A thermotectonic framework for the growth and stabilization of the eastern Kaapvaal craton, southern Africa

Abstract

Continents are an amalgamation of crust and mantle lithosphere assembled over -4 Gyr and are therefore our best record of the early Earth. Exposures of rocks -3.0-3.7 Ga cover -20,000 km2 of eastern S. Africa and Swaziland, and provide a record of the continental assembly and subsequent stabilization of the eastern Kaapvaal craton. This thesis uses structural, geochronological, thermochronological and isotopic constraints to examine the tectonothermal processes responsible for the growth and stabilization of this portion of Mesoarchean lithosphere. Field mapping was focused on terrane-bounding shear zones and syntectonic plutons, and in combination with ID-TIMS U-Pb zircon geochronology and Sm-Nd analysis, places sub-Myr constraints on the timing, distribution, and kinematics of magmatism and deformation during growth and modification of continental lithosphere. Detailed U-Pb apatite and titanite thermochronological datasets are used in combination with finite difference numerical modeling to determine non-linear temperature-time paths for rocks between -650-300 °C from 3.45-3.08 Ga - providing a sensitive indicator of tectonic and magmatic processes in the middle to lower crust.

(cont.) From 3.2 to 3.3 Ga, multiple microcontinental fragments with distinct age and Nd isotopic characteristics were assembled along an oblique subduction zone boundary, with the Barberton Greenstone belt representing a lithospheric suture zone. During continental assembly and orogeny, strain was partitioned into 3236-3220 Ma syntectonic plutons and terrane-bounding transcurrent shear zones bordering the margins of the previously stabilized ca. 3.66-3.45 Ga Ancient Gneiss Complex. Subsequent 3.2-3.1 Ga reactivation of crustal anisotropies within the lithospheric suture zone - represented broadly by the Barberton Greenstone Belt - controlled the thermal and compositional reorganization of the crust through transtensional tectonics, exhumation of deep-crustal gneiss terranes, and upward migration of granitic batholiths. This final period of crustal modification was responsible for juxtaposing low-grade greenstone supracrustal rocks against middle- to lower-crustal gneiss terranes, and ultimately led to crustal stabilization in the greenstone belt and vicinity. These results support a model in which the stabilization of the Kaapvaal craton was a piece-wise transition resulting from lithospheric thickening and crustal thinning over, hundreds of Myr.