The structure across an area spanning the eastern range-front of the Dunstan Mountains in the Vicinity of Thomsons Gorge.


Walcott, RC


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Investigation of brittle structures of Thomsons Gorge Area, on the eastern slopes of the Dunstan Mountains, Central Otago, has shown that strain is widely distributed throughout the range. The uplift of the mountains is a result of the growth of a macroscopic fold constrained at depth on the eastern side by a blind Rangitata (D4) fault. The schist (in the mountains)-sediment (in the valley) contact is largely sedimentary. The contact is along antithetic clay-rich horizons that may or may not be at the schist-sedimentary boundary.
The schist in Thomsons Gorge Area is textural grade four Haast Schist. Eight schist lithologies are identified; three varieties of greyschist, four varieties of greenschist and two varieties of quartzite. Greenschist is abundant south of Thomsons Gorge. Six Generations of structures are recognized in the schist. The first four generations (D1-D4) involved ductile deformation and are related to the Mesosoic Rangitata Orogeny. The fifth generation involved pervasive brittle extensional shearing and is related to regional extension during late Cretaceous time. The sixth and ongoing generation is related to the Kaikoura Orogeny.
The sixth deformation generation has resulted in the growth of the Dunstan Mountains. Uplift north of Thomsons Gorge is 500m greater than to the south of the gorge and the area is more heavily denunded. Associated with the denudation is the deposition of Vinstan Gravel in the Manuherkia Valley. Uplift and warping of schistosity has occured by the reactivation of a large number of D4 and D5 fractures and faults on both sides of Thomsons Ridge (informal name). Reactivated faults generally trend northwest and slickenside lineations on these faults show deformation is mainly by strike/slip motion. The orientation of faults in the sediments bordering the range-front indicate a dextral sense of displacement.
Defining the trace of the macroscopic fold is the Cretaceous peneplain on schist basement which occurs both at the itop of Thomsons Ridge and at the schist-sediment contact at the base of the ridge at Tinkers Diggings. Four varieties of intensely altered schist occur at the latter locality; White, Green, Blue and Orange altered schists. X.R.D analyses has shown that their mineralogy consists predominantly of smectite and kaolinite and thus identifies them as part of the peneplain.
Tinkers Breccia (new name) crops out between altered schist basement and steeply dipping to overturned Manuherikia Group sediments at Tinkers Diggings. It is an auriferous sedimentary (channel) deposit composed of peneplain derived clay, altered schist clasts, plastically deformed quartz and albite clasts, sarsen stones, quartz pebbles and tectonically incorporated Manuherikia Group and gold. The gold is alluvial in origin and extremely pure (fineness 999-1000). X.R.D. and microprobe analyses identified a ferrous smectite-vermiculite in Tinkers Breccia, the second recorded occurence of the clay in Otago. As the smectite-vermiculite, requires a reducing environment for formation it is suggested that the alteration of peneolain schist is a post-Manuherikia feature.
Manuherikia Group immediately overlies schist basement and two lithological sequences are identified, a silt-dominated, and an overlying pebble-dominated sequence. The silt-rich lithological sequence has taken up most of the strain incurred by uplift of the Dunstan Mountains. Deformation in the sediments is occurring by pure dip-slip synthetic thrusting (horse stacking) away from the range-front and antithetic thrusting (horse and en echelon stacking) close to the range-front, and thus con1trasts with the schist.
A maximum principle stress orientated east-west can account for the variation in slip direction.

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90 leaves. Ill, diagms, photos, Map (folded in pocket); 30 cm.


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Walcott, RC, “The structure across an area spanning the eastern range-front of the Dunstan Mountains in the Vicinity of Thomsons Gorge.,” Otago Geology Theses, accessed May 22, 2024,

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