Geology, Quaternary structure, fault rocks and fluid flow, Fox Peak Range, Eastern Southern Alps

James, Zoe Elizabeth, 1974-

rangeparallel Fox Peak Fault Zone was studied in order to examine the geology, structure, fault rock mineralisation and fluid flow within the area, and to correlate Fox Peak Range deformation with regional Southern Alps tectonics. The Fox Peak Range is composed of Triassic to Jurassic aged weakly to moderately metamorphosed (TZ1 to TZ2b) Torlesse Terrane basement. Cover sediments which unconformably overlie basement in the south of the field area include a transgressive-regressive sequence that was deposited during the submergence of the South Island during the Eocene to Miocene time period. Pliocene aged non-marine greywacke gravels present at the top of the stratigraphic section mark the emergence and uplift of the Southern Alps at this time. Widespread Pleistocene glaciation resulted in a sequence of glacial deposits which occupy much of the Fairlie Basin to the east of the Fox Peak Range. Recent river deposits occur in association with the Opihi and North and South Opuha Rivers. The Fox Peak Fault Zone consists of 'a series of north - south striking faults that may be divided into an upper fault system situated on the flanks of the range, and a lower, range-bounding fault system. The upper faults are recognised chiefly by topographic depressions, and lack kinematic indicators. However, they are interpreted in this study to be old range-bounding faults that were replaced by new (lower) faults during the process of range uplift. The lower fault traces displace basement, Pleistocene glacial terrace surfaces and alluvial fans. The faults range from linear to lobate in character, and accommodate both thrust and dextral strike-slip motion. The ve1tical displacement of dateable surfaces, and the dextral horizontal offset of watercourses in the north of the field area, allows the calculation of time-averaged vertical and horizontal displacement rates of 1-1.5 mm I yr and 2-2.5 mm I yr, respectively. Multiple faulting events in the Cloudy Peaks area are evident by the correlation between increasing displacement and terrace age. Uplift of the Fox Peak Range is estimated to have commenced at 2 Ma. Drainage patterns suggest that the Fox Peak Range developed after the uplift of the Two Thumb Range to the west, probably as a response to the eastward migration of the deformation front resulting from the oblique collision of the Pacific and Australian plates at the Alpine Fault plate boundary. Fault zones within the Fox Peak area consist of carbonate cemented gouge and breccia. Discrete shear planes occurring within the zones preserve slickensides, the orientation of which suggest a range of fault motion from dip-slip to strike-slip. Based on cathodoluminescence studies, authigenic calcite within fault rock was precipitated by several fluids of varying chemistry. The geochemistry of a series of authigenic green and white phyllosilicates occurring within fault rock was analysed. The phyllosilicates are formed by the non-oxidised alteration of muscovite and chlorite. The alteration of the original phyllosilicates, and possibly the precipitation of part of the calcite, is a result of the downward percolation of ground water into the fault zones. Oxygen and carbon stable isotope analysis of fault rock calcite indicates that the calcite was precipitated by a mixture of meteoric I basinal fluids and rock -exchanged fluids. The temperature of the fluid is estimated at 90-150°C. The rock-exchanged fluids, originating at depths of several kilometres, gained access to shallow levels by the vertical permeability accompanying faulting. Basinal fluids and basinal I meteoric mixes enter the fault zone either by downward percolation through the sedimentary pile, or by mobilisation due to basement overthrusting. Variation in fluid isotopic composition within the field area may be due to local variation in overlying sediments and temperature at the calcite precipitation. The Fox Peak Fault Zone may be correlated with other north-south striking, obliquely deforming fault zones (such as the Ostler Fault Zone) that compose the fold-and-thrust belt of the outboard of the Southern Alps orogen. Deformation and fluid flow within the orogen may be predicted by analogue and numerical modelling of the orogen as a three-dimensional critical wedge facilitating considerable fluid circulation.

vii, 123 leaves : ill., (some col., one folded), maps ; 30 cm.

1998James

Abstract Only

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