The Neoarchaean Limpopo Orogeny: Exhumation and Regional-Scale Gravitational Crustal Overturn Driven by a Granulite Diapir

Abstract

Integrated geological, geochronological and geophysical data from the Neoarchaean Limpopo Complex (LC) and adjacent granite-greenstone terrane of the Northern Kaapvaal Craton (NKVC) demonstrates the following three key features. (1) The LC-NKVC tectonic boundary defined by the Hout River Shear Zone (HRSZ) played a fundamental role in establishing high-grade metamorphic domains in the Southern Marginal Zone (SMZ) of the LC. These domains are characterized by their specific retrograde P-T evolution, (2) Near-isobaric southward thrusting of a hot allochthonous SMZ nappe with imbedded steeply plunging folds and steep shear zones against and over the NKVC along the HRSZ before ca. 2.68 Ga occurred as a consequence of steep exhumation within the Central Zone (CZ) at 2.72–2.62 Ga. (3) Initial exhumation of the CZ to mid-crustal levels before ca. 2.68 Ga was associated with isoclinal folding and melt-weakened domes and directed the southwards channelling of the hot nappe. Final exhumation of the CZ to the upper crust at 2.65–2.62 Ga was driven by granitic diapirism and the formation of shear-related, mega-scale closed structures. These features favour an intracrustal Neoarchaean Limpopo orogeny as the result of a granulite diapir triggered by mantle heat-fluid flow underneath the CZ at 2.72–2.62 Ga. The data is neither compatible with continent collisional models nor with an Archean accretionary orogen along the northern edge of the Kaapvaal Craton that is linked to the collision of the Pietersburg Block with the CZ.