Horizontal vs. vertical tectonics : analysis of large-scale structures related to the deformation history of the Archean Barberton Greenstone Belt

Schmitz, Matthias GND

One of the major unresolved questions in Precambrian geology is the nature of tectonic processes during Earth’s early history and the timing of the transition to modern-style plate tectonics. The Barberton Greenstone Belt (BGB) of South Africa and Eswatini features prominently in this discussion because it represents, along with the Pilbara region of Australia, the prime geological archive of the late Paleoarchean (ca. 3.5-3.2 Ga). This time period may mark the transition from a pre-plate tectonic setting to Phanerozoic-style plate tectonics. The cuspate-lobate geometry of the BGB, together with its general structural make-up (defined by folding style, stratigraphic fill and strain distribution) appears to represent a non-actualistic Archean tectonic style characterized by vertical rather than horizontal displacements, as known from modern plate tectonics. A compilation of geological data from the entire greenstone belt demonstrates its heterogeneity and complex deformation history is compared with own investigations in this work. A critical comparison of suggested tectonic settings to recent observations shows that no pure plate-tectonic scenarios are applicable. The temporal and spatial heterogeneity of deformation, the relative greenstone-down sense of shear along many of its contacts to the adjacent plutons, and the overall synclinal structure of the BGB emphasize a non-plate-tectonic setting dominated by vertical movements. Local subsidence due to folding, tilting and sagging of thick, dense greenstone regions into an incompetent granitoid middle crust during partial convective overturn plausibly explains the rotation of the enormous Onverwacht Anticline, the characteristic folding pattern and the temporally and spatially heterogeneous deformation history of the BGB.


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