Mesozoic-Cainozoic volcanism of Australia

Tectonophysics, 48 (1978) 413-427 0 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

MESOZOIC-CAINOZOIC

VOLCANISM

413

OF AUSTRALIA

F.L. SUTHERLAND Departmenl

of Mineralogy

and Petrology,

The Australian

Museum,

Sydney

(Australia)

(Received for publication January 5, 1978)

ABSTRACT Sutherland, F.L., 1978. Mesozoic-Cainozoic volcanism of Australia. In: E. Scheibner (Editor), The Phanerozoic Structure of Australia and Variations in Tectonic Style. Tectonophysics, 48: 413-427. Mesozoic-Cainozoic volcanism was concentrated on the youngest eastern Australian craton. Basaltic activity (with some felsic fractionation) has predominated over Mesozoic interludes of calcalkaline volcanism (rhyolites, dacites, trachytes, andesites) and more isolated shoshonitic activity (now represented by appinitic, syenitic, granitic and lamprophyric complexes). Epeirogenic movements and associated sea-floor spreading and erogenic episodes at the continental margins, initiated and controlled much of the volcanism. Basin edges, faults, lineaments and their intersections were important in locating sites of volcanism; some fundamental structural lines have focussed volcanism over 300-600 km. The eastern Mesozoic basaltic volcanism shows a late Jurassic N-S trend from undersaturated to saturated compositions, with increasing intensity of melting towards a major Tasmanian-Antarctic thermo-tectonic event. A late Jurassic-late Cretaceous E-W trend may extend from possible ‘kimberlites’ through shoshonitic to calcalkaline activity with increasing proximity to erogenic movements along the New Zealand ‘Geosyncline’. Cainozoic basaltic volcanism reflects the NNE drift of Australia under Atlantic-Indian-southern Ocean sea-floor spreading, with a debatable role for subduction along the Tasman Sea margin. The ultimate mechanisms of volcanism are not clearly understood. Drift of cratonic structural weaknesses over thermal anomalies in the mantle, with generation of magmas from a geochemically zoned Lower Velocity Zone under influence of uplifts, lithospheric thickness and periodic release of thermaf energy, seems to partly explain observed patterns of E. Australian volcanism.

INTRODUCTION

Australia consists of Australia, 1976a), the Mesozoic time. Later episodes of calcalkaline cessively isolated from Evans, 1975; Sutherland,

three cratonic regions (Bureau of Mineral Resources youngest eastern craton being completed in earliest volcanism was predominantly basaltic, with waning to shoshonitic activity, as the continent became sucGondwanaland by sea-floor spreading (Veevers and 1975; McDougalI and Wellman, 1976).

414

The Mesozoic-Cainozoic volcanism is concentrated on the Pacific side of the eastern c&on, but overlaps the western edge and inliers of central cratons (Figs. 1 and 2). Xenoliths and geochemical evidence from the volcanics suggest that ‘granulitic’ basement extends below much of the eastern craton from Queensland to Tasmania (Wass and Irving, 1976; Ewart et al., 1976; author’s unpublished data). Only minor volcanics lie on the Indian Ocean side of western cratons. The cratonic volcanism may be controlled by: (1) tectonic events at the Australian-Papua New Guinea-Pacific plate margins and marginal spreading basins and at major rifts and spreading floors of the Indian and Southern Oceans; (2) deep-seated structural lines in the Australian cratons and lithospheric thickness; (3) thermal anomalies in the mantle, geochemical zoning 01 the Lower Velocity Zone and degrees of partial melting. These ideas have been advanced recently by Wellman and McDougall (1974a), Sutherland (1975, 1977a), Scheibner (1974, 1976) and Green (1976, 1977). This paper incorporates recent work and describes the volcanism in relation to cratonic structures and Mesozoic-Cainozoic tectonic events. The time scale applied is the modified scale used by McDougall and Wellman (1976), Wellman and McDougall (1974b) and Wellman (1974). E. GONDWANA

VOLCANOES

This stage relates to late transitional erogenic and epi-erogenic faulting of the E. Gondwanaland cratons, before significant dispersal by sea-floor spreading.

During the Triassic, Australian volcanism possessed both late-stabilization (calcalkaline) and post-cratonic (basaltic) affinities in the eastern craton. Early to mid-Triassic volcanism in Queensland was associated with tensional tectonics in a late erogenic cycle that formed partly fault-bounded depressions, graben-like in the Abercorn Trough and Esk Basin and along a continental edge in the Gympie Basin (Day et al,, 1974). The volcanism, predominantly silicic to intermediate with some basaltic activity, formed sequences of lavas, ignimbrites and tuffs up to 1000 m thick and lahar deposits up to 900 m thick; layered gabbroic intrusions have been interpreted as sub-volcanic magma chambers (Stevens, 1969; Day et al., 1974). Post-erogenic transitional uplift and granite intrusion (late Middle Triassic) deformed these sequences and initiated new basin structures. Widespread late Triassic silicic, intermediate and mafic volcanism accompanied faulting at the north end of the Ipswich and Nambour Basins. Rare eudialyte-pectolite phonolite intrudes folded Esk Basin volcanics (Carr et al., 19761, but its precise age is unknown. Triassic volcanism elsewhere was isolated: it postdated the Kubor erogenic

415

domain in Papua New Guinea (dacitic; Bureau of Mineral Resources Australia, 1976b), accompanied intrusion of igneous complexes between major faults near Benambra, N.E. Victoria (trachytic; Tattam, 1976) and formed basaltic vents and flows in the Sydney Basin and hinterland in New South Wales (Valiance et al., 1969; Hamilton, 1971). Possible extrusive equivalents of late-Triassic intrusions such as calcaikaline microgranites in the Lorne Basin {McDougall and Wellman, 1976) may have been lost through erosion. Volcanic wackes in late Triassic sequences in Tasmania (Hale, 1962) may be partly derived from external volcanism located in adjacent Gondwanan blocks (Elliot, 1975).

Rifts and tensional faulting developed across the Australian cratonic margins in the Jurassic, particularly along the Indian and Antarctic boundaries (Griffiths, 1971; Johnstone et al., 1973). Volcanism in the southern rift region around the Otway Basin formed basaltic sequences up to 600 m thick, which are mainly mildly undersaturated potassic suites, with subordinate alkaline and tholeiitic rocks (191-153 m.y.; Ellenor, 1975; Tattam, 1976; McDougall and Wellman, 1976). A tholeiite on Kangaroo Island (170 m.y.) lies at the intersection of Duntroon Basin faulting and the major Torrens lineament. The voluminous tholeiitic intrusions of Tasmania (165 m-y.) were associated with complex rifting and faulting, but any eruptives are now stripped (Sutherl~d, 1966). Magma pressure estimations on the intrusions suggest sufficient energy for eruption (Leaman, 1975) and secondary mineral assemblages suggest original burial under a thick pile of eruptives (Sutherland, 197713). Phonolitic and monochiquitic plugs and dykes in central northern Victoria (153-146 m.y.) probably represent remnants of former undersaturated volcanism in these areas. Mafic and ultramafic breccia pipes and small intrusions in the Eastern Highlands, Sydney Basin and Cooma district in New South Wales are dated between 193 and 163 m.y. (McDougall and Wellman, 1976) and many are located on major east-west and meridional fracture zones identified from ERTS-1 imagery (e.g. Lachlan Lineament, Fig. 1; Scheibner, 1974, 1976). Volcanic provinces and associated intrusions on the western margin of the Gunnedah-Sydney Basin structures are also closely related to lineaments and fractures. They include the Garawilla alkali basalts and alkaline rocks and the Mittagong complexes (195-177 m.y.). The latter form syenitic, tholeiitic, felsic and rarer undersaturated rocks, distinctive from the Cainozoie alkali basalts in the area. All the later Jurassic volcanism (170-146 m-y.) may stem from a long term supercontinental thermo-tectonic cycle centred on eastern AntarcticaTasmania (Sutherland, 1975). It trends with decreasing intensity of melting from Tasmanian (Ferrar) tholeiites northwards through transitional tholeiitic, felsie and alkali basaltic volcanism at the South Australian-Victorian

f

417

rifts into more South Wales.

undersaturated

CRATONIC

VOLCANISM

DRIFT

This stage relates floor spreading. India-Australia m.y.)

drift

to dispersal

volcanism

volcanism

in northern

of Gondwanan

(late

cratons

Jurassic-late

Victoria

and New

by episodes

Cretaceous,

of sea-

150-90

Sea-floor spreading between Australia and India occurred between 150 and 80 m.y. (Veevers and McElhinny, 1976). Some eastward drift component from this spreading may explain the tectonic setting, in which volcanism formed both basaltic and non-erogenic shoshonitic-calcalkaline phases (Fig. 1). The basaltic activity is known on the rifted margins to the opening sea floors (Ashmore and Scott Reefs adjacent to late Jurassic spreading; tholeiite in the Perth Basin adjacent to early Cretaceous spreading, min. 90 m.y.: Veevers and Evans, 1975; McDougall and Wellman, 1976). Silicic to intermediate calcalkaline and silicic to mafic shoshonitic activity extended down the eastern cratonic margin and dates between 137 and 91

Fig. 1. Mesozoic volcanism and related structural and tectonic features (193-90 m.y., E. Gondwanan cratons, pre-Tasman Sea opening). The Australian map (a) shows areas of volcanism in relation to the cratons, main Mesozoic rifting, Indian Ocean spreading and erogenic activity along the eastern margin. In the main map, volcanics and associated intrusives are shown as black areas. Predominantly calcalkaline activity is marked ca, predominantly shoshonitic activity, sh, and predominantly mafic-felsic basaltic activity is unspecified. The main Queensland activity is enlarged in insets (b: left side, Triassic volcanism; c,d: right side L. Cretaceous volcanism and intrusion). Areas of Queensland volcano-elastic sediments outside the insets are indicated by UT% (U. Triassic fine tuffs), MJ (M. Jurassic intermediate volcanic detritus, Eromanga Basin) and UJ (U. Jurassic ‘volcanic’ bentonites). Designated U. Triassic-L. Jurassic volcanism includes Papua New Guinea dacites (PNG), Garawilla mafic province (G), Mittagong mafic-felsic province (M), Benambra trachytes (B) and Kangaroo Island tholeiite (K). Designated U. Jurassic-U. Cretaceous intrusive-extrusive complexes include King Island (KI), Western Tasmania with an area of lamprophyric dyke swarm (W), Cygnet (C). Cape Portland (CP), Mt. Dromedary (MD) and Lord Howe Rise rhyolite (R, Deep Sea Drill Site 207, restored to pre-Tasman position). Distributions of E. Australian alluvial diamonds and suggested eastern limit for diamondiferous ‘kimberlite’ sources (DK) are largely based on data provided by D.M. Colchester (personal communication, 1977). Designated structural features (main map) include Sydney Basin margin (SB), Gunnedah Basin margin (GB), Duntroon Basin faulting (DB), Torrens Lineament (TL), Lachlan Lineament (LL), Garawilla-Lorne Basin lineament (G-LB), and main tensional dykefault trends associated with Tasmanian tholeiitic intrusion and volcanism (TTY). Designated localities (map a) include Perth Basin (P), Ashmore Reef (A) and Scott Reef (R).

418

m.y. (Sutherland, 1973; Sutherland and Corbett, 1974; McDougall and WeIlman, 1976). In Queensland, volcanism was predominantly silicic to intermediate calcalkaline and quite voluminous. Andesitic activity along the margin of the Maryborough Basin dates around 133 m.y. (Green and Webb, 1974). A strong volcanic line extended over 300 km along the coast between Bowen and Yeppoon; dacitic to andesitic eruptives (110-115 m.y.) filled an unstable fault-bounded trough (Whitsunday Basin) and rhyolites and dacites erupted around Port Clinton (Webb and McDougall, 1968; Paine, 1969; Murray, 1974a). Further south, shoshonitic activity predominated forming appinitic, syenitic, granitic and lamprophyric complexes. Only rare volcanics are found in the small complexes (Cape Portland, N.E. Tasmania), but extrusives were probably unroofed by erosion in other cases. In New South Wales, the activity rose on east-west fracture zones (incipient transform faults for later Tasman Sea spreading: Ringis, 1975; Scheibner, 1976) and examination of the Tasman spreading patterns of Hayes and Ringis (1973) suggests that fracture zones also controlled Tasmanian complexes (Fig. 1). Rhyolites on the now submerged but continental Lord Howe Rise (94 m.y.; McDougall and Van der Lingen, 1974) also fall in line with a Tasman fracture zone. This line was closed up against eastern Tasmania at that time, with the rhyolites lying slightly east of the meridian passing through the Mt. Dromedary complex on the New South Wales margin. Migration

and extent

of eastern

volcanism

Some eastward migration of volcanism across the eastern craton is suggested by older granitic bodies and complexes (125 m.y.) intruding the Connors Arch and Bowen Basin structures inland of the Bowen-Yeppon volcanic line, and by dates on the shoshonitic bodies across S.E. Australia (137 m.y., King Island, W. Tasmania; 98 m.y., Cygnet, S. Tasmania; 103-91 m.y., Cape Portland, N.E. Tasmania; 94 m.y., Mt. Dromedary, N.S.W., Lord Howe Rise). Other associated activity may have been ‘kimberlitic’ and the source of alluvial diamonds found in deep leads in eastern Australia (Fig. 1). This would fit the trend of silicic to intermediate calcalkaline to more mafic shoshonitic activity, passing away from an unstable New Zealand margin and towards stabilized Australian cratons (Sutherland, 1973). Known ‘kimberlitic’ breccia pipes in eastern New South Wales are virtually nondiamondiferous; the diamondiferous kimberlites probably lay west of a line through the Inverell-Albury areas, below which the Lower Velocity Zone and kimberlitic source region probably became deep enough to reach pressures of the diamond stability field (D.M. Colchester, personal communication, 1977). The area of projected kimberlitic activity is now partly covered by JurassicCretaceous beds of the Great Australian and Murray Basins, but kimberlitic rocks and lamprophyric carbonatites are described to the southwest near Terowie, South Australia (Colchester, 1972; Tucker and Collerson, 1972).

419

Extrapolation from the generalized model of kimberlitic pipe and dyke relationships (Hawthorne, 1975), suggests erosion of approximately 2 km depth of cover since the Terowie emplacements; stratigraphic data suggests 6.3 km of erosion since the late Cambrian-Ordovician orogeny in the area and hence a probable mid- to late Mesozoic age for this activity (D.M. Colchester, personal communication, 1977). The trend from possible kimberlitic to shoshonitic and calcalkaline activity may reflect increasing proximity to the late Jurassic-early Cretaceous Rangitata Orogeny along the New Zealand ‘Geosyncline’ (Griffiths, 1975). Drift of Australia eastwards against the Pacific margin, through episodes of early Indian Ocean spreading, may have initiated these erogenic movements, creating flanking epeirogenic tension and volcanism. E. Australasia-Australia 90-55 m. y.)

drift

volcanism

(late

Cretaceous-early

Tertiary,

Spreading of the Tasman Sea between Australia and the Lord Howe RiseNew Zealand side of Australasia (85-60 m.y.; Hayes and Ringis, 1973) interrupted the India-Australia drift pattern. This isolated the eastern craton from the more unstable Pacific margin. New marine magnetic profiles in the SW. Tasman (Roots, 1977) indicate slightly older sea-floor and earlier age of opening, in this region at least. The opening is taken at approximately 90 m.y. for this discussion, but its precise timing relative to the earlier shoshonitic-calcalkaline activity is uncertain. Volcanism adjacent to Tasman spreading (Fig. 2) was isolated and basaltic except for an elongate zone of more voluminous mafic to felsic and calcalkaline (?) activity between Mackay and Rockhampton in Queensland (70 m.y.; Murray 197413; Sutherland, 1977a). The latter was associated with fault blocks on the east margin of the Bowen Basin, but its ultimate origin is uncertain. Subduction of Tasman sea floor inherent in the interpretations of magnetic anomalies by Hayes and Ringis (1973), Ringis (1975) and Roots (1975) would reach its greatest overlap towards this area, but uplifts related to transform and marginal fractures bounding the Cato Trough spreading provide an alternative mechanism. Strong late Cretaceous folding and faulting took place in the Maryborough Basin, where it intersects the line of the Cato fracture zone, and may be due to basement fault displacements (Day et al., 1974). After the Tasman-Cat0 Trough spreading and during time of Coral Sea spreading, regularly spaced cells of basaltic volcanism (60-53 m.y.) formed down the edge of the eastern craton (Sutherland, 1977a). Two of the main cells were located on major lineaments (Fig. 2), near Walcha on a WNW-ESE trans-Australia “gravity pattern” discontinuity (O’Driscoll, 1975) and near Ipswich on the ENE-WSW Cobar-Inglewood ERTS lineament (Scheibner, 1974).

420

JOLCANIC RAIL TREND!

Qld.

: i

\

DAMPIER LORD RIDGE. 30

HOWE

421

Antarctic--Australia

drift

volcanism

(early

Tertiary-Present,

53-O

m.y.)

Thermal anomalies associated with sites of volcanism at the eastern cratonic margin and with spreading of the SE. Papua-Coral Sea Basin may have initiated some volcanic trails as Australia began north-northeasterly drift under a new regime of major spreading in the Southern and Indian Oceans (55-53 m.y.; Wellman and McDougall, 1974a; Veevers and McElhinny, 1976; Sutherland, 1977a). Eastern

craton

(older

volcanism)

The general trends of volcanic migration probably reflect the relative sum of drift vectors from Atlantic-Indian-Southern Ocean spreading, retarded by rotational inertial and tidal drag effects on the lithosphere. An apparent trail bend in the late Oligocene may reflect removal of an easterly drift component from South Atlantic spreading when Africa became stationary. ‘Central volcanoes’, which include basaltic and felsic activity, show southward migration of activity (66 mm/yr av.; Wellman and McDougall, 1974a). Some

Fig. 2. Latest Cretaceous-Cainozoic volcanism (black and stippled areas) and related structural and tectonic features, E. Australia (90-O m.y., post-Tasman spreading). Volcanism during Tasman Sea spreading (90-60 m.y.) is enclosed in hatched lines (Mackay-Rockhampton provinces, MR; Gippsland-3acchus Marsh provinces, G) with Kempsey area, K. Main areas of basaltic volcanism and sea-floor spreading structures, active prior to Southern Ocean opening (60-53 m.y.) are indicated by crossed circles (S.E. Papua, SEP; Coral Sea Basin-Louisiade Fracture Zone, CSB-LF; Ipswich field, I; Walcha-Barrington province, W; Gippsland Basin provinces, G). Arrowed lines represent ‘volcanic trails’, generated from main 60-53 m.y. thermal anomalies and associated spreading structures, following post-53 m.y. trends of the westernmost ‘central volcano’ line (after Sutherland, 1977a). ‘Central volcano’ provinces (open circles) have ages of felsic activity in m.y. (33 Cape Hillsborough, 32 Nebo, 31 N. Clermont, 29 S. Clermont, 27 Springsure, 24 Glass Houses, 23 Toowoomba-Main Range, 21 Tweed, 19 Ebor, 18 Nandewar, 16 Comboyne, 16-14 N-S Warrumbuilgles, 12 Dubbo, I I Orange, 6 Macedon; after Wellman and McDougall, 1974a and author’s unpublished Nebo data). Bass Basin and Tasmania basalt provinces (Eocene-M. Miocene) are indicated by stippled areas (B-T). Extended volcanic lines with probable basement structure control (close dashed lines) include leucite-lavas (13-6 m.y.) and S.W. Bassian-Tamar-S. Tasmanian basalts (BTS). Young volcanic provinces (8--O m.y.) are enclosed by short dashed lines and the Jugiong kimberlites are located by an asterisk (J). Tasman Sea Basin (4000 m bathymetric contour) and Cato Trough sea floors are separated by the Cato transform fracture (CF), with positions of Tasman Sea central spreading ridge, trends of magnetic anomaly 32 and transform fractures after Hayes and Ringis (1973). Major lineaments and faults shown include a Trans.Australia gravity discontinuity (TA-GD), the Cobar-Inglewood ERTS lineament (C-I), Sydney--San Cristobal bathymetric lineament (SS) and Paimerville FauIt (PF). The Western Australian inset shows areas of lamproite activity (and alluvial diamonds) in relation to the Fitzroy rift, Kimberley Basin marginal faulting and the Tram-Australian gravity discontinuity lineament (TA-GD).

422

apparent migrations of volcanism extrapolate on to transform fractures associated with sea-floor spreading (Coral Sea, Southern Ocean) or back to uplifted zones in mobile regions at the cratonic margin (Papua New Guinea). Trail trends generated from 55-53 m.y. ‘volcanic’ thermal anomalies and associated spreading structures are shown in Fig. 2, following the apparent migration trends shown by the most western ‘central volcano’ line. These arbitrary trails project on the general areas of post-53 m.y. volcanism (excluding N. Queensland and W. Victorian younger volcanism). Eruptive ages observed in a number of provinces show reasonable matches with ages predicted from Australian drift on Southern Ocean spreading rates (50-74 mm/yr over the last 50 m.y.). Some provinces do not match predicted ages on these particular trails and the full extent to which Australian volcanism can be explained by drift over magma sources is uncertain. Further dating of provinces and more detailed examination of different ‘volcanic trail’ models are needed to test their potential applicability. Scatter in the sites of volcanism along trails may indicate broad underlying magma sources. Particular sites are commonly located on structural weaknesses, including basin margins, faults and intersections of lineaments interpreted from ERTS imagery; ‘central volcano’ spacings in Queensland seem to match approximate underlying lithospheric thickness (Wilkinson, 1974; Scheibner, 1976; Sutherland, 1977a). Some linear features appear to control eruptive sites over considerable lengths (Fig. 2). K-rich ultrabasic volcanoes (leucite lavas, 13-6 m.y.) fall along a linear zone extending for 600 km down through New South Wales into Victoria and probably mark a deepseated basement fracture associated with an eastern margin of the Australian shield (Cundari, 1973; Wellman and McDougall, 1974b; Birch, 1976). A fundamental basement fault, identified along the Tamar rift in Tasmania, provided a locus for Tertiary volcanism (Sutherland, 1971; Williams et al., 1975); this can be extrapolated for 550 km along the S.W. margin of Bass Basin and down through S.E. Tasmania. Eastern-Central cratons {younger volcanism) Volcanism became more restricted in late Miocene time and was mainly associated with areas of tensional uplift, faulting and crustal creep in western Victoria and northern Queensland (Wellman and McDougall, 1974a; Gunn, 1975; Sutherland, 1977a). Critical to this is possible rotation and subduction of the Tasman sea floor under the intervening area of extinct volcanism, though there is little evidence for this from rocks on the eastern Australian margin itself. Wholesale Cainozoic subduction is suggested by Roots (1977) as an explanation for apparent E-W younging in age of basaltic volcanism. However, Sutherland (1977a) explains the trend by easterly components of Australian drift (Indian-Southern Ocean spreading) and would confine any subduction, if it occurred, to a main late Cretaceous Tasman spreading phase and a more limited late Cainozoic phase (post-7 m.y.?) following formation of

423

Tasman Sea guyot chains. The role of subduction and volcanism on the eastern Australian craton will remain a problem, until conflicts in geophysical interpretation of Tasman Sea structures (Weissel et al., 1976) are resolved. Isolated young volcanism borders the region of uplift and compressive seismicity on the E. Australian-Tasman margin (Cleary, 1973). Pleistocene basalts erupted where the northern bounding Cato fracture transform would intersect S.E. Queensland and late Tertiary to Quaternary basalts (N.E. Victoria) and kimberlitic diatremes (S. New South Wales) lie along the southwestern side. The kimberlites are found near the Jugiong shear zone and contain mantle xenoliths that suggest an abnormally high geothermal gradient and a source region unlikely to lie within the diamond stability field (Ferguson et al., 1977). Western era tons The only identified volcanism (Fig. 2) is high K-ultramafic leucite lamproites (early Miocene, Wellman, 1973). They lie in a major NNW-SSE rift (Fitzroy Trough; Geological Society Australia, 1971) and also occur besides the parallel major trans-Australia “gravity pattern” discontinuity lineament of O’Driscoll (1975). Alluvial diamonds are found with ‘kimberlitic’ prospects in the west Kimberley region (Byrne, 1977) and are also reported from nearby, presumably young, a~~stralite-bearing gravels in King George River (J.F. Lovering, personal communication, 1977). The age of this diamondiferous ‘kimberlitic’ activity is unknown, but conceivably it may be associated with Mesozoic-Cainozoic events. Causes of volcanism The origin of the volcanism remains a problem. The apparent SSW migration of activity can be attributed to drift over thermal anomalies in the mantle and/or to propagation of fractures as considered for oceanic island chains (Wellman and McDoL~gall, 1974a; Duncan and McDoug~l, 1976; Sutherland, 197’7a). In Australia’s case, any involvement with propagating fractures along volcanic migration paths finds no consistent expression in the major structural and gravity trends observed in the eastern craton by O’Driscoll (1975) and Wellman (1976). The SSW trend of volcanic migration is however reflected by the major NNE-SSW Sydney-San Cristobal bathymetric lineament (O’Driscoll, 1974). This bounds the S.E. Australian margin and Coral Basin structures (Fig. 2) and is possibly linked to the general movement of Australia under the Southern-Indian Ocean spreading. Tapping of a world-wide geochemically zoned Lower Velocity Zone can explain geochemistry of Australian volcanic provinces, without necessity of mantle plumes or ‘geochemical hot spots’ (Green, 1977). Australian voicanism is certainly associated with pre-existing major fracture zones and their structural intersections, but some fundamental fractures in the volcanic belts show no significant volcanism (e.g. Palmerville Fault, N. Queensland). Dy-

424

namic drift of cratonic fracture zones over mantle thermal anomalies, whether initiated by or associated with uplift, marginal sea-floor spreading, propagating fractures or other mantle processes, seems to at least partly explain the Australian pattern of volcanism. The role of build-up and dissipation of sub-crustal thermal energy in relation to lithospheric thickness, fracture zones, epeirogeny and spacing of volcanic episodes in time and place is not yet clearly understood. Australia is volcanically inactive at present, but its past record suggests that cratonic volcanism has not terminated. ACKNOWLEDGEMENTS

Assistance with data was provided by J. Ferguson, Bureau of Mineral Resources, Canberra; D.M. Colchester, New South Wales Institute of Technology, Sydney; J.F. Lovering, Geology Dept., University of Melbourne, and E.S.T. O’Driscoll, Western Mining Corp. Ltd., S. Australia. The paper was read by S.Y. Wass, School of Earth Sciences, Macquarie University, Sydney, and E.A.K. Middlemost, Geology Dept., University of Sydney. I. McDougall, Research School of Earth Sciences, Australian National University, corresponded on some aspects of Cainozoic volcanism.

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