Extrusional tectonics in the core of a transpressional orogen; the Kaoko Belt, Namibia

Abstract

The Pan-African Kaoko Belt of NW Namibia provides a well-exposed example of material flow in the evolving middle and lower crust during sinistral transpressional orogenesis. The Kaoko Belt is a composite metamorphic belt with shear zone bounded zones of contrasting metamorphic style that were metamorphosed at approximately the same time (575–550 Ma). It is divided into a Barrovian-style Escape Zone with inclined nappes and thrusts that verge outward towards the foreland, a steep high-grade Orogen Core that experienced intense wrench-style deformation and, in the hinterland, an older (650 Ma) exotic Coastal Terrane that was reworked at moderate-T/low-P conditions during transpressional orogenesis. The kinematic-lineation array (defined by stretching lineations and shear-sense indicators) shows variation in degree and polarity of lineation obliquity to the grain of the belt, lineation plunge and different polarity of vergence along the plunge, with respect to the overall sinistral shear sense. The gross regional lineation pattern defines an arcuate array from near orogen-parallel in the Orogen Core to higher-angle obliquity across the Escape Zone, reflecting lateral escape towards the orogen margin. Two high-grade lobes within the Orogen Core are coincident with oblique-inclined, upward flow trajectories based on the kinematic-lineation array, and represent oblique extrusion of lower-crustal material into middle-crustal levels. An intervening low-grade metamorphic ‘trough’ within the Orogen Core and the Coastal Terrane coincides with acute oblique downward and outboard-directed apparent flow trajectories. These kinematic patterns are consistent with barometric constraints for peak metamorphism of 5·2 kbar and 4·5 kbar in the low-grade ‘trough’ and Coastal Terrane and 8·0 kbar in the two high-grade lobes. Integration of the kinematic and metamorphic datasets shows that oblique, shallowly inclined extrusion trajectories occurred within the Orogen Core of this classic transpressional orogen, and that stretching lineations approximately reflect the particle paths experienced. Across-orogen metamorphic gradients indicate a marked contrast between the Escape Zone and Orogen Core. The Escape Zone experienced an inverted Barrovian-style high-P/moderate-T metamorphism and low thermal gradients of 20°C/km. The Orogen Core experienced moderate-P/high-T metamorphism and high thermal gradients of 30–40°C/km. Clockwise P–T paths were experienced in all parts of the Kaoko Belt. Tight and shallow conduction-dominated P–T paths were experienced in all Orogen Core domains, involving heating-dominated prograde paths followed by decompression, consistent with extrusional tectonics. Steeper and more open advection-dominated P–T paths were experienced in the crustal over-thrust Escape Zone, which experienced low thermal gradients.