On the kinematics and timing of Rodinia breakup: a possible rift–transform junction of Cryogenian age at the southwest cape of Congo Craton (northwest Namibia)

After tilt correction for Ediacaran thick-skinned folding, a pair of Cryogenian half grabens at the autochthonous southwest cape of Congo Craton (CC) in northwest Namibia restore to different orientations. Toekoms sub-basin trended east-northeast, parallel to Northern Zone (NZ) of Damara belt, and was bounded by a normal-sense growth fault (2 290 m throw) dipping 57° toward CC. Soutput sub-basin trended northwest, oblique to NZ and to north-northwest-trending Kaoko Belt. It was bounded by a growth fault (750 m down-dip throw) dipping steeply (~75°) toward CC. Soutput growth fault could be an oblique (splay) fault connecting a Cryogenian rift zone in NZ with a sinistral transform zone in Kaoko Belt. A transform origin for the Kaoko margin accords with its magma-poor abrupt shelf-to-basin change implying mechanical strength, unlike the magma-rich southern margin where a gradual shelf-to-basin change implies a mechanically weak extended margin. A rift−transform junction is kinematically compatible with observed north-northwest−south-southeast Cryogenian crustal stretching within CC. Post-rift subsidence of the CC carbonate platform varies strongly across the south-facing but not the west-facing shelf. A sheared western CC margin differs from existing Kaoko Belt models that posit orthogonal opening with hyper-extended continental crust. Carbonate-dominated sedimentation over southwest CC implies palaeolatitudes ≤35° between 770 and 600 Ma.

Evolution of the south-eastern hinterland in the South Atlantic Neoproterozoic Orogenic System – the Coastal Terrane of the Kaoko Belt (NW Namibia) revisited

<p>The Coastal Terrane of the Kaoko Belt in Namibia was originally defined as a Neoproterozoic arc terrane that originated outboard of the attenuated Congo Craton margin. Early (~650–630 Ma) igneous activity and high-grade metamorphism were interpreted as connected with subduction of the Adamastor Ocean and related arc magmatism. Protoliths of metasedimentary lithologies were interpreted as juvenile clastic sediments originating from the arc erosion. Later deformation (~580 Ma) was associated with lower amphibolite-facies conditions during thrusting over the Congo Craton margin.</p><p>Our research, however, suggests different evolutionary scenario.  The structurally lowermost part of the metasedimentary complex contains amphibolites and orthogneisses with U–Pb zircon ages between ~820–785 Ma, interpreted as metamorphosed syn-sedimentary bimodal volcanics. Detrital zircon ages from associated metamorphosed clastic sediments show identical patterns as observed in the metasedimentary cover of the underlying Congo Craton. Towards the structural hanging wall, the metasediments are devoid of metavolcanic rocks, and their detrital zircon age spectra are comparable with those from flysch sediments in the eastern, less metamorphosed parts of the Kaoko Belt.</p><p>The structurally lowermost part of the Coastal Terrane shows signs of partial melting broadly coeval with intrusion of ~650 Ma (U–Pb zircon) granitic–dioritic/gabbroic rocks. The temperature and depth of this migmatization event remains unconstrained, because the original mineral assemblages were overprinted during thrusting over the Congo Craton margin.</p><p>The thrusting period is characterized by solid-state reworking and partial retrogression of the migmatites in the lower part, and by pervasive metamorphism in the upper part, of the metasedimentary complex. Lu–Hf age (583 ± 2 Ma) of garnet from reworked migmatite shows that the garnet-bearing mineral assemblage represents conditions of thrusting, which were determined at ~660–670°C and 5.5–6 kbar. The ~580 Ma (and beyond) period of deformation started with development of flat-lying metamorphic fabric, later overprinted by folds with step axial planes, steep cleavage and isolated shear zones with general N–S to NNW–SSE trend. The associated intrusions show steep magmatic fabric, which transits into solid-state deformation in bodies close to the base of the Coastal Terrane.</p><p>Rather than an arc, the Coastal Terrane probably represents the inner part of an early Neoproterozoic rift. This interpretation is supported by the zircon provenance data and the presence and age of the bimodal volcanic rocks. The early, ~650–630 Ma magmatic activity and migmatitization coincides with the early period of rift inversion that took place along the western edge of the rift system in the Dom Feliciano Belt (Brazil and Uruguay). At this period, the former rift centre was established as the high-grade hinterland system of the developing Kaoko–Dom Feliciano–Gariep orogen. Inversion of the eastern rift edge started at ~580 Ma, as recorded in the Coastal Terrane, and continued up to ~550 Ma, which is the timing of the metamorphic peak in the Kaoko Belt foreland.</p><p>Financial support of the Czech Science Foundation (GACR 18-24281S) is appreciated.</p>

Early extensional detachments in a contractional orogen: coherent, map-scale, submarine slides (mass transport complexes) on the outer slope of an Ediacaran collisional foredeep, eastern Kaoko belt, Namibia

The existence of coherent, large-scale, submarine landslides on modern continental margins implies that their apparent rarity in ancient orogenic belts is due to non-recognition. Two map-scale, coherent, pre-orogenic, normal-sense detachment structures of Ediacaran age are present in the Kaoko belt, a well-exposed arc–continent collision zone in northwestern Namibia. The structures occur within the Otavi Group, a Neoproterozoic carbonate shelf succession. They are brittle structures, evident only through stratigraphic omissions of 400 m or more, that ramp down to the west with overall ramp angles of 1.1° and 1.3° with respect to stratigraphic horizons. The separations of matching footwall and hangingwall stratigraphic cut-offs require horizontal translations >20 km for each detachment. One of the detachments is remarkably narrow (5 km) in the up-dip direction, just one fourth of its translation. The other detachment is stratigraphically dated at the shelf–foredeep transition, when the passive margin was abortively subducted westward, in the direction of submarine sliding. Trenchward sliding on the foreslope occurred concurrently with deep karstification of the autochthonous carbonate succession to the east, presumably due to forebulge uplift and (or) conjectural basin-scale base-level fall. We expect that similar detachments exist in other orogenic belts, and failure to recognize them can lead to misinterpretations of stratigraphy, sedimentary facies, and paleogeography.