Compressional Deformation of Oceanic Lithosphere in the Central Indian Ocean: Why It Is Where It Is

Abstract

The prominent example of compression of oceanic lithosphere in the central Indian Ocean south of India and Sri Lanka does not occur where recent models for the state of stress in the Indo-Australian Plate predict maximum horizontal compressive stress. The Afanasiy-Nikitin seamount group, which erupted in Late Cretaceous or early Tertiary time, is centrally located in the region where deformation is best developed. We suggest that critical wavelength components in the deflection caused by the emplacement of these seamounts were preferentially amplified when north-south directed compression was applied to the northern part of the Indo-Australian Plate in the late Miocene. To test this hypothesis, we develop a simple two-dimensional model for compression of a thin elastic plate overlying an inviscid fluid, where the plate contains a pre-existing deflection. The deflection caused by seamount loading is obtained by approximating the topography of the Afanasiy-Nikitin Seamounts as a bi-variate Gaussian distribution. The 2-km peak-to-trough amplitude and 200-km average wavelength characteristics of the broad-scale crustal deformation and the observed east-west-trending pattern of free-air gravity anomalies are best matched in the modeling with an applied horixontal compression between 1.5-2.0 × 10 13 N/m, a plate with an effective elastic thickness of 10-15 km at the time of compression and 5 km at the time of seamount emplacement. Furthermore, infilling of topographic lows of the deformation by Bengal Fan sediment rather than seawater is a major factor in promoting lithospheric deformation by horizontal compression.

Growth of the Afanasy Nikitin seamount and its relationship with the 85°E Ridge, northeastern Indian Ocean

“…Firstly, the undisturbed sequence of seafloor spreading anomalies 32n.1 through 34 (figure 3 and discussed in Krishna and Gopala Rao 2000;Krishna 2003), strongly suggests that the main plateau age is approximately the same age as the ocean floor beneath the seamount . This is in agreement with estimates of low elastic plate thickness (T e ) of 2-5 km for the seamount (Karner and Weissel 1990;Paul et al 1990). All data are consistent with the ANS main plateau being constructed at a midocean ridge simultaneously with the generation of new oceanic crust (figure 10a) and remained as a ridge crest feature (closely associated with Marion Dufresne, Lena and Ob seamounts emplaced on the Antarctic plate, see discussion below).…”

“…Two shallow edifices (at 1600 and 2050 m) have the morphology of a guyot, suggesting that they were formed close to sea-level during the phase of late-stage volcanism and topping the 'main plateau'. • Seafloor spreading anomalies 34 through 32n.1, low (2 -5 km) T e values (Karner and Weissel 1990;Paul et al 1990) and ∼18 km thick crust beneath the ANS (Krishna 2003) suggest that the main plateau of the ANS was formed in an initial phase of volcanism in an on-ridge setting at 80-73 Ma. Based on present results and published plate reconstruction results of the Indian Ocean, we believe that the Conrad Rise hotspot has emplaced both the main plateau of the ANS and Conrad Rise (including the Marion Dufresne seamount) together during the period 80-73 Ma, close to the India-Antarctica spreading ridge system.…”

“…There are a number of lines of evidences to suggest that the ANS and 85 • E Ridge arose from different mantle sources. Flexural analysis (Karner and Weissel 1990) and admittance analysis of the gravity data (Paul et al 1990) for the ANS provide elastic plate thickness (T e ) values of 5 and 2-5 km, respectively. These low T e values together with 18 km thick crustal model (Krishna 2003) indicate that the seamount initial load was emplaced at a mid-ocean ridge (India-Antarctica Ridge) concurrently with the generation of new oceanic crust.…”

“…Previous work using rock samples and geophysical data has suggested that the ANS was formed in two phases, one close to the age of the underlying oceanic crust, the other phase being poorly constrained, but much later. Analyses of gravity and bathymetry data of the ANS (Karner and Weissel 1990;Paul et al 1990) determined a low (2-5 km) elastic plate thickness (T e ) below the ANS and was interpreted as indicating that the seamount was emplaced close to the spreading center. Subsequently, Sborshchikov et al (1995) studied the petrochemical data of the ANS and found that the main plateau of the seamount, comprised of basalts and picrite-basalts, formed on young oceanic crust during the late Cretaceous.…”

Fissure control on volcanic action in the Pacific

“…In a similar fashion, in the eastern Indian Ocean, compressional deformation resulting from the imbalance of forces between ridge push and trench pull at the Indonesian trench caused buckling and sediment deformation in the interior of a portion of the Indian plate (Karner and Weissel, 1990;Karner et al, 1993).…”