Geophysical Research Letters

New aeromagnetic data from the western Enderby Basin and consequences for Antarctic-India break-up

Authors


Abstract

[1] In order to enhance our understanding of the continental break-up between East India and Antarctica, a Japanese/German aerogeophysical survey was conducted off eastern Dronning Maud Land and western Enderby Land, Antarctica. The systematic survey with a line spacing of 20 km provides strong constraints on the timing of the initial break-up of India/Antarctica compared to the conjugate margin south of Sri Lanka. While the continent-ocean transition is very well marked by pronounced positive and negative magnetic anomalies, no hints on the existence of magnetic spreading anomalies north of this zone has been found in the new data set. Thus, in contrast to most recently published models, the data only support kinematic models, which include a final break-up of Sri Lanka/India from our investigated area some 90-118 Ma within the Cretaceous Normal Superchron.

1. Introduction

[2] The dispersal of Gondwana starting at approximately 160 Ma was accompanied by various magmatic events caused by large thermal anomalies in the upper mantle. The only portion of Gondwana that had common boundaries with all present day southern hemisphere continents (South America-SAM, Africa-AFR, Madagascar-MAD, India-IND, Australia-AUS, New Zealand-NZ) is Antarctica (ANT), thus making it possible to investigate if the break-up processes produced coherent signals/geological features on- and offshore in order to better understand the driving forces of continental break-up. Therefore, one important constraint is the knowledge of the seafloor spreading history of the Gondwana fragments in as much detail as possible, and its relation to onshore volcanism, which proceeded or accompanied the break-up process.

[3] In the past decade, models for the timing and geometry of the dispersal of South America, Africa [Jokat et al., 2003; König and Jokat, 2006, 2010], Australia and New Zealand from Antarctica have constantly been refined by new seismic and magnetic data. An exception is the Indian sector of Antarctica (Figure 1). Here, no consensus has been achieved about the timing and geometry of the drift, mainly because of the absence of clear and continuous magnetic spreading anomalies along ANT and eastern India. This resulted in one class of models, which interpreted the variable magnetic signals as being created during the Cretaceous Normal Superchron (CNS) (120-84 Ma). In the absence of magnetic spreading anomalies, these models [Royer and Coffin, 1992; Banerjee et al., 1995] related the initiation of the break-up process to the first occurrence of volcanic rocks onshore Eastern India (Rajmahal Traps, 118 Ma) [Banerjee et al., 1995] and the southern Kerguelen Plateau (118 Ma) north of the East Antarctic coast. The second class of models propose a much older dispersal of IND and ANT, some 135 Ma [Ramana et al., 1994a, 1994b; Nogi et al., 1991, 1996; Ramana et al., 2001; Desa et al., 2006; Gaina et al., 2007]. Mesozoic magnetic anomalies back to chron M9o (130 Ma) [Gaina et al., 2007] to chron M11 (>134 Ma) [Ramana et al., 2001] off ANT have been identified in similar or same magnetic data sets within our research area (Figure 1). One of the problems with the latter models has been that onshore massive volcanism occurred approximately 18 Myr (IND-Rajmahal Traps; ANT-Kerguelen Plateau), after the first formation of oceanic crust along the conjugate margins. Gaina et al. [2007] propose an intermediate model. These authors agree that most of the oceanic crust off Eastern India was formed during the CNS, but identified M-series back to M9o off ANT. These anomalies formed around an extinct spreading axis, which was located in the Enderby Basin off ANT. A two-stage spreading model was introduced, with parts of the Kerguelen Plateau (Figure 1; Elan Bank) still connected to India while the M-series formed off Enderby Land. During the CNS, finally, the Elan Bank drifted also away from eastern India, leaving Late Cretaceous oceanic crust behind. This model includes the observation that magnetic M-series could not be found off eastern India.

Figure 1.

Overview map. The dashed box indicates the location of the aerogeophysical survey. ETOPO bathymetry is gray shaded. The Japanese Antarctic Station Syowa is marked. Abbreviations: EB-Elan Bank, GR-Gunnerus Ridge, KFZ-Kerguelen Fracture Zone.

[4] At this stage, it is important to note that throughout the entire geophysical literature about the break-up of ANT-IND, all authors report problems with making magnetic anomaly interpretations. As a consequence, different age models were published based on the same data sets. Without new magnetic data, the general problem of widely spaced ship tracks as well as generally random-orientated ship tracks could not be solved for the IND-ANT drift history. Thus, the major purpose of our survey north of Enderby Land was a) to conduct a systematic survey with a line spacing of 20 km, and b) to map in greater detail the Mesozoic magnetic anomalies in order to refine/confirm published break-up models. An aerogeophysical survey around the Japanese Antarctic Station Syowa (at −69°S, 39,58°E) was conducted from January 09th to 29th, 2006 with the fixed wing aircraft Polar 2. In this contribution, we will report about the results of the aeromagnetic investigations off Enderby Land, which is believed to be the conjugate margin of Sri Lanka.

2. Experiment Set-Up and Data Processing

[5] The flight altitude varied between 150 and 250 m above sea level and between 3000 and 3600 m above land. The data were acquired at a mean speed of 248 km/h (69 m/s). Within 15 days and 23 flights, more than 22825 kilometres of new aeromagnetic data were acquired. The line spacing was 20 km, and the magnetic field was recorded with a sampling rate of 1s.

[6] Standard data reduction was applied to the measurements, including regional IGRF 2005 (International geomagnetic reference field) and diurnal corrections. Diurnal variations were taken from the permanent recordings of a base station located at Syowa station. The variation of the total magnetic field was calculated out of these components using a declination of −49°12,2′ and an inclination of −63°36,1′, which were the values for epoch 2006.0, given by NIPR (National Institute of Polar Research, Japan). A quiet level value of 102 nT was then subtracted from all base data. This value was calculated as the mean value for the three quietest days of January, detected by statistical analysis of all given base data of that month (January 04th / 08th / 30th). Thereafter, a 120 s time filter was applied to the base data in order to suppress high frequency variations, and to remove the regional trend of the variations. The aeromagnetic data were then corrected by the processed base station data. Because of the different main flight levels across the sea and over land, we compiled two separate magnetic maps. Here, only data acquired over sea are shown. They are upward continued to 250 m. Both data sets were filtered by a 15 s time filter afterwards, and levelled using LCT® software (Fugro Ltd.). To achieve this, a levelling polynom of the 8th order was applied and the lines were weighted in such a way that the flights flown at the magnetically very noisy day (January 23rd) were assigned only 10% of the weight of the other flights.

[7] This levelling procedure reduced the average mistie at crossing points from 13,9 nT ± 9,5 nT to 3,8 nT ± 3,2 nT for the data set across the sea (Figure 2).

Figure 2.

Misties before and after levelling.

[8] The description of the data will be restricted to the area north of the present day coastline as the recorded magnetic field there is relevant for providing constraints on the break-up of ANT and IND. Furthermore, the full resolution of the magnetic data at a flight level of 250 m is needed for the interpretation, rather than a general flight level of 3600 m for both data sets.

3. Magnetic Field

[9] The first prominent and continuous magnetic anomaly is visible parallel to the continental margin in our survey area, and has along-strike variable, positive anomalies ranging from 200 nT to 630 nT at maximum (Figure 3). Around 40°E, the southern part of the generally long wave length anomaly shows high frequency signals, which might be caused by shallow intrusions into the extended continental crust. Seawards north of this positive anomaly pattern, a negative anomaly with amplitudes ranging between −90 nT and −170 nT along strike is also visible (Figure 3a). Just north of this prominent positive/negative anomaly pattern, a broad positive magnetic anomaly is observed (Figure 3b, white boxes). The pattern is reasonably well constrained, and is slightly oblique compared to the strike of the continental shelf break and its prominent anomaly pattern. Origins of these magnetic anomalies are possibly normal magnetized oceanic crust and/or excess magmatic intrusions into the extended continental crust during the initial break-up. The anomalies north of this area towards the Gunnerus Ridge (GR) (Figure 3, west of 40°E) vary in general between −100 nT and +160 nT with local maxima/minima at +270 nT and −200 nT, respectively. East of 40°E, the field becomes very smooth with values ranging between −70 nT to +70 nT at maximum. The amplitudes are, in general, decreasing towards the north and increasing towards the GR, which might be due to increased magmatism during the break-up closer to GR. The magnetic field close to the GR is difficult to level because of its location at the rim of the aeromagnetic network.

Figure 3.

(a) Wiggle representation of the magnetic data. Flight altitude is 250 m. The chron identification of Gaina et al. [2007] is added as well as the seismic line GA-229/35 [Stagg et al., 2004] (yellow line). ETOPO bathymetry is plotted with a contour interval of 200 m. Abbreviations: GR-Gunnerus Ridge, XR-Extinct spreading axis. (b) Magnetic field off Syowa Station at a flight altitude of 250 m. The location of the seismic profile [Stagg et al., 2004] (GA-229/35) is indicated as yellow line. The two white boxes might indicate a magnetic signal from the continent-ocean transition or the magnetic pattern represents the youngest Mesozoic magnetic spreading anomaly in this area. ETOPO bathymetry is plotted with a contour interval of 200 m. Abbreviations as in Figure 3a.

[10] North of the prominent positive/negative anomaly pattern close to the coast, neither data representations (Figures 3a, wiggle and 3b, grid) provide an indication for the presence of any clear continuous Mesozoic spreading anomalies as reported by previous investigations covering approximately the same area. The data have variable amplitudes in polarity but no obvious continuity. The variable field starts at present day water depths of around 4000 m.

4. Discussion and Interpretation

[11] The investigated area (Figure 1) is in most reconstructions the conjugate margin to the southernmost tip of IND/Sri Lanka. Published models south of Sri Lanka [Desa et al., 2006] suggest a break-up at approximately 134 Ma (M11). However, this identification is based on random ship tracks with wide line spacing. According to Desa et al. [2006], the spreading south of Sri Lanka ceased more or less at chron M0 and/or slowed down from 35-55 mm/yr to very small rates of less than 6 mm/yr. Around chron C34, a normal spreading regime was reinstalled. Similar break-up ages (M9o, ∼130 Ma) have been recently published [Ramana et al., 2001; Gaina et al., 2007]. The latter authors published the most extensive magnetic data set off Enderby Land, which was conjugate to eastern India. Based on seismic and magnetic data, an extinct spreading axis was identified in the Enderby Basin. Here, the Elan Bank, a proposed continental fragment west of the Kerguelen Plateau, forms the conjugate margin to Enderby Land. The Enderby Basin is, thus, of Early Cretaceous age. In a second phase, the margin of Eastern India formed during the CNS. One of several complications testing this model might be the thick sedimentary column off East India (up to 10 km), and/or total intensity magnetic anomalies observed around the equator. These factors might have modified the magnetic anomalies in a way that no coherent spreading pattern could be identified [Krishna et al., 2009]. In contrast, off Enderby Land the sediments in the oceanic part of our research area are only approximately 1.5 km thick [Stagg et al., 2004] (profile GA-229/35). Thus, even weak coherent M-anomalies, if present, should be visible of a similar quality as observed in the adjacent Riiser-Larsen Sea with a much thicker sediment cover (Figure 1) to the west [Jokat et al., 2003].

[12] For further discussions, it is important to define the landward limit of oceanic crust in our area. Here, the best estimate is based on a seismic reflection profile and the subsequent 2D-gravity modelling [Stagg et al., 2004]. According to Stagg et al. [2004], the continent-ocean boundary lies approximately 150 km seaward along the profile GA-229/35 (Figure 3, yellow line). Corresponding to the simultaneously recorded magnetic data, this location is marked by the termination of a broad positive anomaly just north of a strong negative one. In the absence of any deep seismic sounding data, this interpretation is mainly based on the account of basement variations in the seismic section. Adapting this interpretation and transferring it into our data set, the broad magnetic anomaly marked in Figure 3b is the last portion of extended crust in our research area. North of this anomaly pattern, the diffuse magnetic field is located on oceanic crust. Here, the new data do not show any coherent magnetic signals. For comparison, we have plotted the anomaly identification of Gaina et al. [2007], which is based on three widely spaced profiles, on top of our data (Figure 3). Though, M9n is generally a weak anomaly, there is no coherent anomaly pattern along the marked line. One should suspect stronger and older Mesozoic magnetic anomalies south of M9n to the continent-ocean transition, however, this is not the case.

[13] Thus, the data support models, which favour an ANT-IND separation during the CNS, or very close to it. Based on the new aeromagnetic data we prefer the following interpretations/models:

[14] 1. The pronounced positive/negative anomaly pattern close to the coast marks the continent-ocean transition. Without deep seismic sounding data no precise decision can be made whether the broad positive anomaly just north of this pattern (Figure 3b, white boxes) is part of the continent-ocean transition or represents chron M1n. In the latter case, the continent-ocean transition would be very sharp, which is in accordance with results published off East India [Krishna et al., 2009].

[15] 2. The new data set show a similar diffuse magnetic pattern as reported from vector data gathered north of Syowa Station [Nogi et al., 1996]. However, the explanation of those is different. In this study, we prefer the interpretation that the oceanic crust was formed during the CNS.

[16] 3. The rift-drift transition off Syowa Station started after the emplacement of the Rajmahal Traps at approximately 118 Ma within the CNS. Such a scenario is also supported by rift-related dykes in the Prince Charles mountain [Arne, 1994], which are dated at 100 Ma.

[17] 4. We propose that the first oceanic crust formed with some time delay to the emplacement of the Rajmahal Traps. Denudation studies [Lisker, 2004] along both margins indicate that the last tectonic event terminated around 100 Ma.

[18] 5. We propose that there are no M-series south of Sri Lanka. According to Sreejith et al. [2008], part of the identified M-anomalies [Desa et al., 2006] are located within the southern continent-ocean-transition zone or the spreading was highly asymmetric. The distance from the proposed continent-ocean boundary to chron C34 is ca. 800 km along 83°E. North of Syowa Station this distance is ca. 900 km. This would result into a mean spreading velocity of 25 mm/yr for the IND plate.

[19] 6. In cannot be ruled out that even our magnetic survey is still located within the continent-ocean transition. However, the existing geophysical data base does not support this view, especially as no deep seismic data exist to constrain the onset of oceanic crust in the investigated area.

5. Consequences for Geodynamic Models

[20] Tentatively, we estimate that the oceanic crust in the investigated area formed after the emplacement of the Rajmahal Traps some 118 Ma. If there was a time delay between the emplacement of the traps and the formation of the first oceanic crust in the Enderby Basin its duration is not known. Thus, the oceanic crust in the investigated area might have formed in a time window between 118 Myr to even 90 Myr. This younger break-up age has significant consequences for the plate dynamics in the area and bio-geographical studies. IND was attached to ANT much longer than the some models predict. The data set supports the interpretation of Banerjee et al. [1995] and the reference therein, which state that “no oceanic crust older than 100 Ma was drilled or found off eastern India”. We think that the newly acquired aeromagnetic data are typical for the area west of the KFZ (Figure 1). East of the KFZ, where also the Elan Bank (Figure 1) is located, the trends of the fracture zones might indicate a different spreading regime. Tentatively, taking the available geophysical information into account, we suggest that the KFZ is not a significant geological age boundary, and, thus, that large parts or the entire Enderby Basin was formed during the CNS.

[21] Furthermore, in view of the different data sets and interpretation, we speculate that the ANT-IND break-up process started in the east and propagated towards the Gunnerus Ridge, leaving India much longer connected to Antarctica than other models formerly proposed. Thus, together this allowed an exchange of fauna and flora till the Late Cretaceous between Antarctica and India/Madagascar. Our younger break-up model also agrees well with geological interpretations along both margins.

Acknowledgments

[22] We thank the aircraft crew, the technicians, Daniel Steinhage, and Sven Riedel for conducting the survey in the field. We thank S. Cande and D. Müller for their reviews. We also thank the JARE members and crews of the icebreaker Shirase for their kind help during operations from the JARE-46 to JARE-47. Y.N. is also grateful to Kazuyuki Shiraishi, Kazuo Shibuya, and the members of Department of Earth Science, National Institute of Polar Research, for their understanding and interest in the study.

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