Upper Wilberforce geology Mid-Canterbury, New Zealand


Becker, John Andrew, 1976-


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The upper Wilberforce Valley is a structurally complex area composed of north, northeast and east-west striking fault systems. Topography is strongly controlled by movement on the Main Divide Fault Zone, a major fault system extending regionally northeast immediately to the east of the Main Divide. The Main Divide Fault Zone is composed of regional northeaststriking oblique-reverse faults which dip at c. 50° -northwest. The Main Divide Fault (MDF) steps dextrally about 1 km to the east at Browning Pass forming a dilational jog. Two fault segments of the MDF are linked by a steeply dipping, east-west striking, oblique strike-slip fault. Browning Pass is a topographically low saddle produced by the dextral trans-tension caused by movement along the MDF. With progression south of the Main Divide, structure is dominated by north striking oblique reverse strike-slip faults. Bq!Jl hangingwall and footwall sequences are composed of relatively homoclinal, low grade (TZl, possibly TZ2A in the hangingwall) prehnite-pumpellyite facies greywackes and argillites. Footwall sequences dip c. 50-80° WNW while hangingwall sequences dip c. 50° WNW. The hangingwall is intensely deformed adjacent to major faults, while smaller, locally pervasive subsidiary faults cause most deformation in the footwall rocks. Thermochronological data suggest significant vertical offset during the Cenozoic. The MDF and most other observed structures are the result of a late Cenozoic backthrust off the Alpine Fault. Mineralisation in the form of often wide quartz veins (up to 8 m) characterises both the footwall and hangingwall sequences adjacent the Main Divide Fault. Veins consistently strike north to northeast, dipping at angles of 40-60° WNW. Mineralisation occurs as gold and sulphide associations within white unaltered vein quartz. Veins exist preferentially in argillite sequences where they both conform to- and zig-zag across bedding. Wall-rock directly adjacent to the veins is rarely altered or silicified. Furthermore, the wall-rock is intensely brecciated at the vein margins suggesting extensive hydraulic brecciation. No shearing of wall-rock into veins occurs, nor is there any observable form of significant offset of the veins. The Wilberforce Reefs are suggested to form part of a much larger, regional mineralisation zone striking NNE, with veins dipping at angles of 50-70° WNW. This mineralised z~ne extends at least south to Moa Creek (15 km southwest), and 20 km north to the Alpine Fault at the mouth of the Taipo River. The veins strike directly NNE across the MDF with no apparent change in orientation, offset or strike. Fluid inclusion studies, coupled with arsenopyrite geothermometry suggest vein formation temperatures of 250-300°C, while late stage carbonate from the small faults suggests a mixed meteoric, or isotopically exchanged (metamorphic) fluid source Geochemical data suggest that veins may either be late-stage metamorphic in origin, and hence Mesozoic in age, or similarly, it is possible the data may be interpreted as representing a Cenozoic age. Structural arguments suggest more of a Cenozoic origin.

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157 leaves : ill. (some col.), maps ; 30 cm.


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POLYGON ((171.402514673134505 -42.914616295524247,171.398599103596439 -42.987527292314283,171.310595693794966 -42.986559704073386,171.315878636625882 -42.911885151264329,171.402514673134505 -42.914616295524247))





Becker, John Andrew, 1976-, “Upper Wilberforce geology Mid-Canterbury, New Zealand,” Otago Geology Theses, accessed May 22, 2024, https://theses.otagogeology.org.nz/items/show/338.

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