Australia: The Land Where Time Began

A biography of the Australian continent 

Gondwana Breakup - Antarctica-India-Australia

 In their paper the authors1 present evidence in support of their tectonic interpretation of breakup and early stages of seafloor spreading as India broke from Gondwana and Australia, the study being based on improved coverage of potential field and seismic data between the Gunnerus Ridge and the Bruce Rise, off the margin of east Antarctica. In the Enderby Basin they identified a series of magnetic anomalies, of Mesozoic age, that are ENE-trending, from chron M9o, about 130.2 Ma, to M2o, about 124.1 Ma, and in the Princess Elizabeth Trough and Davis Sea Basin, an M9o to M4o, about 126.7 Ma, that indicates that the separation of India from Antarctica and Australia occurred at roughly the same time. The evidence they provide indicates that south of the Elan Bank microcontinent is an abandoned spreading centre; the time of its extinction has been estimated to correspond to the early expression at the surface of the Kerguelen Plume about 120 Ma at the Southern Kerguelen Plateau. Between chron M9 and M4 the spreading rate increased from west to east, 38-54 mm/yr, along the margin of Antarctica, the authors1 suggesting the inherited structure of the crust, the kinematics of the fragmentation of Gondwana and the proximity of the Kerguelen hotspot, influenced the tectono-magmatic segmentation of the oceanic crust. The Mac Robertson Coast Anomaly (MCA), a magnetic lineation that is oriented E-W, and of high amplitude, that coincides with a landwards step-down of basement seen in seismic reflection data, has been interpreted tentatively as the continental/transitional zone-ocean crust boundary. This margin is suggested to have formed in a metamorphic core complex extension mode with a high strength ratio between upper and lower crust, typical above mantle that is anomalously hot, by the exposure of rocks from the lower crust on the coast. This observation and the presence of the MCA zone, suggests the early surface outpouring/steady state magmatic production of the Kerguelen LIP was predated by a temperature anomaly of the mantle. According to an alternative model the northward ridge jump, limited to the Elan Bank region, but in the West Enderby Basin and its Sri Lankan conjugate margin, seafloor spreading continued. The authors1 suggest that if this was the case, interpretation of the MCA magnetic anomaly as the southern arm of a ridge propagator stopping around 120 Ma is possible.

Discussion

The authors1 suggest insights into the evolution of the margin of East Antarctica, and its relationship with the Kerguelen Plume as breakup and early seafloor spreading was underway, is provided by the data and the new observations provided in this paper. The incipient development of the Kerguelen Plume is suggested by the authors1 to be a likely factor at breakup and early spreading, in the case of the spreading system between the Enderby Basin and the Perth Basin, with varying degrees of lithosphere-mantle and ridge-hotspot interaction. They discus the tectonic and magmatic along-axis margin segmentation associated with the Kerguelen Plume growth and development.

Kerguelen Plume

Evidence has been found indicating that a mantle thermal anomaly related to the Kerguelen Plume before about 118 ± 2 Ma when it formed the Southern Kerguelen Plateau (SKP). A comprehensive overview and summary has been provided of isotopic data that characterises the timing, distribution of magma outflow from the Kerguelen Plume (Coffin et al., 2002). The Bunbury Basalts in Western Australia, dated to about 132 Ma,  are the first surface expression that are related to the Kerguelen mantle source (Coffin et al., 2002) that coincide with the time just prior to the commencement of seafloor spreading about 130 Ma in the Enderby and Perth Basins. In the Perth Basin there has also been minor Neocomian magmatic activity, about 137-127 Ma (Gorton & Deighton, 2002), though in general there has been little rift-related magmatic activity of high volume or reflectors that are seaward dipping (SDRs) in the Enderby basin and the Perth Basin. The authors1 suggest that this indicates minor magmatic activity appears to have been involved in early breakup between Antarctica and India, occurring about 15 My before the earliest surface expression of the SKP, about 118 Ma, and the Rajmahal traps of India about 117 Ma.

 

Conclusions

In the area seaward of the margin of Antarctica, that was a conjugate part of the margin of Southern Greater India, the timing and direction of early seafloor spreading, along the region of the Enderby Basin and the Davis Sea that is largely unknown, has been analysed using a new data compilation. Along-axis breakup and seafloor spreading across the margin has been examined using this data as a basis. The results display a considerable amount of tectono-magmatic variation, along-axis and across-axis, that is related to the spreading rate, production of magma and the distance to the developing Kerguelen Plume. Correlated with the Mac Robertson Coast Anomaly is an example, the prominent magnetic anomaly boundary signal and a sharp basement step not being observed elsewhere in the Enderby Basin, Princess Elizabeth Trough or Davis Sea. In the central Enderby Basin evidence has been found of a fossil spreading centre that was abandoned, that the authors1 suggest could continue to the west of the Kerguelen Plateau, east of Gunnerus Ridge. About 120 Ma the early surface expression of the Kerguelen Plume at the Southern Kerguelen Plateau, and subsequently the formation of the Elan Bank microcontinent, coincided with the estimated extinction time. An alternative explanation is that only in the central Enderby Basin that the ridge jump occurred, as a result of the proximity of the Southern Kerguelen Plateau, though seafloor spreading continued in the western Enderby Basin and conjugate south of the Sri Lanka Basin.

According to the authors1 is is likely that prior to the early surface outpouring/steady state production of magma of the Kerguelen LIP a mantle temperature anomaly was present, though in the Enderby Basin and the Perth Basin there are very few seaward dipping reflectors, which suggests there was no plume present about 130 Ma during the breakup. The authors1 suggest the possibility, based on their observations, that the formation of the Antarctic-Enderby Margin occurred in a metamorphic core complex (MCC) mode, that was likely triggered by a thermal mantle anomaly in the area before the arrival of the head of the Kerguelen Plume.

 

 

Sources & Further reading

  1. Gaina, Carmen, R. Dietmar Müller, Belinda Brown, Takemi Ishihara, and Sergey Ivanov. "Breakup and Early Seafloor Spreading between India and Antarctica." Geophysical Journal International 170, no. 1 (2007): 151-69.
Author: M. H. Monroe
Email:  admin@austhrutime.com
Last updated 25/11/2012


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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading