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Definition of a Cenozoic alkaline magmatic province in the southwest Pacific mantle domain and without rift or plume origin

Carol A. Finn1, R. Dietmar Müller2 & Kurt S. Panter3

1
U.S. Geological Survey, MS 964, Denver Federal Center, Denver, CO 80225, USA
cfinn@usgs.gov

2School of Geosciences, Edgeworth David Bldg. F05, The University of Sydney, 2006, AUSTRALIA
dietmar@geosci.usyd.edu.au

3Department of Geology, Bowling Green State University, Bowling Green, OH
43403-0218, USA
kpanter@bgnet.bgsu.edu

A Cenozoic (< 50 Ma) bimodal, but largely basaltic, mostly alkaline igneous province covers a broad area of continental and oceanic lithosphere in the southwest Pacific Ocean region (Figure 1). It has been conjecturally linked to rifting, mantle plumes, or hundreds of hot spots, but all of these associations have flaws. For example, plate reconstructions demonstrate that the last episode of major regional rifting in west Antarctica, eastern Australia and New Zealand occurred during the Mesozoic break-up of Gondwana. GPS and stress-field measurements show no extension in Australia, New Zealand and much of west Antarctica, suggesting that the widespread magmatism cannot be explained by rifting alone.  Estimates of volumes of magmas erupted in west Antarctica and Australia, as well as magma production rates are low compared to areas associated with plumes.  Uplift and doming typically associated with mantle plumes are also largely absent.  Also, to explain the areal distribution of the volcanism, an unusually large plume would have to underlie the entire southwest Pacific, or there would have to be hundreds of hot spots, which are not observed. Clearly, new models for volcanism are required.

Figure 1. Rayleigh wave 150s group velocity map (~120 km depth) [Larson and Ekström, 2001].  Only long-lived hot spots with long traces [Clouard and Bonneville, 2001; Gaina et al., 2000; Ritsema and Allen, 2003] underlain by low velocity perturbations in the upper mantle are shown. AAD=Australian-Antarctic Discordance; LHR=Lord Howe Rise; RS=Ross Sea; TAM=Transantarctic Mountains, BH=Bellingshausen Sea. Click on image to enlarge.

Comparison of the location of volcanoes and seismic shear wave perturbation models (Figure 1), shows that this alkaline volcanic province occurs in thin (< 80 km) lithosphere [Ritzwoller et al., 2001] (e.g., Figure 2a).  The province correlates with distinct low seismic velocity anomalies (Figure 1) generally restricted to a zone in the mantle between ~ 60 and 200 km depth (e.g., Figure 2a). In places, low velocity perturbations (> 1.2% decrease from PREM), such as those beneath the Lord Howe Rise (Figure 2b), extend to ~ 670 km depth and the Bellingshausen Sea to ~ 800 km depth (Figure 2c). The prominent lower mantle central Pacific low velocity perturbation zone (Figure 2b) does not seem to penetrate the 670 km discontinuity [Ritsema & Allen, 2003; Su et al., 1994] nor extend south to the Antarctic region (Figure 2c). All of these examples show that the low velocity anomaies in the region, generally interpreted to arise from high temperatures, are restricted to the upper mantle.

Figure 2. A) (left) Shear wave velocity perturbation model  derived from inversion of Rayleigh and Love wave data and relative to AK135, Antarctica. right) Red line shows profile location (from Ritzwoller et al., 2001).

B) Shear velocity anomalies from model S20RTS in 180o wide cross-sections through the mantle [Ritsema et al., 1999]. The thick dashed line indicates the 670-km discontinuity. (upper) Section from the Southeast Indian Ridge, across the Tasman Sea, and Kermadec trench. (lower) Section from the Indian Ridge, East Antarctic craton, Bellingshausen Sea, Pacific-Antarctic Ridge and central Pacific.

Geochemical studies show that for most of the region, the magmatism is a result of small degrees of melting (F = 1-3%) of a source enriched in incompatible elements relative to primitive upper mantle. The enrichment may have involved the introduction of volatile-rich fluids or melts into pre-existing upper mantle (e.g., Gamble et al., 1988; Hart et al., 1997; Kamenetsky et al., 2000; Panter et al., 2000; Rocchi et al., 2002; Zhang et al., 1999). This suggests that melting of metasomatized upper mantle can occur without excessive temperatures (Figure 3) and that the low seismic velocities are primarily related to slightly elevated temperatures, water and in places, melt.


Figure 3. McMurdo/SE Australia geotherm (from Berg et al., 1989; O'Reilly and Griffin, 1985), compared to stability fields of amphibole and phlogopite.  Also shown are water-saturated and water-undersaturated solidi and an adiabatic path for asthenospheric mantle. 
The principal characteristics of alkaline magmatism in the SW Pacific must be identified for input into any new models. Striking features of the alkaline province are its longevity (~ 50 Ma) and broad regional extent but low volumes. A primary attribute is the low velocity zone lying between ~ 60 and 200 km depth beneath the region of alkaline magmatism. Lithospheric thickness may play a fundamental role in localizing magmatism in that the volcanism does not occur in regions whose high velocity lids exceed ~ 80 km. We therefore characterize the province based on coincident thin lithosphere hosting largely alkaline magmatism generated from a metasomatized source associated with low seismic velocity anomalies in the upper mantle (black line, Figure 1). The age of the metasomatism is not known but may relate to a combination of Paleozoic-Mesozoic subduction along the Pacific margin of Gondwana and possible plume-related activity in the Jurassic. During Cretaceous break-up of Gondwana, rifting in east Australia and west Antarctica did not result in voluminous magmatism despite thinning and regional extension of continental lithosphere containing metasomatized mantle. This suggests that late Cretaceous-early Eocene regional heating and/or a mantle stirring event is required to allow alkaline magmatism.

Any satisfactory scenario for generation of the SW Pacific alkaline magmatic province must explain the following:  1) broad extent, 2) location in the SW Pacific, 3) low volumes, 4) HIMU-EM1-EM2 signature, 5) duration, 6) timing of onset, and 7) coincidence with volcanic centers with linear age progressions that track plate motion (e.g., the Louisville Ridge, Tasmintid, Lord Howe and Tasmanian seamounts). Localization of specific volcanic centers by pre-existing zones of weakness clearly occurs, but does not seem to drive the system. Several explanations for episodic plate reorganization (Fukao et al., 2001; King et al., 2002) may also provide mechanisms for mantle flow sufficient to cause alkaline magmatism in thin, metasomatized lithosphere. Plate motion history (Lithgow-Bertelloni & Richards, 1998) and seismic tomography studies (Fukao et al., 2001; van der Hilst et al., 1997) propose that high density and velocity subducted slabs lying in the lower mantle detached from the mantle transition zone at various times. These slab detachment events, so-called mantle avalanches, can cause vertical and lateral flow in the entire mantle (Brunet & Machetel, 1998; Christensen, 1997; Pysklywec et al., 2003; Solheim & Peltier, 1994). Detachment of the subducting Pacific slab (now partially entrained in the AAD) beneath Australia and Antarctica by the late Cretaceous (~ 65 Ma) (Lithgow-Bertelloni & Richards, 1998) may have produced viscous flow of warm mantle to the region.

References

  • Berg, J.H., R.J. Moscati, and D.L. Herz, A petrologic geotherm from a continental rift in Antarctica, Earth and Planetary Science Letters, 93 (1), 98-108, 1989.
  • Brunet, D., and P. Machetel, Large-scale tectonic features induced by mantle avalanches with phase, temperature, and pressure lateral variations of viscosity, Journal of Geophysical Research, B, Solid Earth and Planets, 103 (3), 4929-4945, 1998.
  • Christensen, U.R., Influence of chemical buoyancy on the dynamics of slabs in the transition zone, Journal of Geophysical Research, B, Solid Earth and Planets, 102 (10), 22,435-22,443, 1997
  • Clouard, V., and A. Bonneville, How many Pacific hotspots are fed by deep-mantle plumes?, Geology, 29 (8), 695-698, 2001.
  • Fukao, Y., S. Widiyantoro, and M. Obayashi, Stagnant slabs in the upper and lower mantle transition region, Reviews of Geophysics, 39 (3), 291-323, 2001.
  • Gaina, C., R.D. Mueller, and S.C. Cande, Absolute plate motion, mantle flow, and volcanism at the boundary between the Pacific and Indian Ocean mantle domains since 90 Ma, in The History and Dynamics of Global Plate Motions, edited by M.A. Richards, R.G. Gordon, and  R.D. van der Hilst, pp. 189-210, American Geophysical Union, Washington, DC, 2000.
  • Gamble, J.A., F. McGibbon, P.R. Kyle, M.A. Menzies, and I. Kirsch, Metasomatised xenoliths from Foster Crater, Antarctica; implications for lithospheric structure and processes beneath the Transantarctic Mountain front, in Journal of Petrology, vol.1988, edited by M.A. Menzies, and  K.G. Cox, pp. 109-138, Clarendon Press, Oxford, 1988.
  • Hart, S.R., J. Blusztajn, W.E. LeMasurier, and D.C. Rex, Hobbs Coast Cenozoic volcanism; implications for the West Antarctic rift system, Chemical Geology, 139, 223-248, 1997.
  • Kamenetsky, V.S., J.L. Everard, A.J. Crawford, R. Varne, S.M. Eggins, and R. Lanyon, Enriched end-member of primitive MORB melts; petrology and geochemistry of glasses from Macquarie Island (SW Pacific), Journal of Petrology, 41 (3), 411-430, 2000.
  • King, S.D., J.P. Lowman, and C.W. Gable, Episodic tectonic plate reorganizations driven by mantle convection, Earth and Planetary Science Letters, 203, 83-91, 2002
  • Larson, E.W.F., and G. Ekström, Global Models of Surface Wave Group Velocity, Pure & Appl. Geophys, 158 (8), 1377-1400, 2001.
  • Lithgow-Bertelloni, C., and M.A. Richards, The dynamics of Cenozoic and Mesozoic plate motions, Reviews of Geophysics, 36 (1), 27-78, 1998
  • Panter, K.S., S.R. Hart, P. Kyle, J. Blusztanjin, and T. Wilch, Geochemistry of Late Cenozoic basalts from the Crary Mountains:  characterization of mantle sources in Marie Byrd Land, Antarctica, Chemical Geology, 165, 215-241, 2000.
  • Pysklywec, R.N., J.X. Mitrovica, and M. Ishii, Mantle avalanche as a driving force for tectonic reorganization in the southwest Pacific, Earth and Planetary Science Letters, 209, 29-38, 2003
  • Ritsema, J., and R.M. Allen, The elusive mantle plume, Earth and Planetary Science Letters, 207, 1-12, 2003.
  • Ritsema, J., H.-J. van Heijst, and J.H. Woodhouse, Complex shear wave velocity structure imaged beneath Africa and Iceland, Science, 286 (5446), 1925-1928, 1999.
  • Ritzwoller, M.H., N.M. Shapiro, A.L. Levshin, and G.M. Leahy, Crustal and upper mantle structure beneath Antarctica and surrounding oceans, Journal of Geophysical Research, B, Solid Earth and Planets, 106 (12), 30,645-30,670, 2001.
  • Rocchi, S., P. Armienti, M. D'Orazio, S. Tonarini, J.R. Wijbrans, and G. Di Vincenzo, Cenozoic magmatism in the western Ross Embayment:  Role of mantle plume versus plate dynamics in the development of the West Antarctic Rift System, Journal of Geophysical Research, 107 (B9), 10.1029/2001JB000515, 2002.
  • Solheim, L.P., and W.R. Peltier, Avalanche effects in phase transition modulated thermal convection; a model of Earth's mantle, Journal of Geophysical Research, B, Solid Earth and Planets, 99 (4), 6997-7018, 1994
  • Su, W.-J., R.L. Woodward, and A.M. Dziewonski, Degree 12 model of shear-velocity heterogeneity in the mantle, Journal of Geophysical Research, 99, 6945-6980, 1994.
  • van der Hilst, R.D., S. Widiyantoro, and E.R. Engdahl, Evidence for deep mantle circulation from global tomography, Nature, 386, 578-585, 1997
  • Zhang, M., S.Y. O'Reilly, and D. Chen, Location of Pacific and Indian mid-ocean ridge-type mantle in two time slices: Evidence from Pb, Sr, and Nd isotopes for Cenozoic Australian basalts, Geology, 27 (1), 39-42, 1999.

last modified 18th July, 2003

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