Holocene marine terraces as recorders of earthquake uplift: Insights from a rocky coast in southern Hawke's Bay, New Zealand

Abstract

On rocky tectonic coasts, data from Holocene marine terraces may constrain the timing of coseismic uplift and help identify the causative faults. Challenges in marine terrace investigations include: (1) identifying the uplift datums; (2) obtaining ages that tightly constrain the timing of uplift; (3) distinguishing tsunami deposits from beach deposits on terraces; and (4) identifying missing terraces and hence earthquakes. We address some of these challenges through comparing modern beach sediments and radiocarbon ages with those from a trench excavated across three terraces at Aramoana, central Hikurangi Subduction Margin, New Zealand. Sedimentary analyses identified beach and dune deposits on terraces but could not differentiate specific environments within them. Modern beach shells yielded modern radiocarbon ages, regardless of position or species, showing age inheritance and habitat is likely not an issue when dating shells on these terraces. By integrating terrace mapping, stratigraphy, morphology, and radiocarbon ages we develop a conceptual model of coastal uplift and terrace formation following at least two, possibly three, earthquakes at 5490-5070, 2620-2180, and 950-650 cal. yr BP. A high step and time gap between the upper two terraces raises the possibility that at least one intervening terrace is completely eroded. The trench exposure also showed that terrace stratigraphy may differ from that inferred from surface geomorphology, with apparent beach ridges being of composite origin and draping of younger beach deposits on the outer edge of a previous terrace. Dislocation modelling and comparison of marine terrace and earthquake ages from $4 km south and ≤ 73 km north confirms that the most likely earthquake source is the nearshore, landward-dipping, Kairakau Fault. Alternative sources, such as multi-fault ruptures of the Kairakau-Waim arama faults or Hikurangi subduction earthquakes, and/or a combination of the two are also possible and should be examined in future studies.

Upper Plate Faults May Contribute to the Paleoseismic Subsidence Record Along the Central Hikurangi Subduction Zone, Aotearoa New Zealand

“…Attributing specific fault sources to events at Ahuriri Lagoon and other sites along the Hikurangi margin is made difficult by wide displacement and age uncertainties, short record lengths, limited spatial preservation, and intertwined signals from subduction and upper plate fault earthquakes (e.g., Clark et al., 2019). Slip on upper‐plate structures clearly control the topography and sedimentary basin structure of the area over 10–100 kyr timescales (e.g., Barnes et al., 2002; Berryman et al., 2011; Hull, 1987; Litchfield et al., 2022; Paquet et al., 2009). Despite this established role of upper‐plate faulting in the structural evolution of the region, previous work has not identified upper‐plate earthquake sources that explain coseismic subsidence at the lagoon.…”

Causes of permanent vertical deformation at subduction margins: Evidence from late Pleistocene marine terraces of the southern Hikurangi margin, Aotearoa New Zealand

“…On the western side, the shore platform is oriented 111/8.8 °NE; it dips in an inland direction, in contrast to the modern-day shore platform which is nearhorizontal. Ninis et al (2022) inferred that this dip reflects localised deformation by the Ōhāriu Fault. Alternatively, this dip could be the result of an erroneously high elevation point (e.g., a preserved stack) at the coastal extent of the terrace.…”

“…The majority of margin-parallel motion (>50->70%) is taken up by slip on several predominately dextral strike-slip upper-plate faults in the exposed forearc; these include the Wairarapa, Wellington, Ōhāriu, and Shepherds Gully/Pukerua faults (Figure 2). The Wellington Fault (slip rate 5.5 ± 2.3 mm/yr-New Zealand Community Fault Model (NZ CFM), Seebeck et al (2022)) has a near-vertical dip and exhibits minimal dip-slip motion where it crosses the coast, while the Wairarapa [slip rate 11 ± 3 mm/yr-Seebeck et al (2022)] and Ōhāriu faults [slip rate 1.5 ± 0.5 mm/yr-Seebeck et al (2022)] dip Frontiers in Earth Science frontiersin.org steeply northwest and host a larger component of dip-slip motion. Many of these faults may be listric at depth (Henrys et al, 2013), and the Wairarapa Fault may connect at a relatively shallow depth (~5-10 km) with the nearby Wharekauhau Thrust Fault [slip rate 2.5 ± 1 mm/yr-Seebeck et al (2022)] depending on their respective geometries.…”

“…The Wellington Fault (slip rate 5.5 ± 2.3 mm/yr-New Zealand Community Fault Model (NZ CFM), Seebeck et al (2022)) has a near-vertical dip and exhibits minimal dip-slip motion where it crosses the coast, while the Wairarapa [slip rate 11 ± 3 mm/yr-Seebeck et al (2022)] and Ōhāriu faults [slip rate 1.5 ± 0.5 mm/yr-Seebeck et al (2022)] dip Frontiers in Earth Science frontiersin.org steeply northwest and host a larger component of dip-slip motion. Many of these faults may be listric at depth (Henrys et al, 2013), and the Wairarapa Fault may connect at a relatively shallow depth (~5-10 km) with the nearby Wharekauhau Thrust Fault [slip rate 2.5 ± 1 mm/yr-Seebeck et al (2022)] depending on their respective geometries. Most (~80%) of the marginperpendicular motion is believed to be accommodated by slip on the subduction interface and connected imbricate thrusts in the offshore accretionary wedge (Darby and Beavan, 2001;Nicol and Beavan, 2003;Wallace et al, 2004;Nicol et al, 2007), the closest of which, at ~5 km from the coast, is the west-dipping, dextral reverse Palliser-Kaiwhata Fault (e.g., Barnes and Mercier de Lepinay, 1997;Barnes et al, 1998;Barnes and Audru, 1999;Mountjoy et al, 2009) (vertical slip rate 5.0 ± 2.0 mm/yr (Seebeck et al, 2022) (Figure 2).…”

“…In this section we apply the marine terrace shore platform mean planar attitudes from Ninis et al (2022) and create profiles across each shore platform and corresponding sea cliff, in order to determine the shoreline angle elevations of the late Pleistocene terraces preserved along the Wellington south coast. We then use the reconstructed shoreline angle elevations, corrected for sea level during the relevant, formative highstand, to determine absolute tectonic uplift and calculate uplift rates.…”