| Popis: |
The geological history of the northernmost part of the Mozambique Orogenic Belt, exposed in north-central Kenya, is described. A basement of ∼ 1200-Ma (Kibaran) migmatites is overlain by altered sediments with lateral and vertical facies changes defining eight lithostratigraphic groups. There is no evidence for any Archaean-early Proterozoic crust within the mapped part of the orogenic belt. A tectonothermal (Samburuan-Sabachian) event in the amphibolitegranulite facies at ∼ 820 Ma produced major recumbent folds with ductile thrusting to interleave the basement, metasedimentary cover and slices of oceanic metavolcanic complexes. There was syntectonic emplacement of crustal melt granites and metabasic dykes. We interpret this tectonothermal event to be related to plate collision, oblique to the north-south orogenic strike, between the Archaean Tanzanian craton in the west and an eastern Kibaran craton. A one-dimensional manifestation of the suture between these cratonic areas is possibly exposed at West Pokot in western Kenya. Post-collisional greenschist-amphibolite facies (Baragoian-Barsaloian) deformation at between 620 and 570 Ma produced regional upright folds and vertical ductile strike-slip shear zones striking subparallel to the orogenic strike. The metamorphism culminated during the intrusion of syntectonic granites. High-level open folding and brittle shears record the final events of the orogeny. Uplift and cooling, which extended beyond the confines of the orogen, are dated by ubiquitous mineral ages of ∼ 500-480 Ma. Whereas the roughly contemporaneous (Pan-African) Nubian Shield to the north preserves large segments of island arc/ophiolitic material, in the Mozambique Orogenic Belt this volcanic material was mostly consumed during the Upper Proterozoic oblique continent-continent collision. The roots of the collision zone are now exposed in the highgrade Mozambique Orogenic Belt and contrast with the higher exposure levels seen in the Nubian Shield. |
