Seabed Fluid Flow

7.01 – Chemical Composition and Mineralogy of Marine Sediments

In this chapter, a review of the composition of marine sediments is presented, first addressing pelagic sediments in general, and then considering several specific types of sediments, each dominated by hydrogenous components. In each section, a review of the mineralogy of sediments is followed by an examination of their geochemistry using enrichment factors and factor analysis.

9.1 – Chemical Composition and Mineralogy of Marine Sediments

This is the first detailed report and analyses of deformation from the W part of the Deccan large igneous province (DLIP), Maharashtra, India. This deformation, related to the India–Seychelles rifting during Late Cretaceous–Early Paleocene, was studied, and the paleostress tensors were deduced. Near N–S trending shear zones, lineaments, and faults were already reported without significant detail. An E–W extension was envisaged by the previous workers to explain the India–Seychelles rift at ~64 Ma. The direction of extension, however, does not match with their N–S brittle shear zones and also those faults (sub-vertical, ~NE–SW/~NW–SE, and few ~N–S) we report and emphasize in this work. Slickenside-bearing fault planes, brittle shear zones, and extension fractures in meso-scale enabled us to estimate the paleostress tensors (directions and relative magnitudes). The field study was complemented by remote sensing lineament analyses to map dykes and shear zones. Dykes emplaced along pre-existing ~N–S to ~NE–SW/~NW–SE shears/fractures. This information was used to derive regional paleostress trends. A ~NW–SE/NE–SW minimum compressive stress in the oldest Kalsubai Subgroup and a ~N–S direction for the younger Lonavala, Wai, and Salsette Subgroups were deciphered. Thus, a ~NW/NE to ~N–S extension is put forward that refutes the popular view of E–W India–Seychelles extension. Paleostress analyses indicate that this is an oblique rifted margin. Field criteria suggest only ~NE–SW and ~NW–SE, with some ~N–S strike-slip faults/brittle shear zones. We refer this deformation zone as the "Western Deccan Strike-slip Zone" (WDSZ). The observed deformation was matched with offshore tectonics deciphered mainly from faults interpreted on seismic profiles and from magnetic seafloor spreading anomalies. These geophysical findings too indicate oblique rifting in this part of the W Indian passive margin. We argue that the Seychelles microcontinent separated from India only after much of the DLIP erupted. Further studies of magma-rich passive margins with respect to timing and architecture of deformation and emplacement of volcanics are required.

Near N–S paleo-extension in the western Deccan region, India: Does it link strike-slip tectonics with India–Seychelles rifting?

Manganese nodules occur as two-dimensional deposits in abyssal plains of all major oceans. In the Clarion-Clipperton Zone of the northeast equatorial Pacific alone, the amount of nodules is estimated to 21 billion tons indicating the huge potential of this deposit type. Apart from manganese, metals of economic interest are nickel, copper, and cobalt, but the nodules also contain interesting amounts of molybdenum, titanium, lithium, and the rare earth elements. Therefore they are also called as polymetallic nodules.

Composition, Formation, and Occurrence of Polymetallic Nodules

The Indian Ocean Nodule Field: petrotectonic evolution and ferromanganese deposits

Hydrosweep mapping of crust in the Central Indian Ocean Basin reveals abundant volcanoes ocurring both as isolated seamounts and linear seamount chains parallel to flow lines. Their shapes, sizes and overall style of occurrence are indistinguishable from near-axis seamounts in the Pacific. Evidence from seamount morphology, distributions and petrography of dredged samples suggests that they were generated near the fast-spreading Southeast Indian Ridge at 50–60 Ma. If so, this style of near-axis seamount generation may be a result of fast-spreading rate rather than a peculiarity of the present Pacific spreading ridges. In fact, the results of several recent studies, taken together, suggest that the style of axis/near-axis seamount volcanism varies systematically as a function of spreading rate.

Basinal seamounts and seamount chains of the central Indian Ocean: probable near-axis origin from a fast-spreading ridge

Previous studies on the distribution and morphology of ancient seamount chains (> 50 Ma) in the Central Indian Ocean basin (CIOB) indicated their generation from the fast spreading Southeast Indian Ridge. Here we describe the petrology of some of these seamounts. Fresh glass veneers of pillow basalts from these seamounts were analyzed by electron microprobe. The rocks show a low content of TiO2 and FeO and moderate Mg# suggesting slow eruption of a minimally fractionated N-MORB magma. In terms of chemistry and morphology, CIOB seamounts are indistinguishable from seamounts across slow spreading MAR and fast spreading EPR.

Petrology of seamounts in the Central Indian Ocean Basin: Evidence for near-axis origin

parameters, both natural (atmospheric, physical, chemical and geological) and anthropogenic (pollution, tourism, deforestation). We submit here an assessment of changes in shoreline pattern of 14 islands in Indian Sundarbans (ISD) since 1979. Using multi temporal satellite images of LANDSAT, we found that as many as four islands within ISD have lost area in excess of 30%. While the area loss for another three islands has been between 10 and 30%, five islands show minor loss of area (<10%).The shoreline variations in two other islands, however, are of extreme nature. While the Lohachar Island on the river Hoogly was completely submerged by the end of the last century, the neighboring Nayachar Island on the same river has gained more than 30 km 2of land area.An interesting difference in nature of land loss is seen between western and eastern group of islands. Over the last 32 years, the total loss of area in six western islands (Ghoramara, Sagar, Jambudwip, Mousuni, Namkhana, Lothian) has been 23.64 km 2, while another six islands from eastern Sundarbans (Surendranagar, Dhanchi, Bulchery, Chulkati, Dalhousie and Bhangaduani) have lost about 53.85 km 2of land area. The role of vari ous forcing parameters, such as long shore current, abnormal tidal heights, subsi dence, tilting of crust and sub -surface geomorphology, in effecting changes in shoreline pattern in 14 Islands of the Indian Sundarbans are discussed.

Decadal Changes in Shoreline Patterns in Sundarbans, India

The nearly 7,500 km long coastline of India is remarkably varied and dynamic and displays diverse rock-based, sediment-based and coral-based landforms. Coastal processes along the Indian coasts are controlled largely by monsoons. The Arabian Sea coast differs from the Bay of Bengal coast in several respects. The east coast is wider, with several large deltas, large lagoons, one of the world’s largest mangrove wetland (Sundarbans) and long stretches of sandy beaches backed by dunes or ridges. In comparison, the west coast is more indented with rocky headlands, intervening sandy bays and multiple estuaries. Cliffs and associated features are relatively more common. A large saline marshland and lagoon-barrier complexes (kayals) are some of the noteworthy features along the west coast.

Ranadhir Mukhopadhyay

Papers

The Indian Ocean Nodule Field: petrotectonic evolution and ferromanganese deposits

Basinal seamounts and seamount chains of the central Indian Ocean: probable near-axis origin from a fast-spreading ridge

Petrology of seamounts in the Central Indian Ocean Basin: Evidence for near-axis origin

Decadal Changes in Shoreline Patterns in Sundarbans, India

The Indian Coastline: Processes and Landforms