CONTINENTAL MARGIN TECTONICS AND THE EVOLUTION OF SOUTH EAST AUSTRALIA

1971 ◽  
Vol 11 (1) ◽  
pp. 75 ◽  
Author(s):  
J. R. Griffiths
Keyword(s):  
Continental Margin ◽  
Rift Valley ◽  
Structural Evolution ◽  
Initial Crack ◽  
Alkali Basalts ◽  
Australian Plate ◽  
Differential Movement ◽  
Mid Ocean Ridge ◽  
Regional Tectonic ◽  
Ocean Ridge

Following recent advances in geotectonics, a new approach can be applied to the study of the development of continental margins.A continental margin begins to form as an older continental craton breaks up. The initial crack develops into a rift valley, which becomes filled with thick clastic and volcanic deposits. As separation continues a new mid-ocean ridge is formed, and the two plates begin to drift apart more rapidly. At this stage the structural evolution of the margins is virtually complete, and marine sediments are deposited unconformably across the fault troughs.The continental fragments in the south west Pacific can be reassembled as a part of the ancient continent of Gondwanaland. Gondwanaland began to break up in the mid-Jurassic. A rift valley developed along the line of the present southern coast of Australia, through the Otway Basin. Two subsidiary tensional splays gave rise to the Elliston and Robe-Penola Troughs. Clastic sediments stripped from the cratonic highlands, and alkali basalts, occur in the rift grabens. Faulting and deposition continued throughout the Lower Cretaceous. About mid-Cretaceous a marine transgression from the west entered the subdividing rift valley. In the Eocene a new mid-ocean ridge formed and the Australian and Antarctic plates began to separate more rapidly. After this, quiet marine sedimentation occurred on the continental shelf and slope.The Bass and Gippsland Basins began to develop in the Cretaceous as differential movement occurred between the main Australian plate and a partially detached Tasmanian sub-plate. In the Upper Cretaceous the Gippsland Basin became open towards the evolving Tasman Sea, as New Zealand detached. The Tasmanian sub-plate ceased fo exist after the Eocene, becoming firmly fixed to the Australian plate. Later readjustments have occurred giving rise to further limited movements, mainly in the Gippsland Basin.The integration of detailed geological work and a regional tectonic analysis has been successfully applied to south east Australia and it is probable that a similar approach would yield fruitful results applied elsewhere.

Platinum-group element geochemistry of intraplate basalts from the Aleppo Plateau, NW Syria

2012 ◽  
Vol 150 (3) ◽  
pp. 497-508 ◽  
Author(s):  
GEORGE S.-K. MA ◽  
JOHN MALPAS ◽  
JIAN-FENG GAO ◽  
KUO-LUNG WANG ◽  
LIANG QI ◽  
...  
Keyword(s):  
Partial Melting ◽  
Middle Miocene ◽  
Primitive Mantle ◽  
Element Geochemistry ◽  
Alkali Basalts ◽  
Melt Extraction ◽  
Mid Ocean Ridge ◽  
Order Of Magnitude ◽  
Platinum Group ◽  
Ocean Ridge

AbstractEarly–Middle Miocene intraplate basalts from the Aleppo Plateau, NW Syria have been analysed for their platinum-group elements (PGEs). They contain extremely low PGE abundances, comparable with most alkali basalts, such as those from Hawaii, and mid-ocean ridge basalts. The low abundances, together with high Pd/Ir, Pt/Ir, Ni/Ir, Cu/Pd, Y/Pt and Cu/Zr are consistent with sulphide fractionation, which likely occurred during partial melting and melt extraction within the mantle. Some of the basalts are too depleted in PGEs to be explained solely by partial melting of a primitive mantle-like source. Such ultra-low PGE abundances, however, are possible if the source contains some mafic lithologies. Many of the basalts also exhibit suprachondritic Pd/Pt ratios of up to an order of magnitude higher than primitive mantle and chondrite, an increase too high to be attributable to fractionation of spinel and silicate minerals alone. The elevated Pd/Pt, associated with a decrease in Pt but not Ir and Ru, are also inconsistent with removal of Pt-bearing PGE minerals or alloys, which should have concurrently lowered Pt, Ir and Ru. In contrast, melting of a metasomatized source comprising sulphides whose Pt and to a lesser extent Rh were selectively mobilized through interaction with silicate melts, may provide an explanation.

scholarly journals Zircon survival in shallow asthenosphere and deep lithosphere

2020 ◽  
Vol 105 (11) ◽  
pp. 1662-1671
Author(s):  
Anastassia Y. Borisova ◽  
Ilya N. Bindeman ◽  
Michael J. Toplis ◽  
Nail R. Zagrtdenov ◽  
Jérémy Guignard ◽  
...  
Keyword(s):  
Tholeiitic Basalt ◽  
Alkali Basalts ◽  
Mid Ocean Ridge ◽  
Basaltic Melt ◽  
Fast Transport ◽  
Basalt Composition ◽  
Oceanic Asthenosphere ◽  
Ocean Ridge ◽  
Low Pressures

Abstract Zircon (ZrSiO4) is the most frequently used geochronometer of terrestrial and extraterrestrial processes. To shed light on question of zircon survival in the Earth's shallow asthenosphere, high-temperature experiments of zircon dissolution in natural mid-ocean ridge basaltic (MORB) and synthetic haplobasaltic melts have been performed at temperatures of 1250–1300 °C and pressures from 0.1 MPa to 0.7 GPa. Zirconium measurements were made in situ by electron probe microanalyses (EPMA) at high current. Taking into account secondary fluorescence effects in zircon-glass pairs during EPMA, a zirconium diffusion coefficient of 2.87E-08 cm2/s was determined at 1300 °C and 0.5 GPa. When applied to the question of zircon survival in asthenospheric melts of tholeiitic basalt composition, the data are used to infer that typical 100 mm zircon crystals dissolve rapidly (~10 h) and congruently upon reaction with basaltic melt at pressures of 0.2–0.7 GPa. We observed incongruent (to crystal ZrO2 and SiO2 in melt) dissolution of zircon in natural mid-ocean ridge the basaltic melt at low pressures <0.2 GPa and in the haplobasaltic melt at 0.7 GPa pressure. Our experimental data raise questions about the origin of zircon crystals in mafic and ultramafic rocks, in particular, in shallow oceanic asthenosphere and deep lithosphere, as well as the meaning of the zircon-based ages estimated from these minerals. The origin of zircon in shallow (ultra-) mafic chambers is likely related to the crystallization of intercumulus liquid. Large zircon megacrysts in kimberlites, peridotites, alkali basalts, and carbonatite magmas suggest fast transport and short interaction durations between zircon and melt. The origin of zircon megacrysts is likely related to metasomatic addition of Zr into the mantle as an episode of mantle melting should eliminate them on geologically short timescales.

Mantle heterogeneity beneath oceanic islands: some inferences from isotopes

1993 ◽  
Vol 342 (1663) ◽  
pp. 91-103 ◽  
Keyword(s):  
Alternative Hypothesis ◽  
Melting Zone ◽  
Oceanic Islands ◽  
Alkali Basalts ◽  
Mauna Loa ◽  
Mid Ocean Ridge ◽  
Mantle Sources ◽  
Hawaiian Plume ◽  
Mantle Helium ◽  
Ocean Ridge

Radiogenic isotopes in oceanic basalts are extremely useful as tracers of long-lived heterogeneities in the Earth's mantle. Helium isotopes provide unique information in that high 3 He / 4 He ratios are indicative of relatively undegassed mantle reservoirs (i.e. mantle with high time-integrated 3 He/(Th + U) ratios). An alternative hypothesis is that high 3 He / 4 He ratios may have been produced by ancient melting events, if the solid/melt partition coefficient (K d ) for He is greater than that for Th and U (i.e. yielding relatively high He/(Th + U) in the residue of melting). However, the distribution of helium within basaltic phenocrysts, and olivine/glass helium partitioning within mid-ocean ridge basalts, suggest that helium behaves as an incompatible element during melting (K d (olivine/glass) < 0.0055), which strongly supports the hypothesis that high 3 He / 4 He ratios are derived from undegassed mantle reservoirs. Isotopic measurements of He, Sr, and Pb in Hawaiian volcanoes lavas demonstrate that the mantle sources have changed on extremely short timescales, between 100 and 10 000 years before present. The preferred explanation for these variations is that they represent heterogeneities within the Hawaiian mantle plume, combined with late stage melting in the lithosphere for post shield alkali basalts. Helium isotopic data from Kilauea, Hualalai and Mauna Loa suggest that the plume is presently located beneath Kilauea (and Loihi seamount), and constrain the melting zone of the Hawaiian plume to be less than 40 km in radius.

Dives of AUV "r2D4" to Rift Valley of Central Indian Mid-Ocean Ridge System

2007 ◽  
Author(s):  
Tamaki Ura ◽  
Kensaku Tamaki ◽  
Akira Asada ◽  
Kei Okamura ◽  
Kenji Nagahashi ◽  
...  
Keyword(s):  
Rift Valley ◽  
Mid Ocean Ridge ◽  
Ocean Ridge ◽  
Ridge System

Late stage rifting of the Laurentian continent: evidence from the geochemistry of greenstone and amphibolite in the central Vermont Appalachians1This article is one of a series of papers published in CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology.

2012 ◽  
Vol 49 (1) ◽  
pp. 43-58 ◽  
Author(s):  
Raymond Coish ◽  
Jonathan Kim ◽  
Nathan Morris ◽  
David Johnson
Keyword(s):  
New England ◽  
East African ◽  
Alkali Basalts ◽  
Iapetus Ocean ◽  
Afar Region ◽  
Mid Ocean Ridge ◽  
African Rift ◽  
Type 3 ◽  
Ocean Ridge

Metamorphosed mafic rocks from west-central Vermont crop out in tectonic slices of the Stowe Formation within the Rowe–Hawley Belt of New England. The rocks include greenstone and amphibolite, which are interpreted to have been basaltic flows and gabbroic intrusions, respectively. Even though the rocks have been metamorphosed to greenschist or amphibolite facies, their igneous origins can be deciphered through careful use of geochemistry. Three geochemical types have been identified. Type 1 and 2 samples have geochemical characteristics similar to those found in mid-ocean ridge basalts (MORB), except that they have slightly elevated light rare-earth element (LREE) concentrations and are higher in Nb/Y ratios. Their Nb/Y ratios are similar to basalts found in Iceland and parts of the Afar region of the East African Rift. Types 1 and 2 are similar to metabasalts of the Caldwell and Maquereau formations in southern Quebec. The less-common type 3 samples have highly enriched LREE and are high in Nb/Y and Zr/Y ratios, similar to some alkali basalts from Afar and Iceland. Detailed analysis of the geochemistry suggests that greenstones and amphibolite from the Stowe Formation formed as basaltic eruptions during very late stages in rifting of the Rodinian continent that eventually led to formation of the Iapetus Ocean. This interpretation is consistent with tectonic models of the Vermont and Quebec Appalachians.

Geophysical studies of the continental margin northeast of Newfoundland

1968 ◽  
Vol 5 (3) ◽  
pp. 483-500 ◽  
Author(s):  
D. K. B. Fenwick ◽  
M. J. Keen ◽  
Charlotte Keen ◽  
A. Lambert
Keyword(s):  
Continental Crust ◽  
Continental Margin ◽  
Magnetic Anomalies ◽  
Ocean Basin ◽  
Labrador Sea ◽  
Mobile Belt ◽  
Magnetic Survey ◽  
Continental Plate ◽  
Mid Ocean Ridge ◽  
Ocean Ridge

A seismic and magnetic survey has been made of an area that straddles the continental margin northeast of Newfoundland from the edge of the shelf to the ocean basin of the Labrador Sea. The results bear on the question of the extension of the mobile belt of the Appalachian System in Newfoundland to the margin. Our seismic studies show that the crust is approximately 30 km thick beneath the edge of the shelf northeast of Newfoundland, and thins to approximately 12 km at the foot of the slope. Seismic studies by Lamont Geological Observatory suggest that the mobile belt continues to the upper part of the slope; our studies support this. Large magnetic anomalies run nearly parallel to the edge of the shelf over the slope and rise, at right angles to the strike of the Appalachian System. Their amplitude is 1400 gammas and the belt of anomalies is 180 km wide. It runs continuously for at least 220 km. One interpretation is that the anomalies owe their origin to the juxtaposition of a magnetic continental plate 30 km thick against a thinner magnetic oceanic plate 10 to 12 km thick. We suggest that the continental crust is particularly magnetic, and the 'shelf-edge' anomalies particularly large, because it represents the basic mobile belt of the Appalachian System terminating beneath the lower part of the continental slope. The contrast in magnetization between non-magnetic oceanic mantle and magnetic continental crust is due to this, and perhaps also to higher temperatures in the oceanic mantle. They could be particularly high if the Labrador Sea is the site of a mid-ocean ridge. Some evidence from the magnetic survey suggests that a fault with dextral slip runs on the continent side of the margin in a northeasterly direction. It does not continue into the ocean basin.

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