Geodynamics of the Amirante Ridge and Trench Complex, Western Indian Ocean

Abstract

The formation and evolution of the ∼600 km long arcuate Amirante Ridge and Trench Complex (ARTC) is a significant geomorphic-structural feature in the Western Indian Ocean (WIO). The WIO contains evidence of at least two major magmatic episodes followed by continental rifting within the span of a little more than 20 million years. This involved the splitting of Madagascar from India at around 85 Ma and then separation between India and the Seychelles at 64-63 Ma as a possible consequence of two powerful volcanic eruptions from the Marion and Reunion hot spots, respectively. Formation and evolution of the ARTC represents this tumultuous period in the Indian Ocean, approximately between 85 and 60 Ma (Late Cretaceous-Early Tertiary).We integrated geophysical, palaeomagnetical, and petrological data to examine three existing models that attempt to explain the formation of ARTC. In contrast, our study hints at several stages of extension and compression responsible for its formation. Our integrated data also suggest that the Carlsberg Ridge may have played a prominent role in the evolution of the ARTC that seems to have formed through a ridge-jump process after the conjugate spreading centres -Mascarene and Palitana ridges formed earlier during the India-Madagascar separation -ceased spreading because of violent eruption of the Reunion hot spot at around 65 Ma. The eruption disturbed the plumbing system of magma ascent, resulting in cessation of spreading along the conjugate spreading centres, forcing a ridge jump.A collage of seismic refraction and reflection, free-air gravity, magnetic anomaly data, and Ar dating of rocks indicates that as the Carlsberg Ridge swept the Seychelles towards south, the crust between Madagascar and the Seychelles was increasingly compressed, with the abandoned northern Mascarene spreading centre absorbing the maximum stress. With continued compression, the western limb of the abandoned spreading ridge was thrust below the eastern limb to a limited degree. This partial subduction agrees with the gravity and seismic results. Our new study also accounts for the anomalous presence of 14 km-thick oceanic crust beneath the ARTC and its characteristic difference in petrology with other established subduction zones in the world.

A relook into the crustal architecture of Laxmi Ridge, northeastern Arabian Sea from geopotential data

“…During the period 89 and 85 Ma, as Indian plate passed over the Marion plume, Madagascar separated from India (Agarwal et al 1992;Storey et al 1995) leading to conjugate spreading along the Mascarene and Palitana Ridges. By integrating seismic refraction and reflection, free-air gravity, magnetic anomaly data, and Ar dating of rocks, Mukhopadhyay et al (2012) opined that spreading along the Palitana Ridge split the Laxmi Ridge and Seychelles from India (between 79 and 66 Ma). Further, Samal et al (2012) from studies of high resolution seismic, free-air gravity and well data in the west coast of India suggested that the gravity low along the Laxmi Ridge is due to the down-warping at the lower crust as a result of excessive magmatic material below the ridge.…”

Geochemical heterogeneity along the Vema Fracture Zone, Indian Ocean: Mixing of melts from the Reunion plume and the Central Indian Ridge

“…These discontinuities are expected to have profound effects on ridge petrology, melt dynamics, and evolution of the seafloor (Gregg, Behn, Lin, & Grove, ; Iyer & Ray, ; Klein & Langmuir, ; Langmuir, Bender, & Batiza, ; Lin & Morgan, ; Macdonald, Scheirer, & Carbotte, ; Mukhopadhyay & Iyer, ; Mukhopadhyay, Iyer, Ray, Karisiddaiah, & Drolia, ; Ray, Iyer, Banerjee, Misra, & Widdowson, ; Tolstoy, Harding, & Orcutt, ). Also the northern Indian Ocean floor was perforated in the geological past by at least four major mantle plumes, two plumes apiece contributed in the making of the eastern and the western continental margins of India (Mukhopadhyay, Karisiddaiah, & Ghosh, ; Todal & Edholm, ). Among these, while the Marion and Reunion plumes contributed significantly to the formation of western continental margin of India, the other two, Kerguelen and Crozet plumes, influenced the construction of the eastern continental margin.…”

“…In fact between 90 and 85 million years (Ma), the first episode of flood basalt volcanism occurred along the western margin of the north‐moving Indian Plate (then positioned in the southern hemisphere) when the Marion Plume erupted. This volcanism prompted the separation of Madagascar from India (Agrawal, Pandey, & Negi, , Mahoney, Duncan, Khan, Gnos, & McCormick, ; Mukhopadhyay et al, ). The second episode of basalt volcanism in the north‐western Indian Ocean had been from the Reunion plume.…”

“…Both these eruptions were slow and effusive. The entire eruption from Reunion plume on the Indian sub‐continent started at about 74 Ma and can be divided into clearly distinguishable four episodes—moderate to high rate of eruption of lava between 74 and 68 Ma, a slow surge between 68 and 66 Ma, a sudden extremely rapid spurt between 66 and 65 Ma, and again a slow to moderate rate of upsurge between 65 and 62 Ma (Chatterjee et al, 2013; Mukhopadhyay et al, ).…”

The Indian Coastline: Processes and Landforms

“…The western continental margin has several ridge-like features, which run parallel to the coast and also have significantly controlled the sedimentation pattern along the margin. The architecture of this margin seems to have been affected by three events-two episodes of continental separation associated with episodic eruptions from two plumes-the older Marion Plume in the south and the younger Reunion in the north, and the Deccan volcanism (Mukhopadhyay et al 2012). …”