The metamorphic architecture of a transpressional orogen: the Kaoko Belt, Namibia

Abstract

The Kaoko Belt in Namibia represents the deeply eroded core of a classic sinistral transpressional orogen with a half flower structure centred on the crustal-scale Purros Mylonite Zone. The Kaoko Belt consists of three NW-trending structural zones each with distinct kinemetamorphic style. The Eastern Kaoko Zone contains upright-folded, Neoproterozoic Damara Sequence shelf carbonates. The Central Kaoko Zone comprises an inverted Barrovian metamorphic series within large-scale east-vergent nappes, whereas the Western Kaoko Zone is predominantly of high metamorphic grade and intruded by numerous granitoids. The Western Kaoko Zone has orogen-parallel panels of distinctly different metamorphic grade separated by strike-slip ductile shear zones with overall isograd pattern being indicative of extrusional tectonics in the orogen core. The Kaoko Belt evolved through three distinct phases of a protracted Pan-African Orogeny in the late Neoproterozoic to Cambrian: (1) an early Thermal Phase (early M2) was responsible for pervasive partial melting, high-grade parageneses and granite emplacement between 580 and 570 Ma; (2) the main deformation Transpressional Phase (580–550 Ma) reworked early M2 parageneses in the pervasive orogenic fabric producing M2 assemblages that formed as a result of progressive sinistral transpression that evolved from wrench-style to high-angle convergence accompanying foreland-vergent thrusts and nappes; (3) the post-transpression Shortening Phase generated upright, open folds during north–south shortening (530–510 Ma). In the Western Kaoko Zone, peak metamorphic conditions were attained during early M2 at moderate to high average thermal gradients (29–40°C/km) and were intensely reworked by lower-grade pervasive fabrics during M2. (Average thermal gradient is simply the calculated metamorphic temperature divided by the calculated depth assuming a density of 2•8 g/cm3. It should not be confused with the instantaneous thermal gradients in the vicinity that an assemblage formed, or imply that the thermal gradients are time equivalent.) In the northern part of the Western Kaoko Zone, immediately adjacent to the Purros Mylonite Zone, the amphibolite-grade Khumib Terrane experienced peak M2 metamorphism at 573°C and 5•4 kbar. Along strike to the south the granulite-grade Hoarusib Terrane experienced peak early M2 conditions at 843°C and 8•1 kbar and M2 reworking at approximately 560–580°C and 4•8 kbar. In the western margin of the orogen, the Coastal Terrane experienced early M2 metamorphism at sillimanite–K-feldspar–melt grades and was reworked during M2 at muscovite–biotite grade. In the Central Kaoko Zone, metamorphic grade increases towards the west to higher structural levels. Peak metamorphic matrix assemblages formed during pervasive deformation in the Transpressional Phase (M2) at conditions in the range of 530–690°C and 8•5–9•0 kbar with consistently low average thermal gradients (17–23°C/km). Clockwise P–T paths were experienced in both the Central Kaoko Zone and Western Kaoko Zone. Garnet Sm–Nd geochronology indicates that matrix parageneses, early M2 in the Western Kaoko Zone and M2 in the Central Kaoko Zone, formed at the same time within uncertainties (576 ± 15 Ma). This indicates that the thermal peak was contemporaneous across the belt, even though deformational phases of equivalent structural style were diachronous across the Kaoko Belt.