Isostasy and crustal structure of the Chagos-Laccadive Ridge, Western Indian Ocean: Geodynamic implications

K.M. Sreejith¹, P. Unnikrishnan², M. Radhakrishna²

¹Geosciences Division, ISRO- Space Applications Center, Ahmedabad, India; sreejith81@gmail.com

²Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai India; parakkal_u@yahoo.com ; mradhakrishna@iitb.ac.in

This webpage is a summary of Sreejith K. M., P. Unnikrishnan and M. Radhakrishna (2019) Isostasy and crustal structure of the Chagos-Laccadive Ridge, Western Indian Ocean: Geodynamic implications, J. Earth Syst. Sci., 128:15, DOI: 10.1007/s12040-019-1161-2

Overview

The massive eruption of the Deccan flood basalts over western India during the Late Cretaceous is thought to have resulted from Reunion plume activity and India–Seychelles breakup (Courtillot et al. 1988). Subsequent volcanism on the Indian plate as it moved over the plume is thought to have led to the formation of the Laccadive Ridge, Maldive Ridge and Chagos Bank in the West Indian Ocean. Together these are known as the Chagos-Laccadive Ridge (CLR) (Figure 1).

Important outstanding questions are:

Figure 1: (Left) Residual mantle Bouguer anomaly map of the Chagos-Laccadive Ridge area. (Middle) Basement map of CLR and adjoining oceanic region. The positions of overlapping blocks (B1–B8) are shown. Positions of seismic refraction stations and seismograph stations are shown as black triangles and white rectangles respectively.Overlapping rectangles B1-B8 represent analysis windows for flexural modelling and coherence analysis. (Right) Variations in Te (blue) and degree of underplating (red) along the ridge (graphical representation of Table 2 of our paper).

Seismic velocities are largely inconclusive regarding the nature of crust beneath the CLR (Naini & Talwani, 1983). However, the presence of magmatic underplating beneath the entire hotspot track has been revealed by receiver function studies (Gupta et al. 2010; Fontaine et al. 2015). Based on geochemical evidence, Torsvik et al. (2013) proposed that the Chagos Bank, Maldive and Laccadive ridges, along with continental fragments from the southern Mascarene plateau, were part of a microcontinent ‘Mauritia’ that was thinned, fragmented and concealed during Cretaceous–Cenozoic times.

Transfer function analysis of bathymetry and gravity data can be used to estimate the Effective Elastic Thickness (Te) and ratio of subsurface to surface loading (f) thereby providing constraints on the mode of isostatic compensation and crustal structure including the degree of magmatic underplating (e.g., Watts 2001 and references therein). Previous isostatic studies over the CLR were either carried out using sparse ship-borne data or limited to the southern part of the CLR (Ashalatha et al. 1991; Tiwari et al. 2007; Chaubey et al. 2008). In the present study, we investigate variations in isostatic compensation mechanism along the CLR using 3D flexural modelling of satellite gravity and bathymetry data. We also provide unbiased estimations of variations in the degree of underplating along the CLR along the ridge by modelling the residual mantle Bouguer anomaly and topography coherence function.

We infer that the northernmost segment of CLR, the Laccadive Ridge, is characterised by a low Te value of 3 km and associated with a subsurface loading equal to that of the surface loading (degree of underplating ~ 100%). However, towards the south, the Maldive Ridge and the Chagos Bank Ridge have fairly uniform Te of 8–10 km with a very low degree of underplating (10-20%).

Geodynamic Implications

Te values largely depend on the strength of the lithosphere at the time of emplacement (Watts, 2001). A low Te generally suggests either volcanic emplacement near a spreading centre or emplacement on a strong lithosphere followed by thermal rejuvenation due to prolonged interaction of a hotspot with oceanic crust, e.g., Cape Verde and Magellan Seamounts (Young & Hill, 1986; Smith et al., 1989). Indeed, non-uniform melt production and/or plume-lithosphere interactions can cause spatial variations in Te and the crustal thickness as reported for the Ninetyeast Ridge (Sreejith & Krishna, 2013 & 2015). However, plate tectonic models suggest that the Reunion hotspot could not have erupted on-ridge until ~47 Ma (Duncan, 1990) and the velocity of the Indian plate was exceptionally high from ~66 Ma till India-Eurasia collision at 52 Ma. As a result, the low Te for the Laccadive Ridge cannot be explained by either on-ridge or off-ridge emplacement on oceanic lithosphere.

We propose that lithospheric stretching during early Tertiary rifting between India and Seychelles followed by Reunion hotspot volcanism must have led to weakening of lithosphere. The Laccadive Ridge is a continental sliver underplated by magmatic rocks. Large subsurface loading occurred during the early Tertiary at the base of the thick continental crust. Low loading in the south might be due to lesser magma input. The southern portion of the CLR was emplaced on a fast-moving Indian plate that might have hampered prolonged interaction between the hotspot and the lithosphere for significant subsurface loading process. The Maldive Ridge and the Chagos Bank might have been emplaced on the flanks of the Central Indian Ridge.

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