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(BGB)是了解早期地球地壳形成过程的关键区域之一，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式弧形岩浆作用、地体增生或巨量岩浆作用、地壳分异作用和部分对流翻转作用。对地球化学和地层学数据的不同解释加剧了这一争议，而缺乏全面的结构分析阻碍了这一问题的解决。尽管弧吸积论者将大规模逆冲和横卧等斜褶皱的存在作为大幅水平缩短的证据，但该带的向斜几何学、外部边界的瓣尖几何学、各种内部地层学特征以及同位素地球化学的结果表明，BGB可能是受PCO影响的较大绿岩盖层序列的结构残留物。一个尚未试验的区分这些模型的方法是评估与BGB填充物中最年轻的碎屑单元的构造几何和地层学有关的假设。同构造Moodes群(3.22Ga)：(1)弧形吸积模型预测褶皱将在浅倾斜至陡峭的轴面上具有浅倾铰，而PCO模型预测弧形宽向斜和窄而紧的背斜位于陡峭倾斜的高度非圆柱形铰链上-这种风格在以水平构造为主的造山带中找不到。(2)弧形吸积模型预测，填充物应从受侵蚀的老地壳演化到年轻的弧形地壳，主要来自碰撞地点，并垂直于造山带的长度输送，而PCO模型预测，沉积充填物将主要沿向斜长轴输送，并来自于由于基底折返而逐渐变得更老的物源。(3)弧形吸积模式下的同沉积应导致沉积相、颗粒/碎屑粒度分布、扩散方向和厚度变化的不对称且基本上是单向的模式，而PCO模式将预测区域径向内向模式的优势。我们建议在大量前人专业知识的基础上，在整个盆地范围内详细研究莫迪群的地层学、物源组成、古水流历史和褶皱几何形状，并将这些与下伏火山岩和深成岩的结构特征联系起来，以此来约束BGB的构造演化。我们的结果将有助于更好地约束弧吸积构造或PCO是BGB构造发展的主导机制，并有助于解决关于早期地球构造过程的长期争论。

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.