Rupert Sutherland

Prediction of Emperor-Hawaii seamount locations from a revised model of global plate motion and mantle flow

The bend in the Hawaiian-Emperor seamount chain is a prominent feature usually attributed to a change in Pacific plate motion ∼47 Myr ago. However, global plate motion reconstructions fail to predict the bend. Here we show how the geometry of the Hawaiian-Emperor chain and other hotspot tracks can be explained when we combine global plate motions with intraplate deformation and movement of hotspot plumes through distortion by global mantle flow. Global mantle flow models predict a southward motion of the Hawaiian hotspot. This, in combination with a plate motion reconstruction connecting Pacific and African plates through Antarctica, predicts the Hawaiian track correctly since the date of the bend, but predicts the chain to be too far west before it. But if a reconstruction through Australia and Lord Howe rise is used instead, the track is predicted correctly back to 65 Myr ago, including the bend. The difference between the two predictions indicates the effect of intraplate deformation not yet recognized or else not recorded on the ocean floor. The remaining misfit before 65 Myr ago can be attributed to additional intraplate deformation of similar magnitude.

The Australia‐Pacific boundary and Cenozoic plate motions in the SW Pacific: Some constraints from Geosat data

Finite poles of Australia-Pacific rotation are calculated using a three-plate (Australia-Antarctica-Pacific) model and published Geosat data analyses of the Indian and Pacific Oceans (GEOS-3P solution). Features identified on maps of Geosat data from SW of New Zealand are used to determine a new best fit finite pole for the prerift (mid Eocene; ∼45 Ma) configuration of the Australia-Pacific plate boundary and to refine Eocene-Miocene motions on the plate boundary (∼45–20 Ma; GEOS-NZ results). Inverting the Australia-Pacific best fit finite rotation to find relative displacements between east and west Antarctica since the Eocene implies dextral oblique extension in the Ross Embayment and almost pure dextral movement in the Queen Maud Range. Early-mid Eocene convergence predicted through Antarctica conflicts with geological evidence and remains an unresolved problem.

BASEMENT GEOLOGY AND TECTONIC DEVELOPMENT OF THE GREATER NEW ZEALAND REGION : AN INTERPRETATION FROM REGIONAL MAGNETIC DATA

Abstract The basement geology of New Zealand is composed of early Palaeozoic terranes of the Western Province, that are separated from late Palaeozoic–Mesozoic Eastern Province terranes by a suite of Carboniferous–Cretaceous arc-related igneous rocks (Median Tectonic Zone, MTZ). The Stokes Magnetic Anomaly System (SMAS) is associated with MTZ rocks and volcanogenic basement terranes of the Eastern Province. Offshore, it can be traced north along the northern margin of the New Caledonia Basin, and correlative Eastern Province rocks are found in New Caledonia. It can also be traced south across the Great South Basin, until a significant ENE-trending tectonic boundary is encountered on the central Campbell Plateau. This boundary is defined by linear gravity and magnetic anomalies (Campbell Magnetic Anomaly System, CMAS), and narrow fault-bounded sedimentary basins. If the sources of CMAS anomalies are correlative with those of the SMAS, then magnetic data require a ∼400 km dextral offset of basement rocks by faults along the northern margin of the CMAS prior to 80 Ma. The geometry of correlative Western Province and MTZ rocks in Marie Byrd Land supports the hypothesis that CMAS anomalies are sourced by MTZ-correlative rocks. A NNE-trending boundary on the central Challenger Plateau marks a change from high amplitude magnetic anomalies to weakly magnetic basement, and appears to represent a fundamental change in crustal character. The western Challenger Plateau and Lord Howe Rise (south of 30°S) are characterised by high amplitude magnetic and gravity anomalies with a NW-trending fabric, but the source of magnetic anomalies is unresolved. The magnetic character, combined with Cretaceous reconstruction, supports basement rock correlations with the east Lachlan Fold Belt or New England Fold Belt in Australia, rather than Western Province rocks in New Zealand. The magnetic signature of marginal ocean crust around New Zealand also offers clues into the regions tectonic history. Negative magnetic anomalies adjacent to the Campbell Plateau and Lord Howe Rise, and in the New Caledonia Basin, suggest that seafloor formation started during chron 33r (79–83 Ma). A linear positive magnetic anomaly in the outer Bounty Trough may be anomaly 33, which was isolated by a ridge-jump, and is consistent with separation of the Bollons Seamount continental fragment by ocean crust. Alternatively, it may be anomaly 34 and represent the earliest ocean crust formed between New Zealand and Marie Byrd Land. Magnetic lineations in the southern South Fiji Basin suggest that at least one ridge–ridge–ridge triple junction was active during its opening, and imply that Cenozoic ocean crust is younger in the east. At the southern edge of the South Fiji Basin, there is a significant tectonic boundary, named here the van der Linden Fault. It can be traced for ∼500 km and may have been a leaky transform during basin formation.

Plate boundary deformation in South Island, New Zealand, is related to inherited lithospheric structure

Abstract The Alpine Fault is the main active structure in the oblique continental collision zone of South Island, New Zealand. It is continuous at the surface for ∼800 km and accommodates ∼70% of current plate motion. A 460 km offset of basement rocks suggests it has accommodated >50% of plate displacement since 45 Ma. Geophysical, geological, and contemporary kinematic data are successfully modeled by slip on a narrow Alpine Fault zone extending into the lower crust. The data are consistent with plate boundary deformation occurring on either northwest- or southeast-dipping shear zones in the upper mantle, or with widely distributed strain in the upper mantle. We propose that the position of the Alpine Fault and any associated lithospheric discontinuity is controlled by an Eocene passive margin that separated Palaeozoic continental lithosphere of the Challenger Plateau from much younger oceanic lithosphere. Strike-slip motion since ∼25 Ma has translated the passive margin into the continental collision zone, resulting in subduction of oceanic lithosphere beneath South Island and progressive localisation of shear strain near the Alpine Fault. The Eocene rift boundary formed by exploiting Cretaceous oceanic transform faults and an older discontinuity within the New Zealand continent. We propose that a precursor to the Alpine Fault could be as old as Palaeozoic. Inherited structure has clearly controlled the first-order deformation pattern through New Zealand since at least 100 Ma and demonstrates that ancient faults can play an important role in determining lithospheric-scale patterns of deformation at continental plate boundaries.

Quaternary slip rate and geomorphology of the Alpine fault: Implications for kinematics and seismic hazard in southwest New Zealand

Glacial landforms at 12 localities in 9 river valleys are offset by the southern end of the onshore Alpine fault. Offsets cluster at ∼435, 1240, and 1850 m, consistent with evidence for glacial retreat at 18, 58, and 79 calendar ka. The peak of an offset fluvial aggradation surface is correlated with the Last Glacial Maximum at 22 ka. Displacement rates derived from features aged 18, 22, 58, and 79 cal. ka are 24.2 ± 2.2, 23.2 ± 4.9, 21.4 ± 2.6, and 23.5 ± 2.7 mm/yr, respectively, with uncertainties at the 95% confidence level. The joint probability, weighted mean, and arithmetic mean of all observations pooled by rank are 23.1 ± 1.5, 23.2 ± 1.4, and 23.1 ± 1.7 mm/yr, respectively. We conclude that the mean surface displacement rate for this section of the Alpine fault is 23.1 mm/yr, with standard error in the range of 0.7–0.9 mm/yr. The reduction in estimated long-term slip rate from 26 ± 6 mm/yr to 23 ± 2 mm/yr results in an increase in estimated hazard associated with faulting distributed across the rest of the plate boundary. Model-dependent probabilities of Alpine fault rupture within the next 50 yr are in the range 14%–29%. The 36 ± 3 mm/yr of total plate motion (NUVEL-1A) is partitioned into 23 ± 2 mm/yr of Alpine fault dextral strike slip, 12 ± 4 mm/yr of horizontal motion by clockwise block rotations and oblique dextral-reverse faulting up to 80 km southeast of the Alpine fault, and 5 ± 3 mm/yr of heave on reverse faults at the peripheries of the plate boundary.

Characterizing the seismogenic zone of a major plate boundary subduction thrust: Hikurangi Margin, New Zealand

The Hikurangi subduction margin, New Zealand, has not experienced any significant (>Mw 7.2) subduction interface earthquakes since historical records began ∼170 years ago. Geological data in parts of the North Island provide evidence for possible prehistoric great subduction earthquakes. Determining the seismogenic potential of the subduction interface, and possible resulting tsunami, is critical for estimating seismic hazard in the North Island of New Zealand. Despite the lack of confirmed historical interface events, recent geodetic and seismological results reveal that a large area of the interface is interseismically coupled, along which stress could be released in great earthquakes. We review existing geophysical and geological data in order to characterize the seismogenic zone of the Hikurangi subduction interface. Deep interseismic coupling of the southern portion of the Hikurangi interface is well defined by interpretation of GPS velocities, the locations of slow slip events, and the hypocenters of moderate to large historical earthquakes. Interseismic coupling is shallower on the northern and central portion of the Hikurangi subduction thrust. The spatial extent of the likely seismogenic zone at the Hikurangi margin cannot be easily explained by one or two simple parameters. Instead, a complex interplay between upper and lower plate structure, subducting sediment, thermal effects, regional tectonic stress regime, and fluid pressures probably controls the extent of the subduction thrusts seismogenic zone.

Cenozoic bending of New Zealand basement terranes and Alpine Fault displacement: A brief review

Abstract It is now possible to accurately reconstruct late Eocene‐Recent positions of the Australian plate with respect to the Pacific plate. Rates of plate displacement through New Zealand were slow in Eocene‐Oligocene time, and increased rapidly in Miocene time as the pole of rotation migrated southeast. Reconstruction of Maitai Terrane rocks suggests they had a smooth and continuously curved geometry through New Zealand in late Eocene time, which was probably similar to their Late Cretaceous configuration. This analysis implies that most or all Alpine Fault offset occurred after 45 Ma, and suggests about half the Eocene‐Recent plate displacement was partitioned onto the Alpine Fault, and half was accommodated by distributed crustal shear. Estimates of Pliocene and Quaternary Alpine Fault displacement rate suggest the proportion of plate boundary displacement partitioned onto the Alpine Fault may have increased with time, displacement rate, and total displacement.

Drilling reveals fluid control on architecture and rupture of the Alpine fault, New Zealand

Rock damage during earthquake slip affects fluid migration within the fault core and the surrounding damage zone, and consequently coseismic and postseismic strength evolution. Results from the first two boreholes (Deep Fault Drilling Project DFDP-1) drilled through the Alpine fault, New Zealand, which is late in its 200–400 yr earthquake cycle, reveal a >50-m-thick “alteration zone” formed by fluid-rock interaction and mineralization above background regional levels. The alteration zone comprises cemented low-permeability cataclasite and ultramylonite dissected by clay-filled fractures, and obscures the boundary between the damage zone and fault core. The fault core contains a <0.5-m-thick principal slip zone (PSZ) of low electrical resistivity and high spontaneous potential within a 2-m-thick layer of gouge and ultracataclasite. A 0.53 MPa step in fluid pressure measured across this zone confirms a hydraulic seal, and is consistent with laboratory permeability measurements on the order of 10?20 m2. Slug tests in the upper part of the boreholes yield a permeability within the distal damage zone of ?10?14 m2, implying a six-orders-of-magnitude reduction in permeability within the alteration zone. Low permeability within 20 m of the PSZ is confirmed by a subhydrostatic pressure gradient, pressure relaxation times, and laboratory measurements. The low-permeability rocks suggest that dynamic pressurization likely promotes earthquake slip, and motivates the hypothesis that fault zones may be regional barriers to fluid flow and sites of high fluid pressure gradient. We suggest that hydrogeological processes within the alteration zone modify the permeability, strength, and seismic properties of major faults throughout their earthquake cycles.

A model of active faulting in New Zealand

Active fault traces are a surface expression of permanent deformation that accommodates the motion within and between adjacent tectonic plates. We present an updated national-scale model for active faulting in New Zealand, summarize the current understanding of fault kinematics in 15 tectonic domains, and undertake some brief kinematic analysis including comparison of fault slip rates with GPS velocities. The model contains 635 simplified faults with tabulated parameters of their attitude (dip and dip-direction) and kinematics (sense of movement and rake of slip vector), net slip rate and a quality code. Fault density and slip rates are, as expected, highest along the central plate boundary zone, but the model is undoubtedly incomplete, particularly in rapidly eroding mountainous areas and submarine areas with limited data. The active fault data presented are of value to a range of kinematic, active fault and seismic hazard studies.

Transpressional development of the Australia‐Pacific boundary through southern South Island, New Zealand: Constraints from Miocene‐Pliocene sediments, Waiho‐1 borehole, South Westland

Abstract Clast compositions of Waiho‐1 conglomerate horizons record dextral movement of the borehole site past different basement terranes on the opposite side of the Alpine Fault. Metavolcanic and volcanogenic metagreywacke clasts in sediments aged c. 4.5 Ma indicate deposition adjacent to exposures of Caples, Dun Mountain—Maitai, or Brook Street basement terranes. Sediments aged c. 4 Ma contain Torlesse Terrane and Haast Schist clasts. Relocation of the Waiho‐1 borehole site using published finite rotations, combined with the observed change in provenance at c. 4.5 Ma, requires that the source rocks had relationships similar to those observed at the present day, that is, that most curvature of basement terranes in southern South Island, New Zealand, was already in place at 4.5 Ma. Major increases in sedimentation rate recorded by Waiho‐1 borehole at c. 12 and 5 Ma are linked to changes in the Australia‐Pacific plate vector. Subsidence before 12 Ma may have been related to thermal subsidence of the Emera...