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

Abstract

Theoretical studies of the seismic cycle at convergent plate boundaries anticipate that most coseismic deformation is recovered, yet significant permanent vertical displacement of the overriding plate is observed at many subduction margins. To understand the mechanisms driving permanent vertical displacement, we investigate tectonic uplift across the southern Hikurangi subduction margin, Aotearoa New Zealand, in the last ∼200 ka. Marine terraces preserved along the Wellington south coast have recently been dated as Marine Isotope Stage (MIS) 5a (∼82 ka), 5c (∼96 ka), 5e (∼123 ka) and 7a (∼196 ka) in age. We use these ages, together with new reconstructions of shoreline angle elevations, to calculate uplift rates across the margin and to examine the processes responsible for their elevation. The highest uplift rate—1.7 ± 0.1 mm/yr–and maximum tilting—2.9° to the west–are observed near Cape Palliser, the closest site to (∼50 km from) the Hikurangi Trough. Uplift rates decrease monotonically westward along the Palliser Bay coast, to 0.2 ± 0.1 mm/yr at Wharekauhau (∼70 km from the trough), defining a gently west-tilted subaerial forearc domain. Locally, active oblique-slip upper-plate faults cause obvious vertical offsets of the marine terraces in the axial ranges (>70 km from the trough). Uplift rates at Baring Head, on the upthrown side of the Wairarapa-Wharekauhau fault system, are ∼0.7–1.6 mm/yr. At Tongue Point, uplift on the upthrown side of the Ōhāriu Fault is 0.6 ± 0.1 mm/yr. Dislocation and flexural-isostatic modelling shows that slip on faults within the overriding plate—specifically the Palliser-Kaiwhata Fault and the Wairarapa-Wharekauhau fault system—may dominate uplift in their immediate hanging walls. Depending on their slip rate and geometry, slip on these two upper-plate fault systems could plausibly cause >80% of late Pleistocene uplift everywhere along the south coast of North Island. Our modelling suggests that subduction of the buoyant Hikurangi Plateau contributes uplift of 0.1–0.2 mm/yr and uplift due to sediment underplating at Tongue Point and Wharekauhau is likely ≤0.6 mm/yr but could be significantly lower. Earthquakes on the subduction interface probably contribute ≤0.4 mm/yr of late Pleistocene uplift, with ≤10% of uplift due to each earthquake being stored permanently, similar to other subduction zones. These results indicate a significant contribution of slip on upper-plate faults to permanent uplift and tilting across the subduction margin and suggest that in regions where upper-plate faults are prevalent, strong constraints on fault geometry and slip rate are necessary to disentangle contributions of deeper-seated processes to uplift.

The Significance of Interseismic Vertical Land Movement at Convergent Plate Boundaries in Probabilistic Sea‐Level Projections for AR6 Scenarios: The New Zealand Case

“…In contrast, along ∼30% of the coastline including eastern and southern lower North Island and upper South Island, geodetic data show land surface subsidence has been high this century with rates between 3 and 8 mm/y (Figure 2a). However, on longer (millennial) geological timescales these regions have generally been uplifted due to convergence and shortening across the plate boundary and the long-term aggregate effect of large earthquakes (Berryman et al, 2011;Clark et al, 2019;Howell & Clark, 2022;Ninis et al, 2023;Ryan et al, 2021). The most significant and/or damaging earthquake events that have affected New Zealand VLM this century are summarized in Section S1.3 in Supporting Information S1.…”

Integrating onshore and offshore paleoseismic evidence to reveal past large earthquakes on the weakly coupled central Hikurangi margin, New Zealand

“…Although most marginnormal motion is taken up on the subduction interface (Nicol & Beavan, 2003), the margin demonstrates a complex arrangement of back-arc rifting and volcanism within the Taupō Volcanic Zone (TVZ), extensional faulting on the erosional northern margin, and contractional faulting across the accretionary central and southern margin (Collot & Davy, 1998;Nicol et al, 2007;Wallace, 2004). Nearshore thrust faulting within the coastal part of the imbricated accretionary wedge contributes to shortening of the upper plate and coseismic uplift of the coastline (Berryman et al, 2018;Berryman et al, 1989;Litchfield et al, 2020;Ninis et al, 2023). These features splay from the interface in an almost continuous swath along the southern, central and northern margin (Barnes et al, 2002;Mountjoy & Barnes, 2011;Pondard & Barnes, 2010).…”

“…Commonly, the uplift of terraces along the southeast coast is associated with earthquakes on the nearshore reverse faults (e.g. Palliser-Kaiwhata fault) due to the patchy alongstrike extents of each terrace (Berryman et al, 2011;Litchfield et al, 2013;Ninis et al, 2023). However, it is plausible that a spatially heterogeneous slip distribution on the plate interface could replicate the disjointed pattern of uplifted geomorphologies, especially between Flat Point and Porangahau where nearshore faults have not been mapped (Berryman et al, 2011;Clark et al, 2019).…”