No abstract

<p>The bathymetry of the south-west Indian Ocean is dominated by three mid-oceanic ridge systems: the Chagos–Laccadives Ridge, the Central Indian Ridge, and the Mascarene Plateau. Although there have been a number of geophysical and geological investigations over the region, the genesis of these morphological features is still contradictory. Most of the estimations of effective elastic thickness in this region have been carried out in the spectral domain, either by transfer function analysis or by free-air admittance analysis. As these investigations were along some one-dimensional profiles or discrete blocks, spatial variation of the effective elastic thickness was not achieved. Here, we reappraise the estimation of effective elastic thickness in the south-west Indian Ocean by performing the computation in the spatial domain using flexure inversion. During this process, we also estimate the Moho depth throughout the region by two independent processes: gravity inversion, and flexural inversion. The Te values (effective elastic thickness) are estimated in the spatial domain, which match well with the results in the spectral domain obtained with the free-air admittance method. In addition, there is spatial variation of the Te values over the area analyzed. Our estimated Te values are low (1-6 km) along the Chagos-Laccadives ridge, implying its proximity to a spreading ridge at the time of creation. The Te values along the Mascarene Plateau show spatial variation with a seafloor age from north (Te, ca. 4 km) to south (Te, ca. 20 km). These findings substantiate earlier data and suggest that Réunion was created due to intraplate volcanism.</p>

Global estimates of the elastic thickness (Te) of the structure of passive continental margins show wide and varying results owing to the use of different methodologies. Earlier estimates of the elastic thickness of the North Atlantic passive continental margins that used flexural modelling yielded a Te value of ∼20-100 km. Here, we compare these estimates with the Te value obtained using orthonormalized Hermite multitaper recovered isostatic coherence functions. We discuss how Te is correlated with heat flow distribution and depth of necking. The E-W segment in the southern study region comprising Nova Scotia and the Southern Grand Banks show low Te values, while the zones comprising the NE-SW zones, viz., Western Greenland, Labrador, Orphan Basin and the Northern Grand Bank show comparatively high Te values. As expected, Te broadly reflects the depth of the 200-400 • C isotherm below the weak surface sediment layer at the time of loading, and at the margins most of the loading occurred during rifting. We infer that these low Te measurements indicate Te frozen into the lithosphere. This could be due to the passive nature of the margin when the loads were emplaced during the continental break-up process at high temperature gradients.

Indian Ocean subduction zone is one of the most active plate margins of the globe as evident from its vast record of great magnitude earthquake and tsunami events. We use Bouguer admittance (Morlet isostatic response function) in Sumatra-Java subduction zones comprising both the subduction and over-riding plates to determine the lithospheric mechanical strength variations. We determine effective elastic thickness (T e ) for five oceanic windows (size 990 × 990 km 2 ) by analyzing the admittance using Bouguer gravity and bathymetry data. The results show bimodal T e values < 20 km for Sumatra and 20-40 km for Java. The lower bimodal values obtained for Sumatra appears to correlate well with the zones of historical seismicity. This is in sharp contrast with Java subduction zone, which shows higher T e values (20-40 km) and apparently associated with low magnitude earthquakes. We suggest a strong and wide interseismic coupling for Sumatra between the subducting and over-riding plates, and deeper mantle contributing to low strength, shallow focus -high magnitude seismicity and vice versa for Java, leading to their seismogenic zonation.

<p>The elastic thickness (Te) of continents is a matter of much debate. Recent studies have shown that a number of factors control the continental Te, including age, heat flow, and lithospheric thickness. Here, we estimate the Te structure of the whole Indian shield using an improved isotropic fan wavelet land ocean deconvolution methodology, and we compare these results with the global published Te estimates in the Archean, Proterozoic and younger geological provinces. Our study reveals low (0-45 km/0-35 km), intermediate (45-70 km) and high (70-100 km) Te values in the Archean/Quaternary, the Proterozoic, and the Tertiary provinces, respectively, of the Indian shield. This is in contrast with global estimates of Te in similar geological provinces. In the absence of any correlation of Te with any controlling parameters, we propose that the mantle properties, rather than the tectonic history, are responsible for influences on the Te values within the Indian shield. The global positioning system horizontal velocity vectors yielded a locking depth of ca. 20 ±4 km, and the aseismic creep beyond correlates well with the high strength of ca. 70 km to 100 km in the central Himalayan foreland.</p>