Mesoarchean assembly and stabilization of the eastern Kaapvaal craton: A structural-thermochronological perspective

Understanding the construction and stabilization of Archean continental lithosphere has important implications for models of tectonic and thermal regimes in the early Earth, as well as for the subsequent evolution of continents. We provide new constraints for the amalgamation and stabilization of the eastern Kaapvaal craton in the vicinity of the Barberton greenstone belt circa 3.3-3.1 Ga. Isotope dilution thermal ionization mass spectrometry U-Pb geochronology and thermochronology are combined with mapping and structural analysis around the margins of this belt in order to constrain movement on major shear zones that separate high-grade orthogneiss terranes from low-grade supracrustal rocks and to relate this movement to the timing of differential exhumation. New and existing data are consistent with accretion at ∼3.23 Ga within an oblique subduction plate boundary manifested in the BGB as an asymmetric flower-like structural geometry. For >100 Ma following terrane assembly, transform boundary tectonics, manifested partially as transtension on reactivated faults, led to large-scale juxtaposition of middle to lower crustal basement orthogneiss complexes against upper crustal greenstone sequences and episodic emplacement of granitic batholiths. Stabilization of this portion of Archean lithosphere involved at least a three-stage process: (1) creation of a thick and rigid mantle lithosphere during crustal growth, subduction, and terrane assembly from circa 3.30 to 3.23 Ga, (2) generation of a rigid crust during strike-slip and transtensional tectonics coupled with migration of granites and heat-producing elements into the upper crust between circa 3.2 and 3.1 Ga, and (3) erosion and removal of the uppermost crust during regional peneplanation that lasted until circa 2.9 Ga.