Journal and Proceedings of
The Royal Society of New South Wales
Volume 115 Parts 1 and 2 [Issued August, 1982]
CONTENTS
| AUTHORS & TITLES | PAGES |
| King, D.S., Proper Motions in the region of the Galactic Cluster NGC 6087 | 1-4 |
| Lomb, N.R., Precise Observations of Minor Planets at Sydney Observatory during 1981 | 5-8 |
| Rynn, J.M.W. and Lynam, C.J., The Kempsey Earthquake of 6 September, 1979 | 9-12 |
| Korsch, R.J., The Dummy Creek Association: Rim Syncline Deposits | 13-19 |
| Willis, I.L., Description and Interpretation of a Useful Leucogneiss Stratigraphic Marker in the Willyama complex, Broken Hill Block, N.S.W. | 21-32 |
| Rutland, R.W.R., On the Growth and Evolution of the Continental Crust. A Comparative Tectonic Approach [Clarke Memorial Lecture] |
33-60 |
| Craig, D.P., Molecular Crystals and Light: Chemical Reactions in Cages [Liversidge Research Lecture] |
61-67 |
Vol 115 parts 1-2 pp.1-5
Proper Motions in the Region of the Galactic Cluster NGC 6087
David S. King
Abstract. Relative proper motions in the region of the galactic cluster NGC 6087 are determined with the aim of identifying stars which are non-members. The relative proper motions have an average standard error of 0."/century and reveal 74 likely members and 83 likely non-members. Two probabilities of cluster membership are given, the first using only the proper motions and the second inserting positional information.
Vol 115 parts 1-2 pp.5-8
Precise Observations of Minor Planets at Sydney Observatory During 1981
N.R. Lomb
Abstract. Positions of 3 Juno, 11 Parthenope, 39 Laetitia, 51 Nemausa and 148 Gallia obtained with the 23 cm camera are given.
Vol 115 parts 1-2 pp.9-12
The Kempsey Earthquake of 6 September 1979
J.M.W. Rynn and C.J. Lynam
Abstract. At 1308 hrs UTC (1108 pm EST) on 6 September 1979 the Kempsey-Macksville area on the central New South Wales coast experienced the effects of an earthquake. This event, recorded by several seismograph stations in eastern Australia, was located about 25 km north-northeast of Kempsey (30.87°S, 152.98°E). Its magnitude was ML 3.3 ( Cooney Observatory). The earthquake was felt over an area of about 1000 km² with a maximum intensity of MM V reported from Eungai Rail – South West Rocks – Stuarts Point region between Kempsey and Macksville.
Vol 115 parts 1-2 pp.13-19
The Dummy Creek Association: Rim Syncline Deposits
R.J. Korsch
Abstract. The Dummy Creek Association in the New England Orogen consists predominantly of conglomerate derived from local basement highs. It is proposed that the highs were produced by a doming effect of the land surface associated with high level intrusion of the granitic plutons. The conglomeratic sediments were deposited in rim synclines which formed in association with the granitic intrusions. The best example is the Dummy Creek Conglomerate and adjacent Mt. Duval Granite. Volcanic rocks, probably derived from material similar to the granite, covered the conglomerate. Further rise of the pluton brought its top to above the level of the original land surface. Erosion of this thin cover exposed the pluton, which has been more resistant to erosion than the surrounding land surface which is over 300 m below the present exposed top of the pluton. A diapiric mechanism is preferred for the rise of the pluton into the highest levels of the crust.
Vol 115 parts 1-2 pp.21-32
Description and Interpretation of a Useful Leucogneiss Stratigraphic Marker in the Willyama Complex, Broken Hill Block, N.S.W.
I.L. Willis
Abstract. A leucocratic quartzo-feldspathic gneiss (leucogneiss) has been recognized as a useful stratigraphic and structural marker in parts of the Willyama Complex of the Broken Hill Block. The microcline-quartz-albite/oligoclase-biotite + garnet leucogneiss occurs as extensive thin, continuous, conformable horizons within Suite 3 of the stratigraphy of Stevens et al (1980). It has the chemistry and field relations of a rhyolite, and probably formed during a brief period as a series of pulses of air-fall tuff. It can therefore be used as a time-stratigraphic marker. The leucogneiss occurs in a narrow stratigraphic interval which can be traced for many kilometres in the Thackaringa and Triple Chance areas, and which stratigraphically underlies the sequence containing the ore bodies at Broken Hill in the Broken Hill Synform. The leucogneiss is associated with garnet- and pyroxene-rich basic gneisses, and with quartz-feldspar-biotite-garnet ("Potosi"-type) gneiss, an association which has similarities with the overlying "mine sequence" at Broken Hill and its regional equivalents (Suite 4).
Vol 115 parts 1-2 pp.33-60
On the Growth and Evolution of Continental Crust: A Comparative Tectonic Approach
R.W.R. Rutland
[Clarke Memorial Lecture, 23rd July, 1981]Abstract. Aspects of present continental crust structure and composition, and of the geological record, are examined which bear on hypotheses concerning the volume, structure, composition, and stability of continental crust through geological time. The records of both platform covers and orogenic provinces are considered with particular reference to the concept of chelogenic cycles (global thermal cycles during which large areas of continental crust were stabilised as shield areas).
Major unconformities at about 2400 and 1000 Ma (106 years) following episodes of basic dyke injection are of similar character and mark the beginning of new chelogenic cycles when cratonic areas were of maximum extent. The continents were emergent before the development of these unconformities, which were followed by long periods of progressive subsidence. Thus the sea level changes do not support theories of general continental emergence consequent upon rapid continental growth and thickening, either at the end of the Archaean or later.
True cratons were present earlier in the Archaean, but reduction of cratonic areas by remobilisation was much more widespread in the Proterozoic than in the Phanerozoic. These cratonic areas were least extensive during the peaks of thermal activity within the chelogenic cycles, at about 1900-1700 and 400-200 Ma.
It is argued that the areally most extensive Precambrian provinces, both Proterozoic and Archaean, are more closely analogous to Palaeozoic Hercynotype orogenic belts than to the Mesozoic-Cenozoic belts which have generated the more popular models. The Hercynotype orogenies are characterised by areally extensive granitic plutonic episodes which correspond to the peaks of thermal activity within the chelogenic cycles. They are allocated to a tectonic environment between the contemporaneous craton and arc-trench complex and are regarded as largely, if not wholly, ensialic. The role of the Wilson cycle in the development of these provinces is still obscure but it seems doubtful whether the closure of large oceans is a critical factor. These Hercynotype provinces are regarded as being of major significance in the development of continental crust.
In these provinces the present structure and composition of crustal layers has been developed by multi-stage processes. Older provinces have been built on Archaean basement by Proterozoic deformation and plutonism of Proterozoic mobile belt sequences and by subsequent platform deposition, while younger provinces have developed on Precambrian basement through similar processes in the Phancrozoic. During the orogeny the crust may become underplated by basaltic material. These processes of vertical accretion produced a grossly layered crust which can be correlated with deep seismic structure.
Circum-Pacific-type processes of lateral accretion of ensimatic arc-trench complexes may be mainly important for the development of the lower crust, whereas Alpine-Himalayan type collision orogeny is important for crustal thickening in relatively narrow belts and the provision of source areas for platform deposition.
The analogies between the post-Archaean chelogenic cycles suggest that ocean floor spreading was active throughout the Proterozoic and that mantle convection produced widespread intracontinental (ensialic) orogeny as well as circum-Pacific-type orogeny on continental margins. It is not clear, however, whether a major phase of continental fragmentation and dispersal followed the thermal peak at 1850-1650 Ma in the same way as Mesozoic fragmentation and dispersal followed the late Palaeozoic thermal peak.
Archaean granulite-gneiss belts show some analogies with younger Hercynotype provinces, although the protoliths of the granitoids were probably closer in age to the plutons themselves, and more uniformly mafic, than in the younger provinces. Granite-greenstone belts are developed on tectonically highly evolved crust immediately prior to stabilisation as cratons, and are best regarded as more mobile equivalents of younger platform covers. They provide no support for theories requiring numerous microplates in the Archaean.
The major peaks of plutonism during the Archaean are analogous to the peaks in the younger chelogenic cycles. They represent growth of the upper crust at the expense of the lower crust.
The isotopic evidence for a major episode of continental growth at the end of the Archaean is better interpreted as evidence for growth of the upper crust by plutonism derived from a thick basaltic lower crust during a global thermal peak. In the post-Archaean cycles there is increasing likelihood that the lower crust was both chemically differentiated and much older than the upper crust. Thus isotopic signatures of plutonic rocks are likely to reflect increasing contamination by evolved older crust in younger orogenic belts.
In view of the absence of the earmarks of subduction in older Precambrian terrains, the time of commencement of ocean-floor spreading and subduction remains conjectural. It may, however, have occurred before the deposition of the earliest granite~greenstone belts and thus assisted in the tectonic thickening of the early globe-encircling protocrust to form the earliest granulite-gneiss provinces.
Vol 115 parts 1-2 pp.61-67
Molecular Crystals and Light: Chemical Reactions in Cages
D. P. Craig
(Liversidge Research Lecture, 20th May 1982)
Abstract. The kinetics, mechanisms, and stereochemistry of solid state photochemical reactions are different in fundamental ways from reactions in fluids, and constitute a distinct set of problems. Molecules in crystals below the melting point are confined by intermolecular forces to sharply defined regions, or 'cages'. Reactions take place between molecules within such cages in positions and orientations known from crystal structure analysis or in positions related to the perfect crystal structure by dislocations or local disordering.
New concepts are being developed to advance understanding in this field. The best known is that of 'topochemistry', which is that static lattice constraints restrict the products of a photochemical reaction to those preformed in the parent crystal, Also recent theory leads to the proposal that there may be impulsive molecular displacements following light absorption and lasting only a few picoseconds that bring neighbour molecules close together and promote chemical change or excimer formation. The theory of this concept of 'dynamical preformation' is described and possible examples discussed.