|Spreading center type:||Large-scale extinct MOR|
|Time of cessation:||Ca. 79.075 Ma, Chron 33n (estimated from magnetic anomalies identified by Segoufin et al., 2004)|
|Subsequent active spreading center:||Probably the present-day configuration of the Southwest Indian Ridge|
|Cessation style:||Not known|
|Later deformation or volcanism:||None evident|
The precise locations of spreading centers between Africa, Antarctica and India/Australia during the Cretaceous are difficult to confirm due to the influence of the Cretaceous Normal Superchron (CNS) and the presence of thick volcanic and sedimentary sequences to the north of Antarctica and, south and west of Madagascar. While it has been demonstrated that the Southwest Indian Ridge has been continuously active since early stages of Gondwanan dispersal (Leinweber and Jokat, 2012), there is more controversy in the region between 35° and 70° East, especially at latitudes south of 47° S, where the tectonic fabric of the seafloor suggests that several reorganizations may have taken place (Nogi et al., 2004).
Nogi et al. (2011) presented a conference report of continental rocks dredged from the Ob seamount, a volcanic feature that is found to the southeast of the Conrad Rise. A further conference paper entitled the “Petrography and provenance of granitic and sedimentary rocks dredged from the Conrad Rise in the southern Indian Ocean” was presented by Kazuya Kobayashi, Hideo Ishizuka, Yoshifumi Nogi, Hiroshi Sato, Nobuhiko Nakano, Tatsuro Adachi, Yasuhito Osanai, at the Fourth Symposium of Polar Science in Japan, 2013, yet the abstract is unavailable and no publications have been forthcoming. Therefore, there is some suggestion of continental rocks in this part of the Indian Ocean that has formerly been considered to have formed primarily by oceanic spreading and the eruption of large-igneous provinces.
Some studies have noted that the gravity signal of the Ob seamount is unusual and that it is difficult to interpret the crustal structure of this feature (Diament and Goslin, 1986; Nogi et al., 2004). This could be due to the presence of a continental core within the seamount. A stranded continental fragment in this location requires that an additional spreading centre was active between the Antarctic and African/Madagascar plates, rather than a single active ridge operating in the Southwest Indian Ridge throughout development of the southwest of the Indian Ocean, as has previously been proposed (McKenzie and Sclater, 1971; Norton and Sclater, 1979; Marks and Tikku, 2001; Bernard et al. 2005; Leinweber and Jokat, 2012). Similar ridge jumps are now known to have isolated continental slithers in the east, at the Elan Bank and South Kerguelen continental fragments (Gaina et al., 2007; Gibbons et al., 2012), and this style of reorganization is consistent with a complex opening of the southern Indian Ocean during Gondwana dispersal.
Desa et al. (2006) observed significant asymmetry of crust formed during chron C34 between the southern and northern sides of the plate boundary and their interpretation of magnetic anomalies in the southern Indian Ocean require a plate reorganization in the mid-Cretaceous, resulting in the spreading axis remaining in close proximity to the seamounts that make up the Conrad Rise. Cross et al. (2011) proposed an extinct ridge in the region between the Crozet Plateau and the Conrad Rise on the basis of an assessment of crustal thickness in this region, however, as yet no peer-reviewed study is available from their work.
There are several lines of evidence that support a complex plate boundary reorganization in the southwest of the Indian Ocean at ca. 85 Ma, including fracture zone orientations, reported continental fragments and some interpretations of the magnetic anomalies. Thickened volcanic plateaus in the region of the reorganization suggests the influence of a proximal hotspot, which was probably the Marion plume.
We review the linear, segmented and east-west trending feature that is approximately equidistant between the Conrad and Del Cano Rises. The segments have bathymetric relief of around 500 m, with axial troughs that are between 24 and 36 km wide. A negative gravity anomaly is present at each of the segments, ranging from 18 and 33 mGals in magnitude, peak to trough signal. Whilst not a definitive, these gravity lows are similar in form to many of our primary-tier, well-established extinct spreading centers, and are suggestive of an extinct ridge origin for this trough. This spreading center is assessed within our secondary tier of analysis and is thought to be very likely to have been an extinct spreading center.
Bernard, A. Munschy, M., Rotstein, Y. and Sauter, D., 2005, Refined spreading history at the Southwest Indian Ridge for the last 96 Ma, with the aid of satellite gravity data, Geophysical Journal International, v. 162, p. 765-778, doi: 10.1111/j.1365-246X.2005.02672.x.
Cross, A., Kusznir, N. and Alvey, A., 2011, Structure and origin of the Crozet Plateau and Conrad Rise, SW Indian Ocean: Insights from crustal thickness mapping using 3-D satellite gravity inversion, In, American Geophysical Union Conference Proceedings, Fall Meeting 2011, abstract #V51D-2544.
Desa M., Ramana, M.V. and Ramprasad, T., 2006, Seafloor spreading magnetic anomalies south off Sri Lanka, Marine Geology, v. 229, no. 3-4, p. 227–240.
Diament, M. and Goslin, J., 1986, Emplacement of the Marion Dufresne, Lena and 0b seamounts (South Indian Ocean) from a study of isostasy, Tectonophysics, v. 121, p. 253–262.
Gaina, C., Müller, R. D., Brown, B. Ishihara, T. and Ivanov, S., 2007, Breakup and early seafloor spreading between India and Antarctica, Geophysical Journal International, v. 170, no. 1, p. 151–169.
Kobayashi, K., Ishuzuka, H., Nogi, Y., Sato, H., Nakano, N., Adachi, T., Osanai, Y., 2013, Petrography and provenance of granitic and sedimentary rocks dredged from the Conrad Rise in the Southern Indian Ocean: Fourth Symposium on Polar Science, Abstract, OG-P13.
Leinweber, V. T., and Jokat, W., 2012, The Jurassic history of the Africa–Antarctica corridor—new constraints from magnetic data on the conjugate continental margins, Tectonophysics, v. 530, p. 87-101.
Marks, K. M., and Tikku, A. A., 2001, Cretaceous reconstructions of East Antarctica, Africa and Madagascar, Earth and Planetary Science Letters, v. 186, no. 3, p. 479-495.
McKenzie, D. and Sclater, J. G. 1971, The Evolution of the Indian Ocean since the Late Cretaceous, Geophysical Journal International, v. 24, no. 5, p. 437-528, doi: 10.1111/j.1365-246X.1971.tb02190.x.
Nogi, Y. et al., 2004, An Interpretation of the Seafloor Spreading History of the West Enderby Basin between Initial Breakup of Gondwana and Anomaly C34, Marine Geophysical Researches, v. 25, no. 3-4, p. 221–231.
Nogi, Y., Sato, H., Sato, T., Hnayu, T., Kobayashi, S., and Ishuzuka, H., 1996, Magnetic anomaly lineations and fracture zones deduced from vector magnetic anomalies in the West Enderby Basin, Geological Society, London, Special Publications, v. 108, no. 1, p. 265–273.
Nogi, Y. et al., 2013, Origin of the Conrad Rise in the Southern Indian Ocean and breakup process of Gondwana. In Proceedings of Antarctica. p. 33.
Norton, I. O. and Sclater, J. G., 1979, A model for the evolution of the Indian Ocean and the breakup of Gondwanaland, Journal of Geophysical Research, 84(B12), p. 6803-6830.
Segoufin, J., M. Munschy, P. Bouysse, and V. Mendel (2004), Map of the Indian Ocean, sheet 1: ‘Physiography’ sheet 2: ‘Structural map’ scale 1:20.000.000, Commission for the Geological Map of the World, Paris.