October 2017 LIP of the Month

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New dredging data shed light on the complex origins of the Naturaliste Plateau, part of the Kerguelen LIP, southern Indian Ocean

N G Direen*

University Associate, School of Physical Sciences, University of Tasmania, Australia

(*Now at ExxonMobil Upstream Research Company, Spring, Texas, USA)

This brief contribution is abstracted from recent research on the Naturaliste Plateau:

“Naturaliste Plateau: constraints on the timing and evolution of the Kerguelen Large Igneous Province and its role in Gondwana breakup” (Direen et al., 2017) in the Australian Journal of Earth Sciences, Vol. 64 (7).

As well, I will review historical research, and highlight some other recent contributions on the Naturaliste Plateau and its surrounds.

Background and Initial Investigations of the Naturaliste Plateau

The Naturaliste Plateau is the southernmost of a number of submarine plateaus in the Indian Ocean off Western Australia. It has an area of about 90 000km2 and lies in water depths of 2000 to 5000 meters. It is roughly rectilinear, extending about 400 km E-W and 250km N-S. It is bordered by the Perth Abyssal Plain to the north and west, the Diamantina Zone and the Australian-Antarctic Basin to the south, and the Mentelle Basin to the east (Fig. 1).


Figure 1. Location Map, Southeast Indian Ocean (Modified from Fig 1b, Direen et al., 2017)

First reference to the Naturaliste Plateau in the scientific literature appears to date to 1965, when Heezen & Tharp (1965) published their first descriptions of the bathymetric physiography of the eastern Indian Ocean. A single Turonian sedimentary core sample from the northern plateau was reported (Burkle et al., 1967) from a 1966 coring, during transit by the R/V Robert Conrad.

The northeastern margin of the plateau was drilled in October 1972, at Deep Sea Drilling Project (DSDP) site 258 on Leg 26 by the D/V Glomar Challenger (Luyendyk & Davies, 1974) (Fig.2), encountering sediments as old as Santonian in age (Shipboard Scientific Party, 1974). The southern margin of the plateau was then subsequently drilled in December 1972 at DSDP site 264 (Fig.2), by the D/V Glomar Challenger during DSDP Leg 28 (Hayes et al., 1975). The rocks at the base of this borehole consisted of 38 m thickness of Cenomanian or pre-Cenomanian volcaniclastic conglomerates, with very poor recovery of a variety of clast compositions (Shipboard Scientific Party, 1975). Despite the pervasive marine alteration, the Shipboard Scientific Party (1975) concluded:

“Chemical and petrologic study of the volcanic rocks has shown that the composition of the crystalline rocks is largely andesitic and that the vitrophyres range from andesite to rhyolite. The study concludes that the source terrane was probably an island arc or continental margin orogenic belt” (p. 22). Further details of the petrological and geochemical study of these rocks were reported by Ford (1975) in the same volume.


Figure 2. Dredging, coring and drilling locations, Naturaliste Plateau

In 1973, the area was subject to more areally extensive coring investigations on the final research voyage 55 of the USNS Eltanin (Heezen & Tharp, 1973; Cassidy et al., 1977), but this sampling encountered mostly clastic and carbonate sediments. Heezen & Tharp (1973) concluded that some conglomeratic clasts at Eltanin coring Site 55-12 (Fig. 2) probably represented continental crustal-derived material, most likely of granite composition.

Subsequent geophysical investigations, integrating reflection seismic data, magnetic and gravity measurements (Petkovic, 1975; Jongsma & Petkovic, 1977, Markl, 1978), revealed more of the morphology of the Naturaliste Plateau, and the related area to the east, then called the Naturaliste Trough, but now known as the Mentelle Basin (Borissova, 2002; Maloney et al., 2011) (Fig.1). New discoveries included variable sediment thicknesses within a buried rift system, and evidence for multiple internal unconformities within the sediment cover (e.g. Fig. 3). Petkovic & Jongsma (1977) concluded from a synthesis of the gravity observations, and the extant dredging and coring, that the Naturaliste Plateau was a submerged, rifted fragment of Australian continental crust.


Figure 3. Representative interpreted seismic section, Naturaliste Plateau (Modified from Fig 5b, Maloney et al., 2011)

A re-examination of the materials recovered by the USNS Eltanin Cruise 55 was made by Coleman et al. (1982). These authors classified nearly all the igneous cobbles in those cores as altered tholeiitic basalts (not granites cf. Heezen & Tharp, 1973, or andesites cf. Ford, 1975), and interpreted them as having transitional within-plate to MORB-like affinities. Coleman et al. (ibid.) further postulated a correlation of the Naturaliste Plateau basalts with the onshore tholeiitic Bunbury Basalt of southwestern Western Australia, first described by Saint-Smith (1912) and Edwards (1936, 1938). On the basis of the magmatic affinities of the reworked basaltic materials, Coleman et al. (1982) concluded that the exact nature of the basement to the Naturaliste Plateau, previously assumed to be continental, remained unknown. An alternative hypothesis they identified was that the tholeiitic basaltic volcanism might be related to the onset of seafloor spreading at ~122 Ma, corresponding to the age of breakup of Australia and India postulated at that time (Markl, 1978).

Naturaliste Plateau and its relationship to the Bunbury Basalt and the Kerguelen LIP

 

Davies et al. (1989) were the first to link the Kerguelen LIP to various other volcanic edifices around the southern Indian Ocean (Fig.1), including the possibility of the Bunbury Basalts being a part of this widespread system. However, at that point in time, the age of the ?correlative Naturaliste Plateau basalts was unknown, and the age of the Bunbury Basalts very uncertain, with unreliable K-Ar ages spanning the range from 136 to 88 Ma, also depending on the validity of lithostratigraphic correlations (McDougall and Wellman, 1976; Playford et al., 1976). In evaluating these correlations, Storey et al. (1992) in a review concluded that:

“More precise dating of both the Naturaliste Plateau and the Bunbury Basalt are required.”

An attempt to date the Naturaliste Plateau basalt at DSDP Site 264 was made by Pyle et al. (1995). They reported a mean 40Ar/39Ar whole rock age from a clast of basaltic andesite of 100.6 ± 1.2 Ma. Unfortunately Pyle et al. did not comment on the possibility of extreme sample alteration that was noted by Ford (1975), and did not publish a Loss On Ignition measurement with their whole rock major element analysis of this sample. The young age reported by Pyle et al. (1995) relative to the results reported in Direen et al. (2017), and for the Bunbury Basalts by Frey et al. (1996, see next paragraph), suggests that the Pyle et al. (1995) reported age has been affected by seafloor alteration of plagioclase and subsequent Ar loss.

The first reliable and precise geochronology on the Bunbury Basalts was published by Frey et al. (1996). These authors used 40Ar /39Ar geochronology to date the basalt eruption ages to between 130 and 123 Ma, and whole rock, trace and isotope geochemistry to link the Bunbury Basalts to the Kerguelen LIP.

These results indicated that the onset of the Kerguelen LIP, as recorded in the areally minor Bunbury Basalts, was significantly older than the main volume of the Kerguelen Plateau volcanism, then sampled by several Ocean Drilling Program boreholes, which were dated at around 124-118 Ma (Coffin et al., 2002). This age discrepancy led to postulation of an “incubating plume” mechanism for LIP’s (Coffin et al., 2002; Kent et al., 2002; also Kent et al, 1992).

Further correlations of the Kerguelen-Bunbury-?Naturaliste basalts on geochemical grounds were published by Mahoney et al. (1995), and Ingle et al. (2002, 2004) (see also Ingle, 2007, this website).

A further enigmatic discovery regarding the origins of the Naturaliste Plateau was made by the French MARGAU expedition of the R/V Marion Dufresne, as reported by Beslier et al., (2004). Although focused on dredging the poorly known Diamantina Zone (Nicholls et al., 1981) to the south of the Naturaliste Plateau (Chatin et al., 1998, Munschy 1998, Beslier et al., 2004), two dredges were made of the southern margin of the Naturaliste Plateau, recovering samples of granite and gneiss, of unequivocally continental origin.

Direen et al. (2007) undertook an integrated study of all of the earlier datasets, as well as new seismic, gravity and magnetic data acquired by Geoscience Australia, and made a petrophysical study of the rocks of the nearest onshore exposed Australian margin (Leeuwin Complex). They concluded that it was most likely that the Naturaliste Plateau comprised a thinned, rifted continental fragment mantled by an extensive and thick volcanic cover sequence, subsequently onlapped by post-breakup sedimentary rocks.

Expedition of the R/V Southern Surveyor to the Naturaliste Plateau, Voyage SS09/2005

Expedition SS09/2005 of the R/V Southern Surveyor (Fig.4) to the Naturaliste Plateau (Crawford, 2006a) was conceived by Prof. Tony Crawford of the University of Tasmania, and funded by the Australian Research Council, and the Australian Major National Research Facilities grant schemes. The cruise title was “Nature and origin of the Naturaliste Plateau and Diamantina Zone: a key to understanding the assembly and breakup history of eastern Gondwana.”


Figure 4. R/V Southern Surveyor in port at Fremantle, Western Australia immediately prior to departure on voyage of discovery SS2005/09

The cruise objectives were:

  • To investigate the nature and origin of the Naturaliste Plateau (NP) through determining the composition and age of the basement on the steep, southern and northern flanks of the plateau; and
  • To establish the nature and extent of the continent-ocean transition zone to the south of the Naturaliste Plateau, by analyzing the basement rocks from the continent-ocean transition zone between the southern margin of the NP and the Diamantina Zone (DZ),

by comprehensively mapping the Naturaliste Plateau margins using multibeam (SIMRAD 30 Hz) swath mapping (Fig.5), in order to select optimal sites for sampling via chain dredge sampling (Fig. 6)


Figure 5. Swath bathymetry map of the southern and western margins of the Naturaliste Plateau, Cruise SS2005/09


Figure 6. L to R: Cruise Scientist Caroline Tiddy (nee Forbes) and students Galen Pettigrew and Cameron Hamilton examine newly dredged material from Dredge 11, 31st October 2005

The cruise sailed from Fremantle 21 October 2005, returning to port in Fremantle, 12 Nov 2005. Scientific crew comprised the Chief Investigator (A J Crawford), 3 senior scientists (M F Coffin, N G Direen, C J Forbes), 5 students (B Paul, B Cohen, L Mitrovic, G Pettigrew, C Hamilton), and 3 professional staff from Geoscience Australia & CSIRO (C Buchanan; R Beattie, S Thomas) (Crawford, 2006a)

Unfortunately for objective 2, water depths >4000m south of the plateau severely limited the application of the available equipment on the R/V Southern Surveyor to allow comprehensive sampling (see Crawford, 2006a). However, two dredges in this area did find useable material. Our findings on the nature of the continent-ocean transition zone are reported in Halpin et al., (2008).

The remainder of the dredging is fully reported in Direen et al. (2017), including the Supplemental Materials. Preliminary analyses and findings were also reported in Crawford et al. (2006b, c) and Direen et al. (2013).

New data from Naturaliste Plateau

New datasets from the 2005 cruise include 40Ar/39Ar geochronology data on a suite of the basaltic samples (e. g. Fig. 7); Laser Ablation ICPMS U-Pb geochronology data on zircons from samples of granodiorite and rhyolite (e.g. Fig. 8); whole rock geochemical data including major elements, trace elements on a range of felsic to mafic samples (Fig. 9); and whole rock Sr, Nd, and Pb isotopic data (e.g. Fig. 10).


Figure 7. 40Ar/39Ar geochronology data, R/V Southern Surveyor SS2005/09 DR 11-19. See Direen et al. (2017) for more samples and dates.


Figure 8. Laser Ablation ICPMS U-Pb geochronology data R/V Southern Surveyor SS2005/09 DR 11-6. See Direen et al. (2017) for more samples and dates.


Figure 9. XRF whole rock geochemical trace element data, SiO2 v MgO and Zr/Y v Zr/Nb, showing bimodal populations of felsic (low MgO, high SiO2) and mafic (high MgO, low-moderate SiO2) rocks, and near ubiquitous tholeiitic compositions.


Figure 10. Whole rock εHf - εNd isotopic data showing potential mixing arrays from inherited Mesoproterozoic Albany Fraser Orogen crustal compositions, Naturaliste Plateau gneisses (Halpin et al., 2008), and Naturaliste Plateau felsic samples to Cretaceous juvenile mafic igneous samples for Naturaliste Plateau, Rajmahal Traps, Broken Ridge, Bunbury Basalt, Southeast Indian Ridge (SEIR) and the Kerguelen LIP (KP). See Direen et al. (2017) for references, and further details of potential mixing curves.

40Ar/39Ar plagioclase ages from mafic units and U–Pb zircon ages from silicic rocks indicate magmatic emplacement on the Naturaliste Plateau from 130.6 ± 1.2 to 129.4 ± 1.3 Ma for mafic rocks, and 131.8 ± 3.9 to 128.2 ± 2.3 Ma for silicic rocks (2σ) (Direen et al., 2017).

Isotope data indicate the Cretaceous Naturaliste Plateau magmas incorporated a significant component of continental crust, with relatively high 87Sr/86Sr (up to 0.78), high 207Pb/204 Pb ratios (15.5–15.6), low 143Nd/144Nd (0.511–0.512) and primitive-mantle normalized Th/Nb of 11.3 and La/Nb of 3.97 (Direen et al., 2017).

The geochemical results are consistent with the plateau being underlain by continental basement (e.g. Fig 10), as also indicated by prior interpretations of seismic and gravity data as proposed by Direen et al. (2007), corroborated by the dredging of Mesoproterozoic granites and gneisses on the southern plateau flank (Halpin et al., 2008; Beslier et al., 2004).

Implications

These new datasets show the extensive carapace of Naturaliste Plateau igneous rocks have signatures indicative of extraction from a depleted mantle, with trace-element and isotopic values that overlap with ODP sampled Kerguelen Plateau lavas, but reflecting crustal contamination by Australian Proterozoic crust (Direen et al., 2017). These results, together with the area sampled (~90000 km2) show the Naturaliste Plateau contains extensive evidence of the earliest voluminous Kerguelen hotspot magmas, erupting into an area of extended continental lithosphere.

Subsequent work to SS09/2005 Cruise

Our results were anticipated by the work of Zhu et al. (2007, 2008, 2009, 2013), and Xia et al. (2014) who despite a lack of reliable age dating for Naturaliste Plateau at the time, suggested the linkage between the Cona and Comei basalts, of Tibet, and the Kerguelen-Bunbury-?Naturaliste province. We confirm their hypothesis, and postulate the linked system covered an area in excess of 244000 km2.

Maloney et al. (2011) reprocessed vintage seismic data from the Mentelle Basin, and tied seismic to well logs from the earlier DSDP 258 borehole (Fig. 11). Significant seismic events were then mapped around the basin in two-way time, and the main tectonostratigraphic packages identified. One of the significant packages identified and mapped was the Valanginian-Hauterivian unconformity (Fig. 12), on which lie major high velocity mounded buildups, interpreted as stratovolcanoes from the Kerguelen LIP event.


Figure 11. Seismic well tie, DSDP Hole 258 (modified from Maloney et al., 2011, figure 4). Panel to the left of the borehole lithology log shows the original data quality before reprocessing. The seismic panel to the right of the borehole shows the data after reprocessing. The marker horizon highlighted at about 4s TWT marks a reflection doublet that is not resolvable on the original data, which after reprocessing is seismically resolvable across the whole Mentelle Basin. Within the lithological interpretation, vertical hatching represents 114m of Miocene deepwater carbonates and oozes; horizontal hatching 171m of limestones and chalks interlayered with chert; wavy hatching Cretaceous black clays and the basal stippled region represents 11 m of Lower Cretaceous glauconitic sands.


Figure 12. Valanginian-Hauterivian unconformity with high-velocity mounded buildups, interpreted by Direen et al. (2017) as Kerguelen-Naturaliste LIP volcanoes. Figure modified from Maloney et al. (2011).

A derivative 40Ar/39Ar isotopic study of the Bunbury Basalt by Olierook et al. (2016) confirmed the eruptive ages published by Frey et al. (1996), and extended the oldest age of the Bunbury Basalt to 137 Ma, thus confirming a correlative K-Ar age of 136 ± 3 m.y. published by Playford et al. (1976).

Olierook et al. (2017) also recently published 40Ar/39Ar dating of the hydrothermal alteration of altered basaltic clasts at DSDP Site 264 (repeating the work of Pyle et al., 1995), and Eltanin Site 55. At site 264, they reported two 40Ar/39Ar plateau ages of sericite alteration of 127.6 ± 0.5 (p = 0.95) and 124.5 ± 1.0 (p = 0.96). These ages are several million years younger than the igneous crystallization ages that Direen et al. (2017) reported for unaltered rocks dredged from the same area (DR11-19 129.4 ± 1.3 Ma; Fig. 7) using the same techniques. Olierook et al. (2017) also reported an altered clast from Eltanin site 55 yielded an unreliable age around 91 Ma.

Future Work

As of publication date of the article on which this LIP of the Month is based (Direen et al. 2017; in Oct 2017), IODP Leg 369 is in progress (Sept 2017-Nov 2017: http://iodp.tamu.edu/scienceops/expeditions/australia_climate_tectonics.html)

Hobbs et al. (2014) proposed to drill the Naturaliste Plateau and Mentelle Basin; the cruise co-chief scientists are Richard Hobbs and Brian Huber. Drilling of six IODP Sites in the Naturaliste Plateau and Mentelle Basin (Fig. 13) will target Early Cretaceous volcanic rocks and underlying Jurassic(?) sediments, and potentially crystalline basement, to provide information on the timing of different stages of Gondwana breakup and the nature of the various phases of volcanism. This drilling is expected to lead to further improvements in the understanding of the evolution of the Naturaliste Plateau and Mentelle Basin.


Figure 13. IODP Leg 369 Proposed drill hole locations (2017) into Naturaliste Plateau & Mentelle Sub-basin. http://iodp.tamu.edu/scienceops/expeditions/australia_climate_tectonics.html

Acknowledgements

This précis obviously rests upon the hard work of my co-voyagers on SS 09/2005 (A J Crawford, M F Coffin, C J Forbes, B Paul, B Cohen, L Mitrovic, G Pettigrew, C Hamilton, C Buchanan, R Beattie, & S Thomas), and the professional seagoing crew of the R/V Southern Surveyor, as well as the subsequent scientific contributions of my co-authors (B Cohen –Ar/Ar dating; R Maas –Sr, Nd, Hf isotopes; F Frey –igneous geochemistry; J Whittaker –plate reconstructions; M Coffin –LIP tectonics; S Meffre – LAICPMS geochronology; J Halpin, –Australia-Antarctica basement geoscience; & A Crawford -igneous geochemistry & tectonics)

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