Age of Seychelles-India break-up

J.S. Collier^a*, V. Sansom^a, O. Ishizuka^b, R.N. Taylor^c, T.A. Minshull^c, R.B. Whitmarsh^c

^aDepartment of Earth Science & Engineering, Imperial College London, UK, jenny.collier@imperial.ac.uk; victoria.sansom@imperial.ac.uk
^bInstitute of Geoscience, Geological Survey of Japan/AIST, Japan, o-ishizuka@aist.go.jp
^cSchool of Ocean and Earth Science, University of Southampton, UK, rex@noc.soton.ac.uk; tmin@noc.soton.ac.uk; rbw@noc.soton.ac.uk
^*corresponding author: Department of Earth Science and Engineering, Imperial College London, SW7 2AZ, UK email: jenny.collier@ic.ac.uk; Tel: 44 20 7594 6443 Fax: 44 20 7594 7444

This webpage is a summary of the paper: Collier, J.S., V. Sansom, O. Ishizuka, R.N. Taylor, T.A. Minshull and R.B. Whitmarsh, Age of Seychelles–India break-up, Earth Planet. Sci. Lett., 272, 264-277, 2008.

Introduction

Many continental flood basalt provinces are spatially and temporally linked with continental break-up. Establishing the relative timing of the two events is key to determining their causal relationship. For example, models in which the rifting results from uplift above the thermal anomaly responsible for the flood basalts (active rifting, Richards et al., 1989, Campbell & Griffiths, 1990) predict that flood basalt volcanism would significantly pre-date break-up. According to Hill (1991), the time lapse between flood basalt emplacement and break-up depends on the initial thermal structure of the lithosphere, with periods of 10-20 Ma expected for “normal” 200 km thick lithosphere. Alternatively, models in which the extension is due to regional tectonics (passive rifting, White & McKenzie, 1989) predict contemporaneous flood basalt volcanism and break-up. Establishing which of the models is correct has important implications for plate-tectonics. In the active-rifting model, mantle plumes play a strong, if not dominant, role in continental break-up and thus changes in plate motions, whereas in the passive-rifting model plate-boundary forces are more significant.

Whilst our ability to determine the age of onshore flood basalt provinces has improved greatly in recent years it remains difficult to determine the age of the associated rifting. The main reason for this is that much of the rifting-associated volcanic products lie offshore, where they are typically buried beneath thick sedimentary piles that form as the continental margins mature. Thus they are rarely sampled. Identifying the first seafloor spreading magnetic anomaly adjacent to rifted continental margins is commonly the only way to place bounds on the age of continental break-up. Whilst magnetic anomalies have proven a highly successful way of determining seafloor age in mid-ocean settings, their interpretation next to rifted margins is complicated by other magnetic bodies within the ocean-continent transition zone, such as syn-rift and early post-rift volcanic rocks. Therefore because of the likelihood of magmatic signatures unrelated to organized seafloor spreading, it is only possible to accurately determine the age of the first oceanic crust where additional constraints, such as seismic data, exist. Experience has also shown that it is advisable to study both sides of a conjugate margin pair in tandem in order to resolve better interpretational ambiguities and thus determine more accurately when plate separation occurred.

In this webpage we discuss the relative age of the Deccan Traps and the separation of India and the Seychelles. Whilst there has been a growing consensus as to the age of the main phase of the Deccan emplacement (65.5 ± 1 Ma, chron 29r, Courtillot & Renne, 2003), the age of the rifting has remained unclear (Figure 1). Previous workers have identified the oldest seafloor spreading magnetic anomaly between India and the Seychelles to be either 27n (62.0 Ma; Schlich, 1982, Bhattacharya et al., 1994, Miles et al., 1998), 28n (64.1 Ma; Norton & Sclater, 1979, Naini & Talwani, 1982, Miles & Roest, 1993, Chaubey et al., 1998), 29n (65.1 Ma; McKenzie & Sclater, 1971), or even 31n (68.7 Ma, Todal & Edholm, 1998, Shreider, 1998). Therefore there is no agreement as to whether the break-up followed, was contemporaneous with, or preceded flood basalt emplacement. Much of the dispute has centred on the interpretation of anomalies on the Indian side in an area known as Laxmi Ridge/Gop Rift which modern plate reconstructions show was conjugate to the Seychelles at the time of break-up (Royer, 2002).

Figure 1 (a) Summary of previous magnetic anomaly interpretations. The boxes mark the conjugate study areas shown in b and c. SP–Seychelles Plateau; MP–Mascarene Plateau; M–Mascarene Basin; LR–Laxmi Ridge; GR–Gop Rift; LB–Laxmi Basin; C-LR–Chagos-Laccadive Ridge. (b) New magnetic anomaly grid (reduced to the pole) and interpretation of the Seychelles margin. Previous interpretations as indicated in the key are shown for reference. The bold line labelled CD144-line9 is the profile modelled in Figure 2. (c) New magnetic anomaly grid (reduced to the pole) and interpretation of the Laxmi Ridge margin. The bold line labelled CD144-line2+4 is the profile modelled in Figure 2. Click here or on Figure for enlargement.

We performed detailed seafloor magnetic anomaly modeling of the north Seychelles and conjugate Laxmi Ridge/Gop Rift margins in order to determine the age of continental break-up. Unlike previous studies, our models have external constraints on the crustal structure obtained from co-incident wide-angle and reflection seismic data (Minshull et al., 2008; Collier et al., 2009).

Figure 2: Forward models of conjugate magnetic anomaly profiles across the Seychelles and Laxmi Ridge margins (see Figure 1 for line locations). Oceanic crust is shaded black/white according to polarity, continental crust light grey, and intermediate crust dark grey. The natural remnant magnetization strength in A/m is marked in each block and the susceptibility is 0.04 (SI units) throughout. The “break-up” labels mark the inception points of the Carlsberg Ridge. PR–Palitana Ridge.

Data acquisition and analysis methods

We collected magnetic, swath bathymetry, seismic reflection and seismic refraction data during two cruises onboard RRS Charles Darwin–CD134b in November 2001 and CD144 in January/February 2003. In total, 900 km of data were collected at the Laxmi margin and 2800 km at the north Seychelles margin. The new magnetic data were merged with historical measurements to produce an anomaly grid for each study area (Figure 1).

Our conjugate magnetic profiles were interpreted using 2D forward modelling. The models consist of juxtaposed uniformly magnetized blocks, the shapes of which included basement topography from co-incident seismic reflection data but were otherwise rectangular. Each block was designated to be either oceanic, continental or intermediate based on the seismic interpretation. Importantly, we interpret the Gop Rift to be underlain by thick oceanic crust and the Laxmi Ridge to be intermediate (probably heavily intruded continental crust). The oceanic crust was parameterized by a ~1.5 km thick upper crustal layer with natural remnant magnetization of 5-6 A/m and a susceptibility 0.04 SI. The continental crust was assumed to have no natural remnant magnetization, but a susceptibility of 0.04 SI. Intermediate crust of the Laxmi Ridge was allowed to have a magnetized layer of any thickness and strength.

Our work shows that syn-rift volcanics offshore the Seychelles Islands in the form of seaward-dipping reflectors were most likely erupted during chron 28n, and the first organized seafloor spreading at the Carlsberg Ridge also initiated during this period, at 63.4 Ma. The severing of the Seychelles occurred by south-eastward ridge propagation that was completed by the start of chron 27n (~62 Ma). A brief, pre-Deccan phase of seafloor spreading occurred in the Gop Rift, possibly during 32r (~73 Ma). A number of seamounts were also erupted onto the seafloor during 27n-26n, 2-3 Ma after the initiation of seafloor spreading. Initial extension at the margin therefore preceded emplacement of the Deccan traps, and separation of the Seychelles was achieved within the next 3.5 Ma.

Comparison with other margins

Table 1 summarises recent compilations of the relative ages of other continental-flood basalt-rifted continental margin pairs (Hawkesworth et al., 1999, Courtillot et al., 1999, Jourdan et al., 2007). The compilations largely differ in the assignment of the age of the first oceanic crust for the reasons outlined in the introduction. Working with the currently available constraints, the various margins fall into two groups, those with short time lapses (< 10 Ma)–Paraná-Etendeka (5-8 Ma), Madagascar (4 Ma), the North Atlantic Magmatic Province (3-7 Ma), and those with relatively long time lapses (> 10 Ma)–the Central Atlantic Magmatic Province (25, or maybe 10 Ma), Karoo-Ferrar (13-35 Ma), Marie Byrd Land (23 Ma) and Ethiopia (20-32 Ma). Our determination of 3.5 Ma for the Deccan and Seychelles-India rift system confirms it to be the shortest time interval so-far reported for any system worldwide.

Table 1. Summary of previous estimates of the relative age of continental flood basalt provinces and associated continental rifting.

Hill (1991) argued that in the active rifting case, where the crustal extension was triggered by thermal doming above an impacting plume, the establishment of a new ridge would require tens of millions of years, the time required for conduction of heat through the upper lithosphere, unless a ridge jump was involved. In the case of the Deccan, ridge jumping was clearly involved, with spreading stopping in the Mascarene basin between chrons 30 and 27 (Bernard & Munschy, 2000). Arguably a further factor that could have contributed to the rapid break-up at the Seychelles-India margins in the active plume case was that the lithosphere may have been already thinned by the earlier rifting of Greater India from Madagascar at 84 Ma and/or by the interaction with earlier plumes–the Kerguelen and Marion plumes (Kent et al., 1992). However, our work shows that plate reorganisation was underway long before the main Deccan eruption, with the opening of the Gop Rift possibly preceding it by 9 Ma. We therefore conclude that external plate boundary forces were largely responsible for the rifting of the Seychelles from India, and the evidence suggests that a passive, rather than active, rifting model is more applicable in this case.

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