Geophysical Investigation of the Slow-spreading Carlsberg Ridge in the Northwest Indian Ocean: Structure and Thermo-mechanical Characteristics

Abstract:

The Carlsberg Ridge (CR) is a slow-spreading ridge (22-32 mm/yr) in the northwest Indian Ocean that extends from Owen transform near 10°N to the Central Indian Ridge near equator, and defines the boundary between India and Africa plates. We present here the results of a detailed modeling (2-D / 3-D) study of bathymetry and the potential field data (gravity, geoid and magnetic), and the spectral based (admittance/ coherence) analysis to understand the structure, isostasy, rheology and segmentation of the ridge. The study revealed that the CR shows distinctly different geophysical characteristics such as wide and deep axial valley (1.5–3 km) with prominently high residual geoid (degree 10) and mantle Bouguer anomalies in the northwest, and narrow and shallow axial valley type morphology with prominent lows in the southeast. This variation is attributed to less mamgmatic phase with dominant tectonic extension in the northwest, and dominance of magmatic accretion phase in the southeast. It is also noticed that the ridge has only one first order segment along its entire length defined by a well developed transform fault and fracture zone. However, along and across axis residual geoid (degree 10, 30 and 50) and mantle Bouguer anomalies of the CR revealed several 2^nd order discontinuities. Inversion of residual mantle Bouguer anomalies reveled along and across axis variation in crustal thickness having 7-10 km close to the spreading centers, 2-5km at the segment ends, and 4-6 km over the flanks of the spreading segments.The Geoid-Topography Ratio (GTR) values computed over the ridge vary between 0.26-0.46 m/km and suggest increase in the apparent compensation depthfrom ~5.5km in north to 9.5 km in the south. On the other hand, the effective elastic thickness (Te) values decrease from 16 km in the north to 5 km in the south. The joint modeling of geophysical data, and the geoid-age relations gave rise to maximum thickness of thermal lithosphere ~85 km in the CR region.