Tasman Basin Research Articles

Evidence for bacterial palaeoecological origin of mineral magnetic cycles in oxic and sub-oxic Tasman Sea sediments

Sediments from the Tasman Basin in the southwest Pacific Ocean show a cyclic pattern of variation in mineral magnetic properties, including susceptibility and remanence, over the Brunhes Chron. The magnetic pattern is dominated by fluctuations in the concentration of a single domain ferrimagnetic component identified as bacterial magnetofossils by TEM investigation of the magnetic fraction. The intervals of low bacterial magnetite concentration, occurring in glacial periods, are accompanied by a change of the bacterial magnetosome morphology assemblage interpreted as recording a change in bacterial species assemblage. There is no evidence of iron or sulphate reducing conditions during low bacterial magnetite intervals. Concentrations of bacterial magnetite are low when oxygen concentrations in surface sediment pore waters dropped, either as the result of variation in bottom water oxygen concentration or sedimentation rate. The variations in the magnetic signal are therefore interpreted as resulting primarily from bacterial palaeoecological control rather than dissolution or alteration. The bacteria in this deep sea environment are apparently aerophilic and grow near the sediment surface within the oxic zone, implying that magnetic remanence is also acquired near the surface but up to tens of centimetres below where the oxic zone is very thick.

Cyclic development of sedimentary basins at convergent plate margins — 1. Structural and tectono-thermal evolution of some gondwana basins of eastern Australia

The Devono-Carboniferous Drummond Basin of eastern Australia formed by extensional tectonics, most probably in a back-arc setting, as an oceanic plate subducted westward under Gondwana's continental margin. Within this basin a syn-rift sequence and a relatively thick post-rift sequence are recognized, and the latter is separated from the overlying Galilee Basin by a mid-Carboniferous unconformity, which heralds a time of relatively mild compression, uplift and folding. The lower Galilee Basin formed as a foreland, secondary peripheral bulge, or mixed-style basin during the Late Carboniferous to Early Permian time interval. A mid-Permian unconformity separates it from the extensive Late Permian to Mid-Triassic upper Galilee Basin, here suggested to be a platform basin. Nest, the main, Mid- to Late Triassic compressional event led to reverse movement along previously normal faults, folding, uplift and erosion of up to 2 km of section from the Galilee and Bowen basins. During Jurassic and Cretaceous times, the Eromanga/Surat/Carpentaria Basin, a platform basin originally >1.7 million km 2 in extent, developed cratonward of the zone of continuing subduction. Subsequently, the culminating extensional event took place further east, where Lord Howe Rise rifted apart from the continental landmass and drifted eastward as oceanic seafloor spread in the Tasman Basin, and widespread uplift and erosion occurred over the eastern Australian coastal area. New data, re-interpretation of existing data and extensive literature support the interpretation of the Phanerozoic tectonic evolution of eastern Australia in a context of convergent plate margins. Within this context, overall eastward migration through time of the subduction zone and associated morphotectonic entities (e.g. the Warburton, Adavale, Drummond, Bowen and Tasman extensional basins) and vertical stacking of unconformity-bound extensional, foreland and platform basins (e.g. the Drummond, Galilee and Eromanga basins) occurred.

Capricorn and Northern Tasman Basins: Structure and Depositional Systems

The Bureau of Mineral Resources completed a major marine multi-channel seismic reflection survey (MCS) off southeast Queensland in December 1989. The research cruise, using RV 'Rig Seismic: was designed to investigate the structure, stratigraphy and petroleum resource potential of offshore basins along this sector of the Australian continental margin. About half of the 2900 km of seismic data acquired were recorded over the Capricorn Basin and margins of the northern Tasman Basin. The survey included seismic ties to the only deep offshore wells in the region, Aquarius-1 and Capricorn-lA, located in the northern Capricorn Basin. It was the first MCS survey shot in the region since 1974. Sonobuoy refraction, gravity, magnetic and bathymetric data were also collected. The Capricorn Basin evolved in the Late Cretaceous as a failed rift arm at the northern end of the Tasman rift system. From the seismic data, basin development in the Capricorn Basin is seen as typically comprising 0.5 ? 1.0 s twt of syn-rift continental/Irestricted marine deposits (Late Cretaceous-early Palaeogene) overlain by about 1.5 s twt of Eocene-Recent, mainly marine, post-rift sediments. Though basement structures generally follow a north-northwest trend, the structural pattern is complex, comprising a mainly discontinuous series of rift basins of various geometries. Complex structure is also indicated by gravity mapping. Shallow-dipping horizons that extend well into 'basement' may be fault-plane reflections from deep extensional structures, but could also be indicative of large wedges of old (Cretaceous+) sediments or volcaniclastics. In the deep-water southeastern part of the basin, at least 3.6 s twt (4 km) of Late Cretaceous ? Palaeogene section is associated with major half-graben development. A mid-Eocene compressional or transpressional event has reactivated basement structures and produced minor faulting and folding in the section. The event, which also produced significant uplift and erosion, is attributed to intraplate stress resulting from major global plate reorganization that occurred at about 43 Ma. Volcanism was active in the southern Capricorn Basin during Late Oligocene. Volcanic edifices several hundred metres high and 3 to 5 km across are exposed on the seafloor. In the Tasman Basin, a 3-km thick pile of Cainozoic post-breakup sediments overlies oceanic basement and highly extended continental crust at the base of the continental slope to the southeast of Fraser Island. A similar thickness of sediment is present at abyssal depths along the northern transform margin. Large-scale mass movements have taken place on the basin's steep and rugged marginal slopes.

The Cretaceous volcanic-plutonic province of the central Queensland (Australia) coast—a rift related ‘calc-alkaline’ province

ABSTRACTSilicic and minor intermediate and mafic pyroclastics, lavas, and dykes occupy a NW-trending zone through the Whitsunday, Cumberland and Northumberland Island groups, and locally areas on the adjacent mainland, over a distance of more than 300 km along the central Queensland coast. K-Ar and Rb-Sr data indicate an age range of 95–132 Ma, with the main activity approximately between 105–120 Ma; there is, however, evidence for easterly increasing ages. Comagmatic granites, some clearly intrusive into the volcanics, occur together with two localised areas of Triassic potassic granites (229 Ma), that form the immediate basement.The volcanics are dominantly rhyolitic to dacitic lithic ignimbrites, with intercalated surge and bedded tuffs, accretionary lapilli tuffs, and lag deposits. Associated rock types include isolated rhyolitic and dacitic domes, and volumetrically minor andesite and rare basalt flows. The sequence is cut by abundant dykes, especially in the northern region and adjacent mainland, ranging from dolerite through andesite, dacite and rhyolite. Dyke orientations show maxima between NW-NNE. Isotope data, similarities in petrography and mineralogy, and alteration patterns all suggest dyke intrusion to be broadly contemporaneous with volcanism. The thickness of the volcanics is unconstrained, although in the Whitsunday area, minimum thicknesses of >1 km are inferred. Eruptive centres are believed to occur throughout the region, and include at least two areas of caldera-style collapse. The sequences are thus considered as predominantly intracaldera.The phenocryst mineralogy is similar to modern “orogenic” volcanics. Phases include plagioclase, augite, hypersthene (uralitised), magnetite, ilmenite, with less common hornblende, and even rarer quartz, sanidine, and biotite. Fe-enriched compositions only develop in some high-silica rhyolites. The granites range from quartz diorite to granite s.s., and some contain spectacular concentrations of partially disaggregated dioritic inclusions.Chemically, the suite ranges continuously from basalt to high-silica rhyolite, with calc-alkali to high-K affinities, and geochemical signatures similar to modern subduction-related magmas. Only the high-silica rhyolites and granites exhibit evidence of extensive fractional crystallisation (e.g. pronounced Eu anomalies). Variation within the suite can only satisfactorily be modelled in terms of two component mixing, with superimposed crystal fractionation. Nd and Sr isotope compositions are relatively coherent, with εNd + 2·2 to +7·3, and ISr (calculated at 110 and 115 Ma) 0·7031-0·7044. These are relatively primitive, and imply mantle and/or newly accreted crustal magma sources.The two end-members proposed are within-plate tholeiitic melt, and ?low-silica rhyolitic melts generated by partial fusion of Permian (to ?Carboniferous) arc and arc basement. The arc-like geochemistry is thus considered to be source inherited. The tectonic setting for Cretaceous volcanism is correlated with updoming and basin rifting during the early stages of continental breakup, culminating in the opening of the Tasman Basin. Cretaceous volcanism is also recognised in the Maryborough Basin (S Queensland), the Lord Howe Rise, and New Caledonia, indicating the regional extent of volcanism associated with the complex breakup of the eastern Australasian continent margin.

Carboniferous granite basement dredged from a site on the southwest margin of the Challenger Plateau, Tasman Sea

Abstract Discordant zircon fractions from a granite sample dredged off a basement horst on the western margin of the Challenger Plateau yield a 335 ± 7 Ma lower intercept date interpreted as the crystallisation age of the granite. This age, and the modal composition of the granite, is similar to that of the Karamea Suite of Westland and Nelson, New Zealand, and some Tasmanian granites. The concordia upper intercept date of 1747 ± 300 Ma implies the presence of Proterozoic continental crustal material in the source region of the granite. The Challenger granite is distinct from older S‐type granites of southeastern Australia and I‐type granites of northern Victoria Land and Marie Byrd Land, Antarctica. Subsequent to emplacement, the granite was brecciated and hydrothermally altered. A K‐Ar age of 95 Ma on hydrothermal sericite indicates that this event overlapped with a major crustal extension event recorded in Westland and Nelson, and predates the oldest known sea floor in the Tasman Basin by at least 11 Ma.

Age progressive volcanism in the Tasmantid Seamounts

The Tasmantid Seamounts comprise a northerly trending linear chain of submarine volcanoes that extend over more than 1300 km in the middle of the Tasman Basin, located to the east of the Australian continental margin. The volcanoes are situated upon deep oceanic crust of Late Cretaceous/Early Cenozoic age. Several of the volcanoes were built from sea floor depths of more than 4000 m to above sea level, and were then eroded to flat-topped mountains which have subsided to depths as great as 400 m. Basalt samples dredged from Gascoyne, Taupo, Derwent Hunter, Britannia and Queensland Seamounts have been dated by the K-Ar and 40Ar/ 39Ar methods, yielding results in the range 24 to 6.4 Ma, Early to Late Miocene. A progressive younging of the volcanism southward along the seamount chain at an average rate of 67 ± 5mm/year is indicated. The predicted present position of the volcanic focus is at 40.4°S latitude, and between 155° and 156°E longitude, virtually coincident with the epicentre of a recent large earthquake. These results provide strong evidence that the Tasmantid Seamounts represent a hotspot track, effectively recording motion of the Australian plate across the sublithospheric mantle source region for the volcanism. Comparison with results from hotspot traces on the same plate and on the African plate further demonstrate that these hotspots provide a useful frame of reference for plate motions, and that relative movement between individual hotspots must be less than about 5 mm/year.

New Seguenziidae (Mollusca: Gastropoda) from the Tasman, South Pacific, and Southern Antilles Basins

Abstract Asthelys antarctica n. sp. from off the South Shetland Islands is described, together with Halystes (n. gen.) chimaera n. sp., from the Tasman and southern South Pacific Basins. Hadroconus diadematus n. sp., Seguenzia matara n. sp., Sericogyra (n. gen.) metallica n. sp., and S. periglenes n. sp., are described from the New Zealand slope of the Tasman Basin. Asthelys cf. simplex (Watson 1879) is recorded for the first time from the Tasman Basin. Quinnia nom. nov. is proposed as a replacement name for Seguenziella Marshall, 1983 (preoccupied).

Three new limpets of the family Pseudococculinidae from abyssal depths (Mollusca, Archaeogastropoda)

Taxonomic descriptions of three new species of Pseudococculinidae, which are treated anatomically in a concurrent paper by Haszprunar, are given: Amphiplica venezuelensis from the Venezuelan Basin, A. knudseni from the Tasman Basin and Yaquinabyssia careyi from the Cascadia Abyssal Plain off Oregon.

Cenozoic and recent inarticulate brachiopods of New Zealand:Discinisca, PelagodiscusandNeocrania

Abstract Four species of acrotretide brachiopods are present in the Tertiary and Recent of New Zealand. Discinisca sp. is reported from Paleogene rocks on the Chatham Islands. Two new localities for the living deepwater cosmopolitan species Pelagodiscus atlanticus (King) from the Tasman Basin are recorded. The Recent species Neocrania huttoni (Thomson) is shown to be widespread in the Three Kings-Hauraki Gulf-Bay of Plenty region. One fossil species, N. chathamensis (Allan), occurs in rocks of Eocene — Oligocene age near Oamaru, and in Paleogene and Pliocene horizons on the Chatham Islands. The systematics, distribution, and some aspects of the ecology of these species are discussed.

Deep currents of the northern Tasman Sea Basin

Off the southeast Australian cost in the Tasman Basin, north of 40°S, the deeper structure obtained from hydrological data is highly correlated with the surface flow, especially in the region east of Sydney where the East Australian Current forms southwards meanders that often break off to form large, warm-core rings. This indicates that the East Australian Current and its warm-core rings have a dominant influence on the baroclinic flow down to abyssal depths. Typical current shears below 2000 m are estimated to be of order 4 cm s−1 km−1. Further north, between 30 and 32.5°S, the correlation between deep and shallow structure is of similar magnitude and there is a definite correlation between parameters of the deep flow and distance offshore. It is concluded that the correlation is due to an east-west tilt of the surface of no motion. Still further north, no significant correlations exist between the deep structure and either distance offshore or shallow structure parameters. No evidence has been found for a swift, narrow, western boundary undercurrent in the Tasman Sea north of 40°S.

Disconformities and paleoceanography in the southeast Indian Ocean during the past 5.4 million years

Biostratigraphy and magnetostratigraphy are used to identify deep-sea disconformities in the southeast Indian Ocean. The hiatuses are used to define periods of increased bottom-water activity and to indicate deep-sea circulation patterns during the Pliocene and Pleistocene. Seven zones of hiatus are restricted to areas confined by topographic restrictions or by water-depth control on bottom-water flow. An analysis of hiatus frequency reveals that a water depth of about 4,200 m separates disconformities formed by Antarctic Bottom Water (>4,200 m) and Circumpolar Deep Water (< 4,200 m). Hiatus zones in deep basins occur in the Tasman, Emerald, Balleny, and South Indian Basins and east of the Macquarie Ridge. Shallow-water hiatus zones (< 4,200 m) occur at the junction of the Southeast Indian Rise, the Macquarie Ridge, and the Tasman Basin and along the Antarctic Margin. Changes in the extent of these regional hiatuses are examined by analysis of hiatus occurrence in seven time slices during the past 5.4 m.y. Although the timing of hiatuses produced by deep and shallow water masses are not synchronous, the causes of increased bottom-water activity may be correlated with paleoclimatic events. Southern Ocean cooling and the proposed re-establishment of the West Antarctic ice sheet and its fringing ice shelves in the late Gilbert Chron (t ≃ 4.2 to 3.6 m.y. B.P.) were accompanied by increased Antarctic Bottom Water and Circumpolar Deep Water activity as revealed by the frequency of both shallow and deep disconformities. The formation of the Northern Hemisphere Barents Ice Sheet at ∼3.5 m.y. B.P., however, led to a decrease in Antarctic Bottom Water production by inhibiting the formation of saline North Atlantic Deep Water, which combines with cold Antarctic shelf water to form Antarctic Bottom Water. Cool Southern Ocean climatic conditions that culminated in the late Matuyama Chron (t = 1.66 to 0.72 m.y. B.P.) resulted in a widespread disconformity produced by increased Circumpolar Deep Water activity and a more restricted disconformity produced by Antarctic Bottom Water. The areal extent of both deep and shallow disconformities was reduced after the late Matuyama Chron (t⩽0.72 m.y. B.P.) in response to decreased bottom-current activity.

A new abyssal arminacean nudibranch from New Zealand

Heterodorls antipodes n.sp. is described from a single specimen trawled at 1760-1800 m on the slope of the Tasman Basin west of New Zealand’s South Island. Diagnostic features exist in body shape and colour, the gut, and the reproductive system. The genus was previously known only from the North Atlantic. Heterodorididae Verriil & Emerton in Verrill is reduced to a subfamily of Arminidae Iredale & O’Donoghue.

Recent sedimentation in the southeast Indian Ocean with special reference to the effects of Antarctic Bottom Water circulation

In the southeast Indian Ocean the influence of the Anarctic Bottom Water (AABW) on sedimentation is evidenced by the intensity of the water turbidity near the bottom, and by the mineralogy (carbonate and quartz abundances) of the sediments and current markings on the sea floor. In the deep basins south of 50°S (South Indian and Tasman basins), besides high biogenic siliceous opal productivity, the greater influence of AABW is recorded in shallower carbonate compensation depths (<4000 m) than is the case in basins north of 50°S (5000–5200 m). Spreading of the AABW from the regions south of the Southeast Indian Ridge to the regions north of it is through fracture zones in the ridge at about 123–126°E and 145–155°E. These bottom-water passages are characterized by (1) low carbonate, high quartz, or high clay contents in the sediments, (2) distinct current markings suggesting the northward flow, and (3) high bottom-water turbidity. In large basins such as the South Indian and Tasman basins which have moderate-to-high near-bottom turbidity, central portions tend to be less turbid than the margins, suggesting strong marginal circulation. The Emerald Basin, situated east of the Tasman Basin and north of the Indian-Pacific Ridge, has high near-bottom turbidities. This suggests that the AABW in the Emerald Basin is derived directly from the source regions south of the ridge although some of the water may also be spreading from the Tasman Basin.

The southwest pacific area during the last 90 million years

Abstract Maps of the southwest Pacific area at 90, 60, 53, 83, 29, 21, and 10 m.y. are accompanied by brief descriptions of the data and palaeogeographic developments in each major area within the region. There are four stages in. the regional palaeogeographic development: I (80–60 m.y.): Tasman Basin and New Caledonia Trough formed; II (60–53 m.y.): Coral Sea Basin formed; III (53–21 m.y.): A great Melanesian marginal sea formed, bounded by Cainozoic island arcs; IV (21 m.y. to present): Much of northern part of Melanesian marginal sea consumed during retrograde motion of island arcs. These developmental stages cannot be adequately explained by interaction between the Pacific and Australian crustal plates. A comprehensive explanation is given using the zonal spreading model proposed by Crook (1977).

Plate tectonics synthesis: The displacements between Australia, New Zealand, and Antarctica since the Late Cretaceous

A comprehensive study of the last 75 m.y. of plate tectonics history has been undertaken for the region south of 30°S in the South Pacific, Southeast Indian Ocean and the Tasman Sea. Some aspects of plate boundary evolution have been clarified by our compilation and examination of available marine geophysical data. Reidentification of magnetic lineations in the southern Tasman basin shows that the controversial interval of subduction of Tasman basin crust along the east Australian margin that was previously proposed is no longer necessary. A comparison of Cenozoic magnetic lineations from both sides of the easternmost spreading segment of southeast Indian ridge indicates that a portion of the Indian plate younger than anomaly 10 (32 m.y.B.P.) is missing. We suggest that the missing crust was either subducted beneath or captured by the Pacific plate. Older lineations on the Indian plate out to about anomaly 21 have greater along-strike lengths than their counterparts on the Antarctic plate. The difference is due to an interval of crustal accretion at the Indian—Pacific plate boundary in the Early to Middle Tertiary. In the South Pacific, the Antarctic plate may not have extended northeast of the Eltanin fracture zone system prior to anomaly 29 (69 m.y.B.P.). Subduction of oceanic lithosphere was probably occurring beneath the Antarctic peninsula and eastern Ellsworth Land parts of the Antarctic plate at that time. Between anomaly-29 time and a major reorganization of plate boundaries in the Late Oligocene, plate interactions occurred in the central South Pacific between the Pacific, Farallon, Antarctic, Aluk and possibly a fifth plate. Spreading rate calculations for the Early Oligocene indicate that a simple three-plate system involving the Pacific, Farallon and Antarctic plates is difficult to maintain unless highly asymmetric spreading occurred at the Farallon—Antarctic boundary in the Early Oligocene. Further to the southeast in the Bellingshausen basin, spreading occurred along segments of the Antarctic—Aluk plate boundary beginning at about anomaly-29 time. Collisions of segments of this boundary with the trench along the Antarctic peninsula occurred in the Early and Middle Tertiary and resulted in the total disappearance of portions of Aluk plate. The collisions brought Antarctic—Aluk ridge segments into contact with Antarctic—Aluk trench segments with resulting stabilization of the Antarctic continental margin. From the magnetic lineations mapped in the southern oceans, we calculated new finite rotations which determine the relative positions of the Australian, New Zealand, and Antarctic continental fragments at 5–10 m.y. intervals during the last 75 m.y. For the New Zealand region, finite rotations calculated from magnetic lineations in the Tasman Sea and southwestern Pacific indicate that three plates were active during the Late Cretaceous to Paleocene. Previous workers have proposed that one of the three plate boundaries occurred southward between East and West Antarctica approximately along the trend of the Transantarctic mountains. As no direct geologic or geophysical evidence in Antarctica supports this proposal and since sea-floor spreading magnetic evidence shows that an active plate boundary passed through the New Zealand region after the Paleocene, we prefer to carry the required plate boundary northward. The Indian—Pacific poles of relative motion were always close to this boundary during the Cenozoic and thus corresponding evidence for plate interactions in New Zealand continental geology is generally variable and sometimes subtle.

Evolution of the Tasman Sea reappraised

We reexamined available marine magnetics data from the Tasman Sea and reidentified sea-floor spreading anomalies in the southern portion of the basin. From the revised magnetic lineations and fracture zones we calculated new finite rotations which descrobe the evolution of the basin in terms of a simple two-plate spreading system active between about 82 and 60 m.y. ago. Allowing for the probable continental origin of the Dampier ridge, the predicted displacement of the western margin of the Lord Howe rise relative to the eastern Australian margin in the northern Tasman basin is consistent with their observed separation. Thus, the controversial episode of subduction of Tasman basin crust at the east Australian margin proposed in earlier studies is no longer necessary.

Erosion of deep-sea sediments in the Southern Ocean between longitudes 70°E and 190°E and contrasts in manganese nodule development

Using our previously published results, we present trend surface maps of surface sediment age, and the thickness of sediment deposited during the last 0.7 m.y. for the area between the Kerguelen Plateau, Broken Ridge, Australia, New Zealand, and Antarctica. Four separate centers of widespread deep-sea sediment erosion have developed in response to high bottom-water velocities. Two of these, centered on the South Tasman Basin and the western end of the South Australian Basin, respectively, areassociated with extensive manganese nodule development. The manganese nodule fields are almost entirely developed on sediment which averages at least 0.5 m.y. in age. In contrast, the southern and eastern flanks of the Kerguelen Plateau, the Ross Sea and the Campbell Plateau have very limited manganese nodule deposits, despite abundant evidence of widespread active erosion. It is concluded that sediment erosion is a necessary but insufficient requirement for extensive manganese nodule development in the area, and that an important role is also played by submarine volcanism.

Sedimentation patterns and accumulation rates in the Tasman Basin

Sediment accumulation rates determined by nondestructive gamma spectrometry on 22 piston cores taken in the Tasman Basin vary from 0·2 to 2·7 g cm −2 (1000 yr) −1. Near the South Tasman Rise, winnowing keeps the sedimentation rate of foraminiferal ooze low on the rise crests [0·15 to 0·90 g cm −2 (1000 yr) −1] while rates on the flanks and base of the rise are faster [1·4 to 2·0 g cm −2 (1000 yr) −1]. The rateson the Tasman abyssal plainand central basin are controlled by proximity to a sediment source area and local scour. The effect of submarine canyons off the Bass Strait is evident in the high rate of clly accumulation near the terminus of the canyons. Rates in the central basin are low [0·2 to 1·0 g cm −2 (1000 yr) −1] because of bottom currents. The 230Th balance is studied by relating the accumulation rate of 230Th ( = F a) to its rate of production from 230U in the water column ( = F p). Sediment settling through the water column scavenges 230Th to produce an ideal ratio of F atoF pof1·0. Winnowing and bottom transport can alter the F a/F p ratio. Cores from rise crest and the central basin have F a/F pratios< 1·0 while areas of high clay accumulation rate have F a/F p > 1·0. The region as a whole is in balance with respect to 230Th accumulation.

Rayleigh and love waves in the Tasman Basin

Rayleigh and love waves in the Tasman Basin

Age of the rhyolites of the Lord Howe Rise and the evolution of the southwest Pacific Ocean

Drilling at site 207 (DSDP Leg 21), located on the broad summit of the Lord Howe Rise, bottomed in rhyolitic rocks. Sanidine concentrates from four samples of the rhyolite were dated by the 40Ar/ 39Ar total fusion method and conventional K-Ar method, and yielded concordant ages of 93.7 ± 1.1my, equivalent to the early part of the Upper Cretaceous. At this time the Lord Howe Rise, which has continental-type structure, is thought to have been emergent and adjacent to the eastern margin of the Australian-antarctic continent. Subsequent to 94 my ago and prior to deposition of Maastrichtian (70-65 myBP) marine sediments on top of the rhyolitic basement of the Lord Howe Rise, rifting occurred and the formation of the Tasman Basin began by sea-floor spreading with rotation of the Rise away from the margin of Australia. Subsidence of the Rise continued until Early Eocene (about 50 myBP), probably marking the end of sea-floor spreading in the Tasman Basin. These large scale movements relate to the breakup of this part of Gondwanaland in the Upper Cretaceous.