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Temperature and fluid pressure conditions control rock deformation and mineralization on geological faults, and hence the distribution of earthquakes. Typical intraplate continental crust has hydrostatic fluid pressure and a near-surface thermal gradient of 31 ± 15 degrees Celsius per kilometre. At temperatures above 300-450 degrees Celsius, usually found at depths greater than 10-15 kilometres, the intra-crystalline plasticity of quartz and feldspar relieves stress by aseismic creep and earthquakes are infrequent. Hydrothermal conditions control the stability of mineral phases and hence frictional-mechanical processes associated with earthquake rupture cycles, but there are few temperature and fluid pressure data from active plate-bounding faults. Here we report results from a borehole drilled into the upper part of the Alpine Fault, which is late in its cycle of stress accumulation and expected to rupture in a magnitude 8 earthquake in the coming decades. The borehole (depth 893 metres) revealed a pore fluid pressure gradient exceeding 9 ± 1 per cent above hydrostatic levels and an average geothermal gradient of 125 ± 55 degrees Celsius per kilometre within the hanging wall of the fault. These extreme hydrothermal conditions result from rapid fault movement, which transports rock and heat from depth, and topographically driven fluid movement that concentrates heat into valleys. Shear heating may occur within the fault but is not required to explain our observations. Our data and models show that highly anomalous fluid pressure and temperature gradients in the upper part of the seismogenic zone can be created by positive feedbacks between processes of fault slip, rock fracturing and alteration, and landscape development at plate-bounding faults.

The New Zealand Community Fault Model (NZ CFM) is a publicly available representation of New Zealand fault zones that have the potential to produce damaging earthquakes. Compiled through collaborative engagement between New Zealand earthquake-science experts, this first edition (version 1.0) of the NZ CFM builds upon previous compilations of earthquake-source active fault models with the addition of new and modified information. Developed primarily to support an update of the New Zealand National Seismic Hazard Model, the NZ CFM comprises two principal components. The first dataset is a two-dimensional map representation of the surface traces of 880 generalised fault zones. Each fault zone is assigned specific geometric and kinematic attributes, including uncertainties, supplemented with a subjective quality ranking focused primarily on the confidence in assigned slip rates. The second component is a three-dimensional representation of the fault zones as triangulated mesh surfaces that are projected down-dip from the two-dimensional mapped traces to a geophysically-defined maximum fault rupture depth. This article summarises the compilation and parameterisation of the NZ CFM, along with background on its relation to predecessor datasets, and forward applications to probabilistic seismic hazard assessment and physics-based earthquake models currently being developed for Aotearoa New Zealand. [ABSTRACT FROM AUTHOR]

An analysis of earthquakes recorded in southern New Caledonia (SNC) over 14 months during 2018-2019 reveals focal mechanisms consistent with a normal-faulting stress state. The minimum principal stress is perpendicular to the Vanuatu subduction zone (VSZ), which is 200 km away, and is highly oblique to the local topographic ridge of New Caledonia, which may induce additional tension. An MW 7.5 earthquake occurred in VSZ on 5 December 2018, and focal mechanisms appear to be different to those before the big earthquake. Significant increase in seismicity rates in both VSZ and SNC are observed following this large earthquake. A strong correlation between local and subduction zone seismicity rates is confirmed by analyses of seismic records before and after large subduction zone earthquakes 200-350 km away during the period of 2000-2018. The local seismicity rate and seismic hazard in SNC is about four times higher immediately after a large subduction earthquake, and Omori decay returns it to background levels after about 30 days. The triggering mechanisms remains unclear, but our study provides the first observations and a framework for future work. [ABSTRACT FROM AUTHOR]

[1] We use seismic reflection and rock sample data to propose that the first-order physiography of New Caledonia Trough and Norfolk Ridge formed in Eocene and Oligocene time and was associated with the onset of subduction and back-arc spreading at the Australia-Pacific plate boundary. Our tectonic model involves an initial Cretaceous rift that is strongly modified by Cenozoic subduction initiation. Hence, we are able to explain (1) complex sedimentary basins of inferred Mesozoic age; (2) a prominent unconformity and onlap surface of middle Eocene to early Miocene age at the base of flat-lying sediments beneath the axis of New Caledonia Trough; (3) gently dipping, variable thickness, and locally deformed Late Cretaceous strata along the margins of the trough; (4) platform morphology and unconformities on either side of the trough that indicate a phase of late Eocene to early Miocene uplift to near sea level, followed by rapid Oligocene and Miocene subsidence of ~ 1100-1800 m; and (5) seismic reflection facies tied to boreholes that suggest absolute tectonic subsidence at the southern end of New Caledonia Trough by 1800-2200 m since Eocene time. The Cenozoic part of the model involves delamination and subduction initiation followed by rapid foundering and rollback of the slab. This created a deep (>2 km) enclosed oceanic trough, ~2000 km long and 200-300 km across, in Eocene and Oligocene time as the lower crust detached, with simultaneous uplift and local land development along basin flanks. Disruption of Late Cretaceous and Paleogene strata was minimal during this Cenozoic phase and involved only subtle tilting and local reverse faulting or folding. Basin formation was possible through the action of at least one detachment fault that allowed the lower crust to either be subducted into the mantle or exhumed eastward into Norfolk Basin. We suggest that delamination of the lithosphere, with possible mixing of the lower crust back into the mantle, is more widespread than previously thought and may be commonly associated with subduction initiation, such as Cenozoic events in the Mediterranean and western Pacific. doi: 10.1029/2009TC002476

West Antarctica and adjacent seafloor have topographic elevations 0.5-1.2 km greater than expected from models of lithospheric age and crustal structure. Ocean crust near New Zealand has no equivalent depth anomaly, but tectonic subsidence histories from Campbell Plateau petroleum wells show anomalously high subsidence rates during the Paleogene, and total subsidence 0.5-0.9 km greater than expected from rift basin models. Geophysical and geochemical anomalies suggest that upward mantle flow supports the anomalous topography beneath Antarctica, and the Campbell Plateau subsidence history indicates that topographic support mechanisms were long lived (>80 Ma) and recoverable over a period of ~30 m.y. as plate motions moved New Zealand from Antarctica. We construct models of Late Cretaceous and Cenozoic mantle flow with a slab graveyard and upwelling above that is initially rooted at 1000-1500 km depth. Our models match topography and subsidence history anomalies, and are consistent with mantle seismic wave speed anomalies and the geoid. We suggest that when thermally driven slab downwelling ceased ca. 100 Ma, low-density material that was fertilized within a broad zone in the lower mantle during the previous ~400 m.y. of Gondwana subduction was released and able to rise. Mantle upwelling from depths of 700-1500 km, lasting for periods of ~100-200 m.y., with enriched chemistry related to the prior subduction history may be a general process that follows subduction death, and has not previously been recognized. doi: 10.1130/G30613.1

[1] We use seismic reflection and refraction data to determine crustal structure, to map a fore-arc basin containing 12 km of sediment, and to image the subduction thrust at 35 km depth. Seismic reflection megasequences within the basin are correlated with onshore geology: megasequence X, Late Cretaceous and Paleogene marine passive margin sediments; megasequence Y, a ~10,000 [km.sup.3] submarine landslide emplaced during subduction initiation at 22 Ma; and megasequence Z, a Neogene subduction margin megasequence. The Moho lies at 17 km beneath the basin center and at 35 km at the southern margin. Beneath the western basin margin, we interpret reflective units as deformed Gondwana fore-arc sediment that was thrust in Cretaceous time over oceanic crust 7 km thick. Raukumara Basin has normal faults at its western margin and is uplifted along its eastern and southern margins. Raukumara Basin represents a rigid fore-arc block >150 km long, which contrasts with widespread faulting and large Neogene vertical axis rotations farther south. Taper of the western edge of allochthonous unit Y and westward thickening and downlap of immediately overlying strata suggest westward or northwestward paleoslope and emplacement direction rather than southwestward, as proposed for the correlative onshore allochthon. Spatial correlation between rock uplift of the eastern and southern basin margins with the intersection between Moho and subduction thrust leads us to suggest that crustal underplating is modulated by fore-arc crustal thickness. The trench slope has many small extensional faults and lacks coherent internal reflections, suggesting collapse of indurated rock, rather than accretion of >1 km of sediment from the downgoing plate. The lack of volcanic intrusion east of the active arc, and stratigraphic evidence for the broadening of East Cape Ridge with time, suggests net fore-arc accretion since 22 Ma. We propose a cyclical fore-arc kinematic: rock moves down a subduction channel to near the base of the crust, where underplating drives rock uplift, oversteepens the trench slope, and causes collapse toward the trench and subduction channel. Cyclical rock particle paths led to persistent trench slope subsidence during net accretion. Existing global estimates of fore-arc loss are systematically too high because they assume vertical particle paths. Citation: Sutherland, R., et al. (2009), Reactivation of tectonics, crustal underplating, and uplift after 60 Myr of passive subsidence, Raukumara Basin, Hikurangi-Kermadec fore arc, New Zealand: Implications for global growth and recycling of continents, Tectonics, 28, TC5017, doi: 10.1029/2008TC002356.

Cosmogenic nuclide (Be-10) exposure dating of moraine boulders in the Cascade Valley, southwest New Zealand, reveals three phases of glaciation with similar maximum magnitude since 100 ka. In this area, 8-10 lateral moraines were deposited during the Last Glacial Maximum (LGM) at 22-19 ka, and >15 lateral moraines and three end moraines were deposited during recession after the LGM. Also, three exposure ages of 29-33 ka from pre-LGM deposits may indicate increased weathering and erosion at the onset of the LGM in New Zealand, as has been suggested by other studies. An exposure age of 57.8 [+ or -] 2.7 ka from one of the highest moraines, combined with previous studies of cave speleothems, glacial features offset by the Alpine fault, the Vostok dust record, and sediment cores, supports the inference that a significant glacial phase culminated at 66-58 ka. A cluster of five exposure ages from older moraines reveals a glacial phase with at least three advance-retreat cycles at 79.0 [+ or -] 3.9 ka. Correlation between the ages of glacial periods and the timing of Southern Hemisphere summer insolation minima suggests that orbital forcing has played a first-order role in regulating glacial extent in New Zealand. Keywords: glacial deposits, moraines, climate, Milankovitch, Last Glacial Maximum, cosmogenic elements, beryllium.

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 [+ or -] 2.2, 23.2 [+ or -] 4.9, 21.4 [+ or -] 2.6, and 23.5 [+ or -] 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 [+ or -] 1.5, 23.2 [+ or -] 1.4, and 23.1 [+ or -] 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 [+ or -] 6 mm/yr to 23 [+ or -] 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 [+ or -] 3 mm/yr of total plate motion (NUVEL-1A) is partitioned into 23 [+ or -] 2 mm/yr of Alpine fault dextral strike slip, 12 [+ or -] 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 [+ or -] 3 mm/yr of heave on reverse faults at the peripheries of the plate boundary. Keywords: Alpine fault, tectonics, earthquakes, glacial deposits, plate collision, strike-slip faults.

The Alpine Fault is an 850-km-long, continental dextral strike-slip fault that accommodates some 60%-90% (~20-30 mm/yr) of the obliquely convergent motion between the Pacific and Australian Plates in South Island, New Zealand. The southern 230 km of the fault traverses the continental margin off Fiordland and intersects the subduction thrust at the northern end of the Puysegur Trench. Marine seismic reflection profiles and bathymetric data are used to evaluate the late Quaternary structure and 3 m.y. evolution of the fault and five sedimentary basins. The fault offshore is more complexly segmented than its onshore counterpart, and at sedimentary basins has substantial (>1 km) bathymetric relief. Three right-stepping and overlapping active sections are identified on the basis of structural continuity and geomorphic expression. The northern, Milford-Caswell section, is 90 km long and continuous with the southern Westland section on land. The southern, Resolution section is 150 km long and developed from initially discontinuous, right-stepping segments. These sections overlap by 35 km at the large Secretary-Nancy Basin, which is currently being dissected longitudinally by the Nancy section, resulting in a straighter, almost fully linked principal displacement zone. The average strike of the fault differs from the azimuth of Pacific-Australian Plate motion by 11[degrees]-25[degrees]. Nevertheless, on the whole, the structure, tectonic geomorphology, and lateral displacements indicate predominantly dextral strike-slip displacement. The basins have the structural characteristics of pull-apart and releasing-bend basins. They commonly initiated at step-overs in the fault and evolved by development of very oblique cross- and along-basin faults that linked the principal displacement zone, resulting in longer, through-going surface traces. Localized transpressional features are developing contemporaneously with the basins and may relate to constrictional fault geometries and/or a 10[degrees] rotation of part of the fault within the last 3 m.y., rather than to changes in plate motions. The absence of significant regional contraction across the fault indicates that obliquely convergent plate motion is strongly partitioned. The convergence normal to the boundary is accommodated largely on thrust and reverse faults offshore and onshore, to the northwest and southeast of the Alpine Fault, respectively. The position of the Alpine Fault is clearly associated with the position of inherited Eocene rift structures carried in the subducting Australian Plate. Our observations and kinematic model imply a spatial-temporal evolution where the position of the southwestern end of the fault trace is transitory on a time scale of [10.sup.5]-[10.sup.6] years, and the fault matures toward the northeast. The data are consistent with models invoking dextral reactivation of the subducting rift structures and tearing of the Australian Plate in the approximate direction of plate convergence, and/or the loading of shear stresses and strike-slip deformation on the Alpine Fault in response to spatial differences in surface roughness and interplate coupling. Several potential earthquake scenarios on the Alpine Fault are considered, ranging from rupture of individual structural sections associated with earthquakes of magnitude M 7.0-7.8, to larger, onshore-offshore composite ruptures of up to M 8.1. Keywords: Alpine Fault, New Zealand, strike slip, strike-slip basin, pull-apart, structure, bathymetry, subduction, Puysegur Trench, tectonic deformation, plate boundary, seismic reflection, transpression, earthquake potential.

Author(s): Bernhard Steinberger (corresponding author) [1, 4]; Rupert Sutherland [2]; Richard J. O'Connell [3] Hotspots are frequently assumed to be fixed and used as a reference frame for plate motions [...]

Radiolaria and paleomagnetic data from Deep Sea Drilling Project Sites 595 and 596 indicate that Early Cretaceous seafloor was formed at moderate to high latitudes in the Pacific. This inference requires a Cretaceous spreading ridge south of the Pacific-Phoenix Ridge and substantial dextral strike-slip motion at the Gondwana margin. The previously unrecognized southern oceanic plate, called here the Moa plate, was obliquely subducted beneath the Gondwana margin, and material accreted from it is now part of the Eastern province of New Zealand. Keywords: Pacific, tectonics, Cretaceous, radiolaria, New Zealand.