Geoscientists have, for the first time, discovered the origins of Australia's two largest basins: Lake Eyre and the Murray-Darling Basin. The research also implies that in 30 million years' time both basins will cease to exist.
Monash University geoscientist Associate Professor Wouter Schellart, and his colleague Professor Wim Spakman from Utrecht University, have discovered how the floor of an entire ocean basin that was destroyed 70 to 50 million years ago off the North coast of New Guinea is currently located at 800-1200km depth below Central and South-eastern Australia.
Using supercomputers, the researchers found that this dense piece of ocean floor material (called a lithospheric slab) is slowly sinking into the Earth's mantle and is responsible for the formation of the Lake Eyre Basin, one of the Earth's largest internally drained basins and home to the lowest point in Australia at 15m below sea level, as well as the Murray-Darling Basin, home to the largest river system in Australia. With a combined surface area exceeding 2 million square kilometres, both basins are located directly above the deep mantle slab.
The research also predicts that in 30 million years from now, when Australia has moved about 1500km northwards, the fossil slab will be located below the Southern Ocean and, as a consequence, the Lake Eyre Basin and Murray Darling Basin will cease to exist.
Using geological and geophysical data from the New Guinea region, Schellart was able to reconstruct the geological evolution of the region over the last 70 million years, including the motion of the tectonic plates and plate boundaries. He discovered that the occurrence of deep ocean floor rocks, volcanic rocks and deformed rocks, which are currently found in the mountain ranges of New Guinea, point to the existence of a 4000km wide subduction zone. At subduction zones such as these, an oceanic tectonic plate sinks (subducts) into the Earth's interior, the mantle.
With these plate tectonic reconstructions Schellart was able to predict where the fossil subduction zone was during its lifetime some 50-70 million years ago, and therefore where the lithospheric slab disappeared into the mantle. With a global seismic tomography model that makes use of seismic waves to map the internal structure of the Earth's mantle, Schellart and Spakman were able to identify the fossil slab structure below central and south-eastern Australia at a location and depth predicted by the reconstructions.
Paleoarchean ocean crust and mantle excavated by meteor impact: Insight into early crustal processes and tectonics
Authors:
Krull-Davatzes et al
Abstract:
Two major features of Earth's geochemical evolution include (1) the extraction of continental crust from the mantle to produce a depleted upper mantle, and (2) the onset of plate tectonics and shallow melting of the upper mantle to produce oceanic crust (mid-ocean-ridge basalt, MORB). Prior studies have suggested that continental crust extraction and/or plate tectonics began prior to 4 Ga or were not initiated until as late as the Neoproterozoic. Earth's geological record is very incomplete prior to ca. 3 Ga, and the limited terrains available for study have left the nature of the earliest tectonics and crustal compositions unresolved. Here we show that a meteor impact at 3.24 Ga excavated basaltic crust and depleted mantle material. We use high field strength elements and other immobile elements to model the composition of the target rock and meteor and compare it to analyses of spherules condensed from the vaporized meteor and impact target rock. This provides insight into otherwise unpreserved Archean crust. A simple mixture of 15% CV group carbonaceous chondrite, 65% ocean basalt, and 20% depleted MORB mantle (DMM) fully replicates the field results. A significant proportion of this basalt is MORB-like, distinctly different from the tholeiitic rocks that dominate in Archean greenstone belts, and consistent with seafloor spreading. The presence of DMM implies that a significant volume of continental crust was extracted before 3.24 Ga. However, no granite or tonalite- trondhjemite-granodiorite can be incorporated into the model and replicate field results, consistent with the lack of any observed shocked zircons in any of the Paleoarchean impact layers to date.
Neoproterozoic oceanic crust remnants in northeast Brazil
Authors:
Caxito et al
Abstract:
The Borborema Province of northeast Brazil occupies a strategic position in the central portion of West Gondwana, linking three of its major cratonic constituents: the São Francisco–Congo, Amazon, and West Africa cratons. The southern portion of the province, the Riacho do Pontal fold belt, comprises an association of exhalative rocks with metabasalts having transitional mid-oceanic-ridge basalt geochemistry (Monte Orebe Complex) that suggests the preservation of remnants of Neoproterozoic oceanic crust. This view is supported by geophysical data: the Riacho do Pontal fold belt corresponds to the inflexion of a paired positive-negative Bouguer anomaly similar to other Precambrian suture zones. Sm-Nd isotope data for the metabasalts yield a whole-rock isochron age of 819 ± 120 Ma with an initial εNd(t)= +4.4, indicating derivation from a depleted mantle source. The onset of subduction within the Borborema Province (ca. 630 Ma) caused the inversion of basins and obduction of slices of oceanic crust. In this scenario, the Riacho do Pontal fold belt represents a complete late Neoproterozoic plate tectonics cycle involving the collision of the São Francisco craton (lower plate) with the Pernambuco-Alagoas block (upper plate). This interpretation challenges current views that the Borborema Province has acted as a coherent block since Paleoproterozoic time (part of the Atlantica supercontinent), suggesting instead a dynamic setting, where multiple plates interacted during the Proterozoic. The Monte Orebe ophiolite provides a link with other Cryogenian oceanic crust occurrences in central Brazil and West Africa, indicating the preservation of a transcontinental Neoproterozoic suture zone in the heart of West Gondwana.
