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

巴伯顿绿岩带部分对流翻转的作用：通过同构造穆迪群的综合沉积和结构分析将深部地幔和地表过程联系起来

• 批准号: 276861234
• 负责人: Professor Dr. Christoph Heubeck
• 金额: --
• 依托单位: Institut für Geowissenschaften
• 依托单位国家: 德国
• 项目类别: Priority Programmes
• 财政年份: 2015
• 资助国家: 德国
• 起止时间: 2014-12-31 至 2019-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/276861234?language=en
• 关键词: role; Partial; Convective; Overturn; Barberton

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.

尽管非洲南部的巴伯顿绿岩带（BGB）是世界上了解早期地球地壳形成过程的关键地区之一，但其构造演化仍然存在很大争议。现代风格的弧岩浆作用和地体增生或大量岩浆作用、地壳分异和火山高原环境中的部分对流翻转（PCO）已被提议作为最终成员。对地球化学和地层数据的不同解释加剧了这一争议，并且由于缺乏全面的结构分析而阻碍了其解决。尽管弧增生支持者引用大规模逆冲和斜卧等斜褶皱的存在作为水平大幅缩短的证据，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可能是沉积在上的更大绿岩盖层序列的结构残余物。 受 PCO 影响的“唾液酸”基底。区分这些模型的一种尚未经过测试的方法是评估与 BGB 充填物最年轻的碎屑单元（同构造穆迪群 (3.22 Ga)）的结构几何和地层学相关的假设：(1) 而弧吸积模型将预测褶皱在浅至 PCO 模型预测陡倾的轴面会出现弓形、宽阔的向斜和狭窄、紧密的背斜，背斜陡峭、高度非圆柱形铰链——这种类型在以水平构造为主的造山带中不存在。 (2) 弧增生模型预测，充填物应该是从被侵蚀的较老的地壳通过新生的弧衍生地壳演变而来，主要源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着向斜的长轴输送，并且源自由于以下因素而逐渐变得较旧的上抬作用的源物质： 地下室挖掘。 (3) 弧增生模式中的同变形沉积应导致沉积相、颗粒/碎屑尺寸分布、分散方向和厚度变化的不对称且很大程度上单向的模式，而 PCO 模式将预测区域径向、向内模式的主导。我们在此建议以大量先前的专业知识为基础，研究全盆地的地层学、来源 详细介绍了穆迪群的组成、古水流历史和褶皱几何形状，并将这些与底层火山岩和深成岩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确定弧吸积构造或 PCO 是否是 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。