Age progressive volcanism in the Tasmantid Seamounts

Earth and Planetary Science Letters, 89 (1988) 207-220 Elsevier Science Publishers B.V., A m s t e r d a m - Printed in The Netherlands

207

[31

Age progressive volcanism in the Tasmantid Seamounts I a n M c D o u g a l l 1 a n d R o b e r t A. D u n c a n 2 I Research School of Earth Sciences, The Australian National University, G.P.O. Box 4, Canberra, A. C. T. 2601 (,4 ustrafia) 2 College of Oceanography, Oregon State University, Corvallis, OR 97331 (U.S.A.) Received January 5, 1988; revised version received April 28, 1988 The Tasmantid Seamounts comprise a northerly trending linear chain of submarine volcanoes that extend over more than 1300 km in the middle of the T a s m a n 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 4°Ar/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 + 5 m m / y e a r 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 r a m / y e a r .

1. Introduction

The Tasmantid Seamounts or Guyots [1,2] comprise a northerly-trending chain of generally flat-topped, submarine mountains extending over 1300 km in the middle of the Tasman Basin, which lies between the eastern margin of the Australian continent and the submarine Lord Howe Rise (Fig. 1). David [3] previously had recognized that the mountains located about 150 km east of the Queensland coast between about 2 7 ° 2 5 ' S and 28°40'S and charted by S.S. "Britannia" during a cable route survey in 1901, were "giant submarine volcanoes". The Tasmantid Seamounts are built upon oceanic crust that was generated by seafloor spreading processes in the Late Cretaceous/Early Cenozoic as the Tasman Sea marginal basin developed [4-6]. The seamounts rise from abyssal depths ( > 4000 m) to near sea level, and are clearly of volcanic origin from their morphology and from seismic profiling data, confirmed by recovery of basalt from some edifices by dredging [7]. The evidence indicates that most of the volcanoes were built above sea level, subsequently erosionally bevelled to sea level, 0012-821X/88/$03.50

© 1988 Elsevier Science Pubhshers B.V.

followed by subsidence to depths ranging from 90 m for Gascoyne in the south to more than 400 m for Recorder Guyot in the north. Vogt and Conoily [8] suggested that the progressively greater submergence to the north indicated that the volcanoes became older in that direction, interpreted as reflecting northward movement of the Australian lithospheric plate over a fixed magma source in the mantle. Here we present results of a test of the hypothesis that the Tasmantid Seamount chain of volcanoes is a hotspot or plume trail effectively registering motion of the Australian lithospheric plate relative to a magma source deeper in the mantle. This work is based upon K-Ar and 4°Ar/39Ar isotopic age measurements made on basalt samples dredged from a number of the volcanoes, and the results obtained provide strong support for the model. The question as to whether hotspots are fixed relative to one another in the mantle is a topic of continuing debate [9-15] and our results provide an important test of this hypothesis. In this study we adopt the numerical time scale of Harland et al. [16] in which the Mesozoic/

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Fig. 1. Bathymetric map of the Tasman Sea, after Mammerickx et al. [56], showing the Tasmantid Seamounts extending north from Gascoyne Seamount, and the Lord Howe Seamount chain. Isobaths in metres. Filled circles in eastern Australia show locations of central volcanoes and their average K-Ar ages, after Wellman and McDougall [37]. Star in Tasman Basin indicates epicentre of magnitude 6 earthquake of 25 November 1983 [36]. Abbreviations: B = Bank; G = Guyot; I = Island; R = Reef and S = Seamount.

Cenozoic boundary has an estimated age of 65 Ma, the Eocene/Oligocene boundary is given as 38 Ma, the Oligocene/Miocene boundary is at 24.6 Ma, and the Miocene/Pliocene boundary is taken as 5.1 Ma. 2. Regional setting The Tasman Basin formed by development of new oceanic crust through sea floor spreading and

rotation of Lord Howe Rise, which has continental crustal structure [17], away from eastern Australia [4-6,18]. Carey [19] previously had recognized the dilatational origin of the Tasman Basin, the opening of which can be described geometrically as a 22 ° rotation about a pole located at approximately 16 ° S, 143 ° E [5,6]. This occurred over the interval from magnetic polarity chron 33 ( - 7 5 Ma) to polarity chron 24 ( - 5 3

209 Ma) inclusive, based u p o n recognition of the corresponding marine magnetic anomalies whose signatures are preserved in the oceanic crust [4-6,20]. Since the cessation of opening of the T a s m a n Basin, the whole region, including the L o r d H o w e Rise, has been rigidly attached to the Australian plate. The b o u n d a r y with the Pacific plate is located well to the east and north, and the b o u n d a r y with the Antarctic plate is situated far to the south and southwest. The floor of the T a s m a n Basin is an abyssal plain with depths mainly in the range 4500 to 4800 m. There is little topographic relief owing to blanketing by sediments, generally from 400 to

1000 m thick [21]. Deep Sea Drilling Project Site 283 located at 43 ° 54.6'S, 154 ° 1 7 ' E in water depth of 4730 m, reached basaltic basement at s u b - b o t t o m depth of 590 m [22]. The sediments are earliest Paleocene to Late Eocene in age and p r e d o m i n a n t l y silts, clays and siliceous oozes, overlain b y less than 15 m of Plio-Pleistocene zeolitic clays [22]. By means of seismic reflection profiting, Hayes and Ringis [4] identified the extinct axial spreading ridge, n o w almost entirely buried by sediments. The ridge strikes a little north of northwest, and is offset en echelon in a systematic m a n n e r b y fracture zones, former transform faults, t h r o u g h o u t the region (Fig. 2). The trend of the T a s m a n t i d Seamounts is subparallel to the fossil spreading ridge of the Tasm a n Basin sea floor. The volcanoes are built mainly u p o n Late Mesozoic to Early Tertiary (Paleocene) oceanic crust showing a range in age o f - 15 M a (Fig. 2). However, there is no systematic change in age of oceanic crust along the s e a m o u n t chain, as assumed by Vogt and Conolly [8]. Several of the seamounts are situated on or adjacent to fracture zones, which appear to have exerted a local influence on the position of the volcanic activity (Fig. 2). The L o r d H o w e Seamounts form a parallel chain east of the T a s m a n t i d Seamounts, on the flanks of L o r d H o w e Rise (Fig. 1). N e a r the southern end of this chain lies Lord H o w e Island, an eroded basaltic volcano built above sea level between 6.9 and 6.4 M a ago [23]. The chain extends to the north as shallow banks and reefs and to the south as several small seamounts.

3. Morphology and sampling

Fig. 2. Pattern of sea floor spreading in the Tasman Basin mainly after Weissel and Hayes [5] and Shaw [6]. Locations of the Tasmantid and Lord Howe Seamounts also are shown. Fossil sea floor spreading axis (heavy line) is drawn coincident with marine magnetic anomaly 24 ( - 53 Ma), the youngest anomaly recognized. The axis is offset by fracture zones. Sea floor generated between anomaly 24 and anomaly 29 time in the Early Cenozoic (Paleocene) is shown by a lined pattern, bordered by Late Cretaceous sea floor indicated by a dotted pattern. The oldest magnetic anomaly interpreted is number 33, adjacent to southeastern Australia and the Lord Howe Rise. Epicentre of magnitude 6 earthquake of November 1983 is shown by a star.

F r o m G a s c o y n e to D e r w e n t H u n t e r the T a s m a n t i d S e a m o u n t chain trends due north, striking just west of n o r t h from D e r w e n t H u n t e r to Queensland G u y o t , then essentially due north to Recorder G u y o t (Fig. 1). The seamount chain appears to extend further north to K e n n Reef, which is situated on or adjacent to rifted continental crust [24-26]. Individual seamounts are as m u c h as 60 k m in diameter at their base, rising from the sea floor with slopes c o m m o n l y in the range 10 ° to 20 °, locally m u c h steeper. Some seamounts are near circular in plan (Gascoyne, Stradbroke), others are

210

elongate (Taupo, Britannia) subparallel to the trend of the volcanic chain. The volcanoes are rather irregularly spaced and as much as 150 k m apart; in some cases two or more volcanoes are in sufficiently close proximity to one another to produce a markedly elongate structure such as that formed by the Britannia-Queensland group of seamounts. As previously noted, a number of the seamounts have areally extensive platforms developed at their summits with relief of only a few tens of metres. Thus Gascoyne has a roughly circular platform about 10 km in diameter at a depth o f - 95 m, Taupo's platform is at a depth o f - 135 m and e x t e n d s - 40 km north to south and is as much as 15 km wide, Derwent Hunter has an approximately 30 by 20 km oval-shaped platform at about 330 m depth, Britannia and Queensland have remarkably flat summits at a depth of - 400 m, and a semblance of a platform is recognized on Recorder Guyot at about 450 m. Some of the smaller seamounts, including Kimbla, Stradbroke and Moreton are more conical and

probably never rose above sea level. Dredging of the platforms commonly yields unconsolidated calcareous shelly sands, but indurated limestones and basalt cobbles also have been recovered, and indicate a relatively thin veneer of sediment on the summits of the seamounts ([7]; this work). There is universal agreement that the flattopped seamounts have been bevelled by erosional processes essentially at sea level. The recovery of water-worn, rounded basalt cobbles and conglomerates with basalt clasts in a number of dredge hauls by Slater and Goodwin [7] and during the present study provides strong confirmatory evidence for this view. Because of the veneer of sediments, dredging in the present study was targetted for the steeper side slopes of the seamounts at depths of about 1000-1500 m to maximize the potential for recovery of basalt sampies. Basalts used in this study were obtained mainly by dredging of the seamounts on cruises F R 3 / 8 5 and F R 7 / 8 6 of R.V. "Franklin". An additional sample was dredged from Queensland

TABLE 1 Information on basalt samples dredged from Tasmantid Seamounts, Ta s ma n Sea, and utilized in this study Seamount

Sample No.

Dredge No.

Depth range (m)

Location lat. ( ° S)

long. ( o E)

Nature of sample

Gascoyne

85-169, 85-170

F R 3 / 8 5 G-6

36 ° 39'

156 ° 14'

northern flank

600-900

well rounded cobbles about 10 c m a c r o s s

Taupo

85-174A

F R 3 / 8 5 T-4

33006 '

156o17 '

eastern flank

500-750

large block

Taupo

85-175, 85-176

F R 3 / 8 5 T-5

32059 '

156014 '

northeastern flank

Derwent Hunter

85-177

F R 3 / 8 5 DH-1

30056 '

156° 14 '

southwestern flank

600-1000

Derwent Hunter

85-179, 85-181

F R 3 / 8 5 DH-4

30047 '

155021 '

northeastern flank

-1150-1250

- 500-750

small ( < 10 cm) clasts clast 1 5 x 6 x 8 cm large conglomerate block with basalt cobbles up to 15 cm across

Britannia

86-147, 86-151

F R 7 / 8 6 34BD4

28 o 38'

155 o 27 '

western flank of southern peak

Queensland

85-162

C 2 / 8 5 7DB1

27 o 29'

155 o 18'

northern flank

1100-1400

- 1500-1900

pillow basalt fragments and hyaloclastite basalt clast about 10 cm across

211 Seamount by personnel from the Ocean Sciences Institute, University of Sydney, using H.M.A.S. "Cook". Relevant information about sample locations is given in Table 1.

4. Methods--sample selection Thin sections were prepared of all volcanic rock samples that appeared to be relatively massive, unaltered and unweathered. Samples were chosen for dating based on freshness, degree of preservation of high temperature minerals and crystallinity. All the samples are basaltic in character with most containing olivine microphenocrysts, with or without plagioclase and less commonly clinopyroxene as phenocrystic phases. Although a few samples have a relatively well crystallized groundmass of plagioclase, clinopyroxene, olivine and opaques, all contain some glass or poorly crystallized mesostasis ranging from a minimum of about 10% by volume (85-181) to a maximum o f - 80% (85-174A). As much of the potassium in these rocks is likely to reside in the glassy material, those in which the glass or mesostasis was significantly altered were rejected for dating. Nevertheless virtually all samples showed evidence of at least incipent alteration in small areas, and thus no sample was regarded as ideal for dating purposes. In K-Ar dating of volcanic rocks the presence of glass is of concern for two main reasons. First, glass produced by quenching of magma may result in trapping of an argon component derived from the source regions of the magma that can be highly radiogenic, leading to apparent ages that are greater than the true age of eruption. Chilled glassy rims of pillow basalts erupted at depths greater than about 1500 m below sea level commonly exhibit this phenomenon [27]. Second, glasses are unstable and readily devitrify and alter, allowing radiogenic 4°Ar generated by in situ decay of 4°K after cooling to be lost by diffusion and other processes, causing measured ages to be too young. Thus the freshness of the glass or mesostasis in these rocks is of critical importance. As the majority of samples utilized for dating in this study probably were erupted in quite shallow water and possibly even subaerially in some cases, the problem of trapped argon is likely to be minimal. This conclusion applies to samples 85-

169, 85-170, 85-175, 85-177, 85-179, and 85-181, each of which is a water-worn basalt cobble either from a conglomerate or dredged as a free cobble. Most of these cobbles are in the range 5-15 cm in diameter and are typical of those found within modern high energy littoral zones. Sample 85-174A from Taupo Seamount is derived from a large block of glassy basalt which may well have erupted in the submarine environment, but probably at a depth less than that o f - 750 m from which it was dredged, as the seamount has subsided at least 130 m relative to earlier sea level. Samples 86-147 and 86-151, from the flanks of the southern peak of Britannia Seamount, are from the inner parts of pillows which show chilled glassy rims as much as 15 nun thick. The petrology and geochemistry of the dredged basalts will be reported elsewhere, but a preliminary report by Eggins [28] indicates that the rocks range in composition from olivine tholeiite to alkali basalt and hawaiite.

5. Methods--age measurement Samples chosen as the most suitable for age determination were crushed after removal of any obviously weathered or altered material. The 150-250/~m fragment size was used in most cases for the age measurements. As phenocrystic phases may carry some trapped argon, they were removed by heavy liquids; K-Ar ages thus were measured on the essentially phenocryst-free sample. Determinations were made using techniques similar to those described previously [29]; potassium was measured by flame photometry and argon by isotope dilution with high-purity 3BAr as the tracer. Argon was extracted from each sample by radiofrequency heating in vacuo, and the argon isotope ratios of the purified gas were measured in a VG-Isotopes 12 cm radius of curvature mass spectrometer operated in the static mode. Some isotope ratio measurements were made on an AEI MS10 mass spectrometer. Prior to extraction of the argon from a sample it was baked to o n l y 1 0 0 ° C in the vacuum system, in order to minimize the possibility of isotopic fractionation of atmospheric argon associated with the sample. In the present study we have chosen to measure 4°Ar/39Ar total fusion ages on selected samples rather than undertaking step heating experiments

212 to p r o d u c e age spectra. Previous studies [31-33,47,48,57] have shown that age spectra on fine-grained basaltic samples, especially those of tholeiitic c o m p o s i t i o n which also exhibit groundmass alteration, can be irregular. The main cause of this disturbance appears to stem from recoil of 39Ar from relatively K-rich to K-poor phases during neutron irradiation. Geologically meaningful age plateaus and isochrons cannot be extracted from such samples. In these cases an age determined by recombining gas compositions from all steps, that is the total fusion age, is the best estimate of crystallization age. While step heating experiments on these samples would give additional age information, their tholeiitic composition, fine-grained texture and rather minor alteration led us to choose a uniform program of 4°Ar/39Ar total fusion experiments. Comparison between conventional K-Ar and total fusion ages allows assessment of the reliability of the apparent ages. For the 4°Ar/39Ar age measurements, samples were irradiated in position X34 of H I F A R reactor for 60 hours using procedures similar to those described previously [30]. Each sample to be dated was loaded into a machined aluminium container. Centrally located within each of these containers was another small aluminium can in which a hornblende sample (84-70) of accurately known K-Ar age was placed, serving as the neutron dose (flux) monitor. Four samples were loaded into their respective containers, enclosed in 0.2 m m thick Cd shielding and placed in a reactor vessel for insertion into the reactor for the irradiation. Subsequent to irradiation the flux monitors and basalt samples were each fused in an argon extraction fine, and the isotopic composition of the purified argon was measured with the AEI MS10 mass spectrometer. 6. Results and discussion

The K-Ar and 4°Ar/39Ar age data are given in Tables 2 and 3, respectively. Most of the K-Ar measurements were made in duplicate, demonstrating that the precision is about one percent at the level of one standard deviation. The uncertainty in the 4°Ar/39Ar total fusion ages is comparable with that found for the K-Ar ages but ranges to somewhat larger errors in several cases

where the proportion of radiogenic argon is low. From measurement of argon extracted from the flux monitor mineral associated with each sample, together with the small fast neutron flux gradient detected across the two irradiation vessels used, it is believed that the uncertainty associated with the irradiation parameter, J, in the 4°Ar/39Ar dating is less than 0.5%. Overall there is a fairly regular increase in the measured K-Ar ages northward along the volcanic chain. The two dated basalt samples from the same dredge haul on Gascoyne Seamount yielded indistinguishable ages of 6.4 + 0.1 Ma. Essentially concordant apparent ages, averaging 11.3_+0.2 Ma, were obtained on two basalt clasts from one successful dredging attempt on Taupo Seamount; a younger age of 10.5 _+ 0.1 Ma was found for a particularly glassy basalt (85-174A) recovered in a separate dredge haul on this volcano. For Derwent Hunter, measured K-Ar ages on three samples from two separate dredgings ranged from 12.3 to 15.4 Ma. Two samples from the cores of pillow basalts dredged from the southern peak of the Britannia G u y o t yielded discordant K-Ar ages of 17.5 and 20.7 Ma. A basalt recovered from the northern slopes of Queensland Guyot has a measured age of 20.9 _+ 0.2 Ma. Although basalt was dredged from Moreton Seamount it was much too altered and weathered to be used for age determination. Based upon the reasonable premise that each volcano of the chain was built by eruptive activity over a relatively short interval, say on the order of a million years or so, we suggest that the spread of measured ages on samples from the same volcano may be reflecting less than ideal behaviour of the samples as geochronometers. More specifically we interpret the K-Ar ages as minimum estimates in most cases, so that the oldest measured age for each volcano is regarded as the closest estimate to the crystallization age. This interpretation is followed because all samples contain appreciable amounts of glass, which rarely is perfectly fresh and isotropic, so that it is likely that some loss of radiogenic argon will have occurred, possibly even from the least altered samples. As noted previously we think that trapped argon is probably less of a problem than argon loss, because many of the samples are believed to be from lavas erupted in shallow water or subaerially.

213 TABLE 2 Potassium-argon age data on basalt samples from Tasmantid Seamounts, Tasman Sea Sample No.

K

Radiogenic '*OAr

(wt.%)

(10 u m o l / g )

Gascoyne Seamount 85-169

1.077, 1.079

85-170

1.063, 1.064

1.197 1.211 1.186 1.196

52.5 52.4 48.1 49.9

Taupo Seamount 85-174A

1.122, 1.130

85-175

1.098, 1.103

85-176

0.990, 1.011

2.050 2.010 2.180 2.176 1.948 1.931

49.2 49.3 66.4 66.7 46.0 58.2

10.5 10.3 11.4 11.4 11.2 11.1

_+0.1 _+0.1 _+0.1 _+0.2 _+0.2 _+0.2

3.678 3.667 2.148 2.136 3.163 3.217

65.4 63.0 30.1 31.6 48.6 55.2

15.4 15.3 12.4 12.3 13.0 13.2

_+0.2 +0.2 _+0.2 _+0.2 _+0.1 _+0.1

1.554 1.560 1.893 1.849

50.3 44.4 42.0 46.3

17.5 17.6 21.0 20.5

_+0.2 _+0.2 _+0.2 _+0.2

2.859 2.856

47.7 60.9

20.9 + 0.2 20.9 _+0.2

Derwent Hunter Seamount 86-177 1.369, 1.376 85-179

0.995, 0.998

85-181

1.400, 1.402

South Peak, Britannia Seamounts 86-147 0.506, 0.512 86-151

0.516, 0.517

Queensland Seamount 85-162

0.781, 0.787

100 rad. 4°Ar

Calculated age (Ma-+ 1 s.d.)

total 4°Ar

6.39_+0.07 6.46 _+0.07 6.42 _+0.07 6.47 _+0.07

Xe + X'~ = 0.581 × 10 1°/year; )tt~ = 4.962 × 10-1°/year; 4 ° K / K = 1.167 × 10 -4 m o l / m o l . Measurements made on whole rock samples, but with phenocrysts removed (if present), and ultrasonically cleaned. Fragment size usually 150-250 #m. TABLE 3 4°Ar/39Ar analytical data and calculated ages for whole rock basalt samples from T a s m a n t i d Seamounts, T a s m a n Sea Sample No.

Sample weight (g)

J x l 0 -3

Gascoyne Seamount 85-169 0.656 1.168 85-170 0.669 1.161 Taupo Seamount 85-174A 0.628 1.164 85-175 0.585 1.186 Derwent Hunter Seamount 85-177 0.603 1.148 85-179 0.642 1.146 85-181 0.618 1.154 Queensland Seamount 85-162 0.507 1.157

40mr (10 -13 mol/g)

39At (10 -15 mol/g)

37Ar (10 14 mol/g)

36Ar (10-16 mol/g)

4°At * 39At K

100 4°Ar * total40Ar

Calculated age (Ma_+l s.d.)

1019_+1 1788_+1

3927_+5 3903_+3

1243_+1 1119_+3

3047_+ 4 5630_+ 7

3.283 3.433

12.6 7.5

4381_+1 604-+1

3825_+3 3950-+3

958_+1 1149-+1

13940_+13 1238_+ 1

7.057 6.275

6.2 41.0

14.8 -+0.8 13.4 _+0.2

623_+1 929_+1 726-+1

4856_+5 2825_+3 4357-+5

1186_+1 1175_+1 1215-+1

823_+ 1 2437_+ 3 1373-+ 2

8.025 7.759 7.593

62.5 23.6 45.5

16.5 _+0.2 16.0 _+0.3 15.7 _+0.2

606_+1

2535_+2

1349_+1

1086_+ 1

11.736

48.9

24.3 _+0.3

6.90_+0.19 7.18_+0.31

), = 5 . 5 4 3 × 1 0 - 1 ° / y e a r . 39Ar, 37At corrected for decay ( X 3 7 A r = 8 . 2 3 x 1 0 - 4 / h o u r ) . Data as listed not corrected for neutron interferences. Correction factors: (36Ar/37Ar) c, = 3.06 × 10-4; (39Ar/37Ar) ca = 7.27 × 10-4; (4°Ar//39Ar)K = 0.027. A m o u n t s of each Ar isotope derived from measured sensitivity of mass spectrometer. Relative a m o u n t s for a given analysis are precise within uncertainties given at level of one standard deviation, but absolute a m o u n t s m a y have uncertainty of - 5%. Neutron flux monitor 84-70 hornblende has K-Ar age = 98.7_+ 1.0 M a (K = 0.608 wt.%; 4°Ar* = 1.070 × 10 10 m o l / g )

214 The 4°Ar/39Ar total fusion ages measured on eight of the samples show a similar trend of increasing values northward along the volcanic seamount chain (Table 3). Invariably, however, the 4°Ar/39Ar ages are greater than the corresponding conventional K-Ar ages, with an average difference of 19%, ranging from 8% to as much as 39%, the latter result having been found on the very glassy basalt sample 85-174A from Taupo Seamount. Differences of this size are much greater than can be accounted for by the errors associated with the physical measurements and must be explained in some other manner. Interestingly, the three samples from Derwent Hunter Seamount yield 4°Ar/39Ar total fusion ages that spread over only 5%, and thus are nearly concordant, compared with a spread of more than 12% in the K-Ar ages on the same samples. Based upon knowledge of the sensitivity of the mass spectrometer and the amount of sample from which the argon was extracted, good estimates are obtained of the amounts of each argon isotope present in each sample used for 4°Ar/39Ar age measurement. From this information it is inferred that several factors have contributed to the observed difference between the 4°Ar/39Ar total fusion age and the conventional K-Ar age on the same sample. For samples 85-177, 85-179, 85-181 and 85-162 the apparent potassium content of the sample derived from the amount of 39ARK present is significantly lower than that measured by flame photometry. In fact the difference between the 4°Ar/39Ar ages and K-Ar ages can be fully accounted for by this effect. Clague et al. [31] and Seidemann [32] found that during neutron irradiation of slightly altered volcanics significant loss of 39Ar commonly occurs, probably from clays and other alteration products. Dalrymple et al. [33] suggested that in such cases the 4°Ar/39Ar total fusion ages may more nearly approximate the age of eruption of the volcanics, based upon the reasonable assumption that phases losing 39Ar during irradiation are likely to have lost their radiogenic argon (4°Ar * ) over geological time. Results on the samples from Derwent Hunter Seamount might well be interpreted in this way. But we also see evidence in the results from samples 85-169 and 85-170 from Gascoyne Seamount and samples 85175 and 85-174A from Taupo Seamount that the 40Ar* content derived from the 4°Ar/39Ar mea-

surements significantly exceeds the 4°Ar* determined by isotope dilution. In addition each of these samples exhibits greater atmospheric argon content in the 4°Ar/39Ar analysis compared with the conventional K-Ar analysis, and may reflect oxidation effects during the irradiation caused by heating via neutron absorption in the Cd shielding. The apparent higher 4°Ar* content may be a laboratory artifact brought about by isotopic fractionation of the atmospheric argon component in the samples during evacuation in the vacuum system, prior to extraction of the argon by fusion. This phenomenon was recognized by Baksi [34] and McDougall et al. [351 in other altered basaltic rocks, and causes calculated ages to be spuriously old. It is concluded that because of the presence of glass showing incipient alteration in all of the samples from the seamounts, it is likely that none has behaved ideally as a K-Ar geochronometer, highlighted by the difference between the conventional K-Ar and total fusion 4°Ar/39Arages. Both methods of measurement yield useful information, with the conventional K-Ar data generally giving minimum ages, whereas the 4°Ar/39Artotal fusion ages may more nearly reflect the eruptive age of the volcanics. Nevertheless if isotopic fractionation of the atmospheric argon component has occurred, some of the 4°Ar/39Ar ages may be too old. Because it is difficult to be sure which ages are likely to be the most reliable, in subsequent analyses both sets of age data are considered. The results are plotted versus latitude of the sites from which the samples were obtained, as the volcanic chain is essentially meridional (Fig. 3). An increase in apparent age northward along the chain is clearly evident. As there is a reasonable linear relation between age and geographic position, least-squares linear regression analysis was undertaken. The best fit line through the conventional K-Ar results on 11 samples from 5 volcanoes has the following equation: Age (Ma) = 63.2( + 4.8) - 1.573( _ 0.150) X where X is latitude in degrees, and uncertainties are at the level of one standard deviation. The correlation coefficient for the regression is - 0.961. The inverse of the slope yields an estimate for the migration rate of the volcanism of 71 _+ 7

215

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m m / y e a r , and the predicted position for the volcanic focus at the present time is 40.2 ( + 1.3) ° S latitude. Assuming simple northward motion the longitude of the zero age position is between 155 ° and 156 o E. A similar analysis utilizing the 4°Ar/39Ar total fusion ages measured on 8 samples from 4 volcanoes yields a best fit straight line: Age (Ma) = 71.2( + 4.7) - 1.754( + 0.144) X The correlation coefficient i s - 0.980, the inverse of the slope of the line gives a volcanic propagation rate of 63 __ 5 iron/year, and the predicted location of the volcanic focus at the present time is 40.6( + 1.2) o S latitude. As the two estimates for the rate of migration of the volcanism are not significantly different from one another, and the predicted locations of the zero age point also agree well, we can average the results to obtain a mean volcanic migration

rate of 67 _+ 5 m m / y e a r , and a zero age location of 40.4 ° S latitude. These results provide clear and unambiguous evidence for systematic migration of the volcanism southward along the Tasmantid Seamount chain over at least the last 20 Ma. F r o m these data it is possible to predict the age of the undated volcanoes to the north, assuming constant plate velocity. Thus, Recorder Seamount to the north has an estimated age of 25 _+ 2 Ma, Late Oligocene, and if Kenn Reef at 21°S is built upon a volcano its age is estimated as 33 _+ 2 Ma. For the predicted current position of the volcanic focus at 40.4°S there is no direct evidence for volcanic activity, and no significant features are evident on the bathymetric charts that might be ascribed to a submarine volcano. However, of particular interest is that considerable seismic activity has been recorded in the general vicinity of the predicted current position of the volcanic focus. A magnitude 6 earthquake occurred on 25 N o v e m b e r 1983 at an estimated depth of 25 km at a location given as 40.45 ° S, 155.51 ° E [36]. F r o m focal mechanism studies this earthquake was thought to be associated with a northeasterly striking dip-slip fault, the plane of which probably dips to the northwest [36]. The reported earthquake epicentres in this region scatter over a few degrees of latitude and longitude, but the surrounding region is essentially devoid of recorded seismicity. Thus it is concluded that the coincidence of seismic activity and the predicted position of the volcanic focus is reflecting some form of tectonic or magmatic activity at the present time at this site. In view of the age-progressive nature of the volcanism and the seismic data we believe that the evidence for a hotspot or plume origin for this volcanic chain is extremely persuasive. Therefore we interpret the volcanic trace as recording passage of the Australian plate over the stationary, sublithospheric source region for the magmas that built the volcanoes. The inferred motion can be compared with other evidence from the Australian region. In addition comparisons can be made with predicted motions for the Australian plate from hotspot tracks on this and other plates over time to examine the question of reference frames for plate motions and whether or not hotspots are indeed fixed relative to one another in the Earth's mantle.

216

Wellman and McDougall [37] suggested that the central volcano provinces in continental eastern Australia are hotspot-related as there is a progressive younging southward (Fig. 1), with a volcanic propagation rate of 66 ( + 5) m m / y e a r from at least 33 Ma ago (Oligocene). A more recent assessment of the data yielded a rate of migration of the volcanism of 65 (_+ 3) m m / y e a r , with a predicted location of the focus at the present time at 40.8°S latitude [38]. These rates are indistinguishable from those determined from the Tasmantid Seamount chain. Note that there is also a remarkable similarity in the latitudinal position of the volcanism at any given time between the two hotspot traces (Fig. 1). However, this relation does not appear to extend into the Lord Howe Seamount chain, as the shield-building volcanism on Lord Howe Island occurred between about 6.4 and 6.9 Ma ago [23], about 8 Ma younger than the volcanism in the same latitude in the Tasmantid and eastern Australian volcanic chains. The direction and rate of migration of volcanism in the Tasmantid Seamounts and the central volcano provinces of eastern Australia are consistent with the motion of Australia away from Antarctica over the last 35 Ma provided Antarctica is essentially fixed [5]. In the hotspot reference frame this appears to be the case [15]. The question of whether or not hotspots are stationary features in the upper mantle has several implications. First, if hotspots do not move relative to one another the geometry and distribution of ages along volcanic trails that issue from them provide a convenient and unambiguous means of reconstructing past plate positions. Plate motions across convergent margins can be calculated using the fixed hotspot reference frame (e.g. [39,40]) with far less uncertainty than by summing a circuit of relative plate motions. Second, a system of hotspots, fixed over geologically significant time, must reflect a stable, deep-seated pattern of mantle convection [9]. That hotspots do not move at the same velocities as plates implies that their causative mechanism lies below the lithosphere and possibly even below the asthenosphere. Molnar and Stock [14] have recently revived this debate with a critique of fixed hotspots. By combining relative plate motions they conclude that the Hawaiian hotspot has moved with respect to hotspots in the Atlantic and Indian Ocean

basins at 10 to 20 m m / y e a r for the last 65 Ma. This appears to contradict similar analyses by Morgan [10,11] and Duncan [15] who concluded that no motion could be discerned between hotspots in the Atlantic compared with those in the Indian Ocean over the last 100 Ma. Molnar and Stock [14], however, find inter-hotspot motion only from plate circuits through Antarctica. They specifically "neglect deformation between East and West Antarctica" (p. 587). This is in opposition to numerous studies which have proposed a plate boundary through the Ross Sea-Weddell Sea region during Cenozoic time [15,41-44]. A far less controversial test of hotspot fixity (because all intervening plate boundaries are unambiguous and well characterized) is to compare Atlantic and Indian Ocean hotspot positions through time. This was done by Morgan [10,11] and Duncan [15] who found that inter-hotspot motion over these regions, comprising two-thirds of the Earth's surface, was less than 5 m m / y e a r since 100 Ma ago. Recent improvements in relative plate motions and greater age resolution along prominent hotspot tracks allow a more definitive examination to be made (Duncan and Morgan, in preparation). The new age data from the Tasmantid Seamounts can be used in an example of such a test. African plate motion over Atlantic hotspots is now determined in detail from 4°Ar/39Ar total fusion ages on igneous rocks from eleven sites along the Walvis Ridge, a volcanic lineament joining hotspot activity at Tristan da Cunha to flood basalts erupted at the opening of the South Atlantic Ocean (Fig. 4). From historic and youthful activity at Tristan [45] the age of volcanism becomes progressively older to the northeast [46,47] to 125 Ma on the African margin. North and South American plate motions (over hotspots), determined by adding the corresponding relative plate motions to the African motion, match well with the age and geometry of proposed hotspot features such as the New England Seamounts [48] and the Rio Grande Rise [47]. Addition of the relative motion between the Indian and African plates (e.g. [49-51]) to the African motion gives predicted Indian plate motion over hotspots. From 42 Ma to the present Australia has been part of the Indian plate so the Tasmantid Seamounts should follow the predicted path left by the inferred hotspot, if and only if this hotspot has

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remained stationary with respect to the Atlantic hotspots over this period. Fig. 5 illustrates the results of this simple test, showing the predicted paths of the Tasmantid, Lord Howe and Eastern Australian hotspots using African plate hotspot tracks and three published models of Indian-African plate relative motion. We have chosen the present position of the Tasmantid hotspot as in longitude 155 o E, within the estimated range of its location. As can be seen, both the geometry and predicted ages match well with the distribution of volcanism along these lineaments. There is at most a departure of 100 km from the tracks modelled on fixed hotspots, which is within the presumed size of the melting anomaly. Hence we can discern no motion between Atlantic hotspots and those in the Tasman Sea, half-way around the globe, since Late Eocene time (42 Ma). Likewise, Duncan and Clague [52] have most recently demonstrated that hotspots throughout the Pacific basin have been stationary since 65 Ma. Molnar and Stock's [14] result, then, can have one of two explanations: either Pacific hotspots have moved en masse with respect to

fixed hotspots in the Atlantic and Indian Oceans, or there is indeed some poorly known plate boundary within Antarctica which has accommodated relative motion during Tertiary time, and all hotspots are fixed (motion less than 5 mm/year). We believe the second to be the more likely. Finally we note that the systematic increase in depth of the summits of the bevelled volcanoes of the Tasmantid Seamounts northward along the chain cannot be explained in terms of thermal subsidence by cooling of oceanic crust of progressively increasing age to the north, as Vogt and Conolly [8] supposed. The seamounts are built upon crust generated between about 55 and 70 Ma ago, but there is no correlation between crustal age and summit depth. The change in depth of the summits of the seamounts, however, does correlate with the volcano ages and suggests that cooling and subsidence have occurred subsequent to reheating caused during passage of the lithospher e across the hotspot [53-55]. The age-depth curve for the Tasmantid Seamounts indicates subsidence of about 500 m in 25 Ma (the predicted age of Recorder Seamount) which agrees closely with

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Fig. 5. Hotspot tracks for eastern Australia and the Tasman Sea are modelled by assuming that hotspots in this region are fixed with respect to those in the Atlantic. Specifically, African plate motion is determined from dated hotspot tracks in the South Atlantic Ocean (see Fig. 4) and relative motion between the African and Indian-Australian plates is added to it to obtain a predicted motion for the Indian-Australian plate over hotspots. The three hotspot tracks indicated are based upon the model of plate motion of Norton and Sclater [49] shown as a dotted line, Fisher and Sclater [50] shown as a full line, and Molnar et al. [51] given as a dashed line. The agreement in predicted geometry and age progression along these modelled tracks is remarkably close to the observed volcanic lineaments, which confirms that Atlantic and Tasman Sea hotspots have been stationary, at least since Late Eocene time, 42 Ma ago. Crough's [53] proposition that hotspots only partially reset oceanic lithosphere subsidence, to that of approximately 25 M a old lithosphere. For comparison, new oceanic lithosphere would subside 1.6 km in the first 25 M a [58]. In the case of the Tasmantid Seamounts there does not appear to be a large elongate swell (bathymetric high) associated with the chain analogous to that observed a r o u n d the Hawaiian chain. Possibly this is because the Tasmantid Seamount volcanoes are considerably smaller than those comprising the Hawaiian chain, with a correspondingly smaller a m o u n t of energy and mass transfer from the hotspot source into the lithosphere. 7. Conclusions

(1) The Tasmantid Seamounts form a particularly clear example of an essentially linear chain of

volcanoes, extending meridionally over a distance of at least 1300 k m and possibly as much as - 2000 km. (2) Samples from 5 volcanoes of the Tasmantid Seamounts yield ages based u p o n the 4°K to 4°Ar decay scheme in the range 6 to 24 Ma. A progressive and systematic y o u n g i n g southward along the chain indicates that the focus of the volcanism migrated southward at an average rate of 67 _+ 5 mm/year. (3) It is concluded that the volcanic chain is an u n a m b i g u o u s example of a hotspot track, one of the clearest cases k n o w n in the submarine environment. (4) The predicted present location of the hotspot is at 4 0 . 4 ° S latitude and about 1 5 5 ° E longitude, coincident with the epicentre of a recent large earthquake. (5) The hotspot track has effectively recorded the m o t i o n of the Australian plate over the deeper mantle source regions for the volcanism for at least the last 25 Ma. (6) In c o m p a r i s o n with data from other hotspot traces, and in particular linear volcanism in the Atlantic and Indian Oceans, we conclude that the hotspots have been fixed relative to one another to better than about 5 m m / y e a r during the last 25 Ma, and thus provide a convenient frame of reference for plate motions. (7) The systematic increasing depth to the summits of the bevelled volcanoes of the Tasmantid Seamounts northward along the chain can best be explained in terms of thermal contraction of the oceanic crust subsequent to the reheating caused b y the volcanism associated with the construction of each volcano above the hotspot. Acknowledgements

We thank the Australian National Facility Steering Committee, O c e a n o g r a p h i c Research Vessel, for providing access to the R.V. " F r a n k l i n " for dredging of the T a s m a n t i d Seamounts on segments of cruises F R 3 / 8 5 (Chief Scientist: J.A. Church) and F R 7 / 8 6 (Chief Scientist: J.B. Keene). The R.V. " F r a n k l i n " is owned and operated by the C o m m o n w e a l t h Scientific and Industrial Research Organization and we express our gratitude to its staff for their fullest cooperation and assistance. The success of the dredging

219 o w e s m u c h to t h e skill a n d p r o f e s s i o n a l i s m o f C a p t a i n N e i l C h e s h i r e , M a s t e r o f the R.V. " F r a n k l i n " , a n d his crew. W e t h a n k G . H . P a c k h a m , J.B. K e e n e , C.J. Jenkins and T.C. Hubble of the Ocean Sciences Institute, University of Sydney, for providing a sample from Queensland Guyot. R o b y n M a i e r a n d T e r r y D a v i e s assisted w i t h t h e age d e t e r m i n a t i o n s , a n d a g r a n t f r o m t h e A u s t r a l i a n I n s t i t u t e of N u c l e a r S c i e n c e a n d E n gineering enabled the irradiations for the 4 ° A r / 3 9 A r d a t i n g to b e d o n e . We thank G.B. Dalrymple, D.H. Green and F.L. Sutherland for helpful comments. T h i s p a p e r r e p o r t s s o m e o f t h e results o f t h e joint Tasmantid Seamount project between the Australian National University and the University of Tasmania through D.H. Green. Trevor Falloon and Steven Eggins of the Department of Geology, U n i v e r s i t y Of T a s m a n i a , a s s i s t e d w i t h t h e d r e d g i n g o n cruises F R 3 / 8 5 a n d F R 7 / 8 6 , r e s p e c t i v e l y . O n e of us ( R . A . D . ) a c k n o w l e d g e s r e c e i p t o f Visiting Fellowships from the Australian National University and from the University of Tasmania d u r i n g 1985 w h e n m u c h o f this r e s e a r c h w a s d o n e .

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220 30 I. McDougall, K-Ar and 4°Ar/39Ar dating of the hominidbearing Pliocene-Pleistocene sequence at Koobi Fora, Lake Turkana, northern Kenya, Geol. Soc. Am. Bull. 96, 159-175, 1985. 31 D.A. Clague, G.B. Dalrymple and R. Moberly, Petrography and K-Ar ages of dredged volcanic rocks from the western Hawaiian Ridge and the Southern Emperor Seamount Chain, Geol. Soc. Am. Bull. 86, 991-998, 1975. 32 D. Seidemann, 4°Ar/39Ar studies of deep-sea igneous rocks, Geochim. Cosmochim. Acta 42, 1721-1734, 1978. 33 G.B. Dalrymple, M.A. Lanphere and D.A. Clague, Conventional and 4°Ar/S9Ar K-Ar ages of volcanic rocks from the Ojin (site 430), Nintoku (site 432), and Suiko (site 433) seamounts and the chronology of volcanic propagation along the Hawaiian-Emperor chain, in: Initial Reports of the Deep Sea Drilling Project, 55, pp. 659-676, U.S. Government Printing Office, Washington, D.C., 1980. 34 A.K. Baksi, Isotopic fractionation of a loosely held atmospheric argon component in the Picture Gorge Basalts, Earth Planet. Sci. Lett. 21,431-438, 1974. 35 I. McDougall, N.D. Watkins and L. Kristjansson, Geochronology and paleomagnetism of a Miocene-Pliocene lava sequence at Bessastadaa, eastern Iceland, Am. J. Sci. 276, 1078-1095, 1976. 36 D. Denham, The Tasman Sea earthquake of 25 November 1983 and stress in the Australian plate, Tectonophysics 111, 329-338, 1985. 37 P. Wellman and I. McDougall, Cainozoic igneous activity in eastern Australia, Tectonophysics 23, 49-65, 1974. 38 R.A. Duncan and I. McDougall, Volcanic time-space relationships, in: Intraplate Volcanism in Eastern Australia and New Zealand, R.W. Johnson and S.R. Taylor, eds., Australian Academy of Science, 1988. 39 D.C. Engebretson, A. Cox and G.A. Thompson, Correlations of plate motions with continental tectonics: Laramide to basin-range, Tectonics 3, 115-120, 1984. 40 R.A. Duncan and R.B. Hargraves, Plate tectonic evolution of the Caribbean region in the mantle reference frame, Geol. Soc. Am. Mem. 162, 81-93, 1984. 41 P. Molnar, T. Atwater, J. Mammerickx and S.M. Smith, Magnetic anomalies, bathymetry and the tectonic evolution of the South Pacific since the Late Cretaceous, Geophys. J.R. Astron. Soc. 40, 383-420, 1975. 42 D. Jurdy, Relative plate motions and the formation of marginal basins, J. Geophys. Res. 84, 6796-6802, 1979. 43 R.G. Gordon and A. Cox, Paleomagnetic test of the early Tertiary plate circuit between the Pacific Basin plates and the Indian plate, J. Geophys. Res. 85, 6534-6546, 1980. 44 J. Stock and P. Molnar, Uncertainties in the relative positions of the Australia, Antarctica, Lord Howe, and Pacific

45

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48

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50

51

52

53 54

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