Australian Digital Theses Program

Thesis Details
Title	Dynamics of fluid flow and fluid chemistry during crustal shortening
Author	Barker, Shaun
Institution	The Australian National University
Date	2007
Abstract	In this thesis, an integrated structural and chemical approach has been used to investigate the spatial and temporal evolution of fluid chemistry, and fluid flow pathways, during crustal shortening. The Taemas Vein Swarm is hosted in a limestone-shale sequence, the Murrumbidgee Group, in the Eastern Belt of the Lachlan Orogen, in New South Wales, Australia. The Taemas Vein Swarm (TVS) is composed of calcite ± quartz veins, hosted in a series of faults and fractures, which extends over an area of approximately 20 km2. The Murrumbidgee Group is composed of several formations, comprising massive grey micritic limestones, redbed sandstones and shales,and thinly interbedded (10–20 cm scale) limestones and shales. ¶ The sedimentary sequence has been folded into a series of upright, open to close folds, and was probably deformed during either mid-late Devonian, or early Carboniferous, crustal shortening. To the east, the Murrumbidgee Group is overthrust by a Silurian volcanic sedimentary sequence along the Deakin-Warroo Fault System. Crosscutting and overprinting relationships demonstrate that vein growth was synchronous with folding, with different vein types related to different fold mechanisms at various stages of fold growth. ¶ Flexural slip folding led to the development of bedding-concordant veins (hereafter called bedding-parallel veins). Flexural flow in semicompetent to incompetent beds caused en echelon extension vein arrays to grow. Decoupling between beds, and dilatancy at fold hinges led to significant vein growth. In addition, fold lock-up led to limb-parallel stretching, and the growth of bedding-orthogonal extension fractures. ¶ Vein growth is inferred to have occurred in a compressional tectonic regime (i.e. sigma3=vertical). Oxygen isotope quartz-calcite thermometry suggests that veins formed at temperatures of 100–200 oC. The depth of vein formation may have been between about 5 and 8 km. Vein textures indicate that growth of veins occurred during multiple cycles of permeability enhancement and destruction. Subhorizontal extension fractures, and faults at unfavourable angles for reactivation, imply that fluid pressures exceeded lithostatic levels during the growth of some veins. Coexisting extension and shear fractures imply that differential stress levels varied over time. ¶ Flexural slip continued throughout folding at Taemas, despite some fold limbs being at angles extremely unfavourable for reactivation ( > 60). As folds approached frictional lock-up, flexural slip continued to occur when supralithostatic fluid pressures were developed. Therefore, large, bedding-discordant faults were not developed to accommodate strain during folding, explaining a deficiency of larger faults in the TVS. ¶ Infiltration of overpressured fluids occurred into the base of the Murrumbidgee Group, and was channelled into a distributed mesh of small faults and fractures. At the point that a connected ‘backbone’ flow network developed in the TVS, highpressure fluids would no longer be available to allow continuing flexural slip on fold limbs approaching lockup. Thereafter, larger faults would develop, which would adjust the fault population in the TVS to a more ‘typical’ displacement-frequency distribution. This had not occurred in the Taemas area by the time crustal shortening ceased. An abundance of small faults, and fracturing driven by invasion of overpressured fluid, implies that the TVS formed via an ‘earthquake swarm’ process. ¶ Modern analytical techniques, utilising laser ablation sampling technology, allow high-spatial resolution chemical data to be collected from syntectonic veins. Insights into the role that fluid-mineral interface processes may have on the chemistry of minerals grown in syntectonic veins were provided by an experimental study. Moderate sized ( < 1−5 mm) synthetic calcite crystals were successfully grown to investigate the uptake of rare earth elements (REE) into calcite. Changes in crystal morphology are linked to variable solution chemistry, which has important implications for the interpretation of hydrothermal vein textures. High-spatial resolution chemical analyses of synthetic calcite crystals demonstrate significant fluctuations in REE concentrations over distances of < 200 μm within calcite crystals. Time-equivalent regions on different crystal faces have significantly different REE concentrations, indicating that fluctuations in calcite trace element composition cannot be interpreted exclusively in terms of changing ‘bulk fluid’ composition. Rare earth element anomalies (Eu/Eu* and Ce/Ce*) are not significantly influenced by compositional zoning, and may be robust indicators of changes in solution bulk chemistry and fluid oxidation state. ¶ Changes in isotopic ratios (13C, 18O and 87Sr/86Sr), and trace element concentrations in veins from the TVS are related to variations in fluid source, flow pathways and chemical conditions (e.g. trace element complexation, precipitation rate, fluid oxidation) during hydrothermal fluid flow. This integrated structural, textural and chemical approach has direct application to the examination of hydrothermal veins in fracture-hosted ore deposits, and may allow the fluid source and/or chemical conditions conducive to the formation of high-grade ore to be discerned. ¶ Vein 18O compositions systematically increase upwards through the Murrumbidgee Group, caused by progressive reaction of an externally derived, low-18O fluid (of probable meteoric origin) with host limestones. Vein 18O and 87Sr/86Sr compositions vary spatially and temporally within the same outcrop, and within individual veins, which is inferred to be caused by the ascent of packages of fluid along constantly changing flow pathways. Fluid-buffered oxygen isotope ratios at the earliest stages of deformation imply that the TVS formed via an ‘invasion percolation’ process. Fluid pathways are inferred to have changed constantly, with fractures ‘toggleswitching’ between high-permeability and low-permeability states, due to repeated fracture opening and sealing events.
Thesis	01front.pdf 272.6 Kb 02whole.pdf 57119.5 Kb 03Begining-Ch2.pdf 1472.8 Kb 04Chapter3.pdf 7022.3 Kb 05Chapter4.pdf 34930.4 Kb 06Chapter5.pdf 3535.5 Kb 07Chapter6.pdf 12429.9 Kb 08Chapter7-End.pdf 592.4 Kb