The role of Partial Convective Overturn in the Barberton Greenstone Belt: Linking deep crustal-mantle and surface processes through integrated sedimentary and structural analysis of the syntectonic Moodies Group

尽管非洲南部的巴伯顿绿岩带（Barberton Greenstone Belt， BGB）是世界上了解早期地球地壳形成过程的关键地区之一，但其构造演化仍存在很大争议。提出了火山-高原环境下的现代弧岩浆活动和陆块增生或体积岩浆活动、地壳分异和部分对流翻转（PCO）作为端元。对地球化学和地层数据的不同解释加剧了这一争议，而缺乏全面的结构分析也阻碍了争议的解决。尽管弧增生的支持者引用大规模逆冲和平卧等斜褶皱的存在作为大量水平缩短的证据，但带的向斜几何形状、外部边界的叶尖几何形状、各种内部地层特征以及同位素地球化学结果表明，BGB可能是沉积在受PCO影响的“硅质”基底上的大型绿岩盖层演替的构造残余物。一种尚未经过检验的区分这些模型的方法是评估与BGB充填体最年轻的碎屑单元——同构造穆迪斯群（3.22 Ga）——的结构几何和地层有关的假设：(1)弧-吸积模型预测褶皱在浅倾至陡倾的轴向平面上具有浅倾的铰链，而PCO模型预测在陡倾的高度非圆柱铰链上具有弧形的宽向斜和狭窄的紧背斜，这种类型在以水平构造为主的造山带中没有发现。(2)弧-增生模型预测，填充物应由受侵蚀的老地壳演变为幼弧衍生的地壳，主要来自碰撞部位并垂直于造山带长度输送；而PCO模型预测，沉积填充物将主要沿着向斜长轴输送，并来自于基底挖掘过程中逐渐变老的上切源物质。(3)弧-增生同变形沉积模式将导致沉积相、颗粒/碎屑尺寸分布、弥散方向和厚度变化的不对称和大部分单向模式，而PCO模式将预测区域性径向内向模式的优势。在此，我们建议在前人专业知识的基础上，在整个盆地范围内详细研究穆迪斯群的地层学、物源组成、古现代历史和褶皱几何，并将这些与下伏火山和深部岩石的结构特征联系起来，作为约束BGB构造演化的一种方式。我们的研究结果将有助于更好地约束弧吸积构造或PCO是否是BGB构造发展的主要机制，并有助于解决长期以来关于早期地球构造过程的争论。

Even though the Barberton Greenstone Belt (BGB), southern Africa, is one of the key areas in the world for understanding processes of crust formation on early Earth, its tectonic evolution remains highly controversial. Modern-style arc magmatism and terrane accretion or voluminous magmatism, crustal differentiation and partial convective overturn (PCO) in a volcanic-plateau setting have been proposed as end members. This controversy is fuelled by differing interpretations of geochemical and stratigraphic data, and its resolution is hampered by the lack of a comprehensive structural analysis. Whereas arc-accretion proponents cite the presence of large-scale thrusting and recumbent isoclinal folds as evidence for substantial horizontal shortening, the synclinal geometry of the belt, lobe-cusp geometry of external boundaries, a variety of internal stratigraphic features, and results from isotope geochemistry suggests that the BGB may be a structural remnant of a larger greenstone cover succession deposited on ''sialic'' basement that has been affected by PCO.A yet untested way to discriminate between these models is to evaluate hypotheses related to the structural geometry and stratigraphy of the youngest clastic unit of the BGB fill, the syntectonic Moodies Group (3.22 Ga): (1) Whereas the arc-accretion model would predict that folds would have shallowly plunging hinges on shallow- to steeply-dipping axial planes, the PCO model predicts arcuate, broad synclines and narrow, tight anticlines on steeply plunging, highly non-cylindrical hinges - a style not found in orogens dominated by horizontal tectonics. (2) Whereas the arc-accretion model predicts that the fill should evolve from eroded older crust through juvenile arc-derived crust, be largely derived from the site of collision and transported perpendicular to the length of the orogen, the PCO model predicts that sedimentary fill would be predominantly transported along the long axis of the syncline and be derived from source materials that become progressively older upsection as a result of basement exhumation. (3) Syndeformational deposition in an arc-accretion mode should result in asymmetrical and largely unidirectional patterns of sedimentary facies, grain / clast size distribution, dispersal directions, and thickness variation whereas a PCO mode would predict a predominance of regionally radial, inward-facing patterns.We here propose to build on considerable previous expertise and investigate basinwide the stratigraphy, source composition, paleocurrent history, and fold geometry of the Moodies Group in detail and relate these to structural features of the underlying volcanic and plutonic rocks as a way to constrain the tectonic evolution of the BGB. Our results will serve to better constrain whether arc-accretion tectonics or PCO was the dominant mechanism for the tectonic development of the BGB and help resolve a long-standing debate on early Earth tectonic processes.