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Refined Spreading History at the Southwest Indian Ridge for the Last Refined spreading history at the Southwest Indian Ridge for the last 92 Ma, with the aid of satellite gravity data Armelle Bernard, Marc Munschy, Yair Rotstein, Daniel Sauter To cite this version: Armelle Bernard, Marc Munschy, Yair Rotstein, Daniel Sauter. Refined spreading history at the Southwest Indian Ridge for the last 92 Ma, with the aid of satellite gravity data. Geophysi- cal Journal International, Oxford University Press (OUP), 2005, 162, pp.765-778. 10.1111/j.1365- 246X.2005.02672.x. hal-00104269 HAL Id: hal-00104269 https://hal.archives-ouvertes.fr/hal-00104269 Submitted on 13 Feb 2021 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Geophys. J. Int. (2005) 162, 765–778 doi: 10.1111/j.1365-246X.2005.02672.x Refined spreading history at the Southwest Indian Ridge for the last 96 Ma, with the aid of satellite gravity data A. Bernard,1 M. Munschy,1 Y. Rotstein1,2 and D. Sauter1 1Ecole et Observatoire des Sciences de la Terre, Universite Louis Pasteur, 5 rue Rene Descartes, 67084 Strasbourg Cedex, France. E-mail: [email protected] 2The Geophysical Institute of Israel, Holon, Israel Accepted 2005 April 27. Received 2004 July 15; in original form 2003 October 10 Downloaded from https://academic.oup.com/gji/article/162/3/765/2098160 by guest on 13 February 2021 SUMMARY The spreading history of the oceans is modelled mostly by using magnetic anomalies and the fracture zone geometry. The high-quality, satellite-derived gravity data, that became available in recent years, reveal the details of fracture zones, which can be used as flow lines to control spreading models. We have applied this approach to the Southwest Indian Ridge (SWIR) in order to refine its spreading history. This is particularly useful for the period of complex spreading between magnetic anomalies 33 and 23, where the magnetic anomalies alone cannot resolve the detailed spreading history. We find four main stages in the spreading history of the SWIR since 96 Ma, including two that were not noted previously, between 96 Ma and anomaly 33 (76.3 Ma) and between anomalies 23o (51.7 Ma) and 18o (40.1 Ma; o denotes old boundaries of normal magnetization period). We also find that the start of the period of complex spreading was at anomaly 33, somewhat earlier than previously proposed. We discuss the characteristics of the extension that the old transform faults underwent during the complex GJI Marine geoscience spreading phase, in response to the counterclockwise rotation of spreading. New transform faults appeared at that time, considerably widening the transform zones. Key words: Indian Ocean, Southwest Indian Ridge, spreading, transform faults. a long period of very slow spreading, which extend to the present INTRODUCTION time. Consequently, previous, simple, single-rotation pole models The sea floor morphology of the Southwestern Indian Ocean is dom- were replaced by a series of rotations representing a rather complex inated by the Southwest Indian Ridge (SWIR), which extends for spreading history, with significant changes in the direction and rate some 7700 km between the Bouvet triple junction at 55◦S, 0.5◦W of spreading (Patriat et al. 1985; Patriat & S´egoufin1988; Royer and the Rodrigues triple junction at 25◦S, 70◦E(Fig. 1). Spread- et al. 1988). These models were mostly based on the identification ing at the axis of the SWIR started 165 Ma ago with the breakup of the magnetic anomalies and less on the use of fracture zones between Africa and Antarctica (Livermore & Hunter 1996). The as indicators of flow lines of the plate motion. This approach was early development of the Southwest Indian Ocean, from breakup to adopted since the fracture zones, although quite prominent in places, anomaly 34 [83 Ma on the magnetic timescale of Cande & Kent appeared to be quite linear, in a marked contrast with the magnetic (1995)] was poorly resolved until recently. Recent works (Marks & anomalies, which clearly detailed a complex spreading history at the Tikku 2001; Tikku et al. 2002) significantly improved the under- ridge. Some of these works (Patriat et al. 1985; Patriat & S´egoufin standing of the early accretion history and, in particular, resolved 1988) have generally used fracture zones as an overall guide and the overlap problem of Madagascar Plateau. They also defined the placed more importance on their trend, only for periods where location of Madagascar with respect to Africa and Antarctica. In the magnetic anomalies proved to be insufficient for determining contrast, the post-chron 34 evolution of the SWIR appeared to be rotation parameters. They noted some inconsistencies and misfits in well constrained by the early studies of the region (Norton & Sclater their models, but related them to fundamental geological processes, 1979; Patriat 1979; Tapscott et al. 1980; Sclater et al. 1981; Fisher such as regional plate deformation and not to a lack of data. & Sclater 1983; Martin & Hartnady 1986). These generally pro- Royer et al. (1988) were the first to use the earliest satellite al- posed that a single pole of rotation describes the motion between timetry data to improve the fracture zone geometry along the SWIR. Africa and Antarctica during the entire period from anomaly 34 to However, the satellite profiles were not dense enough for precisely the present. When additional magnetic data were collected in the following the traces of conjugate fracture zones, particularly in the region, it became apparent that spreading at the SWIR was not con- complex area between 25◦E and 35◦Ewhere the original trend of stant, but rather quite complex, with periods of rapid spreading and the transform faults might have been overprinted by a younger trend. C 2005 RAS 765 766 A. Bernard et al. 0° 10° 20° 30° 40° 50° 60° 70° -20° -30° -40° Downloaded from https://academic.oup.com/gji/article/162/3/765/2098160 by guest on 13 February 2021 -50° -60° Central Indian Ridge Rodrigues Triple ° -20 Junction AFRICA MADAGASCAR Melville Novara -30° Atlantis Gallieni Southeast Indomed Indian Ridge Discovery -40° PE M Mid Atlantic ES Ridge Bouvet Triple Junction ° -50 Southwest AB Indian Ridge -60° American Antarctic Ridge Enderby Basin ANTARCTICA 0° 10° 20° 30° 40° 50° 60° 70° Figure 1. Ver tical gradient gravity map (VDGM; see Rotstein et al. 2001) of the SWIR, based on the free-air gravity data of the Geosat and ERS1 satellites (Sandwell & Smith 1997) showing in detail the transform fault and fracture zone pattern of the SWIR. Also marked on the map are the locations of the bathymetric features discussed in the text. AB = Andrew Bain FZ; M = Marion FZ; PE = Prince Edward FZ and ES = Eric Simpson FZ. C 2005 RAS, GJI, 162, 765–778 Refined spreading history at the Southwest Indian Ridge 767 Using Seasat data was clearly an important step towords improv- data coverage is not uniform throughout the region and particularly ing the details of the tectonic fabric of the world ocean floor (e.g. south of the SWIR, in the Enderby Basin, new data were required to Sandwell & Schubert 1982; Gahagan et al. 1988). However, the qual- better constrain the reconstruction models [Fig. 2, and also compare ity of the maps at that time was not sufficient to reveal the details with fig. 1 in Royer et al. (1988)]. of the complex pattern of fracture zones associated with the SWIR In the IODCP database, all the magnetic anomaly profiles were because of their dense distribution and nonlinear nature. Since then, treated by removing the appropriate IGRF (Mandea et al. 2000) and the Geosat and ERS1 altimeter missions enabled the construction by filtering out the long wavelength anomalies. Only clear anomalies of a uniform and dense 2 gravity anomaly grid (McAdoo & Marks were picked, wherever possible, at intervals of about 10 Ma. These 1992; Sandwell & Smith 1997). The 2 satellite-derived free air grav- include for the Late Cretaceous—Present, anomalies 1, 3y, 5o, 6o, ity map now unveils many details of the structure and segmentation 13y, 18o, 20o, 21o, 22o, 23o, 24o, 25y, 26o, 29o, 31y, 32y, 33y, 33o of the SWIR (Marks et al. 1993; Fig. 1). All the fracture zones that and 34y (Fig. 2 gives the chron/age correspondence; y and o denote were previously observed as bathymetric features (Fisher & Good- the young and old boundaries of the normal magnetization period, willie 1997) are apparent on this gravity map and can now be traced respectively). with significantly more detail and accuracy. Additional small frac- ture zones can now be recognized between the main fracture zones, Satellite altimetry data Downloaded from https://academic.oup.com/gji/article/162/3/765/2098160 by guest on 13 February 2021 achieving a higher resolution of bathymetric detail. Since this work was done, a 1 satellite grid became available but is not expected to Recent satellite altimetry data (Sandwell & Smith 1997), which change the results that are presented in this work. combine Geosat and ERS1 data, represent a significant improve- The availability of this new data set and the observation that the ment as compared to the bathymetric data, mainly due to the uni- details of the accretion on the SWIR since the Late Cretaceous can be form and close spacing of the satellite tracks.
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