Studies in low grade metamorphism in southern New Zealand : the metamorphic and structural geology of the Oturehua-Hawkdun Range area, Central Otago.


Grady, Alexander Edwin.


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Three groups of rocks, defined on litho-stratigraphic grounds, crop out in the region around Oturehua. The Hawkdun Greywacke is a thick unit of argillite, coarse and fine greywacke, and sporadic conglomerate lenses, of probable age in the range Carboniferous to Jurassic. The Haast Schist Group is a series of dominantly quartz-albite-epidote-muscovite-chlorite schists, of possible age in the range Carboniferous to Jurassic. They are sub-divided on mineralogical and textural grounds (after Hutton and Turner, 1938) into Chlorite Sub-zones 2, 3 and 4 and the Biotite Zone. Each of these Sub-zones and the Biotite Zone may be further sub-divided generally into massive schist, laminated schist, and greenschist. The Tertiary sediments comprise a series of intermontaine fluvial, and glacial pediment gravels, sands, silts and clays with occasional lignitic coal seams near the base. They range in age from possibly Eocene to Pleistocene.
The Hawkdun Greywacke and Haast Schist Group have undergone regional metamorphism under conditions ranging from those of the prehnite-pumpellyite metagreywacke facies, in the Hawkdun Greywacke, to those of the greenschist facies in the Haast Schist Group. The prehnite-pumpellyite metagreywacke facies is sub-divided into a prehnite-pumpellyite zone and a prehnite-free zone. Rocks of the greenschist facies comprise those assigned to the quartz-albite-muscovite-chlorite sub-facies and the quartz-albite-epidote-biotite sub-facies. Mesoscopic structures developed in the Haast Schist Group indicate three main phases of deformation, F1, F2 and F3. Development of recumbent isoclinal folds (F1), a sub-horizontal schistosity (S1), and widespread transposition of bedding, accompanied F1 deformation. F2 deformation developed a crenulation cleavage (S2 ) as axial-plane to similar folds (F2). Sporadic occurrence of kink folds (F3 ), in places in zones associated with faults, represent the effects of F3 deformation.
Macroscopic folds of F2 generation are developed on the southern slopes of Blackstone Hill. Changes in vergence of mesoscopic F2 folds in a zone running across southern Blackstone Hill, indicate a major macroscopic structure. The geometry of S1 , S2 and F2 in this zone indicates that the macrostructure developed before F2 deformation and probably after F1 deformation.
Kinematic analysis of F2 folds from Blackstone Hill, compared with similar analyses from the Omakau-Manorburn region (Means, 1966) indicate regional homogeneity of F2 deformation.
Structures in the Hawkdun Greywacke indicate at least two phases (B1 and B2) of folding; both probably of mainly flexural-slip mechanism. Large isoclinal folds may be of B1 generation or earlier in age.
Faults developed are post-metamorphic and comprise three sets. Greywacke is faulted against schist along an easterly striking set which has little topographic expression and little post-Eocene movement. Northeasterly and northwesterly striking fault sets show evidence of post-Eocene throw in excess of three thousand feet. These belong to two sets common over much of Otago. The northwesterly and northeasterly striking fault sets are postulated to have developed as secondary faults in association with the Alpine Fault and are probably of Chinnery's 'type C' and 'type D' secondary faults respectively (Chinnery, 1966). The curvature of the schist belt of Otago is probably controlled by 'type D' faults.
Similarities in mineralogy, lithology and structure, together with structural and textural gradations, and density variations indicate that the Haast Schist Group was probably derived from rocks equivalent to the Hawkdun Greywacke.
Experimental production of kink folds in argillite at 450°c, kilobar PH20, strain rates as low as 0.004 per cent per hour, and total strains as small as 13.5 per cent, is outlined in Appendix A.
Investigation of density variation in the Hawkdun Greywacke and Haast Schist Group, outlined in Appendix B, shows that most density variation reflects mineralogical variation. Density increases slightly between greywacke and schist, but cannot be correlated with degree of deformation or metamorphism. A discussion of change of vergence of early folds, induced by later folding, is given in Appendix C. From the aspect of possible changes in vergence of early folds undergoing refolding, their most important attitude during rotation is the horizontal. Vergence change is induced about this position. For refolding mechanisms of the flexural-slip type, not all second folds are able to induce a change in vergence of early folds. A trigonometric expression is derived, relating:
(i) the initial inclination of early folds,
(ii) the inclination of late fold axes,
(iii) the angle between early and late fold axes, and
(iv) the angle of rotation of early folds, about late fold axes, required to bring early fold axes to the horizontal.
This expression enables prediction of the probability of inducement of change of vergence of early folds in a given situation.
For refolding mechanisms of the simple shear type it is possible to induce a change in vergence of early folds by refolding about late folds of any orientation, except late folds with kinematic a parallel to axes of early folds, and provided that the angle of rotation of early folds is sufficient to bring them to the horizontal.

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xvii, 136 leaves : illus., maps (1 fold. in pocket) ; 27 cm.


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Grady, Alexander Edwin., “Studies in low grade metamorphism in southern New Zealand : the metamorphic and structural geology of the Oturehua-Hawkdun Range area, Central Otago.,” Otago Geology Theses, accessed January 22, 2018,