Abstract

Mesoproterozoic crustal evolution of southwestern Africa involved a complex interplay over 600 million years of subduction, plate-margin or intraplate sedimentation, volcanism and plutonism followed by terrane amalgamation, tectonism, collision, magmatism, metamorphism and final, late-stage isostatic rebound of high-grade terranes. Most terranes are underlain by ~2.0 Ga continental crust but the Areachap and Garies Terranes were juvenile Mesoproterozoic features with respective model ages of ~1.5 to ~1.3 Ga and ~1.25 Ga. Pre- and syntectonic metasediments in northwestern Namibia and in the Namaquan terranes are mid to late Mesoproterozoic in age. Volcanism or plutonism accompanied sedimentation in places. A southward younging of terrane accretion is apparent from about ~1.37 Ga in northwestern Namibia, to ~1.24 Ga(?) in the region of the central Damara Orogen, <1.33 Ga in the Konkiep Subprovince, the latter on the west-dipping Namaqua Front, ~1.2 Ga (O’okiepian episode) in the Kaaien Terrane, to between ~1.11 and ~1.07 Ga in the Bushmanland and Garies Terranes. Widespread intrusion of granite into the base of the crust accompanied collision in northwestern Namibia. Syntectonic granite magmatism was extensive and voluminous during O’okiepian collision in the Gordonia Subprovince. Thereafter, local intermittent magmatism and sedimentation continued in the Rehoboth and Konkiep Subprovinces and the Kaaien, Bushmanland and Garies Terranes. In most regions from the Rehoboth Subprovince to the Garies Terrane there was an ~1.1 Ga period of magmatism and/or metamorphism which Hanson et al. (2004, 2006) relate to plume activity. Redbed successions post-date the youngest magmatic rocks in the Rehoboth and Konkiep Subprovinces. The Bushmanland and Garies Terranes appear to have been subjected to intense deformation, recumbent folding and thrusting between ~1.11 and ~1.07 Ga. Open, upright folding thereafter in these two terranes was accompanied by intrusions of the Spektakel and Koperberg Suites and granulite-facies metamorphism between ~1.06 and ~1.02 Ga. Pegmatites mark the final magmatic event at about 1000 Ma. The high-grade rocks of the Gordonia Subprovince were juxtaposed against low-grade terranes to the north and south during isostatic rebound on major, late-stage, dextral shear zones at or near the margins of the subprovince. Cooling to 300 to 350°C had taken place by 850 to 800 Ma.

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