Elsevier

Gondwana Research

Volume 16, Issue 1, August 2009, Pages 109-118
Gondwana Research

Seismic imaging of the upper mantle under the Erebus hotspot in Antarctica

https://doi.org/10.1016/j.gr.2009.01.004Get rights and content

Abstract

P-wave velocity images are determined under the Mount Erebus volcano, Antarctica by using teleseismic tomography. Our results show a prominent low-velocity (low-V) anomaly of nearly circular symmetry (about 250–300 km in diameter) to about 200 km depth under the Mount Erebus volcanic region, which further extends down to ~ 400 km as a narrow tilted column. The observed low-V anomaly beneath the Mount Erebus volcano can be expression of a thermal anomaly of deep origin. Combining the seismically imaged thermal anomaly with geochemical observation of rift-related volcanism in the region, we consider that the Mount Erebus is a hotspot due to West Antarctic Rift System linked with a mantle plume. In addition, high-velocity anomalies are imaged beneath the East Antarctic craton, being consistent with the previous studies.

Introduction

The Antarctic continent is tectonically divided into the stable East Antarctic (EA) craton and West Antarctic Rift System (WARS) by the Transantarctic Mountains (TAM). The EA craton is a stable Precambrian shield and was a segment of the core of the Gondwanland supercontinent that was assembled during the Neoproterozoic. The EA craton under the polar ice sheet is inferred to include pan-African age belts, Grenville age provinces, Proterozoic domains, and Archean cratons (Yoshida and Santosh, 1995, Fitzsimons, 2000, Fitzsimons, 2003, Santosh et al., 2009, Maruyama et al., 2007, Rino et al., 2008, Veevers and Saeed, 2008, Veevers et al., 2008). The WARS, one of the major active continental rifts on the Earth, consists of several young geological and tectonic units that are rather mobile on a geological time scale (e.g., Roult and Rouland, 1994, Santosh et al., 2001, Fitzgerald, 2002, Elliot and Fanning, 2008, Federico et al., 2009, Vaughan and Pankhurst, 2008). Several hotspots are located in and around the Antarctica and they have played an important role in the tectonic history of the Gondwana breakup (e.g., Windley, 1995). Ross Island in the Ross Sea, West Antarctica (Fig. 1) is entirely volcanic in origin. It hosts four principal volcanoes—Mount Bird, Mount Terror, Terra Nova and Mount Erebus, as well as numerous smaller volcanoes and lava flow. While the first three ones are extinct major volcanic centers, Mount Erebus is an active volcano. As the world's southernmost active volcano and one of the loftiest volcanoes (elevation 3795 m) of the world, the Mount Erebus is the largest volcano by volume (nearly 1035 km2) in the Antarctica continent, and it is less than one million years old. The most recent eruption of the Mount Erebus began in 1972. It has one of the Earth's few long-lived convecting lava lake within a summit crater. The composition of lava within the lava lake of the Mount Erebus volcano is alkali, specifically called anorthclase phonolite, which is common in rift volcanoes (LeMasurier and Thomson, 1990, Kyle et al., 1992, Simkin and Siebert, 1994, de Wit and Anderson, 2003).

Many seismological studies by different researchers were carried out to study the crust and upper mantle structure under the Antarctic region. In the EA craton the crustal thickness varies from 35 to 45 km, while in the Ross Island region the crustal thickness varies from 18 to 25 km (Behrendt, 1999, Bannister et al., 2003, Lawrence et al., 2006b, Watson et al., 2006). The transition zone thickness (266 ± 10 km) under the WARS is slightly larger than the global average (Reusch et al., 2008).

By using data recorded by the sparse seismic network in or around Antarctic or global seismic stations, previous tomographic images show low-velocity (low-V) anomalies at the upper mantle depths beneath the WARS (e.g., Roult et al., 1994, Danesi and Morelli, 2001, Ritzwoller et al., 2001, Zhao, 2001, Zhao, 2007, Zhao, 2009, Kobayashi and Zhao, 2004, Sieminski et al., 2003, Morelli and Danesi, 2004). The low-V anomaly in the upper mantle beneath the WARS was also observed in the recent seismic studies by using a dense local seismic network (Lawrence et al., 2006a, Lawrence et al., 2006b, Watson et al., 2006). The observed low-V anomaly is interpreted as a thermal anomaly either due to the presence of a mantle plume (Behrendt, 1999, Zhao, 2007, Zhao, 2009, Kobayashi and Zhao, 2004, Sieminski et al., 2003, Morelli and Danesi, 2004, Watson et al., 2006) or due to small-scale convection under the WARS (Ritzwoller et al., 2001, Roult et al., 1994).

These earlier seismological studies focused mainly on the problem of the uplift of the TAM and the evolution of the WARS. However, it is still debatable whether the Mount Erebus, the world's southernmost active volcano, is a hotspot or not (e.g., Sleep, 1990, Duncan and Richards, 1991, Zhao, 2007). In this study we focus on understanding the deep structure and evolution of the Mount Erebus through a detailed 3-D P-wave velocity imaging by applying a tomographic method to teleseismic travel times recorded by 42 portable seismic stations in and around the Mount Erebus.

Section snippets

Data and method

We used data from 41 temporary broadband seismic stations of the Trans Antarctic Mountains Experiment (TAMSEIS) and one seismic station of the Mount Erebus Volcano Observatory Seismic Network (ER) (Fig. 1) (Lawrence et al., 2006a, Lawrence et al., 2006b). During November 2001 to December 2003, the TAMSEIS was conducted along three profiles: (1) A 1300-km linear array with 17 seismic stations extending from the central regions of the EA to TAM (array 1), (2) an intersecting 400-km denser array

Analysis and results

We conducted several tomographic inversions using different grids, and preferred a grid spacing of 0.5° and 1.0° in latitude and longitude directions, respectively (Fig. 5). The vertical grid spacing ranges from 40 to 200 km increasing with depth. Inversions were also performed by using various damping and smoothing parameters. The pattern of the tomographic images is generally stable, although there are slight changes in the amplitude of the velocity anomalies.

Fig. 8, Fig. 9 show our optimal

Discussion

The seismic velocity variations revealed by seismic tomography may reflect changes in composition and/or temperature. Large P-velocity reduction (up to 2%) in the upper mantle due to compositional variations is unlikely (Karato, 1993, Sobolev et al., 1997, Faul and Jackson, 2005). However, accounting for first-order effects like anharmonicity (Anderson et al., 1992, Duffy and Anderson, 1989) and anelasticity (Karato, 1993, Sobolev et al., 1997), the amplitude of the low-V anomaly (up to 2%)

Conclusions

We determined a detailed 3-D P-wave velocity structure of the upper mantle beneath the Mount Erebus volcano using a large number of high-quality arrival-time data collected from original seismograms of teleseismic events recorded by portable seismic networks. Our resulting model shows high-velocity (high-V) anomalies beneath the EA craton. Beneath the Mount Erebus volcanic region a significant low-V anomaly of nearly circular symmetry (about 250–300 km in diameter) exists down to about 200 km

Acknowledgments

We thank the IRIS Data Management Center for providing the waveform data used in this study. We are indebted to Dr. Guoming Jiang for his assistance during the analysis and useful discussions. S. Gupta thanks the NGRI director for his encouragement and support and the Department of Science and Technology, Government of India for the BOYSCAST fellowship to study at Tohoku University, Japan. This work was partially supported by a grant (Kiban-A 17204037) from the Japan Society for the Promotion

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