Fellows, D.L. 1986 Prefeasibility study of seismotectonic hazards for the proposed Waiau (Canterbury) hydro development. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EDS 108 61 p.
Abstract: The current knowledge o the geology of the Clarence-Waiau region is summarized for a seismotectonic prefeasibility study of the area. Particular note is made of the structures and deformation features of the region which may affect the possible hydro-electric power scheme. recommendations for further investigations to be carried out are noted. The major points discussed om this prefeasibiity study are summarized below. (1) The regional geology of the area has been studies by a large number of workers, all of whom used differing stratigraphic nomenclature. The naming of Cretaceous-Tertiary aged deposits has been rationalised by Browne and Field (1985) into 8 lithostratigraphic groups with associated formations. Six of these groups are exposed in the project area. This stratigraphy may require further definition to describe tertiary and Quaternary deformation. (2) There is a diverse range of Quaternary age deposits within the region. Most of the glacial, fluvial and marine deposits have been studied in some detail, mainly on a local level. Problems remain in the correlation and dating of deposits and associated surfaces from one river valley to another. Absolute dating of deposits is critical to the Quaternary chronology and correlation, however little suitable dating material is available. (3) North Canterbury / Marlborough and hence the area of the hydro electric scheme is part of s deforming area (Marlborough Shear Belt) of New Zealand which is presently characterised by folding, dextral faulting and uplift. (4) Two styles of deformation characterise the region. The first is strike-slip faulting along north-east and north trending structures. The major northeast trending structures are the Awatere, Clarence, Elliott and Hope faults. These faults are characterised by right lateral strike-slip movement with a minor vertical component, which is believed to have commenced 10 million years ago. They exhibit large right lateral offsets of greywacke and Tertiary aged rocks of up to 20 km. The north trending faults are considered to be subsidiary to the main northeast trends and are mainly located within greywacke. (5) The second deformation style is of complex folding, evidence for which is preserved within Tertiary and early Quaternary deposits. The fold axes generally trend northeast parallel to the major faulting. (6) Evidence for late Quaternary deformation is widespread. Of particular note are the fault traces, and tilted and warped surfaces preserved along the major active faults. Surface deformation associated with active folding in coastal areas is illustrated by the uplift of marine deposits. Although many of the active fault traces have been mapped, the folding in the area is not well documented. (7) Some sites on the active faults have been studied in detail. However, the lack of dating of offset surfaces or lack of confidence in existing data has restricted estimations of faulting recurrence intervals. Recurrence intervals of faulting are a key element in the assessment of seismotectonic hazards in the project area.(8) A section of the Fowlers Fault has been mapped in detail. The remainder of the fault requires investigation. A slip-rate of 1-2 mm/yr has been estimated from a 3.5 m horizontally offset Holocene channel. Assuming the fault rupture increment is 3.5 m a tentative recurrence interval of 2,300 years has been estimated. (9) The Clarence Fault is a complex feature with numerous bifurcating faults. It appears that single displacements and the incremental steps of faulting are variable over the fault length, and that the fault may be segmented. Each segment may be typified by its own characteristic faulting event. The northern segment of the fault is not well understood. The limited data indicates horizontal faulting increments of the order of 7 m right lateral. The limited information on features on the segment associated with the Clarence-Elliott wedge mapped by Keickhefer (1979), indicate the last faulting event was 1.6 m right lateral and modal incremental events of about 2 m. South of the Clarence Fault wedge the information available indicates faulting increments are between 2-3 m right lateral with at least 3 post glacial movements. (10) The deformation features of the Elliott Fault have been mapped. The horizontal and vertical movement of this fault are less than on the Clarence Fault, with an estimated slip rate of 1 mm /yr. (11) The Hope Fault is a complex feature notable for changes in strike and a number of splinter faults. Pull apart basins have formed at local and regional bends on the fault. Many deformation features associated with the fault have been identified, however, some offsets and the ages of particular surfaces and features remain in contention. The fault is last known to have ruptured on 31 August 1888, with an associated magnitude 7.3 earthquake. The event right laterally offset fences by 2.8 m. The exact length of fault rupture is not agreed upon but is probably in the range of 13-50 km. Reported sip rates at localities along the fault vary, indicating both a change in slip rate through time and differing rates along the length of the fault throughout the Holocene
