Structure, mineralisation and fluid flow in the Moonlight Fault Zone: Fan Creek, Queenstown

Menzies, Tristan J

The Moonlight Fault Zone (MFZ) is a large, regionally significant fault in the South Island of New Zealand that acted as a basin?bounding normal fault in the Oligocene and was subsequently reactivated as a high angle reverse fault in the late Miocene. The fault zone is well exposed in and around Fan Creek, near Queenstown. Using detailed field and microstructural observations, this research dissertation documents the structure and fault rock assemblages found along the MFZ in the Fan Creek area, and draws conclusions regarding the exhumation history of the fault zone, deformation processes within the fault rocks, and potential fault weakening mechanisms active during high angle reverse movements. The exposures in Fan Creek consist of four lower greenschist facies metamorphic host rock lithologies and an infaulted package of Oligocene sediments (sandstones and conglomerates). Textural zone IV (Torlesse terrane) phyllites in the hanging wall are juxtaposed against the infaulted Oligocene sediments or textural zone IIB (Caples terrane) phyllites and schists in the footwall. Deformation in the hanging wall is concentrated within foliation?parallel fault breccias that show evidence of multiple cycles of seismic slip and cataclasis alternating with restrengthening and dissolution?precipitation creep processes. The main fault contact is defined by phyllosilicate?rich cataclasites containing a well?defined foliation of aligned and interconnected, platy muscovite and chlorite grains at the schist?sediment faulted contact, and a discontinuous layer of relatively impermeable red fault gouge at the schist?schist faulted contact. Emanating from the main fault plane and tapering away into the immediate hanging wall are large quartz veins, interpreted as the result of build?ups of fluid overpressure in the footwall. Secondary fault strands, separating the metamorphic footwall lithologies, contain complex breccia and vein networks, including quartz veins associated with sulphide mineralization. Brittle shears, some containing green, foliated cataclasites are present in the footwall schists. Results suggest that both elevated fluid pressures and frictionally weak, interconnected phyllosilicates along the main fault trace facilitated Miocene?Recent high angle reverse movements of up to c. 8 km across the MFZ. These processes combined to lower the shear strength of the fault resulting in episodic rupture, the release of fluids into the hanging wall and the onset of fluid?mediated diffusive mass transfer (dissolution-precipitation) processes.

xii, 120 pages A4, some A3

2014Menzies

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