Russell Van Dissen

The Rangipo fault, Taupo rift, New Zealand: An example of temporal slip-rate and single-event displacement variability in a volcanic environment

Geomorphic mapping and paleoseismic studies reveal that the Rangipo fault, the eastern boundary of the southern section of the Taupo rift, New Zealand, has highly variable single-event displacement and slip rate in time. Variability in single-event displacement (0.1–1.2 m since 14 cal. ka) is possibly the result of different fault rupture modes: primary (unsegmented and segmented) and secondary. Variability in fault-slip rate is attributed to interactions with volcanic activity of the nearby Ruapehu volcano (11 km distant), the southernmost and largest andesitic volcanic edifice within the southern Taupo volcanic zone. The Rangipo fault has had a mean slip rate of ∼1.4 mm/yr since ca. 25 cal. ka (calibrated radiocarbon age in thousands of years before present), and a mean slip rate of only ∼0.2 mm/yr since 14 cal. ka. This implies that during the period between 14 and 17–25 cal. ka, the fault had a slip rate of 2–9 mm/yr. This period of increased slip rate coincided with the most voluminous eruptions from Ruapehu volcano in the past 100 k.y. We infer that there is an interaction between the Rangipo fault and Ruapehu volcano, although we cannot confirm the sense of the interaction (i.e., volcano→fault or fault→volcano). The interaction could be related to pulsed rifting, where major rifting occurs in periods of accelerated faulting that coincide with extensive eruptions.

Palinspastic reconstructions of southeastern Marlborough, New Zealand, for mid‐Cretaceous‐Eocene times

Abstract Southeastern Marlborough, New Zealand, preserves many complete sections through the Cretaceous/ Tertiary (K/T) boundary. Attempts to understand the paleogeography of these sections are hampered by the pervasive, Neogene deformation of the area associated with the propagation of the modern Pacific/Australian plate boundary through New Zealand. In this paper, we produce palinspastic maps of southeastern Marlborough for five intervals of Cretaceous and Paleogene time, based on a retro‐deformed, pre‐Neogene geographic model. Retro‐deformation takes account of: displacements on five major faults; distributed, between‐fault shortening; and a uniform, vertical axis, clockwise rotation of 100°. The mapped intervals are: (1) part of the Urutawan‐Motuan (middle‐late Albian, c. 105–102 Ma); (2) the Piripauan (latest Coniacian to late Santonian, 86.5–84.5 Ma); (3) the Early to early Late Haumurian (late Santonian‐Campanian, 84.5–72 Ma); (4) the late Late Haumurian to late Teurian (late Maastrichtian to late Paleocene, 68–58 Ma); and (5) the Waipawan‐Mangaorapan (early Eocene, 55–51 Ma). During the Cretaceous and Paleogene, southeastern Marlborough lay on the generally north‐facing, Pacific margin of proto‐New Zealand. The palinspastic maps record the progressive drowning of what we infer to be a faulted platform, the “Marlborough paleo‐platform”, that formed the eastern boundary of a large embayment, the “Marlborough paleo‐embayment”. In the late Early and early Late Cretaceous, terrigenous clastic sediments were deposited on the platform at mostly shelf to upper bathyal depths. Ngaterian (late Albian‐Cenomanian) and Piripauan (latest Coniacian to late Santonian) paleoshorelines lay within the study area and were oriented northeast‐southwest. Subsequently, regional, passive subsidence of the continental margin resulted in transgression towards the south and southeast and a switch from terrigenous clastic to biogenic sedimentation. By the end of the Cretaceous, much of the Marlborough paleo‐platform was at outer shelf to bathyal depths; by the early Eocene, it lay entirely at bathyal depths. During the latest Cretaceous and Paleogene, the position of the Marlborough paleo‐embayment coincided approximately with a significant boundary in sedimentary regime, separating dominantly biogenic sediments in the east from mixed biogenic‐siliciclastic sediments to the west. The palinspastic maps show internal consistencies that give us some confidence in the new analysis. Differences from previous maps are attributed both to the retro‐deformation and also to variations in the locations, values, and number of data points used to construct isopachs. Locally restoring paleogeography by retro‐deforming structures is likely to be of most use where the amount of deformation is high (e.g., >20% shortening and/or some tens of kilometres of fault displacements), where the isopachs are well constrained by robust data points, and where regional or global controls on sedimentary and biological patterns are significant and of interest.

Constraints on the timing of the three most recent surface rupture events and recurrence interval for the Ohariu Fault: Trenching results from MacKays Crossing, Wellington, New Zealand

Abstract A trench across the Ohariu Fault at MacKays Crossing revealed evidence and provided radiocarbon age constraints for three surface‐rupture events in the last 5300 cal. yr BP. The two older events are dated for the first time at 5270–4410 and 4810–3260 cal. yr BP. The youngest event is correlated to the most recent event, dated elsewhere along the Ohariu Fault, at 1050–1000 cal. yr BP. Using statistical methods incorporating inter‐event times, mean slip rate, and single‐event displacement, and their uncertainties, a best estimate mean recurrence interval of 2200 yr is calculated for the Ohariu Fault, with minimum and maximum 68% and 95% confidence interval limits of 1300 and 3800 yr, and 800 and 7000 yr, respectively. Following the procedures outlined in the Ministry for the Environment Guidelines, these data place the Ohariu Fault in Fault‐avoidance Recurrence Interval Class II (>2000 to ≤3500 yr), with medium to low confidence of classification.

Timing of the most recent surface rupture event on the Ohariu Fault near Paraparaumu, New Zealand

Abstract Thirteen radiocarbon ages from three trenches across the Ohariu Fault tightly constrain the timing of the most recent surface rupture event at Muaupoko Stream valley, c. 2 km east of Paraparaumu, to between 930 and 1050 cal. yr BP. This age overlaps with previously published ages of the most recent event on the Ohariu Fault and together they further constrain the event to 1000–1050 cal. yr BP. Two trenches provide loose constraints on the maximum recurrence interval at 3–7000 yr. Tephra, most probably the Kawakawa Tephra, was found within alluvial fan deposits in two of the trenches.

Post c. 300 year rupture of the Ohariu Fault in Ohariu Valley, New Zealand

Abstract An important component of quantifying seismic hazard and risk in regions such as Wellington is to characterise both the long-term rate of occurrence of the regions major earthquake-generating active faults, as well as potential interactions between faults (e.g., the potential for earthquake triggering). This paper describes paleoseismic data from two trenches (Ohariu Valley) and a natural streambank exposure (Horokiri valley) c. 24 km apart, which constrain the timing and size of recent surface rupture events on the northeast striking Ohariu Fault c. 6 km northwest of Wellington City. One event is recorded at all three sites and radiocarbon age constraints indicate it correlates with the 1050–1000 cal. years BP event previously identified elsewhere along the fault. A younger, smaller (decimetre-scale) event is recorded in one of the Ohariu Valley trenches, and the timing is constrained by two radiocarbon ages to post-310 cal. years BP. This event may either be a small, primary Ohariu Fault rupture, or a triggered event associated with a large earthquake on a nearby fault. If it is a triggered rupture, then a possible mechanism is dynamic triggering associated with one of the recent large-great earthquakes on the Wellington (post-300 yr), Wairarapa (AD1855), or Awatere (AD1848) Faults. Small rupture events do not necessarily contribute to the recurrence interval classification of the Ministry for the Environment Active Fault Guidelines, but they could be important for sensitive structures crossing the fault and for lifelines crossing multiple active faults.

Reassessment of slip rate and implications for surface rupture hazard of the Martinborough Fault, South Wairarapa, New Zealand

Abstract New mapping of the Martinborough Fault utilising high resolution LIDAR data and orthophotos defines a c. 1.3 km long active fold trace consisting of a broad (50–150 m wide) warp crossing dissected alluvial fans. GPS survey profiles across the fold‐scarp reveal vertical separations ranging from 1.2 to 6 m on two fan surfaces. Published OSL ages imply a Porewan age for the upper fan (surface age c. 55 ka). We calculate a revised vertical slip rate of c. 0.1 mm/yr (previously published rates range between 0.3 and 0.67 mm/yr) and a recurrence interval of c. 15 000 yr from these new data. This places the fault within the Ministry for the Environment Active Fault Guidelines Recurrence Interval Class V (>10 000 to <20 000 yr), to which a medium level of confidence is assigned.

A reassessment of abruptly fanning bed dips on the northwestern limb of the Huangarua Syncline, Wairarapa, New Zealand

Abstract Trench and outcrop data are used to reinterpret abruptly fanning bed dips described by Lamb & Vella in Pleistocene conglomerates on the northwestern limb of the Huangarua Syncline, Wairarapa. These data indicate that c. 70% (c. 40°) of the reported change in bed dip occurs across an angular unconformity. The conglomerate above the unconformity, which is significantly younger than the underlying lower Te Muna Formation (c. ≤ 1.6 Ma), is correlated with the Ahiaruhe Formation (c. 0.015–0.5 Ma). Pleistocene stratigraphy was influenced by discontinuous sedimentation across the entire northwestern limb of the syncline during folding.