The Evolution of the Damara Orogenic System: A Record of West Gondwana Assembly and Crustal Response

The Damara Orogenic System is a well-exposed orogenic junction that preserves a rich record of West Gondwana assembly and crustal processes in classic examples of transpression (Kaoko Belt) and bivergent collisional orogenesis (Damara Belt). Both belts show typical orogenic cycles in common with orogenic belts universally: from rifted passive margin sequences, subduction at continental margin arcs with back-arc basins, collision, crustal overthickening, outwedging of orogenic margins, detachment of subducted lithosphere, upper-plate lithospheric thinning and eventual collapse. There is no controversy here, and, like all orogens, kinematic and metamorphic response is dynamic and strongly zonal, the patterns of which are strong evidence for crustal architectures and tectonic history. Large relational data sets are required to characterize these patterns of orogenic response, to test robustness, accuracy and internal consistency, and to build crustal and tectonic models. For this summary of the Damara Orogenic System, two large-scale internally consistent relational data sets have been integrated: (1) age-calibrated deformation histories have been correlated across the whole system on the basis of stress fields and absolute age constraints in common; and (2) deformation structures have been correlated with mineral growth and patterns of metamorphic response characterized by P–T evolutions, metamorphic maps and field gradients, quantified using a large data set of PT determinations. Collision of the Rio De La Plata Craton at ~590 Ma resulted in west over east obduction of the Coastal Terrane arc over the Congo Craton passive margin. The Kaoko Belt subsequently evolved through ~45° clockwise rotation of the stress field, showing progressive transpressional orogenesis, steepening and strain partitioning, resulting in a strike-slip shear system between ~550 and 530 Ma. Collision in the Damara Belt at ~555–550 Ma involved subduction of the Kalahari Craton margin, with a highly attenuated Congo Craton passive margin in an upper-plate setting. The Kaoko and Damara orogenic fronts were both operating between 550 and 530 Ma, and with the same northwest-southeast shortening direction. At ~530–525 Ma a stress switch to north-northwest-south-southeast shortening resulted in transtensional reactivation of the Kaoko Belt shear zones, rapid exhumation and cooling, terminating orogenesis in this belt. At this time, main phase orogenesis, burial and metamorphism peaked in the Damara Belt, and subsequent contraction in this belt dominated the stress field, which evolved through a roughly 70° clockwise rotation to northeast-southwest shortening by ~512–508 Ma. Barrovian metamorphism in the southern orogenic margin was diachronous, 530–522 Ma in the north to 517–514 Ma in the south, and accompanied ongoing contraction. Deep burial to 9.5–11.5 kb followed by rapid isostatic readjustment gave successive foreland propagating exhumation events at <522 and <517 Ma, by southward transport of crustal wedges along basal thrusts. Outwedging was accommodated by top down to the north transport of hanging walls at higher structural levels, indicated by major metamorphic discontinuities and extensional structures, resulting in extensional telescoping of the southern margin. In contrast, medium-P/high-T granulite facies metamorphic conditions persisted in the orogen core from 540 to 505 Ma, following low ΔP/ΔT clockwise P–T paths indicating moderate burial and stable high-heat flow conditions, best explained by the detachment of subducted lithospheric during collision. A rapid stress switch to east–west directed shortening along the orogen at ~508 Ma generated crossfolding in the orogen core and northern margin. This stress field is inconsistent with any plausible trajectory between the Rio De La Plata, Congo and Kalahari Cratons and is interpreted as a far-field effect from the orogenic margin of Gondwana at that time: arc collisions in the Ross Orogen. This established a north–south extension direction exploited by 508–504 Ma decompression melts in many parts of the system, and at ~505–500 Ma triggered gravitational collapse and extension of the thermally weakened orogen core, resulting in a broad bivergent core complex, rapid exhumation and cooling from 700 to 400 ℃ between 500 and 470 Ma.