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We have analysed 11 years of geodetic data from 30 stations distributed over the Otago Fault System in the South Island of New Zealand. Velocities were estimated from time series corrected for coseismic displacements from the 2004 Macquarie Island and 2007 George Sound earthquakes and the coseismic and the short term postseismic deformation associated with the 2009 Dusky Sound earthquake. By dividing the corrected time series in half we were able to demonstrate the existence of a systematic difference between the pre- and post-earthquake velocity fields, associated with a longer term viscoelastic transient related to the 2009 Dusky Sound earthquake. In the northern part of our study area, the geodetic strain rate data are consistent with elastic strain accumulation on the Alpine Fault while in the south and east, the strain rate tensors are consistent with the Otago Fault System. There is a significant change in orientation in the axis of contraction from east to west across the network that corre...

The first phase of the Deep Fault Drilling Project (DFDP-1) yielded a continuous lithological transect through fault rock surrounding the Alpine fault (South Island, New Zealand). This allowed micrometer- to decimeter-scale variations in fault rock lithology and structure to be delineated on either side of two principal slip zones intersected by DFDP-1A and DFDP-1B. Here, we provide a comprehensive analysis of fault rock lithologies within 70 m of the Alpine fault based on analysis of hand specimens and detailed petrographic and petrologic analysis. The sequence of fault rock lithologies is consistent with that inferred previously from outcrop observations, but the continuous section afforded by DFDP-1 permits new insight into the spatial and genetic relationships between different lithologies and structures. We identify principal slip zone gouge, and cataclasite-series rocks, formed by multiple increments of shear deformation at up to coseismic slip rates. A 20?30-m-thick package of these rocks (including the principal slip zone) forms the fault core, which has accommodated most of the brittle shear displacement. This deformation has overprinted ultramylonites deformed mostly by grain-size-insensitive dislocation creep. Outside the fault core, ultramylonites contain low-displacement brittle fractures that are part of the fault damage zone. Fault rocks presently found in the hanging wall of the Alpine fault are inferred to have been derived from protoliths on both sides of the present-day principal slip zone, specifically the hanging-wall Alpine Schist and footwall Greenland Group. This implies that, at seismogenic depths, the Alpine fault is either a single zone of focused brittle shear that moves laterally over time, or it consists of multiple strands. Ultramylonites, cataclasites, and fault gouge represent distinct zones into which deformation has localized, but within the brittle regime, particularly, it is not clear whether this localization accompanies reductions in pressure and temperature during exhumation or whether it occurs throughout the seismogenic regime. These two contrasting possibilities should be a focus of future studies of fault zone architecture.

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.

Lake basins that experience rapid rates of deposition act as high-resolution environmental archives because they produce sedimentary records that have centennial or even decadal resolution. However, identifying target fractions for radiocarbon dating of lake sediments remains problematic because reworked organic material from fluvial catchments can produce anomalously old radiocarbon ages. This study determines the extent to which reworked material from catchment soils impacts radiocarbon dates on pollen and other organic concentrates by comparing radiocarbon dates produced by these techniques against a chronostratigraphic marker in cores from Lake Mapourika, New Zealand. Pollen preferentially preserved and reworked from catchment soils was identified using soil palynology. A technique was then developed to remove reworked pollen types from pollen concentrates extracted from lake sediment. Identification and removal of reworked pollen from pollen concentrates produced ages that were consistently closer to the age of the chronostratigraphic horizon than other organic concentrates. However, these dates were still between 736 and 366 calendar years older than expected. The only organic fractions that reliably reproduced the age of the chronostratigraphic horizon were terrestrial leaf macrofossils, although terrestrial leaf macrofossils isolated from megaturbidite deposits, which are formed by high-energy depositional events, also provided anomalously old ages. The results indicate that leaf material extracted from hemipelagite, which accumulates gradually, is likely to be the only organic fraction to produce reliable chronology in lakes where a component of sedimentation is driven by the fluvial system. The results also demonstrate the importance of conducting a detailed investigation of physical sedimentology before selecting material for radiocarbon dating lake sediments.

We combine recently acquired airborne light detection and ranging (LiDAR) data along a portion of the Alpine fault with previous work to define the ways in which the plate-boundary structures partition at three different scales from 6 to 10 0 m. At the first order ( 6 –10 4 m), the Alpine fault is a remarkably straight and unpartitioned structure controlled by inherited and active weakening processes at depth. At the second order (10 4 –10 3 m), motion is serially partitioned in the upper ∼1–2 km onto oblique-thrust and strike-slip fault segments that arise at the scale of major river valleys due to stress perturbations from hanging-wall topographic variations and river incision destabilization of the hanging-wall critical wedge, concepts proposed by previous workers. The resolution of the LiDAR data refines second-order mapping and reveals for the first time that at a third order (10 3 –10 0 m), the fault is parallel-partitioned into asymmetric positive flower structures, or fault wedges, in the hanging wall. These fault wedges are bounded by dextral-normal and dextral-thrust faults rooted at shallow depths (

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

Large earthquakes in mountain regions commonly trigger extensive landsliding and are important drivers of erosion, but the contribution of this landsliding to long-term erosion rates and seismic hazard remains poorly understood. Here we show that lake sediments record postseismic landscape response as a sequence of turbidites that can be used to quantify erosion related to large (moment magnitude, M w > 7.6) earthquakes on the Alpine fault, New Zealand. Alpine fault earthquakes caused a threefold increase in sediment flux over the ∼50 yr duration of each postseismic landscape response; this represents considerable delayed hazard following earthquake-induced strong ground motion. Earthquakes were responsible for 27% of the sediment flux from the lake catchment over the past 1100 yr, leading us to conclude that Alpine fault earthquakes are one of the most important drivers of erosion in the range front of the Southern Alps.

Strata and fault relationships revealed in five trenches excavated across the recent trace of the Alpine fault at the Haast, Okuru, and Turnbull Rivers, South Westland, New Zealand, record the three most recent surface‐faulting events. Using back‐stripping techniques to remove the three faulting events and the sedimentary units associated with the faulting restores the cross‐sections to gravel‐bed floodplains at the Haast and Okuru Rivers, at about A.D. 750. Horizontal and vertical offsets of stream channels and terrace risers reveal characteristic displacements of about 8–9 m dextral and up to 1 m vertical per event. Cumulative dextral displacement is 25±3 m in the past three events. The most recent surface‐rupture event was probably in A.D. 1717, and the next prior events were about A.D. 1230±50 and about A.D.750±50. The timing of these events is consistent with past large‐great earthquakes on the southern section of the Alpine fault inferred from off‐fault data, but there are fewer events identified in trenches. Our three‐event dataset indicates the average surface‐rupture recurrence interval for the South Westland section of the fault is about 480 years, much longer than the current elapsed time of 295 years. Therefore, the Alpine fault in South Westland may not be close to rupture as is often speculated.

Low-grade albite- and peristerite-mylonites occur as a northeast-tapering, 200–300 m wide zone structurally above basal cataclasite on the Alpine fault (New Zealand). They form an inverted metamorphic sequence grading eastward into an ∼1-km-wide, amphibolite-facies−derived mylonite zone in the hanging wall. Their mineralogy matches low-grade protoliths that occur 100–110 km to the southeast. This displacement is compatible with distributed lateral and vertical shear within the mylonite zone since inception of significant convergence on the plate boundary in the late Neogene.

Measurements of 10Be concentrations in quartz‐rich boulders exposed by the uplift of anticlinal ranges in Central Otago, New Zealand, are used to investigate the fault propagation styles and rates for the underlying blind reverse faults. 10Be ages along Little Rough Ridge reveal the propagation rate for this fault to be between zero (i.e., not propagating at all, if the ages reflect emergence of a fault of fixed length through a sloping surface) and a maximum of 8.1 mm/yr, if the ages reflect true lateral growth. Regardless of the propagation rate, Little Rough Ridge has established its length rapidly compared with the rate at which it has accumulated displacement. On Raggedy Range, the 10Be ages, together with structural data, imply that the ridge reached its current length by the merging of several small fault segments. This study emphasises the importance of combining 10Be measurements with geomorphic observations. Without understanding the geomorphological context, 10Be exposure ages are diff...

The Alpine Fault accommodates around 60–70% of the 37 mm/yr oblique motion between the Australian and Pacific plates in the South Island of New Zealand. Uplift on the fault over the past 5 Ma has led to the exhumation of the deep-seated mylonite zone alongside the present surface trace. Shear strain estimates in the mylonites reach 200–300 in the most highly strained rocks, and provide an integrated displacement across the zone of 60–120 km. This is consistent with the amount of displacement during the last 5 Ma, suggesting that displacement on the fault is localised within a 1–2 km wide ductile shear zone to depths of 25–30 km. Existing geodetic data, together with Late Quaternary slip rate and paleoseismic data, are consistent with the steady build-up and release of elastic strain in the upper crust driven by ductile creep within a narrow mylonite zone at depth. Faults of the Otago Fault System form a parallel array east of the Alpine Fault and accommodate c. 2 mm/yr contraction. Long periods of quiescence on individual structures suggest episodic, or “intermittently characteristic”, behaviour. This is more consistent with failure on faults within an elastico-frictional upper crust above a ductile lower crust. Localisation of crustal deformation may be initiated by inherited weaknesses in the upper crust, with downward propagation of slip causing strain weakening within the ductile zone immediately beneath. Inherited structures of great length focus a greater amount of displacement and hence more rapidly develop underlying zones of ductile shear.

The NNE‐striking faults of the Nokomai valley, Southland, New Zealand, are the southern extension of the Nevis‐Cardrona Fault System, and have similar deformation style. This fault zone defines the boundary between broad, smooth, central Otago ranges and basins dominated by a deformed regional unconformity cut into the basement, and rugged mountains and deeply incised bedrock valleys to the west. Nokomai valley topography is dominated by bounding ranges: Slate and Hector Ranges to the west, and Garvie Range to the east. These ranges are rising on NNE‐striking oblique reverse faults, which dip beneath the ranges in the upper and lower Nokomai valley. The middle reaches of the Nokomai valley are cut by a complex set of NE‐ENE strike‐slip faults with some reverse component. These faults pass through the headwaters of eastern tributaries of the main Nokomai River, which itself passes through a bedrock gorge. Structural depressions have developed in the upper and lower Nokomai valleys, where the obliq...

A 400 ± 100 m offset of Lake McKerrow, South Westland, New Zealand, combined with dated (15.6 ka) glacial lake silts, requires an Alpine Fault displacement rate of 26 ± 7 mm/yr. Moraines associated with Hokuri Creek (assumed to be 17 ± 2 ka) are offset by 440 ± 40 m and require a displacement rate on the Alpine Fault of 26 ± 6 mm/yr. Slickensides, fault exposure, and offset topography are consistent with an almost pure dextral sense of movement on a vertical or subvertical fault. Locally, a small vertical component of up‐to‐the‐west movement is observed. Folding in late Quaternary sediments indicates active tilting of sediments at up to 0.4°/ka and variations in local uplift/subsidence rates of up to 4 mm/yr. At one locality c. 1 km northwest of the Alpine Fault and near the core of an anticline, uplifted shells require an uplift rate of 1.4 ± 0.5 mm/yr relative to sea level. Displaced river channels provide estimates of the last two coseismic displacements on the fault of 9 m (penultimate) and 8...

Recent mapping of the Alpine Fault trace in the Paringa region has revealed the existence of an extensive Haast Schist‐derived thrust nappe resting on Western Province basement rock and moraine. Erosion of the nappe by the Paringa River and its tributaries, however, has resulted in eastward propagation of the active fault zone, forming southeast‐dipping thrust faults linked by swarms of steeply dipping strike‐slip faults. Late Quaternary sediments of the Paringa Formation have been intensely deformed along a newly developed zone of shortening and uplift on the northeast side of Paringa River. Marine, fluviatile, lacustrine, and terrestrial sediments record progressive uplift east of the Alpine Fault. The occurrence of lake deposits rhythmically interbedded with forest horizons may have resulted from damming of the Paringa River behind the zone of rapid uplift. The uplift rate for this region over the last 16 ka has been calculated at 13.7 ± 1 mm/yr. After removing the effects of tilting due to lo...

Stratigraphic sections through sag ponds developed along the Alpine Fault trace between Milford Sound and John O'Groats River record drastic changes in sedimentation. Swamp or forest-floor material has been inundated by up to 0.5 m of cataclasite-derived sand and gravel. Two events, inferred to represent degradation of newly created scarps on the Alpine Fault, occurred at, or later than, 230 ± 50 yr B.P., and 1980 ±60 yr B.P., respectively. At John O'Groats River, large silver beech trees growing on the Alpine Fault scarp have been topped at heights of 8–15 m above ground level by movement on the fault. Unbroken beeches have mean ages calculated at 266 +90, -60 years. On a regional scale, a colluvial fan-forming event at Kaipo River and rotational slumping at Gorge Plateau are broadly synchronous with inferred fault movements near John O'Groats River. It is suggested that a large seismic event, related to the last major movement on the Alpine Fault, occurred in the middle 17th to early 18th century.

[1] In situ cosmogenic 10Be and 26Al measurements on sedimentary quartzites, together with analysis of abandoned river terraces, can be used to show how drainage responded to the lateral propagation of a late Quaternary anticline forming above a blind reverse fault in Central Otago, New Zealand. A close link between the progression of cosmogenic ages and the tectonic geomorphology allows us to confirm that uplift and propagation rates on the anticline are in the ranges 0.08–0.12 mm yr−1 and 0.8–1.5 mm yr−1, respectively, over the last 550,000 years. The agreement between the isotopic and geomorphological evidence in turn requires that minimum 10Be exposure ages as great as 660 ka are not in steady state with respect to erosion. This is a geochemical result of significance, as it opens the possibility of a more regional analysis of landscape evolution in this region, using the same rocks. On an adjacent anticline, thought to be even older on geomorphological arguments, 10Be concentrations give minimum exposure ages in the range 750–1400 ka. These extremely old minimum ages are rare worldwide, as they are usually limited to smaller values by erosion. They are attributable to the very resistant nature of the quartz-rich boulders in which the measurements were made, which in places are shown to have erosion rates lower than 0.5 mm kyr−1.

$^{10}Be$ concentrations measured in silica-cemented Tertiary sandstones in Central Otago have yielded minimum exposure ages of up to 1400 ka, some of the oldest ever recorded outside the dry valleys of Antarctica and some arid deserts. The silica-cemented sandstones outcrop as boulders in a region where their exposure is caused by the growth of anticlinal ridges above blind reverse faults. Initial studies using a combination of in situ cosmogenic isotope measurements and geomorphological observations on one range have demonstrated the potential of this technique to investigate Late Cenozoic fault growth and erosion rates. The best-exposed and preserved occurrence of the quartz-rich boulders is at German Hill on North Rough Ridge, where their stratigraphic context and their sedimentary and diagenetic origin, together with their method of emplacement and preservation on the modern land surface, can be studied in detail. These are all features that affect their suitability for cosmogenic isotope studies of landscape development and are discussed in this article. $^{10}Be$ concentrations in the German Hill rocks yield minimum exposure ages of 600–1300 ka, and it is possible that these concentrations are not in saturated steady state with respect to erosion. These ages imply that the uplift of North Rough Ridge by its underlying fault is a Quaternary phenomenon. This conclusion has implications for the likely earthquake recurrence on the fault, which with an estimated average long-term slip rate of 0.5 mm yr$^{-1}$, is expected to be infrequent.

De Pascale et al. (2014) present a new “bimodal” fault rupture behavior model for the dextral-reverse Alpine fault using evidence from several new, small (

All eukaryote populations accumulate mutations in their mitochondrial DNA (mtDNA) over time, so reproductively isolated populations become characterized by distinct mtDNA lineages. In addition, the degree of genetic differentiation among distinct populations can be used to estimate time elapsed since their isolation. We have identified an informative system for calibrating the mtDNA “clock” by genetically comparing freshwater galaxiid fish populations isolated in different river drainages. Calibration using a range of Quaternary geological events in southern New Zealand shows that the mtDNA divergence rate in galaxiid fishes is between 1% and 2%/100 k.y. up to 250 k.y., with the rate decreasing with increasing age. The estimated divergence rate slows to around 4%/m.y. for the middle Quaternary, although calibration is poor. A calibration curve has been fitted to all data: divergence (%) = −2.2 e −9 t + 2.5 t + 2.2, where t is isolation age (in m.y.). This molecular clock has potential as a dating tool for glacially related and active tectonic events that have caused river drainage changes in the late Quaternary in the Southern Hemisphere, where galaxiids are widespread. An application of this dating tool to an example in northern South Island uses three different species of freshwater-limited fish, and all three data sets imply formation of a drainage divide at 320 ± 110 ka, at about the time of a major glacial advance though the divide (oxygen isotope stage 8).

Concentration of erosional activity along transpressional plate boundaries can significantly alter the pattern of mechanical behavior through the influence of exhumation on crustal strength. Three-dimensional numerical modeling of an obliquely convergent orogen shows that a single oblique plate-bounding structure is stable if asymmetric erosion patterns, such as those observed in orographic mountain belts, pertain, and if Earth9s crust has a strong-on-weak rheology. In early stages of oblique convergence of an initially laterally homogeneous material, lateral (boundary-parallel) strain is accommodated along a near vertical structure and convergent (boundary-normal) strain is concentrated on structures dipping at moderate angles into the orogen. Exhumation of deep crustal material along the convergent structure results in thermal weakening along this dipping structure. When the upper crust beneath the orogen is significantly weakened by exhumation, lateral strain abandons the vertical structure and shifts to the dipping structure, combining with the convergent strain to form a single oblique fault that accommodates the plate motion in the upper crust, as is the case along the Alpine fault, New Zealand. The process of thermal thinning is controlled by advection and occurs on time frames of ∼1–2 m.y. The two components of strain remain separate in the lower crust. During active convergence, exhumation of lower crustal material occurs only along those structures accommodating convergent strain. Consequently, material exhumed from lower regions of ductile deformation, as is the case along the Alpine fault, contains lineations that indicate a greater component of convergence than predicted from the total plate motion. Postorogenic exhumation of the roots of an oblique plate boundary will expose two parallel shear zones, one dominantly convergent and one dominantly strike slip. Widely reported orogen-parallel transport in the late stages of ancient oblique convergence may represent not a change in plate vector, but the exhumation of the lateral transport zone.

Carter, R.M. and Norris, R.J., 1977. Redeposited conglomerates in a Miocene flysch sequence at Blackmount, western Southland, New Zealand, Sediment. Geol., 18: 289319. The Middle Miocene Monowai Formation represents a gravel delta that prograded south into a flysch basin complex developed along the Moonlight Tectonic Zone, southern New Zealand. The delta-slope environment was characterized by a conglomeratic sequence up to 500 m thick. Most of the gravel was moved downslope by mass-transport processes. A complete spectrum exists from synsedimentary slide sheets (up to 10 m thick and 100 m long) that retain pre-sliding sedimentary structures, to more mature mass-transported sediment types in which all original structures have been destroyed. The most distal deposits include ungraded homogeneous pebble conglomerates up to 3 m thick. Some of the more mature redeposited conglomerate--sand--mud units (X--Y--Z sequences) are between 2 and 10 m thick; they comprise a basal X-division of bouldery conglomerate, a middle Y-division of pebbly mudstone or pebbly sandstone, and an upper Z-division of hydroplastically folded mudstone. Though X--Y--Z sequences may have been deposited from very proximal turbidity or fluxoturbidity currents, inertia-flow emplacement seems more likely. An inertia-flow mode of emplacement also seems most probable for the other redeposited sediment types described from the Monowai Formation.

Hydraulic fracturing, induced by thermal expansion of water, is invoked as a common phenomenon by which metamorphic fluid is progressively lost from a sediment pile undergoing metamorphism and subsequent orogeny. On linear thermal gradients, water loss may occur by this mechanism for all gradients greater than 12°C/km at depths greater than 5 to 10 km. During burial on lower gradients, water is retained in the pile and may cause widespread metasomatism.

Small displacements of recently deglaciated surfaces across mesoscopic fractures at several localities adjacent to the Alpine fault allow the determination of the direction (unknown). principal horizontal shortening as 126° ±10°. This is consistent with estimates from geodetic studies, microearthquake fault-plane solutions, and macroscopic faulting, and suggests that the displacements represent the release of accumulated regional tectonic strain. The rapid development of the features following ice removal over the past few centuries suggests a mechanism whereby ice loading allowed the accumulation of tectonic elastic strain in the near-surface rocks, which was subsequently released on unloading.