Richard J. Norris

The obliquely-convergent plate boundary in the South Island of New Zealand: implications for ancient collision zones

Abstract The Alpine Fault of New Zealand forms the western boundary of a zone of distributed deformation formed by the oblique convergence of continental crust belonging to the Pacific and Australian plates. Structural and geodetic data from the Alpine Fault show that a large proportion of the total plate displacement is accommodated by rapid oblique slip on the fault. The remaining displacement is distributed over a 200 km wide zone to the east. The collision may be modelled as a two-sided deforming orogen, with the partitioning of deformation being controlled by erosional differences between the narrow high-strain inboard and broad low-strain outboard sides. In ancient collision zones, little evidence may remain of the nature and amount of displacement on the inboard side. Partitioning of deformation among pre-existing structures complicates interpretation of the outboard zone. Radiometric ages may post-date collision by several tens of millions of years and indicate slow isostatic uplift and unroofing. Collision ages, if preserved, may be recognized by high uplift rates calculated from muscovite-biotite pairs.

The structural evolution of active fault and fold systems in central Otago, New Zealand: evidence revealed by drainage patterns

Abstract Central Otago in New Zealand is an area of active continental shortening in which a peneplain surface cut into schist has been deformed by folds, which are developed above buried reverse faults. We use the drainage patterns in this region to demonstrate various processes in fold (and fault) growth and interaction that would be difficult to identify by other means. In particular we show: (1) how simple asymmetric folds can develop into box folds; (2) how apparently continuous ridges were formed by the coalescing of quite separate propagating fold (and fault) segments; (3) evidence for the relative ages (or relative uplift rates) of adjacent structures; and (4) evidence for the propagation direction of folds (or faults) as they grow. The few quantitative estimates we obtain for fault propagation rates suggest an increase in length of 10–50 m per earthquake on faults about 20 km long. These estimates are very uncertain, but are similar in magnitude to an estimate made in Nevada for a normal fault of similar size and are also similar to predicted estimates from theoretical growth models. They raise the question of whether fault growth, earthquake recurrence rates and climate change can interact to produce semi-regular discrete features in an active landscape.

Cainozoic history of southern New Zealand: An accord between geological observations and plate-tectonic predictions

Abstract Sea-floor spreading data from the Southwest Pacific have recently been used to predict the Cainozoic geological history along the Indo-Australian/Pacific plate boundary. Geologic and sedimentologic data pertaining to this plate boundary where it crosses southern New Zealand, as the Alpine Fault, are summarised and discussed. It is concluded that there is a close accord between the plate-tectonic predictions and South Island Cainozoic geological history. In particular, (1) no Cainozoic plate boundary traversed the New Zealand region prior to 38 m.y. B.P. (late Eocene); (2) transcurrent movement on the Alpine Fault took place largely between ca. 30 m.y. B.P. (middle Oligocene) and ca. 10 m.y. B.P. (late Miocene); and (3) the period 10 m.y. B.P. to present corresponds to a phase of oblique compression, continental collision, and mountain building along the Alpine Fault sector of the plate boundary. There is a close correlation between the sites and histories of Cainozoic sedimentation and this tectonic timetable.

Origin of small-scale segmentation and transpressional thrusting along the Alpine fault, New Zealand

The Alpine fault is the major structural feature of the Australian-Pacific plate boundary in the South Island of New Zealand. Geologic evidence suggests that half to three-quarters of the plate boundary displacement is accommodated by movement on the fault. Detailed investigation of the central section of the Alpine fault has revealed that it consists of oblique thrust sections striking 020°–050° that are linked by subvertical right-lateral faults striking between 065° and 090°. The segmentation is on a scale of 1–10 km. Similarly oriented right-lateral faults are abundant southeast of the Alpine fault and are consistent with stresses induced in an elastic layer by an oblique-slip ductile fault zone below. Propagation of the fault to the surface is predicted to result in an en echelon arrangement of strike-slip and thrust segments. It is suggested that the spatial distribution of segments is affected by the existence of deeply incised valleys in the hanging wall that disturb the stress field to depths of 1–4 km. The segmentation is near surface and does not appear to act as a barrier to the propagation of large earthquakes.

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.

Dewatering of a metamorphic pile

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.

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.

Influence of exhumation on the structural evolution of transpressional plate boundaries: An example from the Southern Alps, New Zealand

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.

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.

Anatomy, structural evolution, and slip rate of a plate-boundary thrust: The Alpine fault at Gaunt Creek, Westland, New Zealand

Minimum slip rates calculated for plate-vector-parallel slickenside trends in cataclasite on the sole of the Alpine fault at Gaunt Creek, Westland, New Zealand, range from 18 to 24 mm/yr. Between half and two-thirds of the total relative motion between the Pacific and Australian plates is being accommodated by movement on a single structure, the Alpine fault. During the past 14 ka, the leading edge of the Alpine fault has changed from a moderately southeast-dipping, oblique reverse fault to a shallowly dipping thrust. The hanging wall (Pacific plate) is composed of a gradational sequence from basal gouge, through pseudotachylite-bearing cataclasite, to progressively more coherent schist-derived mylonite, which has been faulted against subhorizontally bedded, fluvio-glacial gravel in the footwall (Australian plate). During uplift the hanging-wall sequence has been internally sheared and imbricated, producing duplex structures, and retrogressively veined and altered by pervasive hydrothermal fluid flow. Erosion of the exhumed fault zone produced angular, cataclasite- and mylonite- derived, talus-fan breccias, building a west-dipping apron beneath the fault scarp. Wood fragments from near the base off the talus breccias have been 14 C dated at 12,650 ± 90 yr B.P. Progressive tectonic shortening resulted in 180 m of overthrusting of a schist-derived nappe across an irregular talus fan surface composed of its own erosional debris. The structural history of the Alpine fault at Gaunt Creek illustrates the importance of the interaction between fault-induced topography and erosion, and the control these processes exert on the continued tectonic, geometric, and geomorphic evolution of the fault zone.