Gascoyne Abyssal Plains Research Articles

Architecture of volcanic rifted margins: new insights from the Exmouth – Gascoyne margin, Western Australia

Australia's western continental margin formed during Gondwana breakup between Australia – Antarctica and India, leaving behind an evolving plate boundary with significant new magmatic crust accreted to the continental fragments. In this context, the margin segment from the outer Exmouth Plateau to the Gascoyne Abyssal Plain has been repeatedly studied as an example of volcanic margin formation. However, many recent analyses based on selective interpretation of geophysical datasets have come to conflicting conclusions regarding both the tectonic architecture and the age of breakup on this margin. These differences profoundly impact on models for both the global plate circuit and for the timing and extent of magmatism implicated in volcanic breakup processes. We present new interpretations of the distribution of magmatic and pre-rift rock packages in this margin, based on the integrated interpretation of two deep crustal transects with existing seismic-reflection, refraction, gravity, magnetic and geochemical data. Interpretations are constrained by data from sparse Ocean Drilling Program and petroleum-exploration drilling, and dredging. We find evidence for significant accumulation of magmatic rocks and their clastic derivatives infilling extensional fault-controlled basins developed in a broad volcanic-margin transition zone between the outer Exmouth Plateau and true oceanic crust. These rocks have distinctive seismic facies in the form of seaward-dipping reflector sequences, and are dense and magnetised. Most significantly, these packages give rise to potential-field anomalies that have previously been interpreted as due to seafloor spreading. Recognition of these packages in a volcanic-margin transition zone has implications for the recognition of the inboard edge of unequivocal oceanic crust, the oceanic – volcanic-margin boundary. The main locus of igneous activity in the volcanic-margin transition zone off the Exmouth Plateau is spatially offset from a previously recognised high-velocity zone, suggesting that these two phenomena may not be temporally related. Seismically imaged differences in total thinning and partitioning of thinning, between upper and lower crust, provide support for models of depth-dependent thinning previously proposed for this margin.

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.

Rare earth geochemistry of Arenig cherts from the Ballantrae Ophiolite and Leadhills Imbricate Zone, southern Scotland: implications for origin and significance to the Caledonian Orogeny

Rare earth element (REE) data from low to mid-Arenig cherts are used to test competing models for the early Ordovician evolution of the Laurentian margin in the northern British Isles. Cherts from the Ballantrae Ophiolite Complex have chondrite-normalized REE patterns typical of continental margin settings with LREE enrichment, a slight negative Eu anom and shale and chondrite-normalized La/Yb values of 0.97–1.41 and 7.78–11.4 respectively. This pattern, together with a large positive chondrite-normalized Ce anom (1.44–1.70), is virtually identical to that found in radiolarian chert of the Gascoyne Abyssal Plain, in the Timor Sea. Cherts from the Raven Gill Formation within the Leadhills Imbricate Zone, Northern Belt, Southern Uplands have typical continental margin REE patterns, chondrite-normalized Ce anom (0.9–1.21) and Eu anom (0.61–0.79) values indicating that they formed closer to the continental margin than those from Ballantrae. Shale and chondrite-normalized and La/Yb values of 0.95–1.27 and 4.92–13.88 respectively confirm this interpretation. It is concluded that the Ballantrae ophiolite formed in a rifted–arc basin above a northwards dipping, intra-oceanic subduction zone. The modest depth of burial of the Raven Gill Formation precludes it being part of a marginal basin which was subsequently trapped as the Ballantrae Ophiolite was obducted in the late Arenig. The Arenig rocks of the Leadhills Imbricate Zone represent an allochthonous terrane accreted to the western extension of the Midland Valley in Ireland in pre-Caradoc times. Here it formed the basement to the Southern Uplands basin. Palaeontological evidence places this basin adjacent to Pomeroy, Co. Tyrone in the early Caradoc. Sinistral strike-slip faulting, from the late Ashgill transported the Southern Uplands Terrane to its present location, a distance of less than 250 km.

Triassic to Cenozoic evolution of the NW Australian continental margin and the birth of the Indian Ocean (preliminary results of ODP Legs 122 and 123)

The northwest Australian continental margin is a very old, sediment-starved NE Gondwanan margin with the opportunity to study the early structural and depositional evolution during the Triassic to Jurassic rifting stages (Neo-Tethys), the earliest Cretaceous transition from rifting to drifting, and the early tectonic, volcanic, and paleoceanographic history of the Indian Ocean. Ocean Drilling Program (ODP) Legs 122 and 123 drilled eight sites in the area of the Exmouth Plateau and the adjacent abyssal plains: Sites 759–761 and 764 studied the Triassic to Cenozoic evolution of Wombat Plateau, a small sub-plateau of the northern Exmouth Plateau; Sites 765 penetrated almost 1 km of Cretaceous to Cenozoic sediments of the nearby Argo Abyssal Plain and 270 m of the oldest Indian ocean crust; Sites 762 and 763 studied the southern central Exmouth Plateau, whereas Site 766 was drilled into intrusive volcanics with MORB affinity under the adjacent Gascoyne Abyssal Plain at the foot of the western escarpment of the plateau. The major discoveries during the drilling of the two continental margin-deep ocean basin transects led to a better understanding of the early rift history of this margin during late Permian to late Triassic times (stretching, block-tectonic movements, fluviodeltaic/-shallow-water carbonate environments, major Rhaetian carbonate buildup, early rift volcanism); of the rift-drift transition processes (major blockfaulting during Jurassic times with local uplift, rift flank tilting and subaerial erosion forming a post-rift unconformity); of the age and paleoenvironment of the oldest part of the Indian Ocean (possibly not earlier than late Berriasian to early Valanginian breakup, 20 Ma younger than expected); of the »juvenile ocean stage« (transgressive, condensed early Neocomian belemnite sands, calcisphere chalks and bentonites on Wombat Plateau); of the »mature ocean stage« (post-mid-Cretaceous eupelagic chalks); of the geochemical nature and origin of old oceanic crust and the overlying sediments in a »geochemical reference hole«; and of the Mesosoic chrono-(bio-magneto-stratigraphy) and the age and causes of major relative sea level fluctuations, especially during late Triassic and early Cretaceous times.

Late Jurassic‐Early Cretaceous evolution of the eastern Indian Ocean adjacent to northwest Australia

Over 9700 km of new aeromagnetic data were acquired off the northwest coast of Australia and combined with existing magnetic data to map magnetic isochrons in the eastern Indian Ocean. The isochrons were used to constrain a tectonic model of the evolution of the seafloor in the Argo, Cuvier, and Gascoyne abyssal plains. A complete set of anomalies, from M26 through M16, was found in the Argo Abyssal Plain, trending generally N70°E. Spreading commenced in the center of the basin at or prior to M26 and propagated outward until at least M24 time. Anomalies M10–MO, recording the separation of Australia and India, were found in the Cuvier and Gascoyne abyssal plains, with a trend of about N30°E. A significant crustal age discontinuity occurs in the vicinity of the Joey Rise where the two lineation sets converge. Because there appears to be no overlap of isochron ages in the two groups, it is not necessary to postulate that a triple junction existed off northwest Australia as has been previously suggested. At M4–M5 time a 10° clockwise change in spreading direction occurred on the Cuvier‐Gascoyne spreading system. This event triggered ridge jumps that transferred two pieces of the Indian plate to the Australian plate. Overlapping spreading on the forming and dying ridges, curved fracture zones and lineations, as well as fanned lineation trends, suggest that the ridge jumps occurred by ridge propagation and that the transferred lithospheric blocks behaved as microplates for a brief interval of approximately 1–2 m.y.

Sedimentology of the argo and gascoyne abyssal plains, NW Australia: Report on Ocean Drilling Program Leg 123 (Sept. 1–Nov. 1, 1988)

Ocean Drilling Program Leg 123 drilled two sites in the Indian Ocean in order to study the rifting and early spreading of one of the world’s oldest ocean basins.

The sea-floor spreading history of the eastern Indian Ocean

The geologic history of the eastern Indian Ocean between northwest Australia and the Java Trench is known to involve two separate events of rifting and sea-floor spreading. Late Jurassic spreading in the Argo Abyssal Plain off northwest Australia was followed by Early Cretaceous spreading in the Cuvier and Perth Abyssal Plains off west Australia. However, the evolution and interaction of these events has not been clear. Mesozoic sea-floor spreading anomalies have been identified throughout the Argo Abyssal Plain that define a rifting event and subsequent northward spreading on the northwestern Australian margin at 155 m.y.b.p. Magnetic anomalies northwest of the Argo Abyssal Plain indicate a ridge jump to the south at about 130 m.y.b.p. that is approximately synchronous with east-west rifting along the southwestern Australian margin. The Joey Rise in the Argo Plain was probably formed by volcanism at the intersection of this new rift and the spreading ridge to the north. The southern and northern spreading systems were connected through the Exmouth Plateau which was stretched and faulted as spreading progressed. The RRR triple junction was formed at the intersection of the two spreading systems and appears to have migrated west along the northern edge of the Gascoyne Abyssal Plain. Spreading off northwest Australia cannot be easily related to simultaneous spreading in the west central Pacific via any simple tectonic scheme.

Redeposition of Pelagic Sediment by Turbidity Currents; a Common Process for Building Abyssal Plains: ABSTRACT

ABSTRACT Reworked pelagic detritus forms the bulk of graded beds from the upper 10 to 20 m of most abyssal plains. Coccoliths are by far the commonest detrital particle in many of the abyssal plains and occur with discoasters in the fine silt fraction, whereas reworked radiolaria, diatoms and planktonic foraminifera occur in the coarse silt and fine sand fractions. Piston and gravity cores from the Argo and Gascoyne abyssal plains off northwest Australia contain graded beds which consist of pelagic detritus sorted into layers of either radiolaria, diatoms or planktonic foraminifera. These layers clearly define the basal portions of many graded beds of different color shades. The source for most of the sediment must lie in the pelagic oozes of the adjacent abyssal hills and rises and not in the upper continental slope and shelf areas. There is increasing evidence that density currents commonly originate in the more immediate fan valleys and rises flanking the abyssal plains and infrequently on the upper continental slope. End_of_Record - Last_Page 506-------