We examine some aspects of the tectonic evolution of Precambrian cratonic roots beneath South America based on lithospheric distribution of shear-wave velocities. We derive our model by inverting 26,984 fundamental mode Rayleigh wave group velocity dispersion curves, at periods of 9–180 s. We first regionalize our measurements and then invert the result for a 3D S-wave velocity model extending to 200 km depth. Fast velocities beneath the Amazonian and São Francisco cratons, and beneath buried cratonic units in the Parnaíba and Paraná basins, are long wavelength features consistent with previous tomography studies. For the Amazonian craton at 150 km depth, we find an increase of velocities with province age, except for the Maroni-Itacaiúnas province, where we hypothesize that K'Mudku intraplate tectono-thermal events at the middle-late Mesoproterozoic and emplacement of a large igneous province following the breakup of Pangea could have altered at least partially its lithosphere. Our results are consistent with a São Francisco paleocontinent whose borders extend beyond the surface limits of the present São Francisco craton into the neighboring Araçuaí and Brasília belts. Based on slow shear-wave velocities in the upper mantle beneath the Borborema province, consistent with lithospheric thinning, we argue that a possible cratonic root of the São Francisco Paleocontinent beneath this province has likely been eroded away. This analysis is further corroborated by tectonic events that led to the alteration of the Borborema mantle, including hydration in the Paleoproterozoic, rifting in the early-middle Tonian, reworking during Neoproterozoic Brasiliano events, and lithospheric stretching during the breakup of Pangea. Finally, we also image a fast shear-wave velocity structure in the region of the Río de la Plata craton, consistent with magnetotelluric studies.

中文翻译：

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揭开南美洲下方前寒武纪克拉通根源：表面波断层扫描的贡献


我们根据剪切波速度的岩石圈分布研究了南美洲下方前寒武纪克拉通根部构造演化的某些方面。我们通过反转周期为 9-180 秒的 26,984 个基模瑞利波群速度色散曲线来推导出模型。我们首先对测量进行区域化，然后将结果反演为延伸至 200 公里深度的 3D S 波速度模型。亚马逊克拉通和圣弗朗西斯科克拉通下方以及帕尔纳伊巴盆地和巴拉那盆地埋藏克拉通单元下方的快速速度是长波长特征，与之前的断层扫描研究一致。对于 150 公里深度的亚马逊克拉通，我们发现速度随着省份年龄的增加而增加，除了 Maroni-Itacaiúnas 省，我们假设 K'Mudku 在中元古代中晚期发生了板内构造热事件，并就位了一个大型盘古大陆分裂后形成的火成岩省可能至少部分改变了其岩石圈。我们的结果与圣弗朗西斯科古大陆一致，其边界超出了目前圣弗朗西斯科克拉通的地表界限，进入了邻近的阿拉苏伊和巴西利亚带。根据博尔博雷马省下方上地幔的慢剪切波速度，与岩石圈减薄相一致，我们认为该省下方圣弗朗西斯科古大陆的克拉通根可能已被侵蚀掉。这一分析得到了导致 Borborema 地幔改变的构造事件的进一步证实，这些构造事件包括古元古代的水化、早中托通世的裂谷、新元古代巴西利亚诺事件期间的改造以及盘古大陆分裂期间的岩石圈拉伸。 最后，我们还对拉普拉塔克拉通地区的快速剪切波速度结构进行了成像，这与大地电磁研究一致。