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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

1U.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.

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last modified 18th July, 2003

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