FLUID REGIME IN FAULTING DEFORMATION OF THE WARATAH FAULT ZONE, AUSTRALIA, AS INFERRED FROM MAJOR AND MINOR ELEMENT ANALYSES AND STABLE ISOTOPIC SIGNATURES

Geochemical studies have been conducted on the Waratah Fault Zone, a s teep, NE-SW-trending brittle fault zone in the southeastern Lachlan Fo ld 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 faul ted Liptrap Formation and faulted limestones, and a fault core compose d of a fault-melange zone and strongly brecciated limestones. The faul t zone records extensive mineralogical and stable isotope alteration w ithin the deformed turbidites of the Liptrap Formation and within the limestone sequence. For the Liptrap Formation a volume loss of similar to 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 seque nce, are at least an order of magnitude lower than in the fault-melang e zone. Stable isotopic and rare-earth elemental data document the inf iltration of near-surface meteoric water, during brittle deformation a nd alteration of the fault zone. The interaction of fluids with contra sting lithologies led to different alteration processes affecting faul t weakening and/or strengthening. The cyclic change between large-scal e fluid infiltration due to brittle failure (open system) and fault ro ck cementation (closed system) may represent a combined conduit-barrie r system in the limestone sequence. Within the Liptrap Formation, no c ementation 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 infiltrat ion dissolved silica and altered detrital feldspar, resulting in subse quent porosity increase within the fault-melange zone. Our results are consistent with investigations in modem accretionary complexes, where major decollements and subsidiary faults have been shown to be import ant fluid conduits. (C) 1998 Elsevier Science B.V. All rights reserved .