Breakup and early seafloor spreading between India and Antarctica

Abstract

SUMMARY We present a tectonic interpretation of the breakup and early seafloor spreading between India and Antarctica based on improved coverage of potential field and seismic data off the east Antarctic margin between the Gunnerus Ridge and the Bruce Rise. We have identified a series of ENE trending Mesozoic magnetic anomalies from chron M9o (∼130.2 Ma) to M2o (∼124.1 Ma) in the Enderby Basin, and M9o to M4o (∼126.7 Ma) in the Princess Elizabeth Trough and Davis Sea Basin, indicating that India–Antarctica and India–Australia breakups were roughly contemporaneous. We present evidence for an abandoned spreading centre south of the Elan Bank microcontinent; the estimated timing of its extinction corresponds to the early surface expression of the Kerguelen Plume at the Southern Kerguelen Plateau around 120 Ma. We observe an increase in spreading rate from west to east, between chron M9 and M4 (38–54 mm yr–1), along the Antarctic margin and suggest the tectono‐magmatic segmentation of oceanic crust has been influenced by inherited crustal structure, the kinematics of Gondwanaland breakup and the proximity to the Kerguelen hotspot. A high‐amplitude, E–W oriented magnetic lineation named the Mac Robertson Coast Anomaly (MCA), coinciding with a landwards step‐down in basement observed in seismic reflection data, is tentatively interpreted as the boundary between continental/transitional zone and oceanic crust. The exposure of lower crustal rocks along the coast suggests that this margin formed in a metamorphic core complex extension mode with a high strength ratio between upper and lower crust, which typically occurs above anomalously hot mantle. Together with the existence of the MCA zone this observation suggests that a mantle temperature anomaly predated the early surface outpouring/steady state magmatic production of the Kerguelen LIP. An alternative model suggests that the northward ridge jump was limited to the Elan Bank region, whereas seafloor spreading continued in the West Enderby Basin and its Sri Lankan conjugate margin. In this case, the MCA magnetic anomaly could be interpreted as the southern arm of a ridge propagator that stopped around 120 Ma.

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“…The western profile confirms an extremely stretched crystalline continental crust, which thins to 7 km thickness (plus 4 km sediments on top), from the shelf edge to the location of the MCA. It is interesting to note that apart from a few scattered observations close to the marked COBs, major SDR sequences do not seem to exist on the Enderby Land margin (Stagg et al, , 2005, suggesting the lack of a mantle plume at the time of breakup at about 130 Ma (Gaina et al, 2007).…”

“…Most other margins seem to have been formed by processes similar to those proposed for other nonvolcanic margins, such as the IberianNewfoundland conjugate margins (e.g., Sibuet et al, 2007) and the Nova Scotia margin (e.g., Funck et al, 2003), which exhibit widely extended, thinned, and block-faulted continental crust. Whether any updoming of lower continental crust is a possible process, as Gaina et al (2007) suggest for the Enderby Land margin, is difficult to assess because of a lack of drill information.…”

“…The characteristics of the margin off central and eastern Prydz Bay is affected by both the inherited structure of the Paleozoic-Mesozoic Lambert Rift system as well as by magmatic events of the Kerguelen Plateau LIP, probably postdating the initial India-Antarctica breakup by about 10 million to 15 million years (Gaina et al, 2007). Guseva et al (2007) proposed a direct connection between the volcanic Southern Kerguelen Plateau crust and stretched continental crust of Wilhelm II Land.…”

“…Their defined COB lies between 100 km and 170 km oceanward of the shelf edge. Gaina et al (2007) identified spreading anomalies from M0 to M9 east of Gunnerus Ridge from relatively sparse shipborne magnetic data. Most of both magnetic and seismic profiles do not parallel the spreading flow lines and may be biased by the structure and signal of crossing fracture zones.…”

“…Gaina et al (2007) developed a breakup model between India and this East Antarctic sector based on compiled magnetic data of the southernmost Indian Ocean and the structures of the continental margin interpreted from the seismic data. Despite the large amount of high-quality seismic reflection data, the definition of the COB or COT is equivocal due to the lack of deep crustal seismic refraction data with the exception of nonreversed sonobuoy data.…”