Deformation correlations,stress field switches and evolution of an orogenic intersection:The Pan-African Kaoko-Damara orogenic junction,Namibia

摘要

Age calibrated deformation histories established by detailed mapping and dating of key magmatic time markers are correlated across all tectono-metamorphic provinces in the Damara Orogenic System. Cor-relations across structural belts result in an internally consistent deformation framework with evidence of stress field rotations with similar timing, and switches between different deformation events. Hori-zontal principle compressive stress rotated clockwise ~180°in total during Kaoko Belt evolution, and ~135°during Damara Belt evolution. At most stages, stress field variation is progressive and can be attributed to events within the Damara Orogenic System,caused by changes in relative trajectories of the interacting Rio De La Plata, Congo, and Kalahari Cratons. Kaokoan orogenesis occurred earliest and evolved from collision and obduction at ~590 Ma, involving E-W directed shortening, progressing through different transpressional states with ~45°rotation of the stress field to strike-slip shear under NW-SE shortening at ~550-530 Ma.Damaran orogenesis evolved from collision at ~555-550 Ma with NW-SE directed shortening in common with the Kaoko Belt, and subsequently evolved through ~90° rotation of the stress field to NE-SW shortening at ~512-508 Ma. Both Kaoko and Damara orogenic fronts were operating at the same time,with all three cratons being coaxially convergent during the 550-530 Ma period;Rio De La Plata directed SE against the Congo Craton margin,and both together over-riding the Kalahari Craton margin also towards the SE. Progressive stress field rotation was punc-tuated by rapid and significant switches at ~530-525 Ma, ~508 Ma and ~505 Ma.These three events included:(1)Culmination of main phase orogenesis in the Damara Belt,coinciding with maximum burial and peak metamorphism at 530-525 Ma.This occurred at the same time as termination of transpression and initiation of transtensional reactivation of shear zones in the Kaoko Belt.Principle compressive stress switched from NW-SE to NNW-SSE shortening in both Kaoko and Damara Belts at this time.This marks the start of Congo-Kalahari stress field overwhelming the waning Rio De La Plata-Congo stress field,and from this time forward contraction across the Damara Belt generated the stress field governing subse-quent low-strain events in the Kaoko Belt.(2)A sudden switch to E-W directed shortening at ~508 Ma is interpreted as a far-field effect imposed on the Damara Orogenic System, most plausibly from arc obduction along the orogenic margin of Gondwana (Ross-Delamerian Orogen). (3) This imposed stress field established a N-S extension direction exploited by decompression melts, switch to vertical shortening, and triggered gravitational collapse and extension of the thermally weakened hot orogen core at ~505 Ma,producing an extensional metamorphic core complex across the Central Zone.