Northern Australian Margin Research Articles

Using shallow hydroacoustic data to image seafloor mass transport deposits on the North West Shelf of Australia: links to neotectonics

Abstract Mass transport deposits have long been known on the Exmouth Plateau, offshore NW Australia, identified in 2D and 3D industry seismic lines. The expedition SO257 in 2017 collected 30 high-resolution, shallow seismic lines along targeted transects on the northern Australian margin. Many of these imaged mass transport deposits, with the top 700–800 m of the section captured in detail not available with industry seismic data. We present nine new high-resolution seismic lines from three separate areas of the North West Shelf. Slides in the Roebuck Basin show complex anastomosing ductile extensional mechanisms, with multiple slip surfaces and no headscarps or adjacent faults. Slides on the Exmouth Plateau have fault control, with surface fault offsets of up to 300 m, indicating seismicity as a likely triggering mechanism. Slumps along the western margin of Western Australia are more limited in extent, associated with surface notches, with indications of previous activity at depth. All areas show a repeated history of mass transport deposits. The area of the active landslide province offshore of NW Australia is far larger than the individual slides recognized on the Exmouth Plateau.

Tectonic and Geodynamic Evolution of the Northern Australian Margin and New Guinea

Rapid convergence between the Indo-Australian, Southeast Asian, and Pacific plates in the Cenozoic has resulted in a complex tectonic evolution of Australia’s northern margin. A lack of available geologic data leads to large uncertainties, such as the timing of the Sepik collision with the New Guinea margin, currently constrained to sometime between 50 and 30 Ma. Previous work suggested a link between the Sepik collision and a voluminous fast seismic anomaly presently in the mantle beneath Lake Eyre. Following from previous work, this study uses coupled plate reconstruction and numerical geodynamic models to test 50 Ma and 30 Ma collision timings of the Sepik terrane, along with an upper extent back-arc basin, to further refine our understanding of the origin and trajectory of the slab beneath Lake Eyre and address uncertainties in the plate reconstructions. The results of mantle flow models indicate that the Eocene collision timing (~50 Ma) is more likely than an Oligocene collision (~30 Ma). In addition, dynamic topography results support previous suggestions that dynamic subsidence relating to the down-going Sepik slab has influenced the evolution of the Eyre Basin, with up to ~100 m of dynamic subsidence since ~20 Ma. However, further work is required to address numerical issues relating to rapid thermal diffusion of slab material, and to investigate reasonable trench retreat velocities for intermediate (~3000 km) subduction zone lengths. This work highlights the role of numerical experiments in understanding transient plate-mantle processes and their effect on basin evolution.

New constraints on the timing of flexural deformation along the northern Australian margin: Implications for arc-continent collision and the development of the Timor Trough

Numerous extensional faults offset the passive margin strata of the northern Bonaparte Basin. This extensional deformation has been attributed to lithospheric flexure of the descending Australian Plate, in an overall convergence setting. Here we use an extensive 2D and 3D seismic dataset calibrated with well biostratigraphy and strontium (Sr) isotope age data to constrain the timing of deformation along the northern Australian margin during the Neogene. Analysis of fault throw and differential thickness variations give new insights on the propagation and slip history of the faults. Along-dip throw profiles exhibit ‘D’ shape distributions, skewed towards the top. Positive throw gradients above the throw maxima, coinciding with intervals of growth strata, indicate multiphase fault activity. Results indicate that post-rift extensional deformation initiated during the latest Miocene (ca. 6Ma). The development of the modern Timor Trough (as a foreland basin) and Cartier Trough also commenced during this period. A second episode of increased tectonic activity occurred around the Pliocene–Quaternary boundary (ca. 3Ma), and the deformation continued intermittently to the present-day. These new results are in agreement with the timing of initiation of collision between the Australian Plate and the Banda Arc and uplift of the Timor Island, recently derived from stratigraphic analysis in Timor. These regional tectonic events have profoundly affected the paleogeography of the Timor Sea and may explain major changes in oceanic circulation and climate during the Neogene.

Spatial variations in the effective elastic thickness of the lithosphere in Southeast Asia

As a proxy for long-term lithospheric strength, detailed information on lateral effective elastic thickness (Te) variations can aid in understanding the distribution pattern of surface deformation and its response to long-term forces. Here we present high-resolution maps of spatial variations of Te for the complex SE Asian region by analyzing the coherence of topography and Bouguer gravity anomaly data. We find that after considering the gravity deficit of less dense sediment, the recovered Te maps are more representative of the geology, particularly in elongated rift basins. The results show that the Te variation pattern in SE Asia, in general, agrees well with its tectonic provinces and major tectonic boundaries. The oceanic basins, the Indosinian suture zones between the Indochina and Sibumasu blocks, and the Makassar Strait are characterized by low Te, while moderate and high Te values are recovered in the Khorat plateau, West Burma, the Singapore Ridge, the Con Song Swell, Borneo, the northern Australian margin and the Molucca Sea. The Te pattern in the south Indonesian margin is complicated by the approach and collision of oceanic plateaus and seamounts with the fore-arc region. The heterogeneous strength features are consistent with the complex assemblage of different tectonic units, and significant deformation during Cenozoic tectonic events. In the Indochina Peninsula, the extruded displacement during the India-Eurasia collision might have been partitioned and absorbed by the combined mechanism of the extrusion and viscous tectonic models. As a result, the offshore displacements of the major strike-slip faults in the South China Sea are much smaller than originally assumed, thus having less effect on the development of the South China Sea than other mechanisms such as the slab pull of the proto-South China Sea. Since the displacement driven by the boundary tectonic forces has been greatly absorbed and decreased by subduction and deformation in the active margins and adjacent weak regions, the motion velocity of the interior regions is greatly lower than the boundary active margins, and they are largely free of seismicity and volcanism. Our results suggest that East Borneo might share a similar crustal basement, and represent a broad tectonic zone of the destroyed Meso-Tethys Ocean extending from West-Middle Java, through East Borneo to northern Borneo of the Sarawak and Sabah. The Indosinian zones between the Indochina and Sibumasu blocks might extend further southeastward across Billiton Island to offshore of southern Borneo, and the Singapore platform and SW Borneo might belong to the same block. The results also show that the internal load fraction F is high in the coastal area of South China, the northern margin of the South China Sea, and the coastal area of Indochina, which, in general, agrees with the distribution of a high-velocity lower crustal layer and Late Cenozoic basaltic rocks.

Palynology of the Late Triassic Ischigualasto Formation, Argentina: Paleoecological and paleogeographic implications

In this study we describe and compare the palynological assemblages from the Carnian Ischigualasto Formation of westernmost Gondwana to those of eastern Gondwana, which have been referred to the Onslow and Ipswich phytogeographic provinces. The palynofloras studied contain many taxa common to both regions, as well as several species that are widely recorded in European Carnian successions and less frequently recovered in the Carnian of the Southern Hemisphere. For the first time in Argentinean assemblages, we recognize Anapiculatisporites spiniger, Cadargasporites granulatus, Cycadopites stonei, Ellipsovelatisporites plicatus, Enzonalasporites vigens, Ovalipollis pseudoalatus, O. ovalis, Patinasporites densus, Quadraeculina anellaeformis, Samaropollenites speciosus, Staurosaccites quadrifidus, “Rimaesporites” aquilonalis, Vallasporites ignacii and Zonalasporites cinctus. The new evidence extends the distribution of the Onslow palynoflora to include Carnian associations formerly assigned to the Ipswich Province. The affinities of Argentinean palynofloras seem to indicate the existence of a similar flora extending from the northern Australian margin (West Timor), through the western Tethys coasts, to westernmost Gondwana during the Carnian. Moreover, the new data reinforce the close relationship between some diagnostic species and humid environmental conditions.

Structure and prospectivity of the northern Australian margin collision zone revealed by new regional seismic data

The interpretation of recently acquired regional-deep, 2D seismic data across the Timor/Tanimbar Trough has provided a new insight into the structure of this continent/arc collision zone. This data is mostly recorded to 10–12 seconds with an 8 km dual sensor streamer, providing increased resolution and improved penetration compared with vintage data, particularly in the deeper part of the section. A major improvement in the new data is the imaging of the Australian continental margin succession continuously from the Australian side of the Timor-Tanimbar Trough for a distance of a few tens of km underneath the accretionary prism on the northwest side of the trough. Structures observed in the accretionary prism itself include fold and thrust belts, imbricate thrusts and extensional half grabens. The detachment surface at its base may provide a regional seal for traps involving Jurassic and Triassic reservoirs in underlying thrusts for which there is some evidence on the seismic data. The improved visibility of the Mesozoic succession below the accretionary prism allows possible Jurassic source rocks to be followed to the northwest below a growing overburden to a depth at which maturity may be expected. There is also a graben, probably including a greater Jurassic thickness, under the Timor Trough.

Deciphering of six blocks of Gondwanan origin within Eastern Indonesia (South East Asia)

The French Indonesian research programs, both conducted on land and offshore in the Banda Sea basins area, have led us to formulate a new interpretation of the East Indonesian geological components. In this area we distinguish, within the three main tectonic plates (Eurasian, Indo-Australian and Philippine–Pacific), seven blocks. Six blocks are deciphered as issued from the north-eastern Gondwanan margin and only one (the Halmahera block) issued from the Pacific plate. Among the Gondwanan blocks, two of them, namely Timor (probably originates from “Greater India”) and Kolonodale (or the Argo block) came from the north-west Australian margin. The Lucipara, Seram and Banggai–Sula blocks originated from the western extension of the Papua-New Guinea Island while the Irian Jaya block is still linked to the North Australian margin. The last one, the Halmahera block, originated from the Pacific plate, moved westward along the North Papua-New-Guinea margin up to its present position in the northern part of the Banda Sea area. Our interpretation is mainly based on the lithostratigraphic succession, the geological evolution and the Early Mesozoic paleoposition of these blocks with respect to the climatic environment deduced from micropaleontological studies. According to previous studies, the Timor and Kolonodale blocks were detached from Gondwana during the Jurassic whereas the Lucipara, Seram and Banggai–Sula blocks were detached from Papua-New Guinea Island during the Neogene period. All these Gondwanan blocks collided with the Eurasian active margin, in the vicinity of Sulawesi Island: Timor and Kolonodale had joined the Eurasian margin by the end of the Paleogene, while Lucipara, Seram and Banggai–Sula collided with Sulawesi Island between the Middle Miocene to Middle Pliocene. After the collisions the last three blocks together with the Kolonodale block did not move again but suffered the opening of North and South Banda back-arc basins, by the Late Miocene. On the contrary, part of the Timor block moved south together with the southern margin of the south Banda basin and, later on during the middle Pliocene, collided with the northern Australian margin. The detachment of these blocks from the Gondwana margin, and their amalgamation with the Eurasian margin, are discussed and illustrated through palaeogeographic maps.

The tilting continent: A new constraint on the dynamic topographic field from Australia

A pronounced latitudinal asymmetry in the present-day morphology of the Australian continental shelf is reflected in Neogene stratigraphic relationships. The northern Australian margin has a broad shelf, typically 200–500 km wide and a Neogene record of stratal onlap. Relative to the continent, sea levels are currently as high as at any stage during the Neogene. In contrast, the southern shelf is typically less than 100 km wide and shows a record of progressive offlap with Neogene palaeo-shorelines commonly many hundreds of kilometres inland, at elevations up to ∼ 250 m above present-day sea level. This continental-scale ‘reciprocal’ stratigraphy implies 250–300 m N-down, SSW-up apparent vertical motion with respect to sea level since the mid-Miocene. The apparent vertical motion can be attributed to variations in dynamic topography and the geoid along Australia's NNE plate circuit; specifically, the movement of the southern margin off the dynamic topography low, geoid low presently centred on the Australian–Antarctic discordance, and the northern margin towards a dynamic topography low, geoid high associated with the subduction realm to the north. Variations in the geoid appear to account for about 10% of the total apparent motion, depending on assumptions about how the geoid field has evolved during Australia's northward motion. This inferred Neogene, continental-scale dynamic N–S tilting rate of ∼15–20 m/myr provides a compelling new constraint on the nature of the Earth's dynamic topographic field.

Microfloristic provincialism in the Upper Triassic Circum-Mediterranean area and palaeogeographic implication

This study represents a contribution to the developing knowledge about the microfloristic provincialism affecting Upper Triassic palynoflora. The compositional differences existing between the Onslow and the Ipswich microfloras are mainly based on the presence, in the Onslow microflora, of a diverse and more varied group of gymnosperm pollen grains, including typical European elements. In this study, the palynological assemblages recovered from Carnian successions of the western Tethyan margin (Sicily, Tunisia, Albany, Libya and Israel) are compared with those of West Timor microfloral assemblages, which have been referred to the Onslow microflora of southern hemisphere. They contain several common taxa, mainly consisting of conifer miospores widely recorded in Carnian European successions and less frequently recovered in the Carnian of western and eastern Australia. The number of Circum-Mediterranean sporomorphs in the Onslow microflora assemblages is wider than previously thought, providing new evidences to extend the distribution of the Onslow microflora to include Carnian associations formerly assigned to the Circum-Mediterranean assemblages. These broad microfloral affinities seem to indicate the existence of a homogeneous microflora that maintains, with minor variations, its composition from western Tethys coasts to the northern Australian margin (West Timor). The parent plant community grew in a coastal environment, along the continental margins; the establishment of an equable climatic regime influenced by warm equatorial currents and suitable humid conditions probably conditioned its diffusion. It has long been recognised that the strong floral provinciality which characterised the Late Triassic world gave way to a more homogeneous flora in the Early Jurassic. The decrease in macrofloral diversity is associated with a less pronounced microfloristic provincialism, which in turn coincides with the rise, to strong dominance, of cheirolepidiaceous conifers (Circumpolles producers). This important microfloristic event occurs in the southern hemisphere during the Early Jurassic, however this study reveals their incipient diffusion during the Norian.

Late Jurassic rifting along the Australian North West Shelf: margin geometry and spreading ridge configuration

The Argo and Gascoyne Abyssal plains in the easternmost Indian Ocean document the last stages of eastern Tethys evolution before the breakup of eastern Gondwana. Thus they provide crucial information not only for modelling the evolution of the eastern Tethys and Proto-Indian Ocean, but also to understand the complex geodynamic history of the North West Shelf. We have revisited the marine magnetic anomaly record of the Argo and Gascoyne Abyssal Plains in combination with other geological and geophysical data from the North West Shelf and southeast Asia. Based on the combined data, we have created a revised plate-tectonic model and a set of palaeogeographic reconstructions for the evolution of the North West Shelf for the early stages after the breakup. The main difference between this model and previously published models is that we have interpreted a complete section of anomalies, M25A – M22A, in the Gascoyne Abyssal Plain, northwest of the Exmouth Plateau. The magnetic anomalies have the same trend as in the Argo Abyssal Plain. Our new plate-tectonic reconstructions show that continental breakup in the Argo and northern Gascoyne Abyssal Plains, east and northwest of the Exmouth Plateau, respectively, started simultaneously in the Oxfordian with M25A identified as the oldest anomaly. In the Gascoyne Abyssal Plain, the oldest anomaly sequence, M25A – M22A (154.5 – 150.4 Ma) indicates that the ‘Argo’ spreading ridge continued around the northern margin of Greater India, and was probably linked with the Somali Basin. Sea-floor spreading continued until M14, separating the West Burma Block and possibly other smaller continental fragments like the Sikuleh Terrane of Western Sumatra from the northern Australian margin. A southward-directed ridge jump at M13 (134 Ma) transferred segments of Australian Plate oceanic crust to the West Burma Plate. Contemporaneously, an anticlockwise change in spreading direction fixed the West Burma Block relative to Greater India until its collision with the southern Eurasian margin.

The Permian of Timor: stratigraphy, palaeontology and palaeogeography

The Permian of Timor in the Lesser Sunda Islands has attracted the attention of palaeontologists since the middle of the nineteenth century because of the richness, diversity and excellent state of preservation of its fauna. These abundant fossil data have been compiled and updated for the present account. The Permian rocks of Timor were deposited on the northern margin of Australia. At the present time the northern margin of Australia, in the region of Timor, is involved in a continent–arc collision, where Australia is colliding with the Banda Arcs. As a result of this collision, Permian rocks of the Australian margin have been disrupted by folding and faulting with the generation of mud-matrix mélange, and uplifted to form part of the island of Timor. Due to this tectonic disruption, it has proved difficult to establish a reliable stratigraphy for the Permian units on Timor, especially as the classic fossil collections were obtained largely from the mélange or purchased from the local people, and do not have adequate stratigraphic control. Detailed systematic, structural, stratigraphic and sedimentological studies since the 1960s have provided a firmer stratigraphic and palaeogeographic background for reconsideration of the significance of the classic fossil collections. Permian rocks on Timor belong either to a volcanic-carbonate sequence (Maubisse Formation), or to a clastic sequence (Atahoc and Cribas formations) in which volcanics are less prominent. The Permian sequences were deposited on Australian continental basement which was undergoing extension with spasmodic volcanic activity. Carbonates of the Maubisse Formation were deposited on horst blocks and volcanic edifices, while clastic sediments of the Atahoc and Cribas formations were deposited in grabens. The clastic sediments are predominantly fine-grained, derived from a distant siliciclastic source, and are interbedded with sediments derived from the volcanics and carbonates of adjacent horst blocks. Bottom conditions in the graben were often anoxic. In the present account, events on Timor during the Permian are related to the regional tectonic context, with the northward movement of Australia leading to the amelioration of the climate from sub-glacial to sub-tropical, together with the separation of crustal blocks from the northern Australian margin to form the Meso-Tethys.

Neogene wrench reactivation of the Barcoo Sub-basin, northwest Australia: implications for Neogene tectonics of the northern Australian margin

The Barcoo Sub-basin forms the southern part of the Browse Basin of Australia’s northwestern margin. Miocene reactivation of older Precambrian through Mesozoic-aged structures has resulted in a present-day complex right-lateral wrench zone. The fault system displays restraining and releasing bends separated by areas of almost pure strike-slip displacement, along its 180 km length. Reactivation and inversion varies along the system, with the southern end structurally highest, offsetting beds up to and including the seafloor. The location of restraining and releasing bends controls the occurrence of oil and gas shows in nine wells drilled in the sub-basin, with Neogene inversion causing trap leakage in a number of cases. Right-lateral motion on the Barcoo Fault system, in contrast to a proposed regional-scale left-lateral movement component on the northwestern margin as a whole, implies strong strain partitioning along the margin with regional tectonic control. We propose that the structural domain boundaries may be controlled in part by the location of the continental collision zone between the northernmost limit of Australian continental crust with the southernmost continental fragments of the Eurasian plate, in the vicinity of the island of Sumba.

Rotation of horizontal stresses in the Australian North West continental shelf due to the collision of the Indo‐Australian and Eurasian Plates

There is a 40° rotation of regional maximum horizontal stress (σhmax) orientation between the western end of the Australian North West Continental Shelf (Carnarvon Basin) and its eastern end (Bonaparte Basin). A total of 625 borehole breakouts covering a cumulative length of 7.7 km in 42 wells in the Carnarvon Basin indicates a σhmax orientation of 090°–100°N. A total of 616 borehole breakouts over 6.8 km in 46 wells in the Bonaparte Basin indicates a σhmax orientation of 055°N–060°N. Together with extant data from the World Stress Map, these results indicate that regional σhmax orientation is 050°–060°N from New Guinea westward through the Bonaparte Basin to the Canning Basin (central North West Shelf). Between the Canning Basin and the Carnarvon Basin, σhmax rotates to 090°–100°N. The parallelism of σhmax orientation in the Bonaparte Basin to the Australia/Banda Arc collisional zone indicates that this collision is not generating significant net push. Rather, the 050°–060°N σhmax orientation of much of the northern Australian margin is probably controlled by the more mature New Guinea orogen to which it is approximately orthogonal. The observed rotation of σhmax can be explained solely by the focusing of the forces balancing ridge push along collisional segments of the northeastern boundary of the Indo‐Australian Plate (such as the New Guinea orogen). Although not required to account for the observed stress rotation, a slab pull force from oceanic Indo‐Australian Plate being subducted beneath the Sunda Arc cannot be dismissed.

Dating of Neogene igneous rocks in the Halmahera region: arc initiation and development

Abstract Potassium-argon ages of Neogene to Recent igneous rocks from the Halmahera region record a history of intra-oceanic arc development since the late Middle Miocene following an earlier phase of collisional plutonism. Arc formation from the Middle Miocene onwards was due to the east-directed subduction of the Molucca Sea plate beneath the Philippine Sea plate as it arrived at the Eurasian margin. The distribution of ages within the Neogene arc indicates a northward migration of volcanic activity during the Late Miocene to Pliocene. Results of the dating work show that after collision with the Australian margin at c. 22 Ma there was a period of volcanic quiescence and limestone deposition before a new arc formed. This arc began erupting at around 11 Ma on Obi as a result of subduction of the Molucca Sea plate. Initiation of subduction is thought to have occurred around 15–17 Ma and may have been responsible for disturbing potassium-argon ages of pre-Neogene rocks. Dates from fresh rocks show that the volcanic front migrated northwards through Bacan and Halmahera throughout the Late Miocene to Early Pliocene. Limestone deposition was curtailed as arc activity migrated north while volcanism died out from the south. No Neogene volcanism younger than 8 Ma is observed in the Obi area while on Bacan subduction-related volcanism ceased at c. 2 Ma. Late Pliocene crustal deformation caused a 30–40 km westward shift of the volcanic front. Quaternary volcanic rocks exposed in Bacan and the extreme south of Halmahera are not direct products of subduction but, rather, display geochemical characteristics of both subduction and fault-related magmatism. These volcanic rocks are distributed along splays of the Sorong fault system. The formation and propagation of the Halmahera arc is a consequence of the clockwise rotation of the Philippine Sea plate as the southern edge moved across the northern Australian margin and impinged on the east Eurasian margin. The ages of initiation of volcanism and subduction track the developing plate boundary as subduction propagated northwards.

Late Ordovician–early Silurian destruction of the Iapetus Ocean: Newfoundland, British Isles and Scandinavia—a discussion

ABSTRACTThe available data from Newfoundland, the British Isles and Scandinavia suggest that by late Ordovician–early Silurian times the ocean separating Laurentia from Eastern Avalonia and Baltica had partly closed with the consumption of intervening oceanic crust. Marine seaways, however, persisted until the middle or late Silurian. Phases of crustal transtension and transpression, predominantly under a major sinistral shear couple, occurred throughout the Silurian and early Devonian until the remnant Iapetus Ocean was completely destroyed. The most appropriate Recent plate tectonic models for Silurian sedimentation between Eastern Avalonia and Laurentia are probably the deep-marine foreland basins between Timor and the northern Australian margin, or between Taiwan and mainland China.