Accretion of the Archean Zimbabwe Craton

The oldest part of the Zimbabwe craton, the Tokwe terrane, preserves evidence for a complex series of tectonomagmatic events ranging in age from 3.6 to 2.95 billion years ago. These events resulted in complex deformation of the Sebakwian greenstones and intervening gneissic rocks. This may have involved convergent margin accretionary processes that led to the development of the Tokwe terrane as a stable continental nuclei by 2.95 billion years ago.

A widespread unit of mixed volcanic and sedimentary rocks was deposited on the Tokwe terrane at circa 2.9 billion years ago. These lower greenstones include mafic and felsic volcanic rocks, coarse conglomerates, sandstones, and shales. The large variation in volcanic and sedimentary rock types, along with the rapid and significant lateral variations in stratigraphic thicknesses that typify the Lower Greenstones, are characteristic of rocks deposited in continental rift or rifted arc settings. The Tokwe terrane was subjected to rifting at 2.9 Ga, leading to the formation of widespread graben in which the Lower Greenstones were deposited. The southeastern margin of the Tokwe terrane may have been rifted from another, perhaps larger fragment at this time, along a line extending from the Buhwa-Mweza greenstone belts to the Mutare belt, allowing a thick sequence of passive margin-type sediments (preserved in the Buwha greenstone belt) to develop on this rifted margin. Age constraints on the timing of the passive margin development are not good, but appear to fall within the range of 3.09 to 2.86 billion years ago. By 2.7 billion years ago, a major marine transgression covered much of the southern half of the Tokwe terrane, as recorded in shallow-water sandstones, carbonates, and iron formations of the Manjeri-type units preserved in several greenstone belts. The Manjeri-type units overlap the basement of the Tokwe terrane in several places (e.g., Belingwe, Masvingo), and lie unconformably over the circa 3.5 and 2.9 billion-year-old greenstones. Regional stratigraphic relationships suggest that the Manjeri Formation forms a southeast thickening sedimentary wedge that prograded onto the Tokwe terrane, in a manner analogous to the Ocoee-Chilhowee and correlative Sauk Sequence shallow-water progradational sequence of the Appalachians, and similar sequences in other mountain ranges. The progradation could have been driven by sedimentary or tectonic flexural loading of the margin of the Tokwe terrane, but most evidence points to the latter cause. The top of the Manjeri-type units represents a regional detachment surface, on which allochthonous units of the southern greenstones were emplaced. Loading of the passive margin by these thrust sheets would have induced flexural subsidence and produced a foreland basin that migrated onto the Tokwe terrane.

The 2.7 billion-year-old greenstones are divided into a northwestern arc-like succession, and a southeastern allochthonous succession. The northwestern arc succession contains lavas with strong signatures of eruption through older continental crust, and the arc appears to be a continental margin type of magmatic province. In contrast, the southern greenstones are allochthonous and were thrust in place along a shear zone that is well exposed in several places, including the Belingwe belt. These southern greenstones have a stratigraphy reminiscent of thick oceanic crust, suggesting that they may represent an oceanic plateau that was obducted onto the Tokwe terrane 2.7 billion years ago. All of the southern greenstones are distributed in a zone confined to about 100 miles (150 km) from the line of passive margin-type sediments extending from Mweza-Buhwa to Mutare. This "Umtali line" may represent the place where an ocean or back arc basin opened between 2.9 and 2.8 billion years ago, then closed at 2.7 billion years, and forms the root zone from which the southern greenstones were obducted. This zone contains numerous northeast-striking mylonitic shear zones in the quartzofeldspathic gneisses. Closure of the Sea of umtali at circa 2.7 billion years ago deposited a flysch sequence of graywacke-argillite turbidites that forms the upper part of the Manjeri Formation, and formed a series of northeast-striking folds.

The latest Archean tectonic events to affect the Zimbabwe craton is associated with deposition of the Shamvaian group, and intrusion of the Chili-manzi suite granitoids at circa 2.6-2.57 billion years ago. These events appear to be related to a collision of the now-amalgamated Zimbabwe craton with northern Limpopo Province, as the Zimbabwe and Kaapvaal cratons collided. Interpretations of the Limpopo orogeny suggest that the Central Zone of the Limpopo Province collided with the Kaapvaal craton at circa 2.68 billion years ago, and that this orogenic collage collided with the southern part of the Zimbabwe craton at 2.58 billion years ago. Deposition of the Shamvaian Group clastic sediments occurred in a foreland basin related to this collision, and the intrusion of the Chilimanzi suite occurred when this foreland became thickened by collisional processes, and was cut by sinistral intracontinental strike-slip faults. Late folds in the Zimbabwe craton are oriented roughly parallel to the collision zone, and appear contemporaneous with this collision. The map pattern of the southern Zimbabwe craton shows some interference between folds of the early generation (related to the closure of the Sea of Umtali) and these late folds related to the Limpopo orogeny.

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