Nicobar Fan Research Articles

The Himalaya-Bengal Fan Denudation-Accumulation System during the past 20 Ma

Mass balances for both denudation in the Himalayas and sediment accumulation in the Subhimalayan basins, including the Bengal deep-sea fan but excluding the Indus fan, yield 7.1 × 106 km3 and 7.4 × 106 km3 (s ± 20%, rock of 2.75 g/cm3 density), respectively, for the past 20 million years. Coarsening and increased sediment accumulation rates in the foreland basin and in the Bengal foredeep indicate accentuated tectonic activity and unroofing in the Himalayas since that time. The sediment volume includes ≥1 × 106 km3 of Neogene Bengal fan sediment that was lost via the Nicobar fan to the Sunda accretionary wedge. In addition, the Indian peninsular rivers contributed about c. 0.6 × 106 km3 of solid load to the basins. Average denudation during the past 20 m.y., as derived from geothermobarometric data and restored cross sections, occurred most rapidly along the High Himalayan crystalline chain (vertical unroofing c. 1000 m/m.y.; northward lateral retreat of southern Himalayan slope, exposed to monsoonal rain, ≤3.5 km/m.y.) and much slower in the Tethyan sedimentary zone to the north (average 150 m/m.y.). The solute loads of the modern Himalayan rivers indicate a mean chemical denudation rate of 17 m/m.y. The distinct decrease in sediment accumulation on the outer Bengal fan between about 7 and 1 Ma (in contrast to the Indus fan) is probably caused by exogenic factors rather than by a significant decline in tectonic activity. Pre-20 Ma sediments in the Subhimalayan basins were derived mainly from the southern margin of the Tibet plateau or from sources outside the study area.

Sediment volume and mass beneath the Bay of Bengal

Rates of sediment accumulation and the amount of sedimentary fill in depocenters lying downstream of erosion in the Himalayas and Tibet can provide some insight into tectonics and geological history. The objective of this paper is to put on record the best estimates which are possible with existing data of the volume and mass of sediments, sedimentary rock and metasedimentary rock beneath the sea floor of the Bay of Bengal. The sedimentary section in the Bay of Bengal is divided into two parts: (1) Eocene through Holocene, sediments and sedimentary rocks which post-date the initial India-Asia collision: volume = 12.5 × 10 6 km 3; mass = 2.88 × 10 16 t; this is most of the Bengal Fan, including its eastern lobe, the Nicobar Fan, plus some of the outer Bengal Delta; (2) Early Cretaceous through Paleocene, pre-collision sedimentary and metasedimentary rocks: volume = 4.36 × 10 6 km 3; mass = 1.13 to 1.18 × 10 16 t; these are interpreted as continental rise and pelagic deposits.

A comparison of velocity and attenuation between the Nicobar and Bengal Deep Sea Fans

The compressional wave velocity and attenuation measurements from a deep source, deep receiver seismic refraction experiment from the Nicobar Fan, Indian Ocean, are presented and compared to a similar set of measurements in the Bengal Fan. The two deep sea fans consist of thick sections of turbidites derived from the same sediment source but have slightly different depositional histories. Travel time inversion of the data into a velocity depth model uses the tau‐zeta linear inversion scheme. We present a revised reparameterization of the travel time data and improved error analysis for the inversion process. Although the velocity structures with depth are morphologically similar, there exist discrete differences between the two stations. In the Nicobar Fan, the initial velocity (assumed) is 1.513 km/s, with an initial gradient of 2.32 s−1. The velocity increases smoothly with depth, while the velocity gradient decreases rapidly to a constant 0.81 s−1. The Bengal Fan station showed the same trend, although the velocity gradient at depth was only 0.67 s−1. The attenuation profiles are similar only in gross structure, with the Nicobar Fan values of Q−1 being a factor of 3 to 4 less than the Bengal station. The differences in velocity and attenuation structure can be attributed to a number of causes, including porosity and sedimentation rates (which are related to the distance from the sediment source) and the presence of a normal fault near the Nicobar Fan site. This fault may act as a conduit for water removal in the presence of overburden pressure. It is believed that porosity changes, due to whatever cause, dominate the lateral and vertical variations of velocity, velocity gradients, and (together with intrabed multiples) attenuation in thick sedimentary sections.

Sediment sound velocities from Sonobuoys: Sunda Trench and forearc basins, Nicobar and Central Bengal Fans, and Andaman Sea Basins

Supplement is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document B83‐007; $2.50. Payment must accompany order.Measurements of mean sound velocities in the first, largely unlithified layers in the seafloor were made using the sonobuoy technique in several areas in the northern Indian Ocean. Older measurements were added to new measurements, and regressions for mean and instantaneous velocity versus one‐way travel time of sound are presented for the central Bengal Fan, the central Andaman Sea Basin, the Nicobar Fan, and the Sunda Trench. New data and regression equations are presented for the Mergui‐north Sumatra Basin and for four forearc basins between Sumatra and Java and the Sunda Trench. Minimum velocity gradients were found in those areas where sedimentation rates were high, and sediments have accumulated in thick sections which have not had time to fully consolidate (porosity in the top of the sediment section has not been fully reduced under overburden pressure). These minimum velocity gradients (just under the seafloor) were found in the four forearc basins where they ranged from 0.34 s−1 to 0.84 s−1 with an average of 0.58 s−1. The near‐surface velocity gradient in the Sunda Trench was 1.33 s−1, but was higher in the adjacent, fossil Nicobar Fan (1.62 s−1). In the surface of the Bengal Fan the velocity gradient was low in the upper fan (0.86 s−1), high in the central fan (1.94 s−1), and again lower in the southern fan (1.18 s−1), which may support sedimentation models calling for bypassing of the central fan and higher rates of accumulation on the southern fan.

New constraints on the tectonic evolution of the eastern Indian Ocean

From marine magnetic anomaly studies, a fossil spreading ridge is identified beneath the Nicobar Fan in the northwestern Wharton Basin. Several north-south-trending transform faults offset this ridge left-laterally east of the 86°E transform fault. Our findings show that this ridge, which was part of the plate boundary between the Indian and Australian plates, ceased its spreading shortly after formation of magnetic anomaly 20 (∼ 45.6m.y. B.P.). Since the breakup of Australia and Antarctica probably occurred sometime between 110 and 90 m.y. B.P., we suggest that the Indian, Australian, and Antarctic plates were moving relative to one another from about 90 to 45 m.y. B.P. A triple junction would have existed in the southeastern Indian Ocean during that period of time. At anomaly 19 time (∼ 45m.y. B.P.), the junction became inactive, and Australia and India became a single plate. The northwest-southeast-trending Southeast Indian Ridge was formed by connecting the India-Antarctica spreading center with the Australia-Antarctica spreading center. Its activity has continued to the present time.

The Bengal Submarine Fan, Northeastern Indian ocean

Bengal Submarine Fan, with or without its eastern lobe, the Nicobar Fan, is the largest submarine fan known. Most of its sediment has been supplied by the Ganges and Brahmaputra Rivers, probably since the Early Eocene. The “Swatch-of-No-Ground” submarine canyon connects to only one active fan valley system at a time, without apprent bifurcation over its 2500-km length. The upper fan is comprised of a complex of huge channel-levee wedges of abandoned and buried older systems. A reduction of channel size and morphology occurs at the top of the middle, fan, where meandering and sheet flow become more important.

Acoustic stratigraphy, structure, and depositional history of the Nicobar Fan, eastern Indian Ocean

The Nicobar Fan is a topographically isolated segment of the Bengal Deep-Sea Fan. Seismic reflection data indicate that the Nicobar Fan may have begun forming in late Miocene to early Pliocene time, but considerably later than the Bengal Fan. Since then, the original basement relief has been gradually inundated and buried by fan sediments, although numerous basement peaks still pierce the surface of the fan. Sediment thicknesses in excess of 1.6 km occur in the northern part of the fan. The fan surface is marked by turbidity-current channels and numerous faults which, in some cases, may have acted as channels. The faults are apparently related to tensional stresses associated with a descending lithosphere and to intraplate seismic activity. Relatively few channels are encountered on the fan, and they are noticeably absent on the northern part. Most of the sediment filling the northern Java Trench consists of depressed fan sediments, although deposition from local sources is evident. Plate motion has gradually isolated the fan from its major source of sediment to the extent that a buried “fossil” fan surface exists in the southeastern portion of the survey area, and much, if not all, of the remaining Nicobar Fan is now receiving only pelagic sediments.

Sediment velocities from sonobuoys: Bengal Fan, Sunda Trench, Andaman Basin, and Nicobar Fan

New measurements of interval compressional wave velocities were made in the first sediment layer using the sonobuoy technique during two expeditions in the Bay of Bengal, in the Andaman Sea, and over the Nicobar Fan and Sunda Trench. Sediment interval velocities from these areas were added to those previously reported, and revised diagrams and regression equations of instantaneous and mean velocity versus one-way travel time are furnished for four areas of the Bengal Fan, and for the Anadman Basin, Nicobar Fan, Sunda Trench. The velocity gradients directly below the sea floor were used to separate the Bengal Fan into four geoacoustic provinces. In the north and west the velocity gradients are 0.86 and 1.28 s/sup -1/, respectively, whereas in the central part of the fan the gradient is 1.87 s/sup -1/. These variations indicate lesser increases of velocity with depth in the sea floor in the north and west, and they are probably due to more rapid deposition, less consolidation, and less lithification near the riverine source areas of the sediments. The near-surface velocity gradients in the other areas are the Andaman Basin, 1.53 s/sup -1/, the Nicobar Fan 1.63 s/sup -1/, and the Sunda Trench, 1.41 s/sup -1/. Themore » linear velocity gradients (from the sediment surface to a given travel time) in 17 areas of the Indian Ocean, Pacific area, Atlantic Ocean, and Gulf of Mexico were averaged at each 0.1 s from 0 to 0.5 s of one-way travel time. These averaged gradients ranged from 1.32 s/sup -1/ at t=0 to 0.76 s/sup -1/ at t=0.5 s. The regression equation for the velocity gradients a, in s/sup -1/, as a function of one-way travel time t, in seconds, is a=1.316-1.117t (for use from t=0 to 0.5 s). These average velocity gradients can be used with sediment surface velocities and one-way travel times (measured from reflection records) to compute sediment layer thickness in areas of turbidites lacking interval velocity measurements in the first sediment layer.« less