Pre‐Miocene and post‐Miocene deformation in the Bannockbum basin, Central Otago, New Zealand

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Abstract
The Bannockburn basin is the southern extension of the Upper Clutha basin, one of a series of prominent northeast‐trending structural and topographic basins in Central Otago. Post‐Miocene structure within the Bannockburn basin is shown up by deformation of an unconformity cut into schist basement beneath Miocene terrestrial sediments. The post‐Miocene deformation has been accommodated largely by flexure, and the structure consists of broad north‐trending synforms and antiforms in schistosity, mimicked by folds in the overlying sediments. The northwestern margin of the basin trends northeast, and the unconformity is steeply dipping or locally overturned, but there is no major fault separating the basin from the range immediately to the west. Minor faults in this area have normal, reverse, or strike‐slip senses of movement. The southwestern margin of the basin trends northwest and is marked by distinct faults which offset the unconformity. The present‐day Bannockburn basin coincides with a complex pre‐Miocene graben structure, within which a complex set of northeast‐ and northwest‐striking faults has juxtaposed differing textural zones within the schist, with the Bannockburn basin lying on the downthrown blocks. This pre‐Miocene structure probably controlled the location of the present basin, and similar structures may have influenced the formation of other Central Otago basins. The characteristic Central Otago, post‐Miocene range and basin structure may thus be inherited from pre‐Miocene extension.