STYLES OF STRUCTURAL DEFORMATION ACROSS CASTLE HILL BASIN, NEW ZEALAND

The immense diversity of structural features at Castle Hill Basin, New Zealand, is telling of its dynamic geologic history. We analyze open, asymmetrical, and overturned folds, as well as doubly plunging periclines, with stereonets and 3D visualization to understand why structures across the basin vary so much. This analysis is done by splitting the basin into domains to further understand the structural controls of the features visible in different parts of the basin.

The structural domains display very different measurements on stereonets. Large reverse faults that bound Castle Hill Basin have formed larger mountains (Craigieburn Range, Torlesse Range, and Flock Hill) surrounding the basin and smaller folds and faults within (Gage, 1970). The orientation of these inner folds and faults often mirrors the mountain ranges close by. Around the edges of the basin, the proximity to a basin-bounding fault was the single best predictor of the orientation of a fold or fault. Towards the middle of the basin, however, some folds display the effects of more than one stress regime. Two individual fold-axes were identified, and this is the reason non-cylindrical folds dominate the northern domain.

This project aims to further our knowledge of the stress that has shaped the folds and faults within Castle Hill Basin. We isolate the effects of two specific basin-bounding faults by providing quantitative measurements that prove that the non-cylindrical folds have two corresponding fold-axes. The different basin-bounding faults and subsequent non-cylindrical folds within Castle Hill Basin can be explained in one of two ways: 1) the basin shows the effects of two individual folding events, or 2) the basin-bounding faults relieved pressure coming from a single stress regime oriented somewhere between the two fold-axes displayed.

Castle Hill Basin is the largest intermontane region of younger strata in mid-Canterbury; a better understanding of the stress regime(s) that formed the basin would be invaluable to a further study of the way stress is accommodated in the Southern Alps of New Zealand.