Fluid regime in faulting deformation of the Waratah Fault Zone, Australia, as inferred from major and minor element analyses and stable isotopic signatures

Abstract

Geochemical studies have been conducted on the Waratah Fault Zone, a steep, NE–SW-trending brittle fault zone in the southeastern Lachlan Fold Belt, Australia. Strike-slip movement along the fault juxtaposed an Early Devonian turbidite sequence (Liptrap Formation) against heavily veined Early Devonian limestones. The fault zone consists of the faulted Liptrap Formation and faulted limestones, and a fault core composed of a fault-melange zone and strongly brecciated limestones. The fault zone records extensive mineralogical and stable isotope alteration within the deformed turbidites of the Liptrap Formation and within the limestone sequence. For the Liptrap Formation a volume loss of ∼28% was calculated for the transition from unaltered sediments to the fault-melange zone. The volume loss in the limestone gouge is much higher (60%). The calculated fluid/rock ratios in the limestone sequence, are at least an order of magnitude lower than in the fault-melange zone. Stable isotopic and rare-earth elemental data document the infiltration of near-surface meteoric water, during brittle deformation and alteration of the fault zone. The interaction of fluids with contrasting lithologies led to different alteration processes affecting fault weakening and/or strengthening. The cyclic change between large-scale fluid infiltration due to brittle failure (open system) and fault rock cementation (closed system) may represent a combined conduit–barrier system in the limestone sequence. Within the Liptrap Formation, no cementation took place, as the conditions for quartz precipitation were unsuitable. Hence, the fault system remains open in respect to fluids (conduit system, only). Additionally, the `permanent' fluid infiltration dissolved silica and altered detrital feldspar, resulting in subsequent porosity increase within the fault-melange zone. Our results are consistent with investigations in modern accretionary complexes, where major decollements and subsidiary faults have been shown to be important fluid conduits.