Peter Wellman

Cainozoic igneous activity in eastern australia

Abstract During the Cainozoic there was widespread basaltic igneous activity in eastern AustraIia along and adjacent to the Eastern Highlands. The activity commenced about 70 m.y. ago, and has continued through the Cainozoic at a nearly constant rate. More than fifty igneous provinces are recognized. Each province consists of similar volumes of volcanic material and crustal intrusives, the volcanism generally lasting less than 5 m.y. and resulting in lavas that cover a region 50–200 km across. Three main types of igneous province are recognized: 1. (1) central volcano provinces, which are composed predominantly of slightly undersaturated to saturated basaltic lavas, but with some felsic flows and intrusions; 2. (2) lava field provinces, which consist of basaltic flows, commonly strongly undersaturated; 3. (3) a single, mafic, strongly undersaturated, high potassium province. Within each type of province the average potassium content of the lavas has not changed with time. Most provinces are consistent with an apparent migration of volcanism westward at 5 mm/year across the Eastern Highlands. This migration is thought to be caused by crustal processes. The distribution and age of the slightly undersaturated provinces suggest a migration of the centre of volcanism southward at 66 ± 5 mm/year. Magmas for these provinces are thought to originate from a magma source or sources, with a limited latitudinal extent, within the asthenosphere. The migration is considered to be related to the movement of the Indian (Australian) lithospheric plate relative to the underlying asthenosphere.

Potassium‐argon ages on the Cainozoic volcanic rocks of New South Wales

Abstract During the Cainozoic there was widespread volcanism, mainly basaltic, in eastern New South Wales. Numerous new K‐Ar ages, together with previously published results, provide information on the age of virtually all the main volcanic provinces, and indicate that the volcanism started about 70 m.y. ago in the Late Cretaceous, and was continuous from about 60 m.y. ago (Palaeocene) until about 10 m.y. ago (middle Miocene). There has been no volcanic activity since 10 m.y. ago. The ages of uplift of the Eastern Highlands are estimated from the relationship of the dated basaltic flows to the topography. A major uplift is deduced some time between the mid‐Cretaceous and late Oligocene, followed by a quiescent period. A further uplift started some time after the middle Miocene, and it continues to the present day. The highland was uplifted differentially both along and transverse to the axis.

Potassium‐argon ages for some Australian Mesozoic igneous rocks

Abstract Potassium‐argon dating shows that Late Triassic to Late Jurassic mainly alkaline igneous activity was widespread in New South Wales and Victoria; it overlapped the Middle Jurassic tholeiitic basaltic activity in Tasmania and South Australia. The more accurate new ages include 205 ± 3 m.y. for microgranodiorites from the Middle Brother in the Lorne Basin, 190 to 177 m.y. for intrusions near Mittagong, 163 ± 8 m.y. for the Myalla Road Syenite near Cooma, 207 ± 4 m.y. for syenitic intrusions near Benambra, Victoria, and 170 ± 5 m.y. for basalt on Kangaroo Island in South Australia. The tholeiitic Bunbury Basalt of Western Australia is at least 90 m.y. old. Much of this magmatism is typical of that associated with the final stages of stabilization of an orogenic zone. The magmatism is thought to be related to the initiation of rifting in this part of Gondwanaland, preceding the separation of the New Zealand‐Lord Howe Rise and Antarctic continental blocks from Australia.

Potassium‐argon ages on the Cainozoic Volcanic rocks of Eastern Victoria, Australia

Abstract The Cainozoic volcanic rocks of eastern Victoria comprise most of the Older Volcanic Series. K‐Ar ages of volcanic rocks in South Gippsland range from middle Palaeocene to middle Eocene; the older ages are to the east. In the Eastern Highlands the majority of lavas are of late Eocene to middle Oligocene age, with some lavas of late Oligocene to Pliocene age. The Cainozoic volcanic activity in Victoria is thought to have been almost continuous since it started in the early Palaeocene, with peaks of activity from Palaeocene to Eocene and from Pliocene to Pleistocene. The uplift of the Eastern Highlands is thought to have taken place in three sub‐equal stages, starting in the middle Mesozoic. The last uplift is inferred to have begun some time after the Oligocene, and to have resulted in less than 300 m of uplift in the central and southern parts of the highlands, and negligible uplift in the northern. In the Oligocene the highlands had a relief of over 1000 m, and a drainage system similar to that ...

On the Cainozoic uplift of the southeastern Australian highland

Abstract The uplift history of the southeastern highlands is determined using middle to late Cainozoic river beds; these are preserved as river gravels overlain by basaltic lavas. Along major rivers in the highlands old river beds have a reasonably constant slope; this slope is similar to that of the present river bed, and similar to a line joining the summits along the valley margin. The approximately parallel relation suggests that the beds of most major rivers remained at a constant height and slope while the highland was uplifted. The relative heights of the river beds of different ages are consistent with uplift at a constant rate during the last 45 m.y. The present level of the pre‐uplift river beds is thought to be just above the present valley margins, so if there was only one uplift and if that was at a constant rate then uplift started about 90 m.y. ago. The total amount of uplift is given by the inferred amount of total river downcutting. Within the highland this is generally about 0.6 km north...

Early Miocene potassium‐argon age for the Fitzroy Lamproites of Western Australia

Abstract Age determinations have been made on three occurrences of Fitzroy Lamproite using the potassium‐argon isotopic dating method. Within each body, well‐crystallised minerals gave consistent ages. It is inferred from these results that the three Fitzroy Lamproite bodies were emplaced 17.0 ± 0.2, 19.5 ± 0.6, and 20.8 ± 2.0 m.y. ago, in the Early Miocene.

Crustal movement determined from repeat surveying — results from southeastern and southwestern Australia

Abstract First order geodetic measurements show that strain rates in southeastern New South Wales are about 50 × 10‐8 y‐1. These strain rates are much greater than those suggested by seismicity. The direction of the principal axis of compression varies with position, but it is consistent with compression axis directions of the earthquake focal mechanisms in the Bowning area, and with the pattern of late Cainozoic vertical displacements suggested by geomorphology. In southwestern Western Australia strain identified from geodetic measurements is patchy in distribution, and is irregular in magnitude and direction. It is found close to areas of historic faulting, in areas of high measured stress, and across the Darling Fault line of weakness separating the Perth Basin from the Yilgarn Craton.