Deep Crustal Studies

Lithospheric rifting is a fundamental process in the growth and evolution of continents, and it is one that has substantial societal relevance by virtue of the global hydrocarbon reserves accumulated within basins formed through rifting. Rifting proceeds from the application of extensional stress to the accumulation and localization of strain until the lithosphere ruptures, whereupon seafloor spreading and production of oceanic lithosphere accommodate most extension. Continental breakup thus constitutes a dramatic expression of two fundamental geological processes: deformation and magmatism. Yet first-order questions exist about every process in the rift-to-drift sequence. We lack a full understanding of both the magnitude and cause of the stresses that drive rifting, the deformational mechanisms by which continental lithosphere responds to those stresses, and the key parameters that control this deformation. Similarly, the role of the rift-related magmas in localizing strain and advecting heat from asthenosphere to lithosphere is poorly understood, as are the controls on mantle melting during extension (e.g., mantle temperature, volatile content, and small-scale convection). Understanding these processes is one of the fundamental goals of earth scientists today.

The project is aimed at detailed study of the SWIM with special emphasis on correlating onshore-offshore structures along this margin. The best observations of complete rifting at lithospheric scale come from seismic studies across rifted continental margins. Any thorough study of continental rupture requires high-quality images of the velocity and impedance structure of the crust and uppermost mantle. These images, furnished by detailed reflection/refraction seismic profiles, provide essential information on patterns of deformation and crustal thinning, crustal composition, subsidence history, magmatic additions, and the onset of seafloor spreading.

Volcanic rifted margins are characterized by voluminous syn-rift volcanism far in excess of that expected for passive decompression melting of normal asthenosphere. These are also commonly identified by wedges of seaward dipping reflectors (SDRs) in seismic records. The wedges, which may have thicknesses of more than 6 km, consist of numerous lava flows and thin inter-bedded sediments. The extrusive region is usually underlain by several km thick body of high seismic velocity (7+ km/s) interpreted to have been added to the base of the crust during the break-up events. SDRs are found on more than 70% of the world’s rifted margins. Such margins are common (e.g. White and McKenzie, 1989; Eldholm et al., 2000), yet we lack a complete understanding of their development, particular in linking the thermal/dynamic models required to explain the effusive magmatism with the mechanical models that describe lithospheric deformation.

While decades of such observations along Indian continental margins have resulted in various hypotheses based on limited sub-surface data coverage, the seismic studies of Indian continental margins have till date not conclusively addressed the degree of complexity involved. This is largely because of the lack of high quality seismic transects across the margin. In recent years loads of seismic data have been acquired by various agencies for different purposes. Taking a cue from such information, a pilot project was initiated at NCPOR, Goa to analyses and interpret MCS data available through Directorate General of Hydrocarbons (DGH), Government of India. The outcome of this project was presented (Nair et al., 2010) before the scientific community which forms the basis of this long-term program. The project envisages high-resolution, deep-penetration seismic reflection/refraction data set along the SW Indian margin to examine the precise nature (such as magma poor or magma rich) of this rifted margin and to provide any insight into the demarcation of the continent–ocean transition (COT) of this area. Moreover, offshore interpretations would be tied up with the onshore studies carried out by extensive DSS studies.

b) Participating Institutions:

National Centre for Seismology, Noida

National Centre for Antarctic and Ocean Research, Goa

c) Implementation Plan:

As the first step in the implementation of this program, it is proposed to carry out the studies along an E-W corridor through the Udipi-Kavali seismic transect in the Southern Peninsular India and linking up with the Arabian Sea Basin off the Laccadive Islands. The high-quality multichannel seismic reflection, gravity, and magnetic data available from the Arabian Sea offshore as well as from the Udipi-Kavali deep-transect which cuts across the entire southern peninsula makes this an ideal corridor for study. In addition, the geological information from the two deep sea drilling sites (DSDP 219 and 221) would offer excellent constraints on the geophysical data. Based on the preliminary results, it is proposed to undertake additional high-resolution MCS and refraction data collection along critical stretches of the WCMI.

d) Deliverables:

The proposed work is expected to usher in new frontiers of geo-scientific research in the country. Using new data as well as new technologies, the Project is anticipated to fill the gap in knowledge about the relationship between onshore and offshore sub-surface structures. The program will provide an opportunity for the Indian Earth scientists to closely understand rifting architecture as well as its geodynamic implication.

e) Budget requirement : 13 Crores

(Rs. In crores)