Scratching the Hinuera Surface: Cryptic faulting in the late Quaternary alluvial plain, Te Tātua o Wairere Fault Zone

Abstract

Between c. 20,000 and 15,000 years BP, precursors to the modern Waikato River deposited alluvium of the Hinuera Formation throughout the Hamilton Basin – a c. 1,600 km2 terrestrial sedimentary basin in the centre-north of New Zealand’s North Island (Māori: Te Ika-a-Māui). Together with clayey silt beds of overlying late Quaternary tephras, the sand and gravel dominated Mid-Pleistocene to Holocene Hinuera Formation infills much of the basin to form the Hinuera Surface, a large alluvial plain, often referred to as a low-angle alluvial fan. Rising above the Hinuera Surface are the low (<c. 100 m) Hamilton Hills, formed of Early to Mid-Pleistocene primary and reworked volcanic sequences, also overlain by late Quaternary tephras. Recent studies have established the presence of paleoliquefaction within the Hinuera Formation – evidence of seismic disturbance – and fault deformation of the adjoining Hamilton Hills. The most recent fault activity identified to date, <c. 50,000 years BP, is within Te Tātua o Wairere Fault Zone, a zone broadly defined by an approximately N-S line of hills across the south-eastern margins of Hamilton City and including portions of the Hinuera Surface. Whilst deformation has been confirmed in cohesive materials of the fault zone’s hills, this study is the first to attempt identification of faulting within the younger, unconsolidated, granular sediments of the <c. 20 ka Hinuera Formation. The study combined desktop analysis of digital models, gravity anomalies, earthquake datasets, and historic aerial imagery with results from field surveys, including geomorphic mapping and electrical resistivity tomography (ERT), and paleoseismic trenching of the Hinuera Surface.

Basin-scale digital modelling showed that Te Tātua o Wairere Fault Zone marks a slight but significant change in Hinuera Surface slope, inferred to mark the eastern boundary of an uplifted fault block. This change is near-coincident with geomorphic evidence for paleochannel constriction and abrupt directional changes, subtle linear breaks in surface elevation, and apparent offsets across paleochannel banks. The same modelling inadvertently highlighted the ultra-low, almost nil angle of slope inherent to the Hinuera Surface (consistently <0.1°) and it is considered that application of a term such as braid-plain is more appropriate to that of alluvial fan. Gravity anomaly analysis did not yield significant results but interrogation of earthquake datasets highlighted a cluster of earthquakes in the Puketaha/Ruakura area – in the fault zone’s extreme north – and seismic reflection survey maps from the 1960s/1970s highlighted prior interpretations of proximal deep (pre-Hinuera Formation) faults. Historic aerial photographs suggested the possibility of fault scarps having once been visible within the fault zone, since obscured or modified beyond recognition by construction of the University of Waikato campus and adjacent developments. ERT surveys established an off-fault reference section which highlighted the essentially planar nature of undisturbed Hinuera Formation and late Quaternary strata at Ruakura. Against the reference section, a number of significant sub-surface resistivity anomalies were identified, including; discontinuities with subtle vertical offsets in shallow high resistivity zones; vertical/sub-vertical zones of low resistivity, and; discrete areas of high resistivity at depth, accompanied by a lack of planar continuity. These ERT anomalies not only aligned with one another but also with known faulting in hills to the south and a hypothesised approximate N-S fault.

Investigations concluded with excavation of a paleoseismic trench on the AgResearch Ruakura Research Farm, exposing a discrete, c. 4 m wide zone of deformation within the upper Hinuera Formation. This deformation was extremely similar to that observed across a confirmed fault scarp within near-identical lithostratigraphy in the Hauraki Basin and includes small listric (normal) fault segments, cryptic granular deformation, and paleoliquefaction. The absence of a through-going fault (pseudo-)plane negates deformation being interpreted as the hypothesised fault-proper but comparison with fault propagation models through granular materials suggests deformation is co-seismic in origin and near-fault. Movement on a deeper, basement fault (possibly identified in the 1960s/1970s but not since explored in any detail) is likely to have been accommodated in a diffuse manner within the Hinuera Formation, with deformation not necessarily following the true fault-plane. The study concludes that evidence of co-seismic deformation within the uppermost Hinuera Formation and for seismic influence of Hinuera Surface geomorphology indicates disturbance must have occurred <c. 15,000 years BP; Te Tātua o Wairere Fault Zone has been active more recently than previous evidence suggested. From the cumulative range of data gathered, an inferred line of faulting is presented which links the deformation at Ruakura with known fault offsets in the south-east of Hamilton City, at Hillcrest Road and Cobham Drive. Significant ERT anomalies and likely tectonogeomorphic expressions in the northern Ruakura area and east of the trenched location remain unresolved and require further investigation to confirm the hypothesised main line of strike. A greater understanding of the nature of Te Tātua o Wairere Fault Zone would be beneficial to seismic hazard risk planning in the area and there is scope for future research to consider the interplay between deeper local fault systems and distributed surface deformation.