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

Abstract

[1] 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 and 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 d 18 O 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 d 18 O fluids. Repeated switching of fractures between highpermeability 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 d 18 O 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 d 18 O over an area of 20 km 2 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.

The role of permeability evolution in fault zones on the structural and hydro-mechanical characteristics of shortening basins

“…Hydraulic fracturing and shear fracturing exert different effects on the formation of fracture networks and the interplay of the mechanisms is complex (Barnhoorn et al, 2010;Cox, 2007Cox, , 2010. In the case of fluid pressure driven fractures, mineral precipitation can also greatly affect hydrologic properties which further complicates the relation (Min et al, 2009).…”

Tectono-thermal evolution of the Palaeoproterozoic Granites–Tanami Orogen, North Australian Craton: Implications from hornblende and biotite 40Ar/39Ar geochronology

“…Added to this is the complexity of mineralisation associated with permeable tectonic structures such faults, shear-zones or fold hinges (e.g. Cox, 2005Cox, , 2007. Such tectonic fabrics commonly have a multistage history of deformation and fluid flow.…”

Post-metamorphic palaeofluid evolution of the Mecsekalja Zone metamorphic complex

“…The Cal EB 1 calcites display a range of stable-isotope compositions too wide to be interpreted as inherited from and determined by a homogenous source with which the fluid reached equilibrium. Oxygen and carbon isotope data of Cox (2007) indicate that externally derived fluids introduced into a developing fracture system may lead to variable isotopic compositions of fluids because of variable buffering by host rock, as also described by Lassey and Blattner (1988). This process can lead to significantly different isotopic compositions within spatially and temporally related veins.…”

“…advective, fracture channelized or pervasive), the fluids role in the activation of the flow path, the stress field's role in the activation, etc. Some exemplary papers that set a similar target to the above approach are those of Rye and Bradbury (1988), Fisher and Brantley (1992), Hilgers and Sindern (2005), Barker et al, (2006Barker et al, ( , 2009, Cox (2007).…”

“…Applications include assessment of fluid-rock interaction (Hilgers and Sindern, 2005) and the evolution of mineralising fluids during fluidrock interaction (Cox, 2007). Bottomley and Veizer (1992) and Rye and Bradbury (1988) assessed recrystallisation of a pre-existing vein system and fluid-rock interaction during nappe stacking, respectively.…”