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 As a result, profi les exhibiting long published and unpublished magnetic data from the Southwest Indian Ridge, we calculated fi nite uninterrupted series of magnetic anomalies rotation poles for A13, A8, and A6, from which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 frequent, which tends to make the forms of A13, A8, and A6 more diffi cult to identify than INTRODUCTION 1997) and unpublished transit ship tracks.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 syn- thetic 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.