Structural and isotopic constraints on fluid flow regimes and fluid pathways during upper crustal deformation: An example from the Taemas area of the Lachlan Orogen, SE Australia

Structural and stable isotope studies of calcite veins and host rocks in a 1000 m thick, Devonian limestone sequence in the Taemas area of the Lachlan Orogen in SE Australia indicate that externally derived fluids migrated through the sequence over a protracted interval during folding and associated reverse faulting at temperatures of ∼200°C. The evolution of fluid pathways was governed by growth of fold-related an fault-related fracture networks at transiently supralithostatic fluid pressures. The internal structures of veins indicate growth during repeated cycles of permeability enhancement and sealing. Systematic increase in δ¹⁸O of vein calcite upward through the limestone sequence, and the presence of an O isotope alteration front in the vein system, is related to buffering of fluid compositions by fluid-rock reaction during upward migration of overpressured, low δ¹⁸O fluids. Repeated switching of fractures between high-permeability and low-permeability states promoted episodic flow and changes in flow paths during progressive deformation. A lack of significant migration of the isotopic front during the flow history is consistent with fluid-rock interaction being associated with numerous, separate pulses of fluid ascending through the percolation network. Irregular variations in fluid δ¹⁸O with time during vein growth are interpreted to be driven by repeated changes in reactive path lengths, reaction rates, and flow rates in a discontinuous flow regime. Systematic upward changes in vein δ¹⁸O over an area of 20 km² requires that on a time-averaged basis, most of the vein system was hydraulically well connected to the external fluid reservoir and that growth of the fracture network occurred by fluid-driven, invasion percolation processes. Repeated failure and sealing events in the fracture system are interpreted to have been driven by episodic migration of fluid pressure waves up through the fracture network, possibly in response to deeper level fault rupture events repeatedly breaching an overpressured fluid reservoir.