman et al., 2023). Such diamonds might subsequently be sampled by kimberlitic magmas that are triggered by lithospheric extension, particularly during supercontinent break-up (Gernon et al., 2023). A direct correlation between the strength and thickness of the lithosphere, kimberlite activity, and seismic velocities can be seen beneath the northwest and southeast parts of the Kalahari craton (Fig. 2B).Our high-resolution SA-RAnis2024 model has detected several fine-scale features of isolated low-Vp anomalies (LVAs) inside the southeast Kalahari craton at 100–200 km depths whose locations are overlapping with the distributions of diamondiferous kimberlites (Fig. 2B) (Tappe e al., 2018; Özaydın and Selway, 2022), possibly indicating lithospheric modification. Available shear wave splitting null measurements (Silver et al., 2001), indicative of either an isotropic medium or a vertical mantle deformation, are consistently located within the observed isolated LVA regions (Fig. 2A). Fossil fabrics of ancient mantle intrusions, having brought the diamondiferous kimberlites to the surface, would tend to develop dominant anisotropy in a vertical orientation and thus can result in the observed null measurements or small azimuthal anisotropy. This is also supported by the relatively high conductivity associated with rising metasomatic fluids from three-dimensional magnetotelluric models of the southern African mantle (Özaydın et al., 2022). Areas with high surface relief on the southeast Kalahari craton (Fig. 2D; Fig. S4) are interpreted to have positive residual topography after removal of the continental crust (Steinberger, 2016). Since the Cretaceous, there is a strong correlation (Fig. 3D) among the unroofing events from thermochronology studies of the Kalahari craton (Stanley et al., 2013), the sedimentation rate of southern Africa (Guillocheau et al., 2012), and the frequency of kimberlite eruptions (Jelsma et al., 2009). This indicates that the aforementioned positive residual topography anomalies of southern Africa can mainly be attributed to cratonic modifications. The infilling of surrounding decompression-melted asthenosphere into the space generated by sinking dense cratonic segments (Fig. 4) would lead to positive residual topography (Hu et al., 2018). Such a mechanism has been globally proposed to explain the topographic anomalies observed in the other cratonic plateaus such as the Transantarctic Mountains in Antarctica (Shen et al., 2018).The large topographic anomaly in the southern African Plateau has also been interpreted as the surface expression of the underlying mantle flow associated with the African superplume, which is usually imaged as a large LVA in the lower mantle (e.g., French and Romanowicz, 2015). However, our SA-RAnis2024 model displays generally positive radial anisotropy (indicative of a horizontal mantle flow; Figs. S1and S3), and the absence of widespread LVAs in the MTZ and shallow lower mantle (Figs. 2 and 3; Figs. S2 and S3), combined with the lack of hot thermal anomalies in the MTZ as inferred from the dominantly normal MTZ thickness (Fig. 2E) (Reed et al., 2016; Sun et al., 2018; Yu et al., 2020), jointly rule out the existence of active mantle upwelling in the shallow lower mantle. Furthermore, the short wavelengths of residual topography also preclude a lower mantle origin of surface topography, which should result in a wavelength of >1000 km (Hu et al., 2018). The African superplume, if it currently exists, is not contributing to the surface elevation of southern Africa.The northwest portion of the Kalahari craton is characterized by continuously thick lithosphere highlighted by a prominent HVA at 200 km depth, relatively low conductivity in the magnetotelluric model (Özaydın et al., 2022), an absence of extensive diamondiferous kimberlites and magmatic activities, and generally negative residual topography (Fig. 2). Together, these are indicative of a preserved or intact cratonic lithosphere. Similar features are also revealed in two other regions (the Irumide Belt and the Mozambique Belt; outlined in Fig. 2B) where diamondiferous kimberlites are exposed at their edges, which may suggest preservation of a craton-like lithosphere.Cratonic modifications have recently been proposed to widely occur under the African plate based on new shear-wave velocity (Celli et al., 2020) and thermochemical (Afonso et al., 2022) models of the upper mantle. However, the existence and origin of detached cratonic segments remains enigmatic due to the limited resolution with depth. Our SA-RAnis2024 model has detected isolated and prominent HVAs at the top of the lower mantle at 700–850 km depths (Figs. 2 and 3; Figs. S1–S3), which have been verified to be reliable and robust based on extensive resolution tests (Figs. S5–S18). These HVAs can be due to either the existence of fossil slabs or foundered lithospheric segments. The most recent subduction in this area can be dated back to the Pan-African Orogeny at 500 Ma (Kröner and Stern, 2004). As the slab sinking rate is globally estimated to be 1–5 cm/yr (Peng and Liu, 2022), with the period of a slab stagnation in the MTZ no more than 60 m.y. (Goes et al., 2017), the most likely candidate for explaining the HVAs in the lower mantle is foundered lithospheric segments from most recent cratonic delamination.Given the absence of widespread isolated HVAs in the lower upper mantle (300–400 km depths) (Fig. S2), the observed HVAs in the top of the lower mantle may represent the foundered lithospheric segments from the final phase of cratonic delamination at ca. 60 Ma, after which there seems to be no further cratonic modification. Such a timing is inferred from the absence of a Cenozoic hotspot (Fig. 1), the termination of extensive kimberlite eruption in southern Africa (Fig. 3D; Jelsma et al., 2009), and the synchronous cessation of topography response to lithospheric delamination (Hu et al., 2018) as suggested by the abruptly dropping rates of denudation, unroofing, and offshore sedimentation (Guillocheau et al., 2012; Stanley et al., 2013). This inference is also consistent with the occurrence of most recent lithospheric thinning under the Kalahari craton in the late Mesozoic, as revealed from mantle xenolith samples (Janney et al., 2010) and joint analysis of kimberlites and velocity anomalies (Celli et al., 2020). We have tried to determine the original locations of these HVAs, taking the one under the southwestern margin of the Kalahari craton as an example. Plate reconstructions from three different models (Heine et al., 2013; Müller et al., 2016; Torsvik and Cocks, 2017) point to a similar position at the southeast Zimbabwe craton (Fig. 2F; Fig. S19) where there is extensive kimberlite activity before 80 Ma and a missing cratonic root (Fig. 3A). Thus, the foundered lithospheric segment under the southwestern margin of the Kalahari craton possibly resulted from the cratonic modification in the Late Cretaceous and subsequent detachment of the lithospheric segment from the southeast Zimbabwe craton at ca. 60 Ma. If we assume that this cratonic segment detaches from a depth of 100 km (Fig. 3A), the sinking rate is estimated to be ~1.2 cm/yr. This is comparable to the minimum rate for currently sinking slabs (Peng and Liu, 2022) and numerical models of cratonic delamination (Wang et al., 2022b).Negative buoyancy is necessary to make these cratonic segments sink into the deep mantle, especially when crossing the 660 km phase boundary (Stixrude and Lithgow-Bertelloni, 2011). The cratonic lithospheric mantle is 0.5%–1.23% denser than the asthenospheric mantle based on analysis of whole lithosphere isostasy (Lamb et al., 2020), topography, and gravity anomalies (Wang et al., 2022a), providing a self-sustained negative buoyancy from gravitational instability. In addition, multiple ancient magmatic events (Fig. 1) have contributed to the refertilization of lithosphere under the southeast Kalahari craton (see the Supplemental Material), in which case cratonic segments with fertile peridotite composition or decreasing depletion degrees tend to sink into the lower mantle as suggested by thermo-chemo-mechanical modeling (Wang et al., 2022b). Some portions of these cratonic segments can be brought back up to shallow depths through mantle convection at hotspots or mid-ocean ridges (Fig. 4). Foundered materials from the Archean Kaapvaal craton have been discovered at the nearby ultraslow-spreading Southwest Indian ridge (Liu et al., 2022).Our new P-wave radial anisotropy and velocity models of southern Africa show a continuous high-velocity anomaly in the lithosphere of the northwest Kalahari craton, suggesting a preserved lithospheric root. In contrast, the southeast Kalahari craton has been modified based on the isolated low-velocity anomalies in the lithosphere, combined with coherent distributions of diamondiferous kimberlites, shear wave splitting null measurements, and positive residual topography. We find evidence of high-velocity anomalies in the uppermost lower mantle that we interpret as delaminated lithosphere based on plate reconstruction and timing of the kimberlitic volcanism.We are grateful to Tim Stern for helpful suggestions on this manuscript. We thank Guoming Jiang, Sinan Özaydın, Bernhard Steinberger, and Trond Torsvik for providing us the modified multi-channel cross-correlation (MMCC) code, kimberlite data, residual topography data, and plate motion model, respectively. Constructive comments from Suzette Timmerman, Nicolas Celli, one anonymous reviewer, and Urs Schaltegger (the editor) greatly improved the manuscript. This research is supported by the National Natural Science Foundation of China (grants 42374054 and 42074052) and the National Key R&D Program of China (grant 2023YFF0803202), and partially funded by the Shanghai Rising-Star Program (grant 22QA1409600), Shanghai Pilot Program for Basic Research, and the Chinese Fundamental Research Funds for the Central Universities.

中文翻译：

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南部非洲太古宙克拉通的改造，沉没的部分落入浅下地幔中


近 30 年来，人们一直在下地幔中发现俯冲板块，但下地幔中是否存在沉没的克拉通段仍不清楚，研究也没有充分进行。我们展示了南部非洲的第一个 P 波径向各向异性断层扫描（我们的模型 SA-RAnis2024），它揭示了保存完好的西北喀拉哈里克拉通根部和修改后的东南卡拉哈里克拉通根部的对比特征。据推断，改造后的克拉通岩石圈的部分已落入浅层下地幔，在那里观察到孤立的高速异常的地震证据。我们在卡拉哈里克拉通西南边缘下方检测到了这种高速异常，该异常可能在大约 10 年前与津巴布韦克拉通东南部分离。 60 Ma 基于板块重建。通过大规模的地幔对流，塌陷的部分可以部分恢复到浅层，并有助于较年轻岩石圈的地球化学非均质性。克拉通是稳定和刚性的地质单元，占大陆陆地面积的60%以上，其改造或破坏已被证明在大陆地形表现以及钻石、金和铂等矿藏位置中发挥着重要作用（Hu et al., 2018）。在克拉通带下方的上地幔和地幔过渡带（MTZ）及其周围海洋区域发现了高地震速度异常（例如，Shen等，2018）。这些被解释为分离岩石圈的碎片，但由于深部地幔中地震成像的分辨率相对较低，这些斑点是否能够沉入高粘度的下地幔中尚不清楚。 人们在有俯冲历史的克拉通地区进行了广泛的研究，例如怀俄明克拉通（美国）和华北克拉通（例如Bedle等人，2021），但在没有古生代后俯冲历史的克拉通下进行了研究，特别是高分辨率下地幔断层扫描研究很少见。在过去 500 多年里，南部非洲下方没有俯冲。 （Kröner 和 Stern，2004）使其成为进行此类研究的天然实验室。南部非洲多期大型火成岩入侵事件（图1）（Zhang et al., 2008; Celli et al., 2020）表明那里的克拉通（即卡拉哈里克拉通和坦桑尼亚克拉通）已经发生了不同程度的改造。与地球化学证据一致，低地震速度揭示了受交代作用显着影响的区域。最近的非洲岩石圈大陆尺度 Vs 模型（图 2C）揭示了过去 200 年来，大多数非洲克拉通下方发生了广泛的岩石圈侵蚀和减薄。 （Celli 等人，2020）。相比之下，正如最新的高分辨率热化学模型所表明的那样，卡拉哈里克拉通的很大一部分被解释为保留了贫乏且相对较厚的岩石圈（Afonso 等人，2022）。这种差异可能归因于不同的地震数据和采用的方法。我们使用所有可用的宽带地震数据创建了第一个关于南部非洲（南纬 5°以南）地幔的 P 波径向各向异性层析成像模型（SA-RAnis2024；Xi，2024），并进一步绘制了喀拉哈里沙漠下方保存的岩石圈与侵蚀岩石圈的地图克拉通（参见补充材料1）。 我们模型中产生的径向各向异性可作为确定地幔流或构造变形的强度（幅度）和方向（水平或垂直）的有效工具。寒冷、厚岩石圈中的耗尽地幔会导致高速异常（HVA）（图 2B；补充材料中的图 S1 和 S2）。含钻石金伯利岩已被广泛用作在喷发时检测厚克拉通岩石圈的代理（Celli 等人，2020），因为它们的钻石负载表明钻石稳定场中的金伯利岩起源。克拉通岩石圈的稳定性可能会因超大陆组装过程中耗尽的大洋岩石圈地幔（携带岩石圈下钻石）附着到大陆龙骨而得到增强（Timmerman et al., 2023）。随后，此类钻石可能会被岩石圈扩张引发的金伯利岩岩浆取样，特别是在超大陆分裂期间（Gernon 等人，2023 年）。在卡拉哈里克拉通的西北和东南部下方可以看到岩石圈的强度和厚度、金伯利岩活动和地震速度之间的直接相关性（图 2B）。我们的高分辨率 SA-RAnis2024 模型已经检测到了几个精细的卡拉哈里克拉通东南部 100-200 公里深度处孤立的低 Vp 异常 (LVA) 的尺度特征，其位置与含金刚石金伯利岩的分布重叠（图 2B）（Tappe 等人，2018 年；Özaydın 和 Selway，2022 年） ），可能表明岩石圈发生了改变。可用的剪切波分裂零值测量（Silver 等，2001）表明各向同性介质或垂直地幔变形，始终位于观察到的孤立 LVA 区域内（图 2A）。 古代地幔侵入的化石结构，将含钻石的金伯利岩带至地表，往往会在垂直方向上形成主要的各向异性，因此可能导致观察到的零测量或小方位各向异性。南部非洲地幔三维大地电磁模型中交代流体上升所产生的相对较高的电导率也支持了这一点（Özaydın 等，2022）。卡拉哈里克拉通东南部具有高地表起伏的区域（图2D；图S4）被解释为在大陆地壳去除后具有正的残余地形（Steinberger，2016）。自白垩纪以来，卡拉哈里克拉通热年代学研究的揭顶事件（Stanley等，2013）、南部非洲的沉积速率（Guillocheau等，2012）和南部非洲的沉积速率（Guillocheau等，2012）之间存在很强的相关性（图3D）。金伯利岩喷发的频率（Jelsma 等，2009）。这表明上述南部非洲的正残余地形异常主要归因于克拉通的改造。周围减压融化的软流圈填充到致密克拉通段下沉产生的空间中（图4）将导致正残余地形（Hu等，2018）。全球范围内已提出这样的机制来解释在南极洲横贯南极山脉等其他克拉通高原观察到的地形异常(Shen et al., 2018)。南部非洲高原的大面积地形异常也被解释为地表异常。与非洲超地幔柱相关的底层地幔流的表达，通常被成像为下地幔中的大型 LVA（例如，French 和 Romanowicz，2015）。 然而，我们的 SA-RAnis2024 模型显示出普遍的正径向各向异性（表明水平地幔流；图 S1 和 S3），并且在 MTZ 和浅下地幔中不存在广泛的 LVA（图 2 和 3；图 S2 和 S3）。 S3），再加上从主要正常的 MTZ 厚度推断出 MTZ 不存在热异常（图 2E）（Reed 等，2016；Sun 等，2018；Yu 等，2020），共同排除了浅部下地幔中存在活跃的地幔上涌现象。此外，残余地形的短波长也排除了表面地形的下地幔起源，这应该导致波长>1000 km（Hu et al., 2018）。非洲超地幔柱（如果目前存在的话）不会对南部非洲的地表高度产生影响。卡拉哈里克拉通的西北部分的特点是连续厚的岩石圈，突出显示在 200 公里深度处有突出的 HVA，大地电磁模型中的电导率相对较低（Özaydın 等人，2022），缺乏广泛的含钻石金伯利岩和岩浆活动，且残留地形总体呈负值（图 2）。这些共同表明克拉通岩石圈保存完好或完整。其他两个区域（伊鲁米德带和莫桑比克带；如图 2B 所示）也显示出类似的特征，其中含金刚石金伯利岩在其边缘暴露，这可能表明保存了类似克拉通的岩石圈。最近有人提出克拉通修改根据新的上地幔剪切波速度（Celli 等人，2020）和热化学（Afonso 等人，2022）模型，非洲板块下广泛发生。 然而，由于深度分辨率有限，分离克拉通段的存在和起源仍然是个谜。我们的SA-RAnis2024模型在下地幔顶部700-850公里深度处检测到了孤立的、突出的HVA（图2和图3；图S1-S3），基于广泛的研究，这些模型被证实是可靠和稳健的。分辨率测试（图S5-S18）。这些 HVA 可能是由于化石板的存在或沉没的岩石圈部分造成的。该地区最近的俯冲可以追溯到 500 Ma 的泛非造山运动（Kröner 和 Stern，2004）。由于全球板片下沉速度估计为 1-5 cm/年（Peng 和 Liu，2022），MTZ 板片停滞期不超过 60 m.y. (Goes et al., 2017)，解释下地幔中 HVA 的最有可能的候选者是最近克拉通拆沉造成的岩石圈部分。鉴于下上地幔中不存在广泛的孤立 HVA（深度 300-400 公里） （图S2），在下地幔顶部观察到的HVA可能代表大约在克拉通拆沉最终阶段沉没的岩石圈部分。 60 Ma，此后似乎没有进一步的克拉通改造。这样的时间是根据新生代热点的缺失（图 1）、南部非洲大面积金伯利岩喷发的终止（图 3D；Jelsma 等，2009）以及岩石圈分层的地形响应同步停止来推断的。 （Hu 等人，2018）正如剥蚀、屋顶脱落和近海沉积率突然下降所表明的那样（Guillocheau 等人，2012 年；Stanley 等人，2013 年）。 这一推论也与地幔捕虏体样本（Janney 等，2010）以及金伯利岩和速度异常的联合分析（Celli 等， 2020）。我们试图确定这些HVA的原始位置，以喀拉哈里克拉通西南缘下的HVA为例。三个不同模型的板块重建（Heine 等人，2013 年；Müller 等人，2016 年；Torsvik 和 Cocks，2017 年）指出津巴布韦克拉通东南部的类似位置（图 2F；图 S19），那里有广泛的80 Ma之前的金伯利岩活动和克拉通根的缺失（图3A）。因此，喀拉哈里克拉通西南缘岩石圈段的塌陷可能是晚白垩世克拉通改造以及随后该岩石圈段从津巴布韦克拉通东南部脱离的结果。 60马。如果我们假设该克拉通部分从 100 公里的深度脱离（图 3A），则下沉速率估计约为 1.2 厘米/年。这与当前板片下沉的最低速率（Peng 和 Liu，2022）和克拉通拆沉数值模型（Wang 等，2022b）相当。负浮力对于使这些克拉通段沉入地幔深部是必要的，特别是当跨越 660 公里的相界（Stixrude 和 Lithgow-Bertelloni，2011）。通过对整个岩石圈均衡性(Lamb等，2020)、地形和重力异常(Wang等，2022a)的分析，克拉通岩石圈地幔比软流圈地幔密度高0.5%~1.23%，为克拉通岩石圈地幔提供了自我维持的基础。由于重力不稳定而产生负浮力。此外，还发生多次古代岩浆事件（图1）。 1) 促进了卡拉哈里克拉通东南部岩石圈的再肥化（参见补充材料），在这种情况下，如热化学机械模型所表明的，具有肥沃橄榄岩成分或逐渐减少的损耗程度的克拉通部分往往会沉入下地幔中（Wang 等人，2022b）。这些克拉通段的某些部分可以通过热点或洋中脊处的地幔对流返回到浅层深度（图4）。在附近超慢速扩张的西南印度洋脊处发现了来自太古宙 Kaapvaal 克拉通的奠基物质（Liu 等人，2022）。我们新的南非 P 波径向各向异性和速度模型显示了南部非洲的连续高速异常。卡拉哈里克拉通西北部的岩石圈，表明存在保存完好的岩石圈根部。相比之下，卡拉哈里克拉通东南部已根据岩石圈中孤立的低速异常，结合含金刚石金伯利岩的相干分布、横波分裂零值测量和正残余地形进行了修改。我们在最上层下地幔中发现了高速异常的证据，根据板块重建和金伯利岩火山活动的时间，我们将其解释为分层的岩石圈。我们感谢蒂姆·斯特恩（Tim Stern）对本手稿提出的有益建议。我们感谢Guming Jiang、Sinan Özaydın、Bernhard Steinberger 和Trond Torsvik 分别为我们提供了修改后的多通道互相关（MMCC）代码、金伯利岩数据、残余地形数据和板块运动模型。 Suzette Timmerman、一位匿名审稿人 Nicolas Celli 和 Urs Schaltegger（编辑）的建设性意见极大地改进了手稿。 该研究得到国家自然科学基金（42374054和42074052）和国家重点研发计划（2023YFF0803202）的支持，并得到了上海市新星计划（22QA1409600）、上海市先导计划的部分资助。基础研究、中央高校基本科研业务费专项资金。