The Coastal Terrane of the Kaoko Belt, Namibia: Outboard arc-terrane and tectonic significance

Abstract

The Coastal Terrane, or westernmost part of the Kaoko Belt outboard of the Three Palms Mylonite Zone, has distinct feldspathic arenite sedimentary sequences, no basement, distinct ɛ Nd sediment signatures excluding the Archaean and suggesting Mesoproterozoic–Neoproterozoic provenance sources, and primitive arc-like geochemical signatures for I-type granitoids. It contains evidence for an older metamorphic event at ∼650–645 Ma not present in any other part of the Kaoko Belt. This metamorphism (M1) was of high- T/low- P granulite to upper-amphibolite facies and includes migmatisation associated with I-type granitic magmatism of arc affinity. Arc growth at 650–640 Ma took place outboard within the Adamastor Ocean while Swakop Group facies turbidite sedimentation continued inboard along the passive margin. Overprinting M1 and M2 fabrics and metamorphic assemblages constrain the docking to have occurred between 650 and 580 Ma. Lack of ophiolite fragments and evidence of intermediate- to high- P metamorphism along the Three Palms Mylonite Zone suggest that it is not a suture and more likely to be part of an arc–backarc wrench–shear system that developed inboard of an E-dipping subduction system where the Adamastor Ocean was subducted beneath the leading edge of the attenuated Congo Craton. Oblique collision between 650 and 580 Ma, due to docking of this outboard Coastal Terrane with arc affinities, caused: (i) crustal scale oblique-overriding of the arc terrane over the passive margin of the Congo Craton; (ii) crustal scale sinistral shear partitioned into two major ductile shear zones; (iii) high- T granulite facies metamorphism in an extruded, shear zone-bounded core; and (iv) outwards- or cratonwards-verging basement-cored fold nappes.