Upper plate deformation and its relationship to the underlying Hikurangi subduction interface, southern North Island, New Zealand

<p>At the southern Hikurangi margin, the subduction interface between the Australian and Pacific plates, beneath the southern North Island of New Zealand, is ‘locked’. It has previously been estimated that sudden slip on this locked portion of the interface could result in a subduction zone or ‘megathrust’ earthquake of Mw 8.0-8.5 or larger. Historically, however, no significant (>Mw 7.2) subduction interface earthquake has occurred at the southern Hikurangi margin, and the hazard from subduction earthquakes to this region, which includes New Zealand’s capital city of Wellington, remains largely unknown. Patterns of uplift at active margins can provide insight into subduction processes, including megathrust earthquakes. With the objectives to i) contribute to the understanding of partitioning of margin-parallel plate motion on to upper plate faults, and ii) provide insight into the relationship of permanent vertical deformation to subduction processes at the southern end of the Hikurangi margin, I investigate flights of late Pleistocene fluvial and marine terraces preserved across the lower North Island. Such geomorphic features, when constrained by numerical dating, provide a valuable set of data with which to quantify tectonic deformation - be they locally offset by a fault, or collectively uplifted across the margin. Fault-offset fluvial terraces along the Hutt River, near Wellington, record dextral slip for the southern part of the Wellington Fault. From re-evaluated fault displacement measurements and new Optically Stimulated Luminescence (OSL) data, I estimate an average slip rate of 6.3 ± 1.9/1.2 mm/yr (2σ) during the last ~100 ka. However, slip on the Wellington Fault has not been steady throughout this time. During the Holocene, there was a phase of heightened ground rupture activity between ~8 and 10 ka, a period of relative quiescence between ~4.5 and 8 ka, and another period of heightened activity during the last ≤ 4.5 ka. Moreover, these results agree with independent paleoseismological evidence from other sites along the Wellington Fault for the timing of ground rupture events. The time-varying activity observed on the Wellington Fault may be regulated by stress interactions with other nearby upper plate active faults. Net tectonic uplift of the southern Hikurangi margin is recorded by ancient emergent shore platforms preserved along the south coast of the North Island. I provide a new evaluation of the distribution and age of the Pleistocene marine terraces. Shore platform altitudes are accurately surveyed for the first time using Global Navigational Satellite Systems (GNSS). From these data I have determine the shore platform attitudes where they are preserved along the coast. The terraces are also dated, most for the first time, using OSL techniques. The most extensive Pleistocene terraces formed during Marine Isotope Stages (MIS) 5a, 5c, 5e and 7a. Because the ancient shorelines are now obscured by coverbed deposits, I use shore platform attitudes to reconstruct strandline elevations. These strandline elevations, corrected for sea level during their formative highstands, have been used to quantify rates of uplift across the southern Hikurangi margin. In the forearc region of the Hikurangi margin, within ~70 km of the trough, uplift observed on the marine terraces along the Palliser Bay coast monotonically decreases away from the trough. The highest uplift rate of 1.7 ± 0.1 mm/yr is observed at the easternmost preserved terrace, near Cape Palliser, about 40 km from Hikurangi Trough. Further to the west, at Lake Ferry, uplift is 0.8 ± 0.1 mm/yr. The lowest rate of uplift, 0.2 ± 0.1 mm/yr, is observed at Wharekauhau, the westernmost marine terrace preserved on the Palliser Bay coast. Overall, the terraces are tilted towards the west, away from the trough, with older terraces exhibiting the most tilting. This long-wavelength pattern of uplift suggests that, in this forearc region of the margin, deep-seated processes, most likely subduction of a buoyant slab in combination with megathrust earthquakes, are the main contributors to permanent vertical deformation. West of Palliser Bay, at a distance of >70 km from the Hikurangi Trough, vertical offsets on the marine terraces are evident across upper plate faults, most notably the Wairarapa and Ohariu Faults. The uplift rate at Baring Head, west and on the upthrown side of the Wairarapa Fault, is as much as 1.6 ± 0.1 mm/yr. At Tongue Point, where the Ohariu Fault offsets the marine terraces preserved there, uplift calculated from the western, upthrown side of the fault is 0.6 ± 0.1 mm/yr. These uplift rates suggest that, in the Axial Ranges, in addition to sediment underplating, movement on the major active upper plate faults also contributes to rock uplift.</p>

Upper plate deformation and its relationship to the underlying Hikurangi subduction interface, southern North Island, New Zealand

<p>At the southern Hikurangi margin, the subduction interface between the Australian and Pacific plates, beneath the southern North Island of New Zealand, is ‘locked’. It has previously been estimated that sudden slip on this locked portion of the interface could result in a subduction zone or ‘megathrust’ earthquake of Mw 8.0-8.5 or larger. Historically, however, no significant (>Mw 7.2) subduction interface earthquake has occurred at the southern Hikurangi margin, and the hazard from subduction earthquakes to this region, which includes New Zealand’s capital city of Wellington, remains largely unknown. Patterns of uplift at active margins can provide insight into subduction processes, including megathrust earthquakes. With the objectives to i) contribute to the understanding of partitioning of margin-parallel plate motion on to upper plate faults, and ii) provide insight into the relationship of permanent vertical deformation to subduction processes at the southern end of the Hikurangi margin, I investigate flights of late Pleistocene fluvial and marine terraces preserved across the lower North Island. Such geomorphic features, when constrained by numerical dating, provide a valuable set of data with which to quantify tectonic deformation - be they locally offset by a fault, or collectively uplifted across the margin. Fault-offset fluvial terraces along the Hutt River, near Wellington, record dextral slip for the southern part of the Wellington Fault. From re-evaluated fault displacement measurements and new Optically Stimulated Luminescence (OSL) data, I estimate an average slip rate of 6.3 ± 1.9/1.2 mm/yr (2σ) during the last ~100 ka. However, slip on the Wellington Fault has not been steady throughout this time. During the Holocene, there was a phase of heightened ground rupture activity between ~8 and 10 ka, a period of relative quiescence between ~4.5 and 8 ka, and another period of heightened activity during the last ≤ 4.5 ka. Moreover, these results agree with independent paleoseismological evidence from other sites along the Wellington Fault for the timing of ground rupture events. The time-varying activity observed on the Wellington Fault may be regulated by stress interactions with other nearby upper plate active faults. Net tectonic uplift of the southern Hikurangi margin is recorded by ancient emergent shore platforms preserved along the south coast of the North Island. I provide a new evaluation of the distribution and age of the Pleistocene marine terraces. Shore platform altitudes are accurately surveyed for the first time using Global Navigational Satellite Systems (GNSS). From these data I have determine the shore platform attitudes where they are preserved along the coast. The terraces are also dated, most for the first time, using OSL techniques. The most extensive Pleistocene terraces formed during Marine Isotope Stages (MIS) 5a, 5c, 5e and 7a. Because the ancient shorelines are now obscured by coverbed deposits, I use shore platform attitudes to reconstruct strandline elevations. These strandline elevations, corrected for sea level during their formative highstands, have been used to quantify rates of uplift across the southern Hikurangi margin. In the forearc region of the Hikurangi margin, within ~70 km of the trough, uplift observed on the marine terraces along the Palliser Bay coast monotonically decreases away from the trough. The highest uplift rate of 1.7 ± 0.1 mm/yr is observed at the easternmost preserved terrace, near Cape Palliser, about 40 km from Hikurangi Trough. Further to the west, at Lake Ferry, uplift is 0.8 ± 0.1 mm/yr. The lowest rate of uplift, 0.2 ± 0.1 mm/yr, is observed at Wharekauhau, the westernmost marine terrace preserved on the Palliser Bay coast. Overall, the terraces are tilted towards the west, away from the trough, with older terraces exhibiting the most tilting. This long-wavelength pattern of uplift suggests that, in this forearc region of the margin, deep-seated processes, most likely subduction of a buoyant slab in combination with megathrust earthquakes, are the main contributors to permanent vertical deformation. West of Palliser Bay, at a distance of >70 km from the Hikurangi Trough, vertical offsets on the marine terraces are evident across upper plate faults, most notably the Wairarapa and Ohariu Faults. The uplift rate at Baring Head, west and on the upthrown side of the Wairarapa Fault, is as much as 1.6 ± 0.1 mm/yr. At Tongue Point, where the Ohariu Fault offsets the marine terraces preserved there, uplift calculated from the western, upthrown side of the fault is 0.6 ± 0.1 mm/yr. These uplift rates suggest that, in the Axial Ranges, in addition to sediment underplating, movement on the major active upper plate faults also contributes to rock uplift.</p>

The effects of storms and a transient sandy veneer on the interannual planform evolution of a low-relief coastal cliff and shore platform at Sargent Beach, Texas, USA

Coastal cliff erosion is alongshore-variable and episodic, with retreat rates that depend upon sediment as either tools of abrasion or protective cover. However, the feedbacks between coastal cliff planform morphology, retreat rate, and sediment cover are poorly quantified. This study investigates Sargent Beach, Texas, USA, at the annual to interannual scale to explore (1) the relationship between temporal and spatial variability in cliff retreat rate, roughness, and sinuosity and (2) the response of retreat rate and roughness to changes in sand and shell hash cover of the underlying mud substrate as well as the impact of major storms using field measurements of sediment cover, erosion, and aerial images to measure shore platform morphology and retreat. A storm event in 2009 increased the planform roughness and sinuosity of the coastal cliff at Sargent Beach. Following the storm, aerial-image-derived shorelines with annual resolution show a decrease in average alongshore erosion rates from 12 to 4 m yr−1, coincident with a decrease in shoreline roughness and sinuosity (smoothing). Like the previous storm, a storm event in 2017 increased the planform roughness and sinuosity of the cliff. Over shorter timescales, monthly retreat of the sea cliff occurred only when the platform was sparsely covered with sediment cover on the shore platform, indicating that the tools and cover effects can significantly affect short-term erosion rates. The timescale to return to a smooth shoreline following a storm or roughening event, given a steady-state erosion rate, is approximately 24 years, with the long-term rate suggesting a maximum of ∼107 years until Sargent Beach breaches, compromising the Gulf Intracoastal Waterway (GIWW) under current conditions and assuming no future storms or intervention. The observed retreat rate varies, both spatially and temporally, with cliff face morphology, demonstrating the importance of multi-scale measurements and analysis for interpretation of coastal processes and patterns of cliff retreat.

Tectonics and shore platform development: Rates and patterns of erosion on recently uplifted mudstone and limestone rocks at Kaikōura Peninsula, New Zealand

&lt;p&gt;On tectonically active rock coasts, there is a dearth of erosion data documenting how rocks adjust (either fast or slow) in response to marine and subaerial processes immediately after coseismic uplift. Here we report erosion rates and evidence of reshaping of shore platform morphology on intertidal- and previously subtidal- rocks at Kaik&amp;#333;ura Peninsula, South Island New Zealand. As a result of the November 2016 Kaik&amp;#333;ura 7.8 (Mw) earthquake, platforms around the peninsula were uplifted by ~1.01 m, extended in width, and a 43-year active erosion monitoring campaign was abruptly halted but an opportunity to record how rocks respond to sudden environmental change like tectonics was presented. High-resolution topographic data obtained from quarterly surveys over four years using the micro-erosion meters (MEM) and Structure-from-Motion Multi View Stereo (SfM-MVS) surveys have provided accurate quantitative rates of erosion and visual representation of surface morphologies. MEM erosion data revealed variations in erosion, weathering and deposition rates across lithology, seasons, tidal positions, and platform elevation after the uplift. Four-years post-uplift erosion data shows a resetting of erosion rates and faster rock breakdown on both mudstone and limestone lithologies compared to pre-uplift rates. Over the 4-year period, surface downwearing rates for all platforms was 2.19 mm/yr, a 99.9% increase from a pre-uplift rate of 1.10 mm/yr. Average lowering rates on limestone, hard mudstone and soft mudstone platforms are 1.31 mm/yr, 2.13 mm/yr and 3.60 mm/yr, respectively. Seasonal trends in erosion rates remain unchanged as higher rates are still experienced during summer than winter seasons due to greater periods of higher temperatures and increased wetting and drying cycles. A year after uplift, previously reported across shore variations where erosion rates decreased from inner/landward margins of the platform to the outer/seaward sections disappeared with higher erosion rates fluctuating across all platform sections. Increased lowering rates on limestone rocks at the inner and outer sections were attributed to greater periods of wetting and drying, and loss of biological cover. These initially rapid rates decreased on the seaward sections after 3 years as a result of bioprotection and increased tidal wetting.&amp;#160; On one of the harder mudstone rocks, a dramatic increase from a pre-uplift erosion rate of 0.43 mm/yr to 19.23 mm/yr (1-year after uplift) and subsequent decline to 1.54 mm/yr after four years is suggestive of isolated incidents of block detachment and erosion. For the first time, we complement MEM data with available SfM-MVS derived orthomosaics to provide evidence of changing rock morphology and processes such as intense granular disintegration, flaking, algal growth, and boring. On tectonically active rock coasts, the strong fluctuations in erosion rates and platform morphological expressions indicate the actions of not only waves, tides, and weathering processes but also tectonics in shore platform development.&lt;/p&gt;