Crustal structure of the Bushveld complex, South Africa from 1D shear wave velocity models: Evidence for complex-wide crustal modification

IF 2.7 3区 地球科学 Q2 GEOCHEMISTRY & GEOPHYSICS
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Abstract

Thirty-nine 1D shear wave velocity profiles, obtained by jointly inverting receiver functions and Rayleigh wave group velocities, are used to investigate the crustal structure of the Bushveld Complex in northern South Africa. Data from teleseismic earthquakes recorded on broadband seismic stations between 1997 and 1999 and 2015–2020 were used to compute P-wave receiver functions. Rayleigh wave group velocities between 5 and 30 s period were obtained from an ambient noise tomography and combined with group velocities between 30 and 60 s period from a published continental-scale surface wave tomography model. Moho depths of 45–47.5 km are found under the center of the complex compared to 40 km thick crust, on average, surrounding the complex, indicating ∼5–7 of crustal thickening. The bottom ∼10 km or more of the lower crust across much of the Bushveld Complex has a Vs ≥ 4.0 km/s, consistent with a mafic composition, whereas in most areas around the margins of the complex the thickness of the mafic lower crust is much less than 10 km. In the upper crust higher velocity structure (Vs > 3.6 km/s) above 15 km depth underlain by lower velocity structure is seen in many locations, suggesting the presence of mafic/ultramafic layering. These results favor the continuous-sheet model for the structure of the Bushveld Complex because the ensemble of 1D models is characterized by three diagnostic features consistent with that model: (1) thicker crust under the center of the complex than away from the complex; (2) a greater thickness of high-velocity (i.e., mafic) layering in the lower crust under the complex compared to away from the complex; (3) high-velocity (i.e., mafic/ultramafic) layering in the upper crust beneath much of the complex. The lack of upper crustal mafic/ultramafic layering beneath some parts of the complex is consistent with the post-emplacement tectonic and magmatic history of the complex.

从一维剪切波速度模型看南非布什维尔德复合地壳结构:整个复合地壳变化的证据
通过联合反演接收函数和瑞利波群速度获得了 39 个一维剪切波速度剖面,用于研究南非北部布什维尔德复合体的地壳结构。宽带地震台站在 1997 至 1999 年以及 2015 至 2020 年期间记录的远震数据被用于计算 P 波接收函数。从环境噪声层析模型中获得了 5 至 30 秒周期的瑞利波群速,并将其与从已公布的大陆尺度表面波层析模型中获得的 30 至 60 秒周期的群速相结合。复合体中心下方的莫霍深度为 45-47.5 千米,而复合体周围的地壳平均厚度为 40 千米,表明地壳增厚了 5-7 千米。在布什维尔德岩群的大部分地区,下地壳底部∼10 千米或更厚的Vs ≥ 4.0 千米/秒,与岩浆成分一致,而在岩群边缘的大部分地区,岩浆下地壳的厚度远小于10 千米。在上地壳 15 千米深度以上的较高速度结构(Vs > 3.6 千米/秒)中,许多地方被较低的速度结构所覆盖,这表明存在黑云母/超黑云母分层。这些结果有利于布什维尔德复合体结构的连续片状模型,因为一维模型集合具有与该模型一致的三个诊断特征:(1)复合体中心下方的地壳比远离复合体的地壳厚;(2)复合体下方的下地壳中高速(即黑云母)分层的厚度比远离复合体的下地壳中高速(即黑云母/超黑云母)分层的厚度大;(3)复合体大部分地区下方的上地壳中高速(即黑云母/超黑云母)分层。建筑群某些部分下的上地壳缺乏黑云母/超黑云母层理,这与建筑群置换后的构造和岩浆历史是一致的。
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来源期刊
Tectonophysics
Tectonophysics 地学-地球化学与地球物理
CiteScore
4.90
自引率
6.90%
发文量
300
审稿时长
6 months
期刊介绍: The prime focus of Tectonophysics will be high-impact original research and reviews in the fields of kinematics, structure, composition, and dynamics of the solid arth at all scales. Tectonophysics particularly encourages submission of papers based on the integration of a multitude of geophysical, geological, geochemical, geodynamic, and geotectonic methods
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