Tasmantid Guyots, the Age of the Tasman Basin, and Motion Between the Australia Plate and the Mantle

Home , Tasman Sea

PETER R. VOGT U.S. Naval Oceanographic Office, Chesapeake Beach, Maryland 20732 JOHN R. CONOLLY Geology Department, University of South Carolina, Columbia, South Carolina 29208 Tasmantid Guyots, the Age of the Tasman Basin, and Motion between the Australia Plate and the Mantle

ABSTRACT sea-floor spreading some time after the Paleo- zoic. The data are too sketchy to be certain The age of the Tasman Sea basement can be about the Dampier Ridge, however, and it may roughly estimated from the 50 m.y. time con- be a line of seamounts similar to the others in stant associated with subsidence of sea floor the Tasman Basin. generated by the mid-oceanic ridge. Present All available magnetic profiles across the Tas- basement depths suggest Cretaceous age, as man Basin (Taylor and Brennan, 1969; Van does sediment thickness. It is further argued der Linden, 1969) have failed to reveal linea- that the Tasmantid Guyots, whose tops deepen tion patterns that might conclusively reflect systematically northward, were formed during spreading and geomagnetic reversals. Nor has Tertiary times by northward movement of the deep-drilling been attempted. Therefore, more Australia plate over a fixed magma source in the indirect evidence must be assembled, and this mantle. As Antarctica was also approximately is one object of our paper. Our other aim is to fixed with respect to the mantle, sea-floor 156 160 spreading between the two continents implies 24° S that the guyots increase in age at a rate of 5.6 yr/cm from south to north. Their northward deepening then yields an average subsidence rate of 18 m per m.y. for the Tasman basement on which the seamounts were extruded. This rate again yields a Cretaceous age for the basement. Great circles constructed normal to the Tas- mantid Seamount chain, and other probable volcanic lineaments reflecting movement of the Australia plate over mantle magma sources, pass near Ethiopia, not far from the Antarctica- Australia spreading pole. This result supports the hypothesis that the mantle sources are approximately fixed with respect to Antarctica. INTRODUCTION Between New Zealand and Australia lies the Tasman Sea, whose western part is the 2,500 fm-deep Tasman Basin (Fig. 1). Both crustal structure (Officer, 1955; Van der Linden, 1967; Conolly, 1969a) and water depths suggest that the Tasman Basin is ordinary oceanic crust formed by sea-floor spreading. Van der Linden (1969) tentatively identified 40 the aseismic Dampier Ridge, situated approxi- 148 156 160 mately midway between the two chains of sea- Figure 1. Volcanic activity in the Tasman Sea area: mounts in the Tasman Sea, as a fossil spreading Seamounts (Conolly, 1969a) and outcrop of east Aus- axis exhibiting a symmetric magnetic signature. tralian Cenozoic volcanic rock provinces (Wallman and others, 1969). Dashed line suggests younging trend and This ridge supposedly developed when Aus- direction presumed due to movement of Australia plate tralia and New Zealand became separated by over mantle.

Geological Society of America Bulletin, v. 82, p. 2577-2584, 5 figs., September 1971 2577

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explore the possibility that the north-south- The Tasmantid Guyots provide an independ- striking island chains and ridges of the Australia ent estimate for the age of the Tasman base- plate (of which the Tasman Sea is a part) reflect ment. Let us accept that long seamount chains the movement of that plate over stationary are caused by differential movement between magma sources in the mantle, according to the the lithosphere plate and the mantle below it hypotheses of Wilson (1965) and Morgan (Wilson, 1965; Morgan, 1971). The north- (1971). south trend of both seamount chains in the Tas- Two north-south seamount chains transect man Sea then implies a north to south or south the Tasman Sea (Fig. 1). In the east, the Lord to north movement of the Australia plate with Howe I chain is capped by coral reefs, but the respect to the underlying mantle. western chain, the Tasmantid Seamounts, in- In particular, because the Tasmantid Guyots cludes 9 guyots whose summits deepen rather deepen northward (Fig. 1), they must increase uniformly from north to south (Fig. 2). The age in age in that directien. That is, the deeper the of the seamounts is unknown; Miocene to Plio- guyot summit, the longer it has been subjected cene foraminiferal ooze dredged from some of to the subsidence of the basement on which it them suggests a minimum age of mid-Tertiary sits. We reject the notion that each guyot in- (Conolly, 1969a). dividually sinks into the plate, partly because Although neither the basin floor nor the sea- the larger ones would sink much faster than the mounts have been accurately dated, both may small ones, and such a correlation is not ob- be used to make independent order of magni- served. In either case, however, a northward tude age estimates that are consistent with sev- increase in guyot age is implied, and this in turn eral other clues. In the following discussion we means, using the Wilson-Morgan theory, that neglect the effect of eustatic sea-level changes the Australia plate is moving northward with and assume that inactive volcanic islands are respect to the underlying mantle (Fig. 4). eroded rather rapidly to the shape of a guyot. Such deduced northward movement is en- tirely consistent with the paleomagnetic and AGE AND SUBSIDENCE OF THE sea-floor spreading data accumulated by many TASMAN BASEMENT workers. McElhinney (1970) summarized this information, and concluded that Australia (and Oceanic basement is typically 2.5 km below hence the adjacent oceanic parts of the Aus- sea level when it is generated by spreading, and thereafter subsides exponentially with a half- Fm PRESENT SEA LEVEL M time of about 50 m.y. (Vogt and Ostenso, 0 0 1967; Sleep, 1970). The Tasman basement is presently 5 to 6 km below sea level (Van der 100 Linden, 1969) implying a total subsidence of 100 2.5 to 3.5 km. Any guyots of the same age as 200 the basement on which they stand must have shared this subsidence, their summits now be- 300 ing 2.5 to 3-5 km below sea level. By contrast O_ 200 400 the Tasmantid Guyots have subsided only 0.1 o to 0.5 km (Fig. 2). As Miocene is a minimum o 500 age of the guyots and seamounts, it follows that 300 the Tasman basement is substantially older than 600 this. This is consistent with the present depth (5 to 6 km) of the basement under the Tasman 700 basin, as deduced by the following reasoning: if 400 we take a depth of 7 km as the ultimate depth of oceanic basement, that is, when thermal equilibrium has been reached, then the exponential subsidence rule takes the form D = Km 0 200 600 1000 1400 7 —4.5e-t/5°, where D is basement depth in kilometers and t is age in millions of years. To Figure 2. Depths to tops of Tasmantid Guyots (Conolly, 1969a) projected on 156° E. meridian. North is sink from 2.5 to 5 or 6 km would have taken at left. Vertical length of bar indicates range of measured the Tasman basement approximately 40 to 70 depths. Straight line is a least-square fit calculated by m.y. (Fig. 3). Malcolm Galloway.

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Figure 3. Schematic interpretation of guyot subsi- ary magma source in the mantle. Post-50 m.y. B.P. sea dence due to subsidence of Tasman sea floor. Guyots floor has been dated by LePichon and Heirtzler (1968) increase in age and summit depth northward as a result and Weissel and Hayes (1971). of northward movement of Australia plate over a station-

tralia plate) moved northward away from a rela- ence of about 30 m.y. between the youngest tively fixed Antarctica during the Cenozoic. (Gascoyne Guyot) and the oldest (Recorder According to magnetic anomalies between Guyot). By extrapolating the straight line fit Australia and Antarctica, these continents sepa- (Fig. 1) to sea level, we predict that if there is rated about 50 m.y. B.P., the average spreading active volcanism along the chain, it occurs some half-rate since then having been about 2.8 300 km south of Gascoyne Guyot above the cm/yr (Weissel and Hayes, 1971). Thus, if An- supposed mantle source. Naturally, this source tarctica has not moved much with respect to the position is uncertain because present sea level mantle (McElhinney and Wellman, 1969; in not necessarily equal to its average value McElhinney, 1970), the Australian plate has over millions of years. Nevertheless, Gascoyne moved north at about 5.6 cm/yr over the Tas- Guyot cannot be extremely old, perhaps sev- man tid magma source. eral million years, and so we tentatively con- As the seamount chain is 1,400 km long, the clude from several lines of evidence that the mantle source hypothesis suggests an age differ- Tasmantid Seamount chain is mid- to upper Ter- tiary in age. MILLION YEARS B.P If we accept the concept of a Tasmantid 50 75 100 125 15O 175 20O magma source over which the Australia plate is riding at about 5.6 cm/yr, the rate of north- 57 ward deepening of the guyot tops (Fig. 1) can RANGE OF AGE OF BASEMENT be used to estimate the average age of the un- 59 UNDER TASMANTID derlying basement. To do this it is necessary to SEAMOUNTS assume there are no gross age differences in the 6.1 age of the basement, so that an average age is meaningful. The rather uniform basement UJ 6.3 depths and the systematic northward deepening

UJ of guyot tops lend some support to this assump- tion. The guyot tops deepen at the rate of 32 m cm/km along the chain, while they supposedly 6.7 increase in age at a rate of 18,000 yrs/km; that is, they are sinking at 32 cm per 18,000 yrs, or 6.9 - 18 m/m.y. A plot of the exponential decay of Km basement depth versus age (Fig. 2) shows that EQUILIBRIUM basement of the order 50 to 70 m.y. old is Figure 4. If the Tasman basement was created by sinking at the observed rate of 18 m/m.y. Be- average mid-oceanic ridge segment, it should be subsid- cause the observed rate is an average one for ing approximately in the exponential fashion shown, the mid- to upper Tertiary, the above age is carrying the Tasmantid Guyots down with it. The in- underestimated on the order of 20 m.y. There- ferred guyot subsidence rate of 18 m/m.y. suggests a Cretaceous age for the basement (subsidence rates of this fore, both the basement depth itself and the magnitude occur along the heavy black line). slope of the guyot tops (Fig. 1) suggest a mid-

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to Upper Cretaceous average age for the Tas- ment with the beginning of breakup and disper- man basin. sion of Pangea (Dietz and Holden, 1971). It seems likely that several tens of million years OTHER EVIDENCE FOR AGE OF THE elapsed between the first continental rifting and TASMAN BASIN the first emplacement of ocean floor. An order of magnitude estimate of basement age can also be made by dividing average sedi- OTHER POSSIBLE MANTLE SOURCES ment thickness (from seismic data) by average UNDER THE AUSTRALIA PLATE sedimentation rate. Tertiary and Cenozoic volcanics characterize Approximately 0.8 to 1 km of soft, mainly the eastern highlands of Australia (Fig. 1), acoustically transparent sediment blankets the which have been uplifted up to 800 m since Lord Howe I rise (Van der Linden, 1969). If early Tertiary (Brown and others, 1968, p. this sediment is calcareous ooze deposited at 309). The volcanic belt roughly parallels the average rates of 1 cm/1,000 yr, then it repre- Tasman Basin seamount chains. On the mantle- sents approximately 80 to 100 m.y. of deposi- source hypothesis a southward younging would tion, giving a minimum age of middle to Upper be expected. No active eruptions have oc- Cretaceous for the oceanic sinking of that part curred during white man's settlement of Aus- of the New Zealand Continent. tralia, but recent eruptions in southern The above estimates for the age of the Tas- Australia occurred at Mount Gambier in South man basin area can be compared with other Australia (4,830 ± 70 yrs B.P.; Gill, 1955) geological developments in the southwest Pa- and at Tower Hill in Victoria (4,315 ± 195 yrs cific. Since parts of the northeastern Pacific B.P.; Gill, 1964). Potassium-argon dates on the Ocean have been shown by deep-sea drilling newer volcanics of Victoria range from 5 to 0.5 (Fischer and others, 1969) to be at least as old m.y. B.P. (McDougall and others, 1966). Fur- as Upper Jurassic (140 m.y. B.P.), by inference thermore, southeastern Australia and Tasmania parts of the remainder of the western Pacific are characterized by unusually high heat flow could well be as old or even older. Hence, as (5° X 5" averages range from 1.9 to 2.9 u. suggested by Dietz and Holden (1971) parts of cal/cm2 sec; von Herzen and Lee, 1969), com- the western Pacific near the Marianas subduc- parable to that observed in the U.S. Basin and tion zone might have a Triassic sea floor. Vogt Range province. Although these clues suggest and others (1971) have correlated western Pa- a mantle magma source presently underneath cific anomalies with the Atlantic Keathley Se- southeastern Australia, limited data on the ab- quence, dated by deep-sea drilling at about 155 solute ages of the major volcanic centers in to 130 m.y. B.P. Hence, the oldest Pacific sea other parts of eastern Australia show considera- floor could be at least Middle Jurassic in age. ble scatter. Wellman and others (1969) Conolly (1969b) discusses the origin of the analyzed the ages and paleomagnetism of three Triassic sedimentation in the Sydney Basin of large volcanoes in northeastern New South eastern Australia. Here the onset of basaltic de- Wales. Nanderwar volcano, dated at 17.5 ± tritus into the basin of sedimentation from an 0.3 m.y. B.P., agrees with our hypothesis, easterly source corresponds with the last phase which predicts 20 m.y. B.P. Liverpool (33.7 ± of sedimentation. The basalts may reflect vol- 0.7 m.y. B.P.) and Barrington (51.6 ± 0.7 canic activity along the initial rift that devel- m.y. B.P) volcanoes are markedly older than oped along this portion of the Australia-New the predicted 15 to 20 m.y. B.P. It seems likely Zealand continent in the Middle Triassic. Even- that they and the Eocene to Paleocene older tual uplift of the basin, caused as a byproduct of volcanics of Victoria (Singleton and Joyce, initial rifting of Australia and New Zealand, is 1969) represent an earlier volcanic episode, suggested as a cause for the cessation of sedi- perhaps related to the separation of Australia mentation. The more or less continuous subse- from Antarctica. The Barrington and Liverpool quent volcanism in eastern Australia, however, ages may be too high, however, because of ar- makes the identification of volcanism associated gon loss; Wellman and others (1969) observe with initial rifting difficult. that the reversal frequency estimated from their Geological evidence from New Zealand and paleomagnetic work on Barrington volcano is eastern Australia suggests that rifting com- inconsistent with the Heirtzler and others menced about the middle of the Triassic (1968) reversal frequency for 51.6 m.y. B.P. (Conolly, 1969b) which gives an upper limit to Rather than invoke omissions in the magnetic the age of the Tasman Basin. This is in agree- anomaly time scale, we suggest the K-Ar date

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may be too old. Proceeding northward, we en- Australia plate over mantle sources approxi- counter ages of 25 to 21 tn.y. B.P. for basalts mately stationary with respect to Antarctica. in southern Queensland (Webb and others, The only result seriously contradicting the 1967) and 27 m.y. B.P. near 24" S., 148° E. "hot spot" conclusions of this paper is the Terti- (listed by Hurley and Rand, 1969). Both of the ary polar wandering curve for Australia de- latter dates are consistent with a movement rate duced by Wellman and others (1969). The of about 5 cm/yr. pole is thought to have moved southwestward In summary, volcanism has been widespread until 25 m.y. B.P., when it abruptly began to during the Cenozoic, but if we consider only move southeastward, with little change in Aus- the youngest activity in each region, there is an tralian paleolatitudes. As the authors acknowl- indication of southward younging at about 5 edge, this result is inconsistent with sea-floor cm/yr. In any case volcanism has been mainly spreading data (LePichon and Heirtzler, 1968) confined to a belt along the eastern margin of unless additional polar wandering occurred. the Australian continent during the Tertiary, Because some or all of our volcanic chains were and the most recent activity is at the southern active 25 m.y. B.P., we believe that the azimuth end of the belt. We propose that this volcanism of the paleomagnetic field measurements, for is associated with the northward migration of some reason, does not reflect movement over the Australian Continent over a mantle magma the mantle. If it did, the volcanic chains would source and away from Antarctica during the appear sharply bent, with the southern parts past 50 m.y. More age-dating and paleomag- bending southeastward. netic studies of the volcanic belt should be un- dertaken to test the hypothesis, that is, to CONCLUSION identify any southward migration of volcanic From various lines of evidence we estimate a activity, which should have proceeded at ap- Cretaceous average age for the crust under the proximately 60 km per m.y. Tasman Basin. In round numbers, we consider There are two other major topographic linea- our estimate no better than 90 m.y. B.P. + 40. tions on the Australia plate (Fig. 4) and both of It is therefore possible that if magnetic linea- these also trend nearly north-south: The tions could be found from future detailed sur- Chagos-Maldive island chain intersects India veys, they might correlate near the lower part near the great flood basalts of the Deccan of the published reversal sequence (Heirtzler Traps. As these basalts were poured out about and others, 1968), which goes back to 80 m.y. 65 m.y. B.P. (Wellman and McElhinney, B.P. In the absence of an established Creta- 1970), we agree with Morgan (1971) and oth- ceous reversal time scale, any lineations in the ers that the Deccan-Chagos-Maldive line also Tasman Sea might be difficult to recognize. A traces out the Tertiary movement of the Aus- late Jurassic (160 to 130 m.y. B.P.) reversal tralia plate with respect to a mantle magma time scale has been published by Vogt and oth- source below the plate. The Ninety East Ridge ers (1971); some of the Tasman Basin might be may be either a chain of seamounts erupted on that old. older sea floor, or else one or more transform The rapid northward movement of the Aus- faults representing a fossil plate boundary. tralia plate since Eocene times (LePichon and We have previously argued that Antarctica Heirtzler, 1968), as well as the relatively fixed has remained roughly stationary with respect to paleomagnetic position of Antarctica (McElhin- the mantle; therefore, both transform faults be- ney, 1970), make the Australia plate particu- tween the Australia and Antarctic plates, as well larly apt for studying the hypothesis of fixed as the volcanic chains on the Australia plate, magma sources in the mantle below the plates. should lie on small circles approximately con- We have shown that the trends of five volcanic centric about a common pole. To test this lineaments is rather consistent with the hypothesis, we constructed great circles normal hypothesis; however, only in three cases is the to the supposed "hot spot trails" (Fig. 5). De- direction of younging known or suggested: the spite considerable scatter, it is clear that the Tasmantid Seamounts, because the northern circles pass near the post-Eocene pole for the guyots have subsided farther; the east Aus- Australia-Antarctic spreading motion (LePi- tralian volcanic belt, because high heat flow and chon and Heirtzler, 1968). This lends support Quaternary volcanism occurs at the southern to our initial premise that the volcanic chains, end; and the Chagos-Maldive-Deccan chain, such as the Tasmantid and Chagos-Maldive- because the Deccan Traps were poured out 65 Deccan belt, were formed by movement of the m.y. B.P. (Wellman and McElhinney, 1970),

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DECCAN CHAGOS CHAIN

NINETY EAST- RIDGE

ANTARCTICA-AUSTRALIA SPREADING POLES (Le Pichont Heirtzler 1968) • PRE-EOCENE • POST EOCENE

CONTINENTAL PLATEAU BASALTS OCEANIC SEAMOUNTS, FRACTURE ZONES PLATE BOUNDARY HIGH HEAT FLOW QUATERNARY 70 VULCANISM 160°E 180 170

Figure 5. Semi-schematic chart of Australia plate, tica has remained approximately fixed with respect to showing north-south volcanic chains used to infer north- the mantle during Tertiary times. East Australian vol- ward movement of plate over mantle, and two major canics appear to be a continental version of seamount fracture zones illustrating movement of Australia plate chain; youngest volcanism (Wellman and others, 1970) with respect to Antarctica. Poles for the latter move- and highest heat flow (von Herzen and Lee, 1969) occur ment are plotted from LePichon and Heirtzler (1968) at southern end, suggesting mantle "hot spot" is pres- and are probably uncertain by at least 5°. Great circles ently below southeastern Australia and Tasmania. Com- graphically constructed normal to major lineaments all pare with Figure 3. pass through same general area, suggesting that Antarc-

clearly pre-dating the sea floor on which the Chagos Archipelago was extruded (LePichon ACKNOWLEDGMENTS and Heirtzler, 1968). For the Lord Howe chain and the Ninety East Ridge, the younging direction is unknown. The most significant test for The writers thank the Geology Department the stationary mantle source hypothesis is the of the University of South Carolina for financial younging rate, presently an unknown or barely assistance during the preparation of the manu- known quantity. In conclusion, therefore, we script, which was written during tenure as a wish to encourage a vigorous program of dat- Visiting Professor by Vogt. Additional financial ing the volcanism, not only on the seamounts, support came from National Science Founda- guyots and reefs, but also in eastern Australia, tion grant G.A. 20415 and GV-28803 and where Wellman and others (1969) and several from the U.S. Naval Oceanographic Office. Mr. other workers have made a substantial begin- Malcolm Galloway kindly calculated the regres- ning. sion for Figure 2.

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