The geology and existing geodynamic models for the c. 3.3 Kromberg mafic–ultramafic sequence of rocks in the Barberton greenstone belt (South Africa) have been the subject of intense debate as, in addition to their regional significance, they have bearing also upon possible Paleoarchean tectonic processes on the early Earth. Models for the origin of the Kromberg sequence of rocks range from formation in an autochthonous, plume-type setting to that of an allochthonous, ophiolite thrust sheet emplaced in a modern-style plate-tectonic regime. This study reports new high-resolution field mapping results, drill core observations and secondary ion microprobe U-Pb detrital zircon age constraints across the Kromberg type-section on both the eastern and western sides of the Komati River. The new observations indicate that the Kromberg type-section represents a ca. 1.65 km thick dismembered and variably silicified mélange of oceanic supracrustal rocks with minor metadunites, which are structurally bound by chlorite-carbonate-quartz shear zones. Field observations and microstructural analysis allow for the rheological behaviour of the Paleoarchean oceanic lithosphere to be explored. It is proposed herein that the observed structural and volcanic geology is compatible with a Paleoarchean oceanic flake tectonic process, involving detachment along a brittle-ductile transition zone and horizontal accretion of thin, rigid, silicified slivers of upper Archean oceanic crust. These accreted allochtonous oceanic fragments would have delaminated from thick, weaker, lower oceanic crust that was recycled into the hot Paleoarchean mantle, and thus removed from the geological record. The model invoked here suggests that, although the well-preserved mafic–ultramafic Kromberg volcanic sequence may share some significant similarities with modern Phanerozoic ophiolites in the field, it most likely did not form by modern-style plate-tectonic processes. The new field observations herein support highly elevated oceanic geothermal gradients at the time due to a hotter, more vigorously convecting, Paleoarchean mantle and crustal geodynamic processes unique to the Paleoarchean era of Earth history in the absence of a global-scale, fully developed plate-tectonic regime.

Geological record of Paleoarchean oceanic flake tectonics preserved in the c. 3.3 Ga Kromberg volcanic type-section, Barberton greenstone belt, South Africa

Viola, Giulio;
2020

Abstract

The geology and existing geodynamic models for the c. 3.3 Kromberg mafic–ultramafic sequence of rocks in the Barberton greenstone belt (South Africa) have been the subject of intense debate as, in addition to their regional significance, they have bearing also upon possible Paleoarchean tectonic processes on the early Earth. Models for the origin of the Kromberg sequence of rocks range from formation in an autochthonous, plume-type setting to that of an allochthonous, ophiolite thrust sheet emplaced in a modern-style plate-tectonic regime. This study reports new high-resolution field mapping results, drill core observations and secondary ion microprobe U-Pb detrital zircon age constraints across the Kromberg type-section on both the eastern and western sides of the Komati River. The new observations indicate that the Kromberg type-section represents a ca. 1.65 km thick dismembered and variably silicified mélange of oceanic supracrustal rocks with minor metadunites, which are structurally bound by chlorite-carbonate-quartz shear zones. Field observations and microstructural analysis allow for the rheological behaviour of the Paleoarchean oceanic lithosphere to be explored. It is proposed herein that the observed structural and volcanic geology is compatible with a Paleoarchean oceanic flake tectonic process, involving detachment along a brittle-ductile transition zone and horizontal accretion of thin, rigid, silicified slivers of upper Archean oceanic crust. These accreted allochtonous oceanic fragments would have delaminated from thick, weaker, lower oceanic crust that was recycled into the hot Paleoarchean mantle, and thus removed from the geological record. The model invoked here suggests that, although the well-preserved mafic–ultramafic Kromberg volcanic sequence may share some significant similarities with modern Phanerozoic ophiolites in the field, it most likely did not form by modern-style plate-tectonic processes. The new field observations herein support highly elevated oceanic geothermal gradients at the time due to a hotter, more vigorously convecting, Paleoarchean mantle and crustal geodynamic processes unique to the Paleoarchean era of Earth history in the absence of a global-scale, fully developed plate-tectonic regime.
2020
Grosch, Eugene G.; Viola, Giulio; Ndlela, Sibusisiwe
File in questo prodotto:
Eventuali allegati, non sono esposti

I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.

Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11585/764933
 Attenzione

Attenzione! I dati visualizzati non sono stati sottoposti a validazione da parte dell'ateneo

Citazioni
  • ???jsp.display-item.citation.pmc??? ND
  • Scopus 3
  • ???jsp.display-item.citation.isi??? 3
social impact