Abstract:
The Auckland Region continues to expand and undergo intensive urbanization and population growth; it becomes crucial to understand the geohazards present in this area. The West Auckland region has explicitly limited studies on the structural geology of the area, with most literature written from a volcanological or paleontological perspective. Geohazards around the Auckland region have been identified to include earthquakes, coastal erosion, and slope instability. Regional fracture and fault networks can be tectonically reactivated; hence, it is crucial to characterize their spatial and temporal patterns, origin, and forecast potential future events. This study aimed to identify dominant structural trends and spatial deformation patterns in Miocene lithologies in West Auckland and locally on Auckland’s North Shore. The aims were achieved using field observations across coastal sites and remote observations via Google Earth mapping across West Auckland and Waiake Bay. Data from relevant literature were also mined to provide supporting and comparative evidence with the data gathered in this study.
Field and remote observations across the entire study reveal a pervasive structural fabric in the Waitematā and Waitākere Group, which exhibits a primary NE trend. The fabric is demonstrated by a wide range of geologic structures (e.g., faults and joints) and geomorphic features (streams and river valleys), with these linear features ranging from outcrop to regional scale. The observed NE-trending structural fabric is likely Miocene and Post-Miocene in age and is importantly reflected in the large-scale faults of Post-Miocene age in Auckland. Its persistence across temporal and spatial scales indicates persistent and ongoing regional tectonic processes favourable for the development of NE-trending structures (e.g., NW-SE-directed extension). The NE-trending structural fabric also matches the regional geomorphic patterns (topographic lineaments, such as streams and river valleys) of West Auckland. Primary NE-trends are distinguished in the small to large-scale geomorphic features such as joints, shore platform lineaments, drainage segments, and topographic lineaments.
The dominant NE trends observed in Miocene to Post-Miocene structures in West Auckland do not match the primary NNW-trending fabric of the underlying Murihiku basement rock, suggesting the observed structures in West Auckland were not derived from the reactivation of Mesozoic structures. Documented regional stress fields in surrounding regions across the North Island (e.g., North Taranaki Basin) since the Late Miocene are consistent with a NE-SW-trending maximum horizontal stress (SH(max)), which likely explains the dominant NE-trending extensional structures in the Auckland region, especially the NE-trending normal faults and
joints. The strong correlation observed between the Miocene structural fabric and recent geomorphic lineaments suggests Miocene structures and past processes continue to have a large influence on modern-day geomorphology, which demonstrates how geologic and surface processes interact across different time scales. The temporal and spatial influence of these structures has persisted from ~25 million years until today.