Australian Tectonic Plates Research Articles

Sunda–Sahul floristic exchange and pathways into the Southwest Pacific: New insights from wet tropical forest trees

AbstractAimRecent investigations on the floristic exchange between Southeast Asia and Australia have shown a clear dispersal directionality bias (West to East) of wet‐adapted plant taxa. However, dispersal routes and directions of wet forest taxa into the South Pacific remain insufficiently known. We here aimed to establish the most likely routes and directions of plant dispersal into the Southwest Pacific islands.LocationSoutheast Asia, East Asia, Australia, Southwest Pacific.TaxonDysoxylum s.l. (Meliaceae). This includes Dysoxylum s.s., Didymocheton, Epicharis, Goniocheton, Pseudocarapa, and Prasoxylon.MethodWe sampled 75% of the species diversity in Dysoxylum s.l., covering the entire distribution range, all genera and major lineages. Phylogenetic relationships of 149 accessions were reconstructed using Bayesian Evolutionary Analysis and two internal constraints. The dispersal–extinction–cladogenesis variant, founder‐event speciation (DEC+J), was used for reconstructing the biogeographic history, and 100 Biogeographical Stochastic Mappings were simulated.ResultsDysoxylum s.l. originated and firstly diversified in the western part of its current distribution range (including Indochina) during the Miocene to Pliocene, followed by an overall eastern range expansion towards Malesia, Australia and the Southwest Pacific in the Pliocene.Main ConclusionsThe south‐eastward expansion of lineages into Wallacea and Australia is in temporal agreement with the convergence of the Asian and Australian tectonic plates since the Miocene. Long‐distance dispersal is the main mechanism that led to the current distribution. Two dispersal pathways into the Southwest Pacific are identified, (1) through New Guinea and the Solomon Islands to Fiji, and (2) from New Zealand to Fiji. For both routes, Fiji was an important secondary source area for dispersal into the Southwest Pacific.

Seismic Constraint on Heterogeneous Deformation and Stress State in the Forearc of the Hikurangi Subduction Zone, New Zealand

Abstract The Hikurangi subduction zone (HSZ) is the collisional boundary between the Pacific and Australian tectonic plates along the eastern coast of the North Island of New Zealand. The region is believed to be capable of hosting large megathrust earthquakes and associated tsunamis. Recent studies observe a range of slip behavior along the plate interface, with a sharp contrast between locked and creeping parts of the megathrust along the margin. This work uses teleseismic scattering data (receiver functions [RFs]) recorded at 53 long-running seismograph stations on the North Island of New Zealand to constrain the structure and mechanical properties of the forearc in the HSZ. We observe directional variations in RF phases at P–S converted delay times (i.e., depths) associated with the overlying forearc crust and note a general correlation with spatial variations in plate coupling as well as other geophysical properties. Our results suggest differences in the nature of crustal deformation (and stress state) along the Hikurangi margin, with evidence of clockwise rotation and/or extension in the northern HSZ, where the overriding forearc crust is uncoupled from the subducting Pacific slab.

GRASS GIS Modules for Topographic and Geophysical Analysis of the ETOPO1 DEM and Raster Data: North Fiji Basin, Pacific Ocean

The paper presents topographic analysis based on the raster data (NetCDF and grid formats) and visualization of the geophysical datasets that resembles the topographic surface. Geographically, the research focuses on the region of North Fiji Basin, South Pacific Ocean. North Fiji Basin is one of the marginal basins located at the converging boundary between the Pacific and Australian tectonic plates and is notable for complex geological settings. Methodology is based on GRASS GIS and includes scripting and cartographic visualization. Data include ETOPO1, GEBCO, EGM96 gravity and geoid raster grids imported to GRASS GIS via GDAL library (‘r.in.gdal’ module) from NetCDF and GRD formats, evaluated and visualized. Several GRASS modules were used as a sequential scripting for displaying and modelling data. Geomorphometric analysis (slopes, curvature, aspect, elevation) was performed by module ‘r.slope.aspect’ based on ETOPO1 grid. Statistical data analysis (histograms, polar diagram) for topographic and geoid gravitational fields were visualized by combination of GRASS modules d.rast, g.region, d.polar, d.histogram, d.grid, d.legend, d.text. Topographic analysis reveal that free-air gravity anomaly mark the continental shelf-slope border showing that generally, geophysical settings are dominated by the topographic effect. These observations are consistent with the inferred prominent role of the geophysical settings. The histograms reveal that topography (trenches, abyssal plains, shelf areas) has values between -5,000 and -1,000 m, and a clear peak with major values between 100 and 350 m above sea level. This distribution indicates that dominating depths of the ocean floor in North Fiji Basin are between -1,500 to -3,000 m. The aspect map of the ETOPO1 initial raw raster grid show that the surface has mostly 50°-110° oriented slope followed by data range in 250°-300° oriented slopes which is well correlating with submarine topography and general relief forms. The steepening of the gravity values between 50 and 55 mGal and 58 to 63 mGal indicates geoid undulations suggesting that bathymetry is controlled by the geological settings in the study area. Console- based scripts of GRASS GIS are presented together with cartographic outputs showing its effectiveness for the geographic spatial analysis.

Geochemical patterns of late Cenozoic intraplate basaltic volcanism in northern New Zealand and their relationship to the behaviour of the mantle

ABSTRACT A major period of subduction-related volcanism in northern New Zealand began about 20 million years ago; its locus of activity migrated southward during the late Cenozoic in response to changes in convergence between the Australian and Pacific tectonic plates. As the volcanic arc migrated, six discrete fields of small-scale basaltic volcanoes were established behind the subduction system. These volcanoes show geochemical characteristics that reveal patterns of mantle behaviour linked to the rapidly evolving plate tectonic environment of Zealandia. Two northern volcano fields began activity about 8 million years ago and have been active into recent times; their chemical compositions reflect partial melting of shallow mantle and their spatial relationships an essentially static source environment. In contrast, four southern fields are dominated by alkalic magmas that likely ascended rapidly from garnet-bearing asthenospheric sources in a northward migrating pattern of discrete temporal episodes (∼1 ma duration) since 3 Ma. The Northland fields were sourced in mantle that became coupled to the lithosphere and have remained active during the last 8 Ma. The Auckland fields are interpreted to reflect independent actively rising asthenospheric mini-plumes linked to mantle convection associated with but geochemically distinct from the subduction system beneath central North Island New Zealand.

Using paleoseismology and tephrochronology to reconstruct fault rupturing and hydrothermal activity since c. 40 ka in Taupo Rift, New Zealand

The Taupo Volcanic Zone (TVZ) in North Island, New Zealand, is the on-land continuation of the Tonga-Kermadec arc formed in the Quaternary at the obliquely convergent boundary of the Pacific and Australian tectonic plates. The central TVZ is a region of intense silicic volcanism and active rifting with a very high heat flux. Within this zone is a dynamic landscape affected by a dense, active fault network, the Taupo Rift. In this rift, the Ngakuru graben hosts fossil hydrothermal systems in an area parallel to numerous active faults including the east strand of Whirinaki Fault that forms a major structure. Using various geoscientific techniques including mapping, stratigraphy, paleoseismic trenching, and tephrostratigraphy, in conjunction with LiDAR-derived DEMs, we reconstruct and date the fault's rupture history along with hydrothermal activity (including silica-sinter development) since c. 40,000 calendar years ago (40 cal. ka) at a site near Hossack Road called “Meade”. Ages for Kawakawa (c. 25.4 cal. ka), Okareka (c. 21.8 cal. ka), Rotorua (c.15.6 cal. ka), Rotoma (c. 9.4 cal. ka), and Taupo (c. 1.7 cal. ka) tephras enabled us to date five identified fault rupture events using the Meade trench excavation. Slip rates of 2.66 ± 0.77 mm/yr (pre-Kawakawa tephra), 0.28 ± 0.04 mm/yr (between c. 25.4 ka and Taupo) and 0.51 ± 0.19 mm/yr (post-Taupo), and the recurrence interval of ∼5500 cal. yr during the last c. 25.4 cal. kyr, all correlate with events of similar ages determined from studies on other trenches on Whirinaki Fault. Intercalated with Tahuna tephra (c. 39.3 cal. ka) and additionally dated at c. 38.9 cal. ka using radiocarbon, the hydrothermal sinter began developing at the Meade site at c. 39 cal. ka and ceased by c. 21.8 cal. ka (marked by Okareka tephra). We examine the causative relationship between fault activity and the development of sinter by comparing the chronology of volcanic eruptions and fault rupturing events with that of sinter formation as recorded in three neighbouring sites, Mangatete, Matthews, and Fitzpatrick. The findings improve understanding of the complex rupture behaviour of faulting and provide evidence for relationships between tectonic and hydrothermal activities, which were additionally influenced by the impacts of climatic change and geomorphic processes on landscape evolution, within the late Quaternary period. The study also exemplifies the unique value of tephrochronology in helping to disentangle complex geological deposits and events in an extremely dynamic part of the Earth's surface (the Taupo Rift).

Paleoclimatology, Paleogeography, and the Evolution and Distribution of Sea Kraits (Serpentes; Elapidae;Laticauda)

The geographic range of sea kraits encompasses one of the geologically most-complex regions of the world. At its center lies Wallacea (the transition between the terrestrial biotas of the Asian and Australian tectonic plates) and the Indonesian Throughflow (nexus of the equatorial marine biotas of the Indian and Pacific oceans). The aim of this study was to elucidate the role of paleogeography, paleoclimatology, and oceanic currents in the evolution and distribution of sea kraits across these major biogeographic crossroads and beyond. A recent assessment of times of taxonomic divergence was projected against paleogeographic reconstructions to produce a parsimonious, hypothetical model of events critical for the origin, dispersal, and differentiation of this taxon. Times and degree of divergence of taxa suggested by recent morphological and molecular studies are in accord with various climatological and geologic events. The model postulates that the distribution of sea kraits was neither greatly affected by tectonics, other than the approach of the Australian Plate to the Asian one, nor dominated by the historic barriers to dispersal of terrestrial fauna across Wallacea, or by the Indonesian Throughflow. Rather, the model suggests that two major factors—paleogeographic alteration of the configuration of land and sea, and the directions of sea currents, past and present—provide an explanation of how these amphibious snakes (1) originated from a terrestrial Asian elapid ancestor, (2) subsequently generated the venomous Australian land snakes and their derivatives the true sea snakes, and (3) differentiated into the species complexes, species, and infraspecific entities of the genus Laticauda.

IDENTIFICATION OF LAND SURFACE TEMPERATURE DISTRIBUTION OF GEOTHERMAL AREA IN UNGARAN MOUNT BY USING LANDSAT 8 IMAGERY

Indonesia located at the confluence of Eurasian tectonic plate, Australian tectonic plate and the Pacific tectonic plate. Therefore, Indonesia has big geothermal potential. One of the areas that has geothermal potential is Ungaran Mount. Remote sensing technology can have a role in geothermal exploration activity to map the distribution of land surface temperatures associated with geothermal manifestations. The advantages of remote sensing are able to get information without having to go directly to the field with a large area, and it takes quick, so that the information can be used as an initial reference exploration activities. This study aimed to obtain the distribution of land surface temperature as a regional analysis of geothermal potential. The method of this research was a correlation of brightness temperature (BT) Landsat 8 with land surface temperature (LST) MODIS. The results of correlation analysis showed the R2 value was equal to 0.87, it shows that between BT Landsat 8 and LST MODIS has a very high correlation. Based on Landsat 8 LST imagery correction, the average of fumarole temperature and hot spring is 240C. Fumarole and hot spring are located in dense vegetation land which has average temperature around 26.90C. Land surface temperature Landsat 8 can not be directly used to identify geothermal potential, especially in the dense vegetation area, due to the existence of dense vegetation which can absorb heat energy released by geothermal surface feature.

Melanesian back-arc basin and arc development: Constraints from the eastern Coral Sea

The eastern Coral Sea is a poorly explored area at the north-eastern corner of the Australian Tectonic Plate, where interaction between the Pacific and Australian plate boundaries, and accretion of the world's largest submarine plateau – the Ontong Java Plateau – has resulted in a complex assemblage of back-arc basins, island arcs, continental plateaus and volcanic products. This study combines new and existing magnetic anomaly profiles, seafloor fabric from swath bathymetry data, Ar–Ar dating of E-MORB basalts, palaeontological dating of carbonate sediments, and plate modelling from the eastern Coral Sea. Our results constrain commencement of the opening of the Santa Cruz Basin and South Rennell Trough to c. 48Ma and termination at 25–28Ma. Simultaneous opening of the Melanesian Basin/Solomon Sea further north suggests that a single >2000km long back-arc basin, with at least one triple junction existed landward of the Melanesian subduction zone from Eocene–Oligocene times. The cessation of spreading corresponds with a reorganisation of the plate boundaries in the area and the proposed initial soft collision of the Ontong Java Plateau. The correlation between back-arc basin cessation and a widespread plate reorganisation event suggests that back-arc basins may be used as markers for both local and global plate boundary changes.

Tectonic classification of vertical crustal motions – a case study for New Zealand

Abstract We investigate the relationship between vertical crustal motion and tectonic block configuration. The study is conducted along the active tectonic margin between the Australian and Pacific tectonic plates in New Zealand with a well-defined tectonic block configuration. For this purpose, the rates of vertical crustal motions relative to the ITRF2008 reference frame are estimated based on processing the GPS data (provided by the GeoNET project) collected at 123 continuous and semi-continuous GPS sites. The numerical results confirmed the uplift of the central Southern Alps at the current rate of 4.5 mm/yr. This tectonic uplift is coupled in the South Island by the subsidence on both sides of the Southern Alps. The detected rates of subsidence in the eastern South Island are typically less than 1 mm/yr. The subsidence in the Buller Region (in the northwest South Island) is 1.4–1.5 mm/yr. Except for the Taupo Volcanic Zone and the upper Raukumara Block (in the central and northeast North Island), the subsidence is prevailing in the North Island. The systematic subsidence up to 9 mm/yr is detected along the Dextral Fault Belt (in the lower North Island). The largest localized vertical displacements (between −10 and 17 mm/yr) in the Taupo Volcanic Zone are attributed to active tectonics, volcanisms and geothermal processes in this region. A classification of these vertical tectonic motions with respect to the tectonic block configuration reveals that most of tectonic blocks are systematically uplifted, subsided or tilted, except for regions characterized by a complex pattern of vertical motions attributed to active geothermal and volcanic processes.

Could Geometry Demystify Patterns of Earth Quakes and Aftershocks? An Answer is Yes

In this article, a pioneering (geometric) approach based on a new what is named here Glued Bivariate Poisson (GBP) distribution is constructed to demystify the 21st century incidences of earthquakes and their aftershocks. A warning risk index for an earthquake or an aftershock to occur is formulated and computed for each of the thirteen major tectonic plates. This article reports a surprise and it is that there exists a geo angle constancy, 1.571 in the results pertaining to earthquakes or aftershocks in all the tectonic plates no matter what varying interlocked “glue level” among the African, Antarctica, Arabian, Australian, Caribbean, Coco’s, Eurasian, Indian, North American, Pacific, Philippine, South American and Scotia tectonic plates. The contents of this article identified higher risk plates, which include Eurasian, India, Pacific and Philippines plates, where earthquakes of Kathmandu, Nepal (on 26 April 2015) and Chich-Shima, Japan (on 30 May 2015) occurred as validations of the contents of this article. An interesting geologic-constancy of 1.571 is noticed based on our statistical approach and an appropriate representative random sampling.

New Insights into the present-day kinematics of the central and western Papua New Guinea from GPS

New Guinea is a region characterized by rapid oblique convergence between the Pacific and Australian tectonic plates. The detailed tectonics of the region, including the partitioning of relative block motions and fault slip rates within this complex boundary plate boundary zone are still not well understood. In this study, we quantify the distribution of the deformation throughout the central and western parts of Papua New Guinea (PNG) using 20 yr of GPS data (1993–2014). We use an elastic block model to invert the regional GPS velocities as well as earthquake slip vectors for the location and rotation rates of microplate Euler poles as well as fault slip parameters in the region. Convergence between the Pacific and the Australian plates is accommodated in northwestern PNG largely by the New Guinea Trench with rates exceeding 90 mm yr−1, indicating that this is the major active interplate boundary. However, some convergent deformation is partitioned into a shear component with ∼12 per cent accommodated by the Bewani-Torricelli fault zone and the southern Highlands Fold-and-Thrust Belt. New GPS velocities in the eastern New Guinea Highlands region have led to the identification of the New Guinea Highlands and the Papuan Peninsula being distinctly different blocks, separated by a boundary through the Aure Fold-and-Thrust Belt complex which accommodates an estimated 4–5 mm yr−1 of left-lateral and 2–3 mm yr−1 of convergent motion. This implies that the Highlands Block is rotating in a clockwise direction relative to the rigid Australian Plate, consistent with the observed transition to left-lateral strike-slip regime observed in western New Guinea Highlands. We find a better fit of our block model to the observed velocities when assigning the current active boundary between the Papuan Peninsula and the South Bismark Block to be to the north of the city of Lae on the Gain Thrust, rather than on the more southerly Ramu-Markham fault as previously thought. This may indicate a temporary shift of activity onto out of sequence thrusts like the Gain Thrust as opposed to the main frontal thrust (the Ramu-Markham fault). In addition, we show that the southern Highlands Fold-and-Thrust Belt is the major boundary between the rigid Australian Plate and the New Guinea Highlands Block, with convergence occurring at rates between ∼6 and 13 mm yr−1.

The gold conveyor belt: Large-scale gold mobility in an active orogen

The Southern Alps of New Zealand are part of an active collisional orogen where metamorphism, hydrothermal fluid flow and the formation of orogenic gold deposits are ongoing. The Southern Alps are forming due to transpressional collision between continental crust fragments on the Pacific and Australian tectonic plates. The plate tectonic rates and geometries, the sources of fluid and broad-scale fluid pathways in the hydrogeological system, and the geochemical compositions of the Torlesse Terrane rock that is being advected through the orogen are well defined so that a mass balance of metal mobility during active orogenic processing in the Southern Alps of New Zealand can be calculated. Advection of a 10km wide×5km deep section of Torlesse rock through the orogen at tectonic rates (0.01m/yr) that is then metamorphosed up to amphibolite facies causes mobilisation of over 11,27t Au, 10.1Mt As, 47,000t Hg, 560,000t Sb and 14,000Mt H2O in 1Myrs. The masses of elements mobilised at the same rate along the length of the Southern Alps (>200km) for 5Myrs would be more than 100 times greater. The metals were mobilised by the metamorphic fluid produced during the orogenic processing of the Torlesse Terrane rocks and the concentrations of Au, As, Hg and Sb in this fluid are calculated to be 0.08, 711, 3, and 40mg/kg, respectively. The mobilised metals form the orogenic gold deposits that occur in the Southern Alps. Different styles of gold deposits form contemporaneously during the active orogenesis of the Southern Alps, including those with a fluid temperature>rock temperature that may appear to have formed after the peak of metamorphism but are instead just the product hydrothermal fluid mineralising rocks on their retrograde metamorphic path. The mass balance shows that there has been orders of magnitude more metal mobilised in the orogen than resides in the currently known deposits. There is a clear potential for large gold deposits occurring in the yet to be uplifted parts of the Southern Alps if there have been efficient enough fluid focusing and metal precipitation mechanisms occurring under the Southern Alps.

The AuScope geodetic VLBI array

The AuScope geodetic Very Long Baseline Interferometry array consists of three new 12 m radio telescopes and a correlation facility in Australia. The telescopes at Hobart (Tasmania), Katherine (Northern Territory) and Yarragadee (Western Australia) are co-located with other space geodetic techniques including Global Navigation Satellite Systems (GNSS) and gravity infrastructure, and in the case of Yarragadee, Satellite Laser Ranging (SLR) and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) facilities. The correlation facility is based in Perth (Western Australia). This new facility will make significant contributions to improving the densification of the International Celestial Reference Frame in the Southern Hemisphere, and subsequently enhance the International Terrestrial Reference Frame through the ability to detect and mitigate systematic error. This, combined with the simultaneous densification of the GNSS network across Australia will enable the improved measurement of intraplate deformation across the Australian tectonic plate. In this paper we present a description of this new infrastructure and present some initial results, including telescope performance measurements and positions of the telescopes in the International Terrestrial Reference Frame. We show that this array is already capable of achieving centimetre precision over typical long-baselines and that network and reference source systematic effects must be further improved to reach the ambitious goals of VLBI2010.

Migration of hydrothermal systems in an evolving collisional orogen, New Zealand

The Pacific–Australian tectonic plate boundary through the South Island of New Zealand consists of the transpressional Southern Alps mountain belt and the transcurrent Marlborough Fault System, both of which have active tectonically driven hydrothermal systems, with topographically driven meteoric incursion and warm springs. The Southern Alps hydrothermal system is relatively diffuse, with little or no fault control, and is channelled through scattered extensional sites beneath the mountains, where gold mineralisation is occurring locally. The hydrothermal activity along the Marlborough Fault System is controlled by the principal faults in well-defined valleys separated by narrow high ridges. Lateral evolution of Marlborough fault strands southwestwards into the Southern Alps has caused diversion of diffuse Southern Alps hydrothermal activity into the structural superimposition zone, where fluid flow is increasingly being controlled by faults. This hydrothermal diversion was accompanied by major topographic reorientation and river drainage reversal in the late Quaternary. Vein swarms now exposed in the remnants of the Southern Alps north of the superimposition zone formed at shallow levels, with some evidence for fluid boiling, from a mixture of meteoric and deep-sourced fluid. These veins, some of which contain gold, are part of an abandoned <1 million-year-old hydrothermal zone beneath the fossil topographic divide of the Southern Alps that has now been dismembered by lateral incursion of the Marlborough fault strands. Observations on this active plate boundary provide some insights into processes that controlled orogenic gold mineralisation in ancient belts, particularly with respect to relationships between hydrothermal fluid flow, structure and topography.

Major speciation accompanied the dispersal of Vireya Rhododendrons (Ericaceae, Rhododendron sect. Schistanthe) through the Malayan archipelago: Evidence from nuclear gene sequences

AbstractThe species within Rhododendron sect. Schistanthe (= sect. Vireya), a taxon unique for its tropical habitat, distinctive leaf scales, and tailed seeds, have diversified greatly in morphological features. The section contains over 300 species, occurring mainly on the Malay Peninsula, New Guinea, and the islands between. An analysis based on the combined sequences of three nuclear genes supports a phylogeny in which sect. Schistanthe (less R. santapaui) is monophyletic, with well‐defined clades corresponding to subsections Euvireya, Malayovireya, Pseudovireya, and Discovireya. Within subsect. Euvireya, molecular‐phylogenetic groupings follow geography more closely than traditional taxonomic groupings based upon morphology. One of the two most derived clades contains exclusively species from New Guinea, Australia, and the Solomon Islands. The results are consistent with a stepwise phylogeographic history of sect. Schistanthe, beginning in Asia, proceeding eastward and reaching New Guinea within the last 15 Ma, when movement of the Australian tectonic plate brought New Guinea into the Malesian domain.

The Moray Eels of the Anarchias cantonensis Group (Anguilliformes: Muraenidae), with Description of Two New Species

The broadly distributed Indo-Pacific moray eel Anarchias cantonensis (Schultz, 1943), heretofore treated as a single species, consists of three species: A. cantonensis, A. exulatus, a new species, and A. schultzi, a new species, based on geographic distribution, coloration, external morphometrics, and vertebral counts. We examined 112 specimens and determined that the true A. cantonensis is distributed throughout the eastern Indian Ocean, western Pacific, and tropical central Pacific, and has a mean vertebral formula of 91/93/104. Anarchias exulatus is distributed anti-tropically in the central and southeastern Pacific and is characterized by a unique mottling pattern and a mean vertebral formula of 100/103/114. A third species, named here as A. schultzi, is characterized by a mean vertebral formula of 96/99/109, a plain brown coloration in adult form, and is restricted geographically to Tonga, New Caledonia, the Solomon Islands, and the Caroline Islands. At the Tonga Islands, which reside on the edge of Australian and Pacific tectonic plates, all three species are present, but maintain their morphological distinctiveness and segregate ecologically.

Water from stone

Where the Pacific and Australian tectonic plates collide under the South Island of New Zealand large quantities of aqueous fluid are produced. But how does this happen? Geophysical and petrological data indicate that it may not be as we thought.

Processing and preliminary interpretation of noisy high-resolution seismic reflection/refraction data across the active Ostler Fault zone, South Island, New Zealand

Abstract In an attempt to understand the structure of active faults as they emerge from bedrock into shallow semi-consolidated and unconsolidated sediments, we have recorded a comprehensive high-resolution seismic reflection/refraction data set across the Ostler Fault zone on the central South Island of New Zealand. This fault zone, which absorbs 1–2 mm/yr of compression associated with oblique convergence of the Pacific and Australian tectonic plates, consists of a series of surface-rupturing N–S trending, west-dipping reverse faults that offset a thick sequence of Quaternary glacial outwash and late Neogene fluvio-lacustrine sediments of the Mackenzie Basin. Our study focuses on a region of the basin where two non-overlapping fault segments are separated by a transfer zone. Deformation in this area is accommodated by offsets on multiple small faults and by folding in their hanging walls. The seismic data with source and receiver spacing of 6 and 3 m and nominal CMP fold of 60 was acquired along twelve 1.2 km long lines orthogonal to fault strike and an additional 1.6 km long tie-line parallel to fault strike. The combination of active deformation and shallow glacial outwash sediments results in particularly complicated seismic data, such that application of relatively standard processing schemes yields only poor quality images. We have designed a pre- and post-stack reflection/refraction processing scheme that focuses on minimising random and source-generated noise, determining appropriate static corrections and resolving contrasting reflection dips. Application of this processing scheme to the Ostler Fault data provides critical information on fault geometry and offset and on sedimentary structures from the surface to ∼ 800 m depth. Our preliminary interpretation of one of the lines includes complex deformation structures with folding and multiple subsidiary fault splays on either side of a ∼ 50° west-dipping primary fault plane.

Fracture lineaments, fault mesh formation and seismicity

Viti Levu, the main island of Fiji, is located in a seismically active area within the Fiji Platform, a remnant island arc that lies in a diffuse plate boundary zone between the Pacific and Australian tectonic plates in the SW Pacific. The upper crust of Viti Levu is dissected by numerous intersecting fault/lineament zones mapped from remote sensing imagery of the land surface (topography, radar and aerial photos) and basement (magnetic) and have been subject to rigorous statistical tests of reproducibility and verification with field mapped fault data. Lineaments on the various imagery correlate with faults mapped in the field, and show spatial continuity between and beyond mapped faults, thereby providing a fuller coverage of regional structural patterns than previously known. Some fault/lineaments zones extend beyond the coastline to the offshore area from the SE Viti Levu study area. Multibeam bathymetry and seismic reflection data show the fault zones occur along and exert control on the location of a number of submarine canyons on the SE slope of Viti Levu. Evidence for Late Quaternary fault activity is only rarely observed in onshore SE Viti Levu (e.g. by displaced shoreline features), and in seismic reflection profiles from offshore. The principal fault sets in Viti Levu represent generations of regional tectonic faulting that pervade the Fiji Platform during and after the disruption of the proto Fijian arc in the Middle to Late Miocene (~15Ma). These fault sets combine to form a complex network of interlocking faults creating a fault mesh that divides the upper crust into a number of fault blocks ranging from ~2-30 km wide. It is inferred that the fault mesh evolved throughout the Neogene as a response to the anticlockwise rotation of the Fiji Platform through progressive development of different fault sets and intervening crustal block rotations. Regional tectonic deformation is presently accommodated in a distributed manner through the entire fault mesh. Low magnitude earthquakes (&lt;M4) occur regularly and may represent ruptures along short linking segments of the fault mesh, while infrequent larger earthquakes (&gt;M4) may result from complex rupture propagation through several linking fault segments of the mesh that lie close to optimum stress orientations. The interpreted model of distributed deformation through the fault mesh for the study area in SE Viti Levu is inferred to be characteristic of the style of active deformation that occurs throughout the entire Fiji Platform.

Evolution of biological dispersal corridors through a tectonically active mountain range in New Zealand

AbstractAim To assess the geological evolution and biogeographical implications of low mountain passes. In particular, we question the common biogeographical belief that major mountain belts form impervious physical barriers to biological dispersal, and that related taxa found on opposites sides of mountains are necessarily a result of vicariant tectonic processes.Location The Southern Alps of New Zealand form a long (500 km) narrow mountain belt at the oblique collisional Pacific–Australian tectonic plate boundary. High mountains were uplifted during the Pliocene (2–5 Ma) and uplift has continued to the present day.Methods We integrate previous work from several disciplines to obtain an overview of inter‐relationships between plate tectonic processes, geomorphology and biogeography along the main mountain barrier in New Zealand, and then extend this approach to other major mountain belts.Results The Southern Alps initially formed a barrier to at least some biological dispersal, including vicariant formation of separate species of freshwater non‐migratory galaxiid fish on either side. However, the high mountain barrier was breached in several places when passive transport of topography occurred, from the low‐erosion rain shadow on the eastern side towards the high‐erosion, high‐rainfall western side. This tectonic transport resulted in the capture of eastern rivers by west‐draining rivers, leaving low passes at the topographic divide. These low‐elevation corridors permitted biological dispersal across the mountains, although continued uplift raises these passes. A new set of passes has formed in the northern part of the mountains where younger faults are cutting across the older mountain topography. These potential dispersal corridors are becoming lower with continued erosion, and more common as the defining structures migrate southwards.Main conclusions Biological dispersal across the Southern Alps may be facilitated by numerous mountain passes, especially via the new passes formed by cross‐cutting faults. More low‐lying corridors existed than is readily apparent now, as old river capture‐related passes have been blocked by ongoing uplift. The dynamic mountain‐building and erosional environment typified by the Southern Alps occurs in all the world’s collisional mountain belts, such as the Andes, Himalayas, European Alps and North American Cordillera. Sister taxa occurring across mountain belts are not necessarily a result of vicariance driven by the rise of the mountains, as numerous passes may have permitted intermittent dispersal. The evolution of low passes may have been more prevalent than is currently appreciated, suggesting that topographically complex mountain ranges might be more effectively viewed as dynamic filters within a probability landscape rather than as static and impervious high‐altitude barriers to all but the rarest of biological dispersal events. In some cases, the biological disjunctions observed across mountains may more directly reflect habitat differentiation driven by orographic mountain development that has limited the probability of trans‐alpine dispersal success.