From Slow to Ultraslow: a Previously Undetected Event at the Southwest Indian Ridge at Ca

From slow to ultraslow: A previously undetected event at the Southwest Indian Ridge at ca. 24 Ma

Philippe Patriat Laboratoire de Géosciences Marines, CNRS-UMR 7154, Institut de Physique du Globe de Paris, 4 place Jussieu 75252 Paris cedex 05, France Heather Sloan Environmental, Geographic, and Geological Sciences, Lehman College, City University of New York, 250 Bedford Park Blvd., Bronx, New York 10469, USA Daniel Sauter Institut de Physique du Globe de Strasbourg, UMR7516 CNRS-ULP, Ecole et Observatoire des Sciences de la Terre, 5 rue Descartes 67084 Strasbourg cedex, France

ABSTRACT survey transects are so well positioned as to Changes in plate motion are thought to be recorded in the trend of fracture zones, even record distinct identifi able magnetic anomaly though fracture zones provide no information about the spreading rate. Using newly compiled sequences. As a result, profi les exhibiting long published and unpublished magnetic data from the Southwest Indian Ridge, we calculated fi nite uninterrupted series of magnetic anomalies rotation poles for A13, A8, and A6, from which we determined a 50% decrease in spreading rate at the SWIR are few, especially on its remote from slow to ultraslow at ca. 24 Ma not accompanied by a signifi cant change in spreading direc- southern fl ank. tion. This spreading rate decrease is concurrent with changes in plate motions at only two of the Anomaly shapes at slow spreading ridges four adjoining plate boundaries. Finally, we discuss the possible relationships of this event with are particularly sensitive to the frequency of other absolute or relative plate motion events that occurred at ca. 24 Ma at the global scale. magnetic fi eld inversions. Since A18 time, magnetic fi eld inversions have been relatively Keywords: mid-ocean ridges, global tectonics, kinematics, magnetic anomalies. frequent, which tends to make the forms of A13, A8, and A6 more diffi cult to identify than INTRODUCTION 1997) and unpublished transit ship tracks. At those of the A21–A18 sequence. We therefore The Southwest Indian Ridge (SWIR) is the slow to ultraslow spreading ridges, complete and began anomaly identifi cation by calculating type example of an ultraslow spreading center easily identifi able magnetic anomaly sequences two models: one with a rate of ~29 km/m.y. to with a current full spreading rate of 14 km/m.y. are rare. They are most often observed along produce anomaly forms that correspond well But how long has this ridge been ultraslow? narrow swaths of seafl oor that form the central with the easily recognizable A21–A18 magnetic Previously presented spreading histories for the part of spreading segments that, due to segment anomaly sequence, the other with the present SWIR indicate faster spreading rates prior to propagation, are not always oriented parallel ultraslow spreading rate of ~15 km/m.y. for anomaly 18 time (A18, ca. 40 Ma; Cande and to fracture zones. It is therefore unusual that the period A5–A0 (e.g., Lemaux et al., 2002; Kent, 1995) than at present, with a decrease from slow to ultraslow sometime between A13 (ca. 33 Ma) and A6 (ca. 20 Ma) (Bergh and NS Norton, 1976; Fisher and Sclater, 1983; Molnar A8A6 A5 A5 A6 A8 et al., 1988; Patriat and Segoufi n, 1988). This CIR 200 100 Africa lack of precision could be attributed to the fact RTJ 0 that magnetic anomalies at ultraslow ridges –100 Magnetic –200 are relatively diffi cult to identify, but it is more SWIR SEIR anomaly (nT) African 2 1155 kkm/m.y.m/m.y. likely due to lack of data combined with the md47 Plate 3 deceivingly linear trends of the SWIR fracture Figure 3 Antarctic Plate 4 zones, which were thought to indicate a single, Figure DR1

Depth (km) 5 simple spreading phase during the last 40 m.y. 100 km At ﬁ rst glance, the smooth curvilinear frac- Profile md47 ture zone trends of the SWIR for ages <40 Ma 200 100 appear consistent with stable plate motion, but 0 newly identiﬁ ed magnetic anomalies tell a differ- –100 –200 ent story. In this work we present evidence that a 100 km dramatic spreading rate decrease occurred along 200 the SWIR ca. 24 Ma and, more generally, that a 100 0 major change in spreading rate can happen with- –100 out apparent change in spreading direction. We Magnetic anomaly (nT) –200 also discuss this event within the context of global 3 29 km/m.y. plate motion at the time, the possibility of “event 4 propagation” along plate tectonic boundaries, and 5

Depth (km) 6 the correlation of the spreading rate change at the A21 A18 A13 A8 A6 A6 A8 A13 A18 A21 SWIR with global plate motion events. Figure 1. Magnetic anomaly profi le md47 compared with variable-spreading-rate synthetic anomaly profi les. Profi le md47 magnetic anomaly forms A21 to A8 correspond DATA ANALYSIS to synthetic profi le calculated with a spreading rate of 29 km/m.y. (bottom). The 15 km/m.y. synthetic profi le (top) matches profi le md47 A6 to A0. Shading indicates the period during This study uses previously collected magnetic which the spreading rate decrease occurred. Map shows location of profi le md47, Figure 3, anomaly profi les (Cannat et al., 2006; Hosford and Figure DR1 (see footnote 1). CIR—Central Indian Ridge; RTJ—Rodriguez triple junction; et al., 2003; Sauter et al., 2001; Sclater et al., SWIR—Southwest Indian Ridge; SEIR—Southeast Indian Ridge.

© 2008 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, March March 2008; 2008 v. 36; no. 3; p. 207–210; doi: 10.1130/G24270A.1; 4 fi gures; Data Repository item 2008054. 207 Schlich and Patriat, 1971). Using these two N 29 km/m.y. 14.5 km/m.y. 14.5 km/m.y. 29 km/m.y. S sets of anomaly identifi cations as anchors, we A profile mdop04_TA B profile md66 adjusted the intervening spreading rate to obtain 200 synthetic anomaly forms that closely resemble 100 observed forms for the remaining time periods 0 -100 (A18–A5). The rate adjustments were made by -200 comparing the synthetic model to the 1100-km- C profile ata9510_TR D profile th99a long profi le md47, the only long profi le oriented 200 perpendicular to the spreading axis that does 100 not cross discordant zones (Fig. 1). For ages 0 -100 ca. 33–26 Ma (A13–A8), the form of the md47 -200 anomalies corresponds to the synthetic profi le 200 Magnetic anomaly (nT) with a spreading rate of 29 km/m.y. For ages 100 <20 Ma (A6 and younger), the 15 km/m.y. syn- 0 -100 thetic profi le is a much better match. We were -200 then able to validate the variable spreading rate A 18 A 13 A 8 A 6 A 5A 5 A 6 A 8 A 13 A 18 model at shorter, ridge-perpendicular profi les 3 (Fig. 2). The synthetic anomaly forms match 4 the observed anomalies well, particularly the 5 A13–A7 sequence, the most diffi cult to identify Depth (km) 6 500400 300 200 100 0 0 100 200 300 400 500 along the SWIR. Having established the validity Distance from the axis (km) of our variable spreading rate model with obser- vations along well-oriented profi les, we used it Figure 2. Magnetic anomaly identifi cations along four profi les (see Fig. 3 for locations) using to identify A6, A8, and A13 along the remaining variable rate model (bottom): 29 km/m.y. for A21 to A6C (24 Ma) and 14.5 km/m.y. for A6 to A1 (present). Shading and model as in Figure 1. profi les compiled between the Prince Edward and Melville fracture zones, making many new or improved identifi cations (Fig. 3). once rotated, fall in the same segment on the ridges at each of the triple junctions (SEIR and SPREADING RATE CALCULATION: conjugate plate, indicating that the plate motion SMAR) remained roughly constant (Patriat FROM SLOW TO ULTRASLOW determination is self-consistent (Fig. 3). and Segoufi n, 1988; Shaw and Cande, 1990). To obtain the spreading rate and direction for Spreading rate and direction were calculated Given these conditions, the velocity triangles the periods A13–A8 and A6–A5, we determined using the stage poles (A13–A8 and A6–A5) for both triple junctions indicate that large the fi nite rotation poles for A13, A8, and A6 deduced from the fi nite rotation poles. For the changes in spreading direction on the CIR (see the GSA Data Repository1) and used a pre- section of the SWIR near longitude 51°E, the and SAAR should have resulted from the 50% viously calculated pole for A5 (Lemaux et al., spreading direction for the period between A13 decrease in spreading rate of the SWIR, a pre- 2002). Finite rotation poles were calculated by and A8 (N16°E) varies only 13° from that cal- diction that appears to be supported by kine- superposing sets of conjugate anomaly identifi - culated for A6 to A5 (N3°E). In sharp contrast matic reconstruction and the satellite-derived cations at as great a distance as possible along to this small change in the spreading direc- gravity map of Smith and Sandwell (1997) the ridge to produce well-constrained poles that tion, the spreading rate for this same section (Fig. 4). Trends of the Egeria fracture zone and accurately describe plate motion without the use decreases ~50%, from 29.5 km/m.y. for the other major fracture zones at this time at the of fracture zone trends (Patriat and Segoufi n, period A13–A8 to 14.2 km/m.y. between A6 CIR shift from WSW-ENE to SW-NE. Spread- 1988). We chose one set of conjugate anomaly and A5, the present ultraslow spreading rate. ing direction deduced from CIR fi nite poles identifi cations close to the Melville fracture zone Assuming that the change in rate was instan- changed from N87°E to N56°E (Patriat and and another at a distance of more than 2000 km, taneous at some time after A8 and before A6, Segoufi n, 1988). The SAAR spreading direc- near the Prince Edward fracture zone. The accu- we calculate that it occurred at 24.2 Ma, A6C tion changed from NW-SE to E-W just before racy of the resulting fi nite poles is determined time (see GSA Data Repository). The close cor- A6 time (20 Ma) when the ridge broke into by calculating 95% confi dence ellipses for all respondence between the data and the synthetic very short segments offset by long transform identifi cations using an accepted method (Royer anomaly forms, which were calculated with an faults (Barker and Lawver, 1988). and Chang, 1991) (Fig. 3; see GSA Data Repos- instantaneous spreading rate change (Fig. 2), Note that the Bouvet triple junction veloc- itory for parameters of confi dence ellipses). The lends support to this assumption. ity triangle has been constructed using rates error ellipses indicate a high degree of accuracy. and directions deduced from the rotation poles The poles produce satisfactory superposition EVIDENCE FROM TRIPLE determined here and so does not take into of magnetic anomalies: Identifi cations made JUNCTION EVOLUTION account postulated relative motion on the Nubia- within a single spreading segment on one plate, At either end of the SWIR is a triple junc- Somalia boundary (Lemaux et al., 2002; Royer tion of divergent plate boundaries, the Rodri- et al., 2006). Our new A13, A8, A6, and A5 guez triple junction to the east where the SWIR identifi cations west of the Andrew Bain fracture 1GSA Data Repository item 2008054, supplemen- tary methods detailing the synthetic model calcula- meets the Central Indian Ridge (CIR) and the zone fi t unexpectedly well with their conjugate tion, time of spreading rate change calculation, fi nite Southeast Indian Ridge (SEIR) and the Bouvet identifi cations when rotated about the newly poles and confi dence ellipse parameters, and new triple junction to the west where it meets the calculated poles. Thus, we fi nd no evidence of magnetic anomaly identifi cations west of Andrew Southern Mid-Atlantic Ridge (SMAR) and the signifi cant differential motion between the east Bain fracture zone, is available online at www. geosociety.org/pubs/ft2008.htm, or on request from South American–Antarctic Ridge (SAAR). and west of the Andrew Bain fracture zone for [email protected] or Documents Secretary, During the period between A13 and A5, the ages ≥11 Ma. This discrepancy with previous GSA, P.O. Box 9140, Boulder, CO 80301, USA. spreading rate and direction of the fastest fi ndings may be explained by differing iden-

208 GEOLOGY, March 2008 36°E 40°E 44°E 48°E 52°E 56°E 60°E 64°E A B CIR A8-A13 a CIR Magnetic Ind. Aus. Anomalies b SWIR 28°S 28°S SMAR Pl. CIR A5-A6 R A6 S. Am. SEI c African SEIR A8 Plate Pl. c SWIR SMAR A13 Scotia Pl. RTJ trace RTJ trace Antarctic Plate A5-A6 SAAR A8-A13 SWIR 32°S 32°S SAAR md47 C 60°E 62°E 64°E 66°E 68˚E 20°S 20°S b FZ

Indomed FZ C IR 36°S 36°S 22°S 22°S FZ d Melville

Novara 24°SA6 24°S

II FZ A8 Discovery FZ 40°S 40°S 60°E 62°E 64°E 66°E 68°E

Z D 18°W 15°W 12°W 9°W 6°W 3°W 0° Gauss FZ Atlantis 60°W 50°W 40°W Eric Simpson FZ azelle FZ Marion FZ Prince Edward FZ G Finite 57°S 57°S rotation allieni F 20°N poles G A5 A6 44°S A8 44°S A13 60°S 60°S 10°N

18°W 15°W 12°W 9°W 6°W 3°W 0° South America 0° Figure 4. Predicted and observed changes 48°S 48°S 36°E 40°E 44°E 48°E 52°E 56°E 60°E 64°E at the Rodriguez and Bouvet triple junctions in response to rate change at the SWIR. Figure 3. Magnetic anomaly identifi cation A6, A8, and A13 plotted on the Southwest Indian A: Location map. The Rodriguez triple junc- Ridge tectonic map. Background bathymetry derived from satellite sea-surface altimeter tion is to the east where the SWIR meets the (Smith and Sandwell, 1997). Anomaly identifi cations appear as solid symbols, and conju- CIR and the SEIR. The Bouvet triple junction is gate rotated anomaly identifi cations appear as open symbols. Dashed lines indicate location to the west where the SWIR meets the SMAR of profi le md47 and Figure 2 profi les. a—profi le mdop04_TA; b—profi le md66; c—profi le and the SAAR. B: Triple junction velocity tri- ata9510_TR; d—profi le th99a; FZ—fracture zone; RTJ—Rodriguez triple junction. Inset angles predict spreading direction change shows location of fi nite rotation poles and 95% confi dence ellipse for Africa-Antarctica rela- at the CIR and SAAR, constant plate motion tive plate motions. Poles and ellipses for A6, A8, and A13 were calculated for this work; the for the SMAR and SEIR assumed. The period A5 pole and ellipse are from Lemaux et al. (2002). A13–A8 is shown in red and A6–A5 in black. C and D: Evidence for plate motion change between A8 and A6 in satellite-derived gravity maps of CIR (C) and SAAR (D). CIR—Central tifi cations of A5 along profi les on the African Several important events occurred on the Indian Ridge; SEIR—Southeast Indian Ridge; plate: We interpret as A5B the anomalies previ- global plate boundary system between 30 and SWIR—Southwest Indian Ridge; SMAR— ously identifi ed as A5 (see Fig. DR1). 20 Ma. Rifting began separating the African Southern Mid-Atlantic Ridge; SAAR—South American–Antarctic Ridge. and Arabian plates at ca. 30 Ma, followed DOES THE SWIR SPREADING RATE by the opening of the Gulf of Aden along CHANGE CORRESPOND TO A GLOBAL the Sheba Ridge, which began spreading at PLATE TECTONIC EVENT? <20 Ma (d’Acremont et al., 2006). Silver et al. the CIR reaches successively the Carlsberg Our fi ndings indicate that a change in plate (1998) proposed an abrupt slowing of absolute and Sheba Ridges. A dramatic spreading rate motion occurred along the SWIR ca. 24 Ma: a motion of the African plate when it collided increase occurred at the Carlsberg Ridge, the 15 km/m.y. decrease of the already slow spread- with Eurasia at ca. 30 Ma. A major PME took boundary between India and Africa (Mercuriev ing rate, but no signifi cant change of spreading place in the Pacifi c at ca. 24 Ma: The Farallon et al., 1996), in conjunction with the opening of direction. It is worth noting that another similar plate split to form the Nazca and Cocos plates the Gulf of Aden along the Sheba Ridge. Trans- plate motion event (PME), change in spreading (Handschumacher, 1976), accompanied by a mission of the SWIR PME appears to dissipate rate, but not direction, has been observed in the marked change in spreading direction appar- along this nascent African-Arabian boundary. Indian Ocean at A31 time (ca. 68 Ma) when ent in both magnetic anomalies and fracture Transmission via the SAAR leads to the Scotia the spreading rate became ultrafast along the zone trends. In the northern Pacifi c the spread- plate boundaries, composed, at A6C time, of a SEIR (Patriat, 1987). While marked changes ing rate decreased dramatically (Cande and complex and evolving system of divergent and in plate boundary geometry and plate motion Kent, 1992), while in the southwestern Pacifi c convergent boundaries (Engles et al., 2005; are observed at the CIR and SAAR, we fi nd the spreading direction changed along the Livermore and Woollett, 1993). As yet we have that little or no change occurred on the SEIR Macquarie Ridge (Cande and Stock, 2004). no direct evidence that the PME was transmit- or the SMAR at the time of the SWIR event, Are these events related to the SWIR PME? ted beyond the boundaries of the Scotia plate to indicating a PME may occur along a particular The answer lies in the tracing of the path of the boundaries of the Pacifi c plate. Ultimately, path affecting some plate boundaries but not plate motion change along the adjoining net- transmission of a PME may be absorbed when others. Whether the SWIR PME was part of a work of plate boundaries and evaluating it in the event path reaches diffuse plate boundaries global PME occurring at ca. 24 Ma can only be light of global absolute plate motion. that typically surround small plates (Gordon, assessed by analyzing evidence of transmission The path of the SWIR PME can be traced 1998), which have been referred to as “buffer along adjoining plate boundaries. along the CIR and SAAR. Transmission via plates” by Anderson (2002).

GEOLOGY, March 2008 209 It is diffi cult to imagine that events in the Barker, P.F., and Lawver, L.A., 1988, South Planetary Science Letters, v. 117, p. 475–495, Pacifi c occurred independently of those in the American–Antarctic plate motion over the doi: 10.1016/0012-821X(93)90098-T. past 50 Ma, and the evolution of the South Mercuriev, S., Patriat, P., and Sochevanova, N., 1996, Indian Ocean. The hypothesis of a relationship American–Antarctic Ridge: Geophysical Evolution de la dorsale de Carlsberg: Evidence between these events assumes a common origin Journal International, v. 94, p. 377–386, doi: pour une phase d’expansion très lente entre 40 for the postulated global event and the possi- 10.1111/j.1365-246X.1988.tb02261.x. et 25 Ma (A18 à A7): Oceanologica Acta, v. 19, bility of its transmission taking several mil- Bergh, H.W., and Norton, I.O., 1976, Prince Edward p. 1–13. lion years. The origin could be the collision of fracture zone and the evolution of the Mozam- Molnar, P., Pardo-Casas, F., and Stock, J., 1988, bique Basin: Journal of Geophysical Research, The Cenozoic and Late Cretaceous evolu- Africa with Eurasia. The sudden halt of African v. 81, p. 5221–5239. tion of the Indian Ocean: Uncertainties in the absolute motion in combination with continuing Cande, S.C., and Kent, D.V., 1992, A new geomag- reconstructed positions of the Indian, African unchanged spreading at the SMAR led to a rapid netic timescale for the Late Cretaceous and and Antarctic plates: Basin Research, v. 1, increase in the westward absolute motion of the Cenozoic: Journal of Geophysical Research, p. 23–40. v. 97, p. 13,917–13,951. Patriat, P., 1987, Reconstitution de l’évolution du South American plate and deformation along its Cande, S.C., and Kent, D.V., 1995, Revised calibra- système de dorsales de l’océan Indien par la western leading edge (Silver et al., 1998), result- tion of the geomagnetic polarity timescale for méthode de la cinématique des plaques: Paris, ing in relative motion change at this boundary. the Late Cretaceous and Cenozoic: Journal of Terres Australes et Antarctiques Françaises, The potential for tracing a continuous PME path Geophysical Research, v. 100, p. 6093–6095, 308 p. from the SWIR PME to Pacifi c reorganization doi: 10.1029/94JB03098. Patriat, P., and Segoufi n, J., 1988, Reconstruction Cande, S.C., and Stock, J.M., 2004, Cenozoic recon- of the Central Indian Ocean: Tectonophysics, is intriguing and remains to be confi rmed by structions of the Australian–New Zealand– v. 155, p. 211–234, doi: 10.1016/0040-1951 global kinematic reconstruction at ca. 24 Ma. South Pacifi c sector of Antarctica, in Exon, (88)90267-3. N., et al., eds., The Cenozoic Southern Ocean: Royer, J.Y., and Chang, T., 1991, Evidence for CONCLUSIONS Tectonics, sedimentation and climate change relative plate motions between the Indian and between Australia and Antarctica: American Australian plates during the last 20 m.y. from Our fi ndings indicate a decrease in the SWIR Geophysical Union Geophysical Monograph plate tectonic reconstructions: Implications spreading rate from slow to ultraslow at ca. 24 Ma 151, p. 5–18. for the deformation of the Indo-Australian with no signifi cant change in spreading direction. Cannat, M., Sauter, D., Mendel, V., Ruellan, E., plate: Journal of Geophysical Research, v. 96, A path of correlative changes in plate motions Okino, K., Escartin, J., Combier, V., and p. 11,779–11,802. can be traced along adjoining plate boundaries Baala, M., 2006, Modes of seafl oor genera- Royer, J.Y., Gordon, R.G., and Horner-Johnson, tion at a melt-poor ultraslow-spreading ridge: B.C., 2006, Motion of Nubia relative to in both directions, although only two of the four Geology, v. 34, p. 605–608, doi: 10.1130/ Antarctica since 11 Ma: Implications for Nubia- immediately adjacent boundaries show any sig- G22486.1. Somalia, Pacifi c–North America, and India- nifi cant adjustment. This absence of evidence d’Acremont, E., Leroy, S., Maia, M., Patriat, P., Eurasia motion: Geology, v. 34, p. 501–504, of plate motion change in the SWIR fracture Beslier, M.-O., Bellahsen, N., Fournier, M., doi: 10.1130/G22463.1. and Gente, P., 2006, Structure and evolution of Sauter, D., Patriat, P., Rommevaux-Jestin, C., zone trends and in spreading at the SMAR and the eastern Gulf of Aden: Insights from mag- Cannat, M., Briais, A., Bergh, H., Boulanger, SEIR may have disguised the importance of this netic and gravity data (Encens-Sheba MD117 D., Deplus, C., Grindlay, N., Isezaki, N., event. Are there common characteristics for plate cruise): Geophysical Journal International, Mendel, V., Mével, C., Thibaud, R., Tisseau, boundaries that remain stable while their triple v. 165, p. 786–803, doi: 10.1111/j.1365-246X. C., Vanney, J.-R., Whitechurch, H., and Yama- junction neighbors respond to PMEs by reorga- 2006.02950.x. moto, M., 2001, The Southwest Indian Ridge Engles, G., Livermore, R., Fairhead, J.D., between 49°15′E and 57°E: Focused accretion nization and plate motion change? and Morris, P., 2005, Tectonic evolution and magma redistribution: Earth and Planetary It seems likely that the SWIR PME is part of of the west Scotia Sea: Journal of Geo- Science Letters, v. 192, p. 303–317. a global event that may have been initiated by physical Research, v. 110, B02401, doi: Schlich, R., and Patriat, P., 1971, Mise en évidence the collision of Africa with Eurasia beginning 10.1029/2004JB003154. d’anomalies magnétiques axiales sur la branche Fisher, R.L., and Sclater, J.G., 1983, Tectonic evolu- ouest de la dorsale médio-indienne: Comptes at ca. 30 Ma. The apparent cause-and-effect tion of the Southwest Indian Ocean since the Rendus de l’Académie des Sciences Série IIa: link between events occurring at 30 Ma and Mid Cretaceous: Plate motions and stability Sciences de la Terre et des Planètes, v. 272, 24 Ma raises another question: What is the of the pole of Antarctica/Africa for the last p. 700–703. duration of the transition from an assumed 80 Myr: Geophysical Journal of the Royal Sclater, J.G., Munschy, M., Fisher, R.L., Weatherall, global event initiated by collision to the fi nal Astronomical Society, v. 73, p. 553–576. P.A., Cande, S.C., Patriat, P., Bergh, H., and Gordon, R.G., 1998, The plate tectonic approxima- Schlich, R., 1997, Geophysical synthesis of the new stable plate motion phase? tion: Plate nonrigidity, diffuse plate bound- Indian/Southern Oceans, part 1: The Southwest Study and comparison of global PMEs may aries, and global plate reconstructions: Annual Indian Ocean, Scripps Institution of Oceanog- represent a new and challenging direction lead- Review of Earth and Planetary Sciences, raphy Reference Series 97-06: San Diego, La ing to a better understanding of the driving v. 26, p. 615–642, doi: 10.1146/annurev.earth. Jolla, Scripps Institution of Oceanography, 26.1.615. University of California, p. 45. forces of plate tectonics. Handschumacher, D.W., 1976, Post-Eocene plate Shaw, P.R., and Cande, S.C., 1990, High-resolution tectonics of the eastern Pacifi c: American Geo- inversion for South Atlantic plate kinematics ACKNOWLEDGMENTS physical Union Geophysical Monograph 19, using joint altimeter and magnetic data: Journal This study was made possible by data collected p. 177–202. of Geophysical Research, v. 95, p. 2625–2644. during transits of R/V Marion-Dufresne. We are grate- Hosford, A., Tivey, M., Matsumoto, T., Dick, Silver, P.G., Russo, R.M., and Lithgow-Bertelloni, ful to Institut Paul Emile Victor and the chief scientists H., Schouten, H., and Kinoshita, H., 2003, C., 1998, Coupling of South American and for their cooperation, especially B. Ollivier for collect- Crustal magnetization and accretion at the African plate motion and plate deformation: ing the data. Most of the profi les west of 45°E come Southwest Indian Ridge near the Atlantis II Science, v. 279, p. 60–63. from the data set of H. Bergh. M. Tivey provided data fracture zone, 0–25 Ma: Journal of Geophysi- Smith, W.H., and Sandwell, D.T., 1997, Global from surveys east of the Atlantis fracture zone. We cal Research, v. 108, p. EMP 9-1–EMP 9-23, seafl oor topography from satellite altim- also thank Steve Cande for insightful discussion and 10.1029/2001JB000604. etry and ship depth soundings: Science, the reviewers, Anne Briais and Sharon Mosher, for Lemaux, J., Gordon, R.G., and Royer, J.Y., 2002, v. 277, p. 1956–1962, doi: 10.1126/science. their thoughtful critique and helpful comments. Insti- Location of the Nubia-Somalia boundary along 277.5334.1956. tut de Physique du Globe contribution 2300. the Southwest Indian Ridge: Geology, v. 30, p. 339–342, doi: 10.1130/0091-7613(2002)030 Manuscript received 11 July 2007 REFERENCES CITED <0339:LOTNSB>2.0.CO;2. Revised manuscript received 5 November 2007 Anderson, D.L., 2002, How many plates?: Geology, Livermore, R.A., and Woollett, R.W., 1993, Seafl oor Manuscript accepted 6 November 2007 v. 30, p. 411–413, doi: 10.1130/0091-7613 spreading in the Weddell Sea and southwest (2002)030<0411:HMP>2.0.CO;2. Atlantic since the Late Cretaceous: Earth and Printed in USA

210 GEOLOGY, March 2008