Mid-crustal geodynamics of the southern central zone, Damara Orogen, Namibia

Abstract

Two areas of well exposed mid-crustal structures in the axial zone of the Pan-African Damaran orogenic belt show that basement has formed domes which have amoeboid forms on the scale of tens of km with steep sided overturned non-planar, non- cylindrical geometry. These are surrounded by open to tight synclinal cover envelopes that converge at depressions between the domes. The domes are found in association with a strong regional WSW moderately plunging lineation. Strain analysis demonstrates that domes have formed in a moderately plunging constrictional field. Structural features which normally indicate polyphase evolution such as mesoscale fold interference patterns are rare and inconsistent. Regional structural form, described morphogically by cylindrical domains, is defined by one fabric S0/S1. Secondary fabric trajectories and mesoscale fold oreintations are controlled by domain scale structure and not regional deformation trends. Dome formation is thus interpreted as being the result of a simultaneous flow and buckling episode within the middle crust. Deformation was extremely ductile. P-T estimates from thermobarometry with Grt-Bt, Grt-Crd-Bt-Sil assemblages indicate that peak metamorphic conditions during deformation were approximately 3.5kbar and 650°C in the cover envelopes and 7kbar 791°C in the basement domes, approaching the amphibolite-granulite transition. Interactive transpressional collision between three cratons during the latest Neoproterozoic-early Paleozoic: namely the Kalahari, Congo and Rio de la Plata caused constriction and extrusion in the Central Zone. The metamorphic gap between basement and cover occurred when the distance between regional isotherms was reduced by thinning at the basement-cover interface. Higher temperatures where preserved at dome cores. At the cooler margins of the Damara Belt thrust tectonics occurred, albeit obliquely with sinistral transpression. After dome formation granite intruded many domal structures.