Zealand Alpine Fault Research Articles

Micro- and nano-porosity of the active Alpine Fault zone, New Zealand

Abstract. Porosity reduction in rocks from a fault core can cause elevated pore fluid pressures and consequently influence the recurrence time of earthquakes. We investigated the porosity distribution in the New Zealand's Alpine Fault core in samples recovered during the first phase of the Deep Fault Drilling Project (DFDP-1B) by using two-dimensional nanoscale and three-dimensional microscale imaging. Synchrotron X-ray microtomography-derived analyses of open pore spaces show total microscale porosities in the range of 0.1 %–0.24 %. These pores have mainly non-spherical, elongated, flat shapes and show subtle bipolar orientation. Scanning and transmission electron microscopy reveal the samples' microstructural organization, where nanoscale pores ornament grain boundaries of the gouge material, especially clay minerals. Our data imply that (i) the porosity of the fault core is very small and not connected; (ii) the distribution of clay minerals controls the shape and orientation of the associated pores; (iii) porosity was reduced due to pressure solution processes; and (iv) mineral precipitation in fluid-filled pores can affect the mechanical behavior of the Alpine Fault by decreasing the already critically low total porosity of the fault core, causing elevated pore fluid pressures and/or introducing weak mineral phases, and thus lowering the overall fault frictional strength. We conclude that the current state of very low porosity in the Alpine Fault core is likely to play a key role in the initiation of the next fault rupture.

Seismic P Wave Velocity Model From 3‐D Surface and Borehole Seismic Data at the Alpine Fault DFDP‐2 Drill Site (Whataroa, New Zealand)

AbstractThe New Zealand Alpine Fault is a major plate boundary that is expected to be close to rupture, allowing a unique study of fault properties prior to a future earthquake. Here we present 3‐D seismic data from the DFDP‐2 drill site in Whataroa to constrain valley structures that were obscured in previous 2‐D seismic data. The new data consist of a 3‐D extended vertical seismic profiling (VSP) survey using three‐component and fiber optic receivers in the DFDP‐2B borehole and a variety of receivers deployed at the surface. The data set enables us to derive a detailed 3‐D P wave velocity model by first‐arrival traveltime tomography. We identify a 100–460 m thick sediment layer (mean velocity 2,200 ± 400 m/s) above the basement (mean velocity 4,200 ± 500 m/s). Particularly on the western valley side, a region of high velocities rises steeply to the surface and mimics the topography. We interpret this to be the infilled flank of the glacial valley that has been eroded into the basement. In general, the 3‐D structures revealed by the velocity model on the hanging wall of the Alpine Fault correlate well with the surface topography and borehole findings. As a reliable velocity model is not only valuable in itself but also crucial for static corrections and migration algorithms, the Whataroa Valley P wave velocity model we have derived will be of great importance for ongoing seismic imaging. Our results highlight the importance of 3‐D seismic data for investigating glacial valley structures in general and the Alpine Fault and adjacent structures in particular.

Fault zone architecture of a large plate-bounding strike-slip fault: a case study from the Alpine Fault, New Zealand

Abstract. New Zealand's Alpine Fault is a large, plate-bounding strike-slip fault, which ruptures in large (Mw>8) earthquakes. We conducted field and laboratory analyses of fault rocks to assess its fault zone architecture. Results reveal that the Alpine Fault Zone has a complex geometry, comprising an anastomosing network of multiple slip planes that have accommodated different amounts of displacement. This contrasts with the previous perception of the Alpine Fault Zone, which assumes a single principal slip zone accommodated all displacement. This interpretation is supported by results of drilling projects and geophysical investigations. Furthermore, observations presented here show that the young, largely unconsolidated sediments that constitute the footwall at shallow depths have a significant influence on fault gouge rheological properties and structure.

Frictional properties and 3-D stress analysis of the southern Alpine Fault, New Zealand

New Zealand's Alpine Fault (AF) ruptures quasi-periodically in large-magnitude earthquakes. Paleoseismological evidence suggests that about half of all recognized AF earthquakes terminated at the boundary between the Central and South Westland sections of the fault. There, fault geometry and the polarity of uplift change. The South Westland AF exhibits oblique-normal fault motion on a structure oriented 052°/82°SE that, for at least 35 km along strike, contains saponite-rich principal slip zone gouges. New hydrothermal friction experiments reveal that the saponite fault gouge is frictionally weak, exhibiting friction coefficients between μ = 0.12 and μ = 0.16 for a range of temperatures (T = 25–210 °C) and effective normal stresses (σn' = 31.2–93.6 MPa). The saponite gouge is rate-strengthening in all velocity steps performed at velocities between 0.01 and 3.0 μm/s, behavior conducive to aseismic creep. A three-dimensional stress analysis shows that the South Westland AF is favorably oriented with respect to the regional stress field for slip within the frictionally weak saponite fault gouge. Geometrically, the fault is severely misoriented for slip in any fault-forming materials with friction coefficients exceeding μ∼0.5. The combination of weak gouges prone to aseismic creep, strong asperities, and low resolved shear stress may impede earthquake rupture propagation along the South Westland Alpine Fault.

Microstructures imply cataclasis and authigenic mineral formation control geomechanical properties of New Zealand's Alpine Fault

The Alpine Fault is capable of generating large (MW > 8) earthquakes and is the main geohazard on South Island, NZ, and late in its 250–291-year seismic cycle. To minimize its hazard potential, it is indispensable to identify and understand the processes influencing the geomechanical behavior and strength-evolution of the fault. High-resolution microstructural, mineralogical and geochemical analyses of the Alpine Fault's core demonstrate wall rock fragmentation, assisted by mineral dissolution, and cementation resulting in the formation of a fine-grained principal slip zone (PSZ). A complex network of anastomosing and mutually cross-cutting calcite veins implies that faulting occurred during episodes of dilation, slip and sealing. Fluid-assisted dilatancy leads to a significant volume increase accommodated by vein formation in the fault core. Undeformed euhedral chlorite crystals and calcite veins that have cut footwall gravels demonstrate that these processes occurred very close to the Earth's surface. Microstructural evidence indicates that cataclastic processes dominate the deformation and we suggest that powder lubrication and grain rolling, particularly influenced by abundant nanoparticles, play a key role in the fault core's velocity-weakening behavior rather than frictional sliding. This is further supported by the absence of smectite, which is reasonable given recently measured geothermal gradients of more than 120 °C km−1 and the impermeable nature of the PSZ, which both limit the growth of this phase and restrict its stability to shallow depths. Our observations demonstrate that high-temperature fluids can influence authigenic mineral formation and thus control the fault's geomechanical behavior and the cyclic evolution of its strength.

A comparison of the use of X-ray and neutron tomographic core scanning techniques for drilling projects: insights from scanning core recovered during the Alpine Fault Deep Fault Drilling Project

Abstract. It is now commonplace for non-destructive X-ray computed tomography (CT) scans to be taken of core recovered during a drilling project. However, other forms of tomographic scanning are available, and these may be particularly useful for core that does not possess significant contrasts in density and/or atomic number to which X-rays are sensitive. Here, we compare CT and neutron tomography (NT) scans of 85 mm diameter core recovered during the first phase of the Deep Fault Drilling Project (DFDP-1) through New Zealand's Alpine Fault. For the instruments used in this study, the highest resolution images were collected in the NT scans. This allows clearer imaging of some rock features than in the CT scans. However, we observe that the highly neutron beam attenuating properties of DFDP-1 core diminish the quality of images towards the interior of the core. A comparison is also made of the suitability of these two scanning techniques for a drilling project. We conclude that CT scanning is far more favourable in most circumstances. Nevertheless, it could still be beneficial to take NT scans over limited intervals of suitable core, where varying contrast is desired.

Damaged beyond repair? Characterising the damage zone of a fault late in its interseismic cycle, the Alpine Fault, New Zealand

X-ray computed tomography (CT) scans of drill-core, recovered from the first phase of the Deep Fault Drilling Project (DFDP-1) through New Zealand's Alpine Fault, provide an excellent opportunity to study the damage zone of a plate-bounding continental scale fault, late in its interseismic cycle. Documentation of the intermediate-macro scale damage zone structures observed in the CT images show that there is no increase in the density of these structures towards the fault's principal slip zones (PSZs), at least within the interval sampled, which is 30 m above and below the PSZs. This is in agreement with independent analysis using borehole televiewer data. Instead, we conclude the density of damage zone structures to correspond to lithology. We find that 72% of fractures are fully healed, by a combination of clays, calcite and quartz, with an additional 24% partially healed. This fracture healing is consistent with the Alpine Fault's late interseismic state, and the fact that the interval of damage zone sampled coincides with an alteration zone, an interval of extensive fluid-rock interaction. These fractures do not impose a reduction of P-wave velocity, as measured by wireline methods. Outside the alteration zone there is indirect evidence of less extensive fracture healing.

Revising the Displacement History of New Zealand's Alpine Fault

A reinterpretation of structural and paleomagnetic data suggests that New Zealand's Alpine Fault accommodates a far greater percentage of geologically recent plate motion than previously thought.

Active tectonics west of New Zealand's Alpine Fault: South Westland Fault Zone activity shows Australian Plate instability

AbstractThe 300 km long South Westland Fault Zone (SWFZ) is within the footwall of the Central Alpine Fault (<20 km away) and has 3500 m of dip‐slip displacement, but it has been unknown if the fault is active. Here the first evidence for SWFZ thrust faulting in the “stable” Australian Plate is shown with cumulative dip‐slip displacements up to 5.9 m (with 3 m throw) on Pleistocene and Holocene sediments and gentle hanging wall anticlinal folding. Cone penetration test (CPT) stratigraphy shows repeated sequences within the fault scarp (consistent with thrusting). Optically stimulated luminescence (OSL) dating constrains the most recent rupture post‐12.1 ± 1.7 ka with evidence for three to four events during earthquakes of at least Mw 6.8. This study shows significant deformation is accommodated on poorly characterized Australian Plate structures northwest of the Alpine Fault and demonstrates that major active and seismogenic structures remain uncharacterized in densely forested regions on Earth.

Focusing of relative plate motion at a continental transform fault: Cenozoic dextral displacement >700 km on New Zealand's Alpine Fault, reversing >225 km of Late Cretaceous sinistral motion

AbstractThe widely accepted ∼450 km Cenozoic dextral strike‐slip displacement on New Zealand's Alpine Fault is large for continental strike‐slip faults, but it is still less than 60% of the Cenozoic relative plate motion between the Australian and Pacific plates through Zealandia, with the remaining motion assumed to be taken up by rotation and displacement on other faults in a zone up to 300 km wide. We show here that the 450 km total displacement across the Alpine Fault is an artifact of assumptions about the geometry of New Zealand's basement terranes in the Eocene, and the actual Cenozoic dextral displacement across the active trace is greater than 665 km, with more than 700 km (and <785 km since 25 Ma) occurring in a narrow zone less than 10 km wide. This way, the Alpine Fault has accommodated almost all (>94%) of the relative plate motion in the last 25 Ma at an average rate in excess of 28 mm/yr. It reverses more than 225 km (and <300 km) of sinistral shear through Zealandia in the Late Cretaceous, when Zealandia lay on the margin of Gondwana, providing a direct constraint on the kinematics of extension between East and West Antarctica at this time.

Slip localization on the southern Alpine Fault, New Zealand

AbstractResults of a detailed field study of the southern onshore portion of New Zealand's Alpine Fault reveal that for 75 km along‐strike, dextral‐normal slip on this long‐lived structure is highly localized in phyllosilicate‐rich fault core gouges and along their contact with more competent rocks. At three localities (Martyr River, McKenzie Creek, and Hokuri Creek), we document complete cross sections through the fault. New40Ar/39Ar dates on mylonites, combined with microstructural and mechanical data on phyllosilicate‐rich fault core gouges show that modern slip is localized onto a single, steeply dipping 1 to 12 m‐thick fault core composed of impermeable (k = 10−20to 10−22 m2), frictionally weak (μs = 0.12–0.37), velocity‐strengthening, illite‐chlorite, and saponite‐chlorite‐lizardite fault gouges. Fault core materials are (1) comparable to those of other major weak‐cored faults (e.g., San Andreas Fault) and (2) most compatible with fault creep, despite paleoseismic evidence of quasiperiodic large magnitude earthquakes (Mw > 7) on this portion of the Alpine Fault. We conclude that frictional properties of gouges at the surface do not characterize the overall seismogenic behavior of the southern Alpine Fault.

South Pacific biogeography, tectonic calibration, and pre-drift tectonics: cladogenesis inAbrotanella(Asteraceae)

Abrotanella is the basal genus in the large tribe Senecioneae (Asteraceae) and has a disjunct distribution in Australasia and South America. A recent molecular phylogeny of the genus was used to investigate whether the main biogeographical patterns in the group could be related to the region's tectonic history in a coherent way. The phylogenetic/biogeographical breaks and overlaps in the genus imply a series of vicariance and range expansion events. Each of these can be related to one of the main tectonic events in the region, including assembly of the New Zealand terranes, crustal extension, and magmatism in Gondwana that preceded seafloor spreading, opening of the Tasman and Pacific basins, and transcurrent movement on the New Zealand Alpine fault. The coincident sequence indicates that pre-drift tectonics and magmatism have been more important for the origin of trans-Tasman and trans-Pacific groups than the final rifting of Gondwana that led to their disjunction. For example, during the pre-drift phase of break-up, the Whitsunday volcanic province of Australia and the Median Batholith of New Zealand formed a large, active igneous belt. Its distribution is aligned with the break between New Zealand–south-eastern Australia clades, and New Zealand–New Guinea clades. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ••, ••–••.

Tectonic tremor and deep slow slip on the Alpine Fault

Tectonic tremor is characterized by persistent, low‐frequency seismic energy seen at major plate boundaries. Although predominantly associated with subduction zones, tremor also occurs along the deep extension of the strike‐slip San Andreas Fault. Here we present the first observations of tectonic tremor along New Zealand's Alpine Fault, a major transform boundary that is late in its earthquake cycle. We report tectonic tremor that occurred on the central section of the Alpine Fault on 12 days between March 2009 and October 2011. Tremor hypocenters concentrate in the lower crust at the downdip projection of the Alpine Fault; coincide with a zone of high P‐wave attenuation (low Qp) and bright seismic reflections; occur in the 25–45 km depth range, below the seismogenic zone; and may define the deep plate boundary structure extending through the lower crust and into the upper mantle. We infer this tremor to represent slow slip on the deep extent of the Alpine Fault in a fluid‐rich region marked by high attenuation and reflectivity. These observations provide the first indication of present‐day displacement on the lower crustal portion of the Australia–Pacific transform plate boundary.

Diverse habitats of pseudotachylytes in the Alpine Fault Zone and relationships to current seismicity

Abstract Pseudotachylytes are ubiquitous within New Zealand's Alpine Fault Zone, occurring as: (i) thin fault veins parallel to existing hanging wall mylonitic foliation; (ii) thicker fault and injection veins around and within metabasite lenses in hanging wall fault rocks and on the footwall–hanging wall boundary; (iii) chaotic injected masses within footwall-derived, granitoid mylonites; and (iv) chaotic injected masses into cataclasites within the fault core. Overall, pseudotachylytes are not volumetrically dominant enough to have formed during all increments of earthquake slip on the Alpine Fault. We propose they were mostly generated during regular moderate magnitude events or during foreshock and aftershock sequences to larger earthquakes. The largest volume pseudotachylytes occur in footwall-derived mylonites (type (iii)). This may indicate that high-stress, anhydrous seismic slip is most common in the footwall. Most types (i), (ii) and (iii) pseudotachylytes formed at or near the base of the seismogenic zone, at temperatures up to 350 °C and at depths of 7–10 km or more. Ductilely overprinted pseudotachylytes represent the down-dip termination of large fault ruptures in a zone that would usually fail by aseismic creep. Type (iv) pseudotachylytes were formed at shallower depths (4–7 km) in a damage zone around the fault principal slip surface. Rare amygdules indicate that the fault zone locally contained free fluids. Supplementary material: Table of host, rock and pseudotachylyte major and minor oxide proportions is available at http://www.geolsoc.org.uk/SUP18490 .

Using geochemical fingerprinting to determine transpressive fault movement history: Application to the New Zealand Alpine Fault

Geochemical fingerprinting of conglomerate clasts from depocenters adjacent to a strike‐slip fault, and of source areas on the opposite side of the fault, provides a method of determining fault movement history in instances where potential piercing points have been removed by erosion or buried in pull‐apart or down‐warp basins. Our example demonstrating this method is the Neogene Maruia Basin adjacent to the New Zealand Alpine Fault, the on‐land Australian‐Pacific plate boundary, which shows Permian Pelorus‐Caples terrane offset by 480 km. Twenty‐six sandstone and schist conglomerate clasts from the Maruia Basin Rappahannock Group were analyzed for major elements and 18 trace elements by X‐ray fluorescence and compared to similar analyses from potential regional source areas. Significant source terranes, the Caples Group and the Torlesse terrane were identified, both located southeast of the fault. Dextral movement on the fault provided a conveyor belt of material to the Maruia Basin, first supplying (at ∼11.5 Ma) Caples sandstone, then Caples schist followed by Torlesse schist (from ∼9.5 Ma), which translates to an average Neogene movement rate of 36–37 mm yr−1. The increase in metamorphic grade reflects accelerated exhumation of source rocks as they passed through a restraining bend adjacent to the Maruia Basin and from later regional uplift from a change in the Australian Pacific plate motion vector and more transpressive movement from ∼5 Ma. This robust method of geochemical fingerprinting could be readily applied to determining movement history of transpressive faults of other regions.

The Alpine Fault biogeographic hypothesis revisited.

The New Zealand Alpine Fault is a major tectonic feature located on the mutual boundary of the Australian and Pacific Plates which is hypothesized to have undergone some 470 km of right-lateral displacement. The Nelson and Westland provinces have moved north-east relative to the rest of the South Island. Many plant and animal taxa show a conspicuous distribution gap in the central South Island, and traditionally this has been explained by glacial extirpation of the central populations. However, the gap is usually filled by a closely related taxon. Furthermore, the taxa involved occupy a wide ecological range, and include intertidal algae, shorefishes, lowland trees, and alpine herbs and insects that thrive around glaciers. The Alpine Fault biogeographic hypothesis [Heads, M., 1998a. Biogeographic disjunction along the Alpine Fault, New Zealand. Biol. J. Linn. Soc. 63, 161-176; Heads, M., 1998b.Coprosma decurva (Rubiaceae), a new species from New Zealand. NZ J. Bot. 36, 65-69.], based on track and phylogenetic analyses, proposes that the disjunction has instead been caused by movement on the fault pulling apart plant and animal populations. Wallis and Trewick (2001) [Wallis, G.P., Trewick, S.A., 2001. Finding fault with vicariance: a critique of Heads (1998). Syst. Biol. 50, 602-609.] provided a critique of this idea and pointed out what they felt were nine problems with it. These problems are answered here and shown to result from Wallis and Trewick's misunderstanding of aspects of geology and biology involved in the hypothesis. In particular, they have confused the age of inception of the fault with the age of movement along it, and, by neglecting the related central taxa in the "gap", have assumed the biogeographic hypothesis to be an uninformative two-area statement, whereas in fact it is an informative three- or four-area statement.

Biogeographic disjunction along the Alpine fault, New Zealand

Eighty taxa (subspecies, species, species groups, genera and families) showing disjunction along the New Zealand Alpine fault (Australian/Pacific plate boundary) are documented and mapped. Four plant divisions, including 14 seed plant families, and four animal phyla, including 13 orders of insects, are represented. The disjunction has usually been explained by glaciers having wiped out central populations. However, the gap is often occupied by a related taxon and many alpine taxa are involved, and so extinction by glaciation seems unlikely as an explanation. It is proposed that the disjunction has been caused by the 480 km of lateral displacement on the fault.

Biogeographic disjunction along the Alpine fault, New Zealand

Eighty taxa (subspecies, species, species groups, genera and families) showing disjunction along the New Zealand Alpine fault (Australian/Pacific plate boundary) are documented and mapped. Four plant divisions, including 14 seed plant families, and four animal phyla, including 13 orders of insects, are represented. The disjunction has usually been explained by glaciers having wiped out central populations. However, the gap is often occupied by a related taxon and many alpine taxa are involved, and so extinction by glaciation seems unlikely as an explanation. It is proposed that the disjunction has been caused by the 480 km of lateral displacement on the fault.

Erosional control on the structural evolution of a transpressional thrust complex on the Alpine fault, New Zealand

The Waikukupa thrust is a 4-km long oblique thrust segment of the New Zealand Alpine Fault that has developed over the last 65,000 years, emplacing mylonite and cataclasite over fluvioglacial gravel. During the last 20,000 years or so, a 2-km long ‘out-of-sequence’ imbricate, the Hare Mare thrust, has formed on the east side of the Waikukupa River valley. The average rate of slip on the two thrusts is estimated at 22–30 mm/year. Analysis of the fault structure in terms of critical wedge theory is consistent with imbrication resulting from a reduction in wedge taper below the critical value due to rapid river erosion. The internal structure of the Waikukupa thrust sheet consists of reverse faults and angular folds forming duplex-like systems, associated with minor strike-slip faults. Within the limits of uncertainty, the structures are consistent with strain accumulating during internal transpressional shear of the thrust sheet while the taper of the latter was subcritical prior to the transfer of fault displacement to the Hare Mare imbricate. The thrust complex forms part of a serrated range front fault system consisting of oblique thrust sections linked by strike-slip faults. We introduce the term ‘serial partitioning’ for this type of partitioned transpressional fault system. Its development is linked to the erosional processes at the range front. An alternative system consisting of parallel thrust and strike-slip faults we here term ‘parallel partitioning’. While angle of obliquity is an important parameter in determining which partitioning model develops, we suggest that erosion rate is also an influential factor.

Seismic imaging of Pleistocene deep-sea cyclothems: implications for sequence stratigraphy

SUMMARY Plate collision along the New Zealand Alpine Fault since the late Miocene has resulted in an abundant supply of temgenous sediment to an adjacent starved late Cretaceous rift-basin, the Bounty Trough. During the late Neogene, alternating pulses of temgenous (glacial) and biogenic (interglacial) sediment were deposited in the trough in the form of deep-sea cyclothems, in sympathy with glacio-eustatic sea-level fluctuations. Individual cyclothems ranging up to 15 m in thickness have been directly imaged by both 3.5 kHz and single channel airgun profiling. They are particularly conspicuous on the continental slope and in areas of the trough under the influence of overbank turbidite deposition from the Bounty Channel. On the abyssal Bounty Fan, approximately 50 cyclothems have been deposited since the late Pliocene, i.e. each cyclothem corresponds to two contiguous oxygen isotope stages. Each cyclothem also corresponds to the offshore conformity of a single 5th or 6th order seismic sequence, with the lower, temgenous portion marking a lowstand systems tract, and the upper, biopelagic part representing the transgressive and highstand systems tracts. Therefore, as a single continental margin sequence is traced seawards it passes from unconformity-bounded shelf and shelf-margin cyclothems, to slope sediments which include thick conformity-bounded cyclothems or autocyclic canyon deposits, to thin conformity-bounded deep-sea cyclothems on the distal rise, and finally to cryptically cyclic biopelagic sediment andor red clay on the abyssal plain.