Deformation and fluid flow during orogeny at the palaeo-Pacific active margin of Gondwana: the Early Palaeozoic Robertson Bay accretionary complex (north Victoria Land, Antarctica)

Abstract

Structural investigations, integrated with X-ray diffraction, fluid inclusion microthermometry and oxygen-stable isotope analyses are used to reconstruct the deformation history and the palaeo-fluid circulation during formation of the low-grade, turbidite-dominated Early Palaeozoic Robertson Bay accretionary complex of north Victoria Land (Antarctica). Evidence for progressive deformation is elucidated by analysing the textural fabric of chronologically distinct, thrust-related quartz vein generations, incrementally developed during progressive shortening and thickening of the Robertson Bay accretionary complex. Our data attest that orogenic deformation was mainly controlled by dissolution–precipitation creep, modulated by stress- and strain-rate-dependent fluid pressure cycling, associated with local and regional permeability variations induced by the distribution and evolution of the fracture network during regional thrusting. Fracture-related fluid pathways constituted efficient conduits for episodic fluid flow. The dominant migrating fluid was pre-to-syn-folding and associated with the migration of warm (160–200 °C) nitrogen- and carbonic (CO₂ and CH₄)-bearing fluids. Both fluid advection and diffusive mass transfer are recognized as operative mechanisms for fluid–rock interaction and vein formation during continuous shortening. In particular, fluid–rock interaction was the consequence of dissolution–precipitation creep assisted by tectonically driven cooling fluids moving through the rock section as a result of seismic pumping. The most likely source of the migrating fluids would be the frontal part of the growing accretionary complex, where fluids from the deep levels in the hinterland are driven trough channelization operated by the thrust-related fracture (fault) systems.