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The Tarava Seamounts: a Newly Characterized Hotspot Chain on the South Paci¢C Superswell

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The Tarava Seamounts: a Newly Characterized Hotspot Chain on the South Paci¢C Superswell Earth and Planetary Science Letters 207 (2003) 117^130 www.elsevier.com/locate/epsl The Tarava Seamounts: a newly characterized hotspot chain on the South Paci¢c Superswell Vale¤rie Clouard a;Ã, Alain Bonneville b, Pierre-Yves Gillot c a Jeune Equipe Terre-Oce¤an, Universite¤ de la Polyne¤sie franc°aise, P.O. Box 6570 Faaa, Tahiti, French Polynesia b Ge¤osciences Marines, CNRS, Institut de Physique du Globe, 4 place Jussieu, 75252 Paris Cedex 05, France c Laboratoire de Ge¤ochronologie (USP-IPGP), Sciences de la Terre, Ba“t. 501, Universite¤ Paris Sud, 91405 Orsay, France Received 12 August 2002; received in revised form 26 November 2002; accepted 12 December 2002 Abstract The Tarava Seamount chain, also known as the Savannah seamounts on some previous marine charts, is located 200 km south of the Society Islands and was surveyed in 1996. It is 700 km long and comprises 18 seamounts, two of which have been dredged and dated. This chain exhibits two main branches: a western branch parallel to Pacific plate motion before 43 Ma and an eastern one parallel to plate motion since 43 Ma. To first order, K^Ar ages obtained on two dredged samples (43.5 þ 0.6 Ma and 35.9 þ 0.5 Ma), morphology and alignments are compatible with a hotspot origin but a large discontinuity exists in the track at 43 Ma, where the chain splits into two distinct alignments. Several volcanic sources could be responsible for these features but a more convincing solution is proposed that involves the influence of the lithospheric stress field on the track of a single hotspot. Considering that the stress field can be separated into a field due to the deflection of the lithosphere under new volcanic load and a pre-existing regional field, we show that volcanoes appear within the width of the hotspot track only where the less compressive component of the horizontal stress field is minimal. This analysis highlights the key role of the lithosphere and of its pre-existing state of stress on the path of the hotspot tracks. ß 2003 Elsevier Science B.V. All rights reserved. Keywords: South Paci¢c Superswell; Tarava Seamounts; hotspot; lithospheric stress ¢eld; de£ection 1. Introduction ascribed to deep hotspots, where the term hotspot refers to any magmatic source deeper than the Since Wilson [1], the origin of linear intraplate moving lithosphere. But as more data were col- volcanic chains with age progressions has been lected, di¡erences between deep hotspot theory (e.g., [2^4]) and observation were increasingly rec- ognized [5]. Speci¢cally, on the Paci¢c plate, the * Corresponding author. Present address: Departamento de analysis of the relationship between oceanic pla- Geo¢sica, Universidad de Chile, Casilla 2777, Santiago, Chile. E-mail addresses: [email protected] (V. Clouard), teaus, linear volcanic chains and hotspots has [email protected] (A. Bonneville), shown that most of the recent intraplate chains [email protected] (P.-Y. Gillot). associated with present-day active hotspots corre- 0012-821X / 03 / $ ^ see front matter ß 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0012-821X(02)01143-3 EPSL 6530 10-2-03 118 V. Clouard et al. / Earth and Planetary Science Letters 207(2003) 117^130 16°S 18°S 20°S 22°S 156°W 154°W 152°W 150°W 148°W depth (m) -5000 -4000 -3000 -2000 -1000 0 Fig. 1. Studied area. The Tarava Seamounts correspond to the seamounts inside the ellipse. They were surveyed in December 1996 by the French R/V l’Atalante. Major magnetic anomalies [15] are represented by white lines. The bathymetry in gray levels is derived from a 30Q grid (see text). spond to short-lived volcanism unrelated to any data collected on a linear volcanic chain south deep mantle plume [6]. In the case of the south- of the Society Islands (Fig. 1). In our discussion, ernmost part of the Austral alignment, because ‘hotspot’ will be used in the general sense of a the age^distance relation between seamounts and localized source of melt in the mantle, below the the hotspot fails, this short-lived volcanism has lithosphere and ¢xed with respect to it, whatever been related to stress in the lithosphere [7].At its depth of origin: just below the lithosphere, at large scale, Steinberger et al. [8] have shown a the transition zone or deeper in the mantle. correlation between hotspots and positive anoma- On a previous bathymetric map [9], the Tarava lies of scalar stress, and these authors hypothe- Seamounts were also known as the Savannah sea- sized that hotspot magma ascends only where mounts, but no o⁄cial name was assigned at that the lithosphere is under tension. To address the time. The name of the chain and of the seamounts question of the respective importance of litho- used in this paper were submitted to the Interna- spheric stress ¢eld and hotspot mechanism, we tional Hydrographic Organization. The complete present in this paper an analysis based on new topography of the chain has been revealed using EPSL 6530 10-2-03 V. Clouard et al. / Earth and Planetary Science Letters 207(2003) 117^130 119 altimetry data constrained by echo-sounding ship on two seamounts have been selected for K^Ar data [10]. The Tarava Seamounts were completely dating: one at 2900 m depth on the eastern £ank charted during ZEPOLYF1 cruise with the of Punu Taipu and one at 1500 m depth on the French R/V l’Atalante [11]. This survey showed eastern £ank of Fafa Piti (Fig. 2). We used the that there are two di¡erent volcanic provinces, method developed by Cassignol and Gillot [18] the Tarava Seamounts themselves to the west and Gillot and Cornette [19]. In this method the and the Va’a Tau Piti Ridges to the east (Fig. precision and accuracy, especially when dating 1). Our study focuses on the Tarava Seamount pillow basalts, depend primarily on the sample chain. To describe the local and regional stress selection and preparation. From careful petro- ¢elds in the Tarava area, we used the main struc- graphic observation, we selected lavas which con- tural directions of both individual volcanoes and tain fresh and preserved microlitic plagioclases in the whole chain. The morphological parameters the groundmass from the inner part of the pillow. are measured from multibeam bathymetric data. Indeed, to reach a precise age value, it is necessary This analysis coupled with radiometric ages on to date a pure mineral phase which crystallized two seamounts leads us to propose a hypothesis while the lava solidi¢ed, avoiding both the early for the origin of the Tarava Seamount chain crystallized minerals responsible for 40Ar inheri- based on the role played by the lithosphere in tance and the deuteric mineral phases, such as controlling the emplacement of this chain. zeolites, clay minerals and oxides. For this, we applied a double separation by means of heavy liquids (methylene iodide diluted in acetone) and 2. Geological setting magnetic separators. Pure plagioclase microlites were extracted from a 30^60 Wm fraction obtained The Tarava Seamount chain is located in the by re-crushing an already selected fraction of 200^ south central part of the Paci¢c plate (Fig. 1) 400 Wm grains of pure groundmass (which elimi- between latitudes 16‡S and 20‡S and longitudes nates all pheno-and xenocrysts). The results of 156‡W and 150‡W. This region is approximately two independent replicate measurements on po- centered on the South Paci¢c Superswell area. tassium and radiogenic argon content are re- The South Paci¢c Superswell [12] is characterized ported in Table 1 where we used the International by a positive depth anomaly of the sea£oor sev- Conventional Constants [20] for the age calcula- eral thousand kilometers in extent with an unusu- tion. ally high concentration of volcanoes [13]. This Finally, we obtained an age of 43.5 þ 0.6 Ma shallow sea£oor is not simply explained by coa- for the Fafa Piti basalt and 36.1 þ 0.5 Ma for lescent swells produced by the di¡erent hotspots, the Punu Taipu basalt. but by a dynamic or thermal e¡ect of an anom- alous asthenosphere on the lithosphere [14]. The eastern part of the Tarava Seamounts lies on a 80 4. Flexural signature of the Tarava Seamounts and Myr old lithosphere [15] and the western part on the Va’a Tau Piti Ridges a lithosphere generated during the Cretaceous Magnetic Quiet Zone [15^17]. The Tarava Sea- The eastern extremity of the Tarava Seamount mounts are located on the Society swell, which chain is elongated by two deep volcanic ridges extends for 300 km from the axis of the Society called Va’a Tau Piti Ridges (Fig. 1), which exhibit chain [14], a recent volcanic alignment due to hot- an east^west trend. Their morphology is su⁄- spot volcanism. ciently distinct from that of the Tarava Seamount to suggest a di¡erent origin. As no basaltic sam- ples have ever been dredged on these ridges, geo- 3. Radiometric ages physical proxies for age, based on the £exural signature, may provide a useful discrimination be- Samples of alkali basalt pillow lavas dredged tween Tarava and the Va’a Tau Piti volcanism. EPSL 6530 10-2-03 120 V. Clouard et al. / Earth and Planetary Science Letters 207(2003) 117^130 155°W153°W 151°W Manuae 2000 Arere 17°S Orio'Mata Otu'eroa 18°S 4000 Honu 4000 Ua'ao Fafa Piti Otaha 4000 19°S (43.5 Ma) 2000 Punu Taipu (35.9 Ma) 20°S Fig.
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