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

点击翻译按钮获取中文摘要

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.