Mechanics of folding in central Otago : an investigation of the recent deformation of the Rock and Piller Range
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Abstract:
Cenozoic deformation of the Central Otago Range and Basin province has been investigated to determine the mechanism of range formation. Mapping in the Rock and Pillar Range revealed macroscopic folds in schistosity which correspond in shape and size to the ranges, suggesting topography is controlled by folding of the schist. This congruence has also been documented in the Raggedy Range, further to the west.
The role of the Hyde Fault in uplift of the Rock and Pillar Range has been investigated. Available evidence suggests there is no large throw across the fault and that faulting is subordinate to folding in the formation of the range. Thus the Central Otago ranges are interpreted as macroscopic folds of the upper crust.
A mechanical framework for the formation of these folds is presented. An elastic-plastic constitutive law for deformation of the Otago Schist has been derived which incorporates the anisotropy of the schist. Plastic components of the model represent slip on schistosity planes. An elastic-plastic plate may buckle under much lower differential stress than a purely elastic plate, but an initial shear on the schistosity planes is required to initiate folding. This may be imposed externally by a fault or internally by misalignment of schistosity planes.
Growth of the folds is controlled by the relative weakness and low friction of the schistosity planes, which allow for easy slip. Folds grow until slip on the schistosity planes is no longer favorable; then faulting across schistosity planes occurs. Scale effects on the initiation and growth of the ranges have been assessed.
Finite difference computer modelling was used to simulate the growth of the ranges from an initial perturbation to finite displacement folds. Deformation is permanent and asymmetric.
Implications for regional tectonics have been assessed. The principal compression direction may be oblique, rather than perpendicular, to the ranges because pervasive lineation of the schist partly controls the orientation of schistosity fold axes. Therefore these folds need not have formed in response to pure compression, but may result from oblique compression.
The role of the Hyde Fault in uplift of the Rock and Pillar Range has been investigated. Available evidence suggests there is no large throw across the fault and that faulting is subordinate to folding in the formation of the range. Thus the Central Otago ranges are interpreted as macroscopic folds of the upper crust.
A mechanical framework for the formation of these folds is presented. An elastic-plastic constitutive law for deformation of the Otago Schist has been derived which incorporates the anisotropy of the schist. Plastic components of the model represent slip on schistosity planes. An elastic-plastic plate may buckle under much lower differential stress than a purely elastic plate, but an initial shear on the schistosity planes is required to initiate folding. This may be imposed externally by a fault or internally by misalignment of schistosity planes.
Growth of the folds is controlled by the relative weakness and low friction of the schistosity planes, which allow for easy slip. Folds grow until slip on the schistosity planes is no longer favorable; then faulting across schistosity planes occurs. Scale effects on the initiation and growth of the ranges have been assessed.
Finite difference computer modelling was used to simulate the growth of the ranges from an initial perturbation to finite displacement folds. Deformation is permanent and asymmetric.
Implications for regional tectonics have been assessed. The principal compression direction may be oblique, rather than perpendicular, to the ranges because pervasive lineation of the schist partly controls the orientation of schistosity fold axes. Therefore these folds need not have formed in response to pure compression, but may result from oblique compression.
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x, 104 leaves : ill. (some col.), maps ; 30 cm.
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1993Salton
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Salton, Gillian Greta., “Mechanics of folding in central Otago : an investigation of the recent deformation of the Rock and Piller Range ,” Otago Geology Theses, accessed May 19, 2025, https://theses.otagogeology.org.nz/items/show/287.