Earth and Planetary Science Letters 203 (2002) 635^650 www.elsevier.com/locate/epsl Paleolatitudes of the Kerguelen hotspot: new paleomagnetic results and dynamic modeling M. Antretter a;Ã, B. Steinberger b, F. Heider a;1, H. So¡el a a Institut fu«r Geophysik, Theresienstr. 41, 80333 Munich, Germany b Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado at Boulder, Campus Box 216, Boulder, CO 80309-0216, USA Received 6 March 2002; received in revised form 28 June 2002; accepted 12 July 2002 Abstract The Kerguelen Plateau, a Large Igneous Province in the southern Indian Ocean, was formed as a product of the Kerguelen hotspot in several eruptive phases during the last 120 Myr. We obtained new paleolatitudes for the central and northern Kerguelen Plateau from paleomagnetic investigations on basalts, which were drilled during ODP Leg 183 to the Kerguelen Plateau^Broken Ridge. The paleolatitudes coincide with paleolatitudes from previous investigations at the Kerguelen Plateau and Ninetyeast Ridge (the track of the Kerguelen hotspot) and indicate a difference between paleolatitudes and present position at 49‡S of the Kerguelen hotspot. We show that true polar wander, the global motion between the mantle and the rotation axis, cannot explain this difference in latitudes. We present numerical model results of plume conduit motion in a large-scale mantle flow and the resulting surface hotspot motion. A large number of models all predict southward motion between 3‡ and 10‡ for the Kerguelen hotspot during the last 100 Myr, which is consistent with our paleomagnetic results. ß 2002 Elsevier Science B.V. All rights reserved. Keywords: geomagnetism; paleomagnetism; Leg 183; paleolatitude; true polar wander; hot spots; mantle plumes; Kerguelen Plateau 1. Introduction Whether or not and to what extent hotspots are ¢xed can give important information about the dy- * Corresponding author. Tel.: +49-89-2180-4231; namics of the Earth’s deep interior where they Fax: +49-89-2180-4205. Until September 2002 at: CEREGE, University of Aix^Marseille 3, P.O. Box 80, Europo“le de probably originate. Furthermore, the hotspot refer- l’Arbois, 13545 Aix en Provence Cedex 4, France. ence frame as it has been used to de¢ne absolute E-mail addresses: maria@geophysik. uni-muenchen.de plate motions is only useful if any motions of hot- (M. Antretter), [email protected] (B. Steinberger), spots are properly taken into account. The subject [email protected] (F. Heider), so¡[email protected] has therefore recently been investigated by several muenchen.de (H. So¡el). researchers: numerical models of motions of hot- 1 Present adress: In¢neon Technologies, 9500 Villach, spots due to mantle £ow have been presented [1,2]; Austria. paleomagnetic data [3,4] and plate reconstructions 0012-821X / 02 / $ ^ see front matter ß 2002 Elsevier Science B.V. All rights reserved. PII: S0012-821X(02)00841-5 EPSL 6360 7-10-02 636 M. Antretter et al. / Earth and Planetary Science Letters 203 (2002) 635^650 [5^8] have been used to argue both for hotspot A way to determine hotspot motion that does mobility and for hotspot ¢xity. Relative hotspot not directly depend on any plate motion models motion between hotspots in the Indian and Atlan- makes use of the characteristic remanent magne- tic oceans has been reported to be a few mm/yr [5]. tization (ChRM) of basalts which have been pro- In contrast, motion of hotspots in the Paci¢c re- duced by the hotspot: if secular variation is prop- gion relative to hotspots in the Indo-Atlantic re- erly averaged out, if the hotspot is ¢xed, if the gions has been found to be a few cm/yr [6]. geocentric axial dipole (GAD) hypothesis holds, Fig. 1. Bathymetric map of the Indian Ocean showing the Kerguelen Plateau. Summary of ODP drill holes on the Kerguelen Pla- teau that recovered volcanic rocks (Leg 119: Site 738; Leg 120: Sites 747, 748, 749, and 750; Leg 183: Sites 1136, 1137, 1138, 1139, 1140). Basalts of bold signed Sites (1138 and 1140) were used to paleomagnetically determine new paleolatitudes, which are presented here. Contour intervals = 500 m. EPSL 6360 7-10-02 M. Antretter et al. / Earth and Planetary Science Letters 203 (2002) 635^650 637 and if the rotation axis does not move with re- spect to the mantle (true polar wander, TPW), the inclination of the ChRM should remain constant and should correspond to the present-day latitude of the hotspot. If that is not the case, one or more of the conditions will not be ful¢lled. This paper contributes to the topic of hotspot motion by looking particularly at the Kerguelen hotspot, and it presents paleomagnetic data as well as modeling results. The Kerguelen hotspot in the southern Indian Ocean has produced the Rajmahal Traps, the Kerguelen Plateau, the Bro- ken Ridge complex and the Ninetyeast Ridge since its origin approximately 120 Ma ago. We present new paleomagnetic investigations of ba- salts of the Kerguelen Plateau (Fig. 1) that were drilled during ODP Leg 183 to the Kerguelen^ Broken Ridge Plateau, yielding new paleolatitudes for the northern Kerguelen Plateau (NKP) and the central Kerguelen Plateau (CKP). Two curves of TPW that were independently derived from continental paleomagnetic data [9,10] are used to compute one component of changes in hotspot latitude. In addition to this, the latitude change of the Kerguelen hotspot due to its motion in large-scale mantle £ow is calculated. Combining both e¡ects, we are able to compute changes in hotspot paleolatitudes and to compare predictions with the paleomagnetic results. 2. Paleomagnetism Fig. 2. (a,b) Characteristic behavior of samples of Site 1138 during alternating ¢eld and thermal demagnetization, respec- tively. The orthogonal vector diagrams [55] show nearly uni- During Ocean Drilling Program (ODP) Leg vectorial decay to the origin after the removal of a small vis- 183, 144.5 m of massive lava £ows were pene- cous overprint. Temperature steps and ¢eld steps are 50^ trated and 69.0 m of core was recovered on the 25‡C and 2^5 mT, respectively. Sample identi¢cations follow- CKP (Fig. 1) (Hole 1138A: 53‡40PS, 75‡35PE). ing conventions of the ODP are as follows: (a) 183-1138A- The basalt sequence can be separated into 21lith- 80R-2, 114^116 cm; (b) 183-1138A-80R-3, 40^42 cm. V: ver- tical component; H: horizontal component; J0 : starting ologic units [11]. 87.9 m of pillow basalts with magnetization. (c) Alternating ¢eld demagnetization of sam- sediment layers between some pillow complexes ple 183-1140A-37R-4, 16^18 cm from Site 1140, showing have been penetrated and 49.1m of core has a magnetization less hard than samples of Site 1138 (a). been recovered on Site 1140, V270 km north of (d) Thermal demagnetization of sample 183-1140A-33R-2, Kerguelen Island on the NKP (Hole 1140A: 104^106 cm from Site 1140 showing a rather large viscous 40 39 component, which, however, could be easily removed after 47‡56PS, 69‡54PE) (Fig. 1). Ar/ Ar radiometric heating to 120‡C. data yield ages of 100.41 þ 0.71 and 34.30 þ 0.59 Ma for Sites 1138 and 1140, respectively [12]. Azimuthally unoriented discrete inch cores (n = 151 for CKP and n = 81for NKP) were drilled EPSL 6360 7-10-02 638 M. Antretter et al. / Earth and Planetary Science Letters 203 (2002) 635^650 from the recovered basalt cores and analyzed in Table 1 the Paleomagnetic Laboratory at the University New paleomagnetic results from the CKP (ODP Site 1138) and NKP (ODP Site 1140) of Munich, Germany. Overlying sediments have K not been used for paleolatitude analysis because Unit num- Sample Mean £ow k 95 ber number inclination of damaged and highly disturbed cores due to n/N the rotary drilling technique. Most of the samples were subjected to detailed thermal demagnetiza- Site 1138 2 7/7 365.0 6912.4 tion. Some samples were also demagnetized using 3 9/9 362.3 392 2.7 stepwise alternating ¢eld treatment in increments 4 6/6 361.5 654 2.9 of 3^20 mT. Thermal and alternating ¢eld results 5 9/8 360.2 937 1.9 are consistent (Fig. 2). During both demagnetiza- 6 7/7 354.6 682 2.5 3 tion techniques, the samples show a univectorial 7 9/8 53.2 664 2.2 8 6/6 362.6 144 6.2 decay after the removal of a small viscous magne- 9 12/10 368.3 306 2.8 tization (Fig. 2a,b for Site 1138; Fig. 2c,d for Site 10 10/10 373.0 3712.5 1140), allowing calculation of a characteristic di- 11 6/6 373.5 2344 2.5 rection with principal component analysis. 12 7/7 371.9 671 2.5 3 All the ChRMs of samples of Site 1138 have a 13 16/16 68.3 141 3.0 14 11/10 364.0 154 3.9 negative inclination. Assuming a southern hemi- 15 0/0 ^ ^ ^ sphere origin, the negative inclinations denote 16 3/3 360.3 82 22.7 normal polarity. We did not recover su⁄cient ori- 17 5/5 352.5 130 7.8 ented material in lithologic Units 15 and 18 of 18 1/0 ^ ^ ^ 3 Site 1138 to obtain reliable paleomagnetic direc- 19 6/6 56.0 187 5.4 20 7/7 351.4 92 6.7 tions. Lithologic Unit 1 of Site 1140 has a nega- 217/7 355.180 7.2 tive inclination, followed by positive inclinations 22 7/6 353.8 171 5.7 downhole in Unit 2^6.