Elsevier

Journal of Geodynamics

Volume 35, Issue 3, April 2003, Pages 289-302
Journal of Geodynamics

Geothermal regime and genesis of the Ninety-East and Chagos-Laccadive ridges

https://doi.org/10.1016/S0264-3707(02)00068-6Get rights and content

Abstract

There are two well-founded hypotheses of the Ninety-East and Chagos-Laccadive Ridges genesis: the hypothesis of the extrusive source disposed at a junction of the spreading axis with transform fault and the hot spot hypothesis. The northward increasing trend of volcanism age and the average formation rate of the ridges calculated from the present deep sea drilling data (9 cm/a) shows their common genesis. Their origin is related to a northward migration of the Indian plate during the Late Cretaceous and Paleogene, with an average rate of 9 cm/a. The features of the age variations along the axes of the Ninety-East and Chagos-Laccadive ridges prove the relation of basement subsidence to the cooling model of the oceanic lithospheric evolution. Heat flow distribution in the vicinity of the ridges fits the same model of their oceanic origin and affiliation to the Indian plate. Assessment of the geothermal data and the basaltic age relate the origin of the Ninety-East and Chagos-Laccadive ridges to the hot spots hypothesis.

Introduction

The Ninety-East and Chagos-Laccadive aseismic ridges are the largest tectonic structures of the Indian Ocean (Fig. 1) . The Ninety-East Ridge separates the Central Indian Basin from the Cocos and West Australian Basins. The ridge has a S–N (slightly to the East) orientation and extends along 90° E more than 4500 km from 32° S to 10° N. The height of the ridge above adjacent deep Basins amounts, on the average, to 2000 m and its summit depth changes from 2000 m in the south to 3000 m in the north. The ridge is connected at an approximate right angle by the Broken Ridge at about 30° S.

The shallowest part of the Ninety-East Ridge is located near 25° S. The Osborn rise is situated at the west side of the ridge near 14° S. It has a roundish shape of ∼200 km in diameter. In the north from 10° N the ridge is buried under the Bengal Fan sediments. The Ninety-East Ridge basement is revealed by seismic investigations up to 12° N (Sclater & Fisher, 1974, Luyendyk, 1977, Perice & Weissel et al., 1989).

The Ninety-East Ridge is morphologically divided into three provinces (first—to the north of 7.5° S, second—between 7° S and the Osborn Rise, third to the south of the Osborn Rise). In the first province, the ridge consists of a series of blocks with north-northeast strike, showing en echelon structures similar to those of the Pacific seamount chains. In the second province, it is of straight-line configuration and has steep symmetrical slopes; the width of the ridge here is about 100 km. In the third province, the ridge is characterized by widening to 200 km and it becomes shallower. Its section is asymmetrical with the east flank steeper. The geological–geophysical investigations during the 58th cruise of R/V “Vityaz” revealed a block structure of the central Ninety-East Ridge. The blocks of the ridge are shifted northeast along faults, which have a prolongation in the Central Indian Basin (Bezrukov and Neprochnov, 1981). From 7° S up to 31° S, the eastern part of the ridge is bounded by subparallel narrow ridges and depressions (Fig. 2). A trench extends along the eastern flank of the Ridge, which is interpreted as a transform fault zone between the Indian and Australian plates.

The average thickness of carbonate sediments on the crest and the slopes of the Ninety-East ridge is 500 m by seismic data (P-wave velocities assumed Vp=2.0÷2.7 km/s). Under the sediments, rocks, occur with velocities Vp=3.9÷4.7 km/s and thickness 2.0÷4.7 km. A layer with Vp=6.7 km/s and thickness 3.0÷5.5 km is lying below. A boundary with Vp=7.7 km/s is at 8.5 km depth below sea-level (Bezrukov and Neprochnov, 1981).

The Ninety-East Ridge is characterized by a quiet sign-variable magnetic field with anomalies, which do not exceed, on the average, 100–200 nT. The magnetic anomaly 32 was identified in the lower part of the ridge north of the equator and traced in the Central Indian Basin (Shreider and Vorob'ev, 1988).

The Ninety-East Ridge is weakly expressed in the gravity field. The free-air anomalies over the ridge crest amount to 40–60 mGal, and are about zero at the equator (Kanaev, 1979).

The Chagos-Laccadive Ridge separates the Central Indian Basin and the Arabian Basin. It extends approximately for 3000 km from 12° S to the western Indian coast at 15° N (Fig. 1). The summit of the ridge rises above the sea level as the Chagos, Maldive and Laccadive coral islands.

The height of the Chagos-Laccadive Ridge relative to the adjacent deep basins, near the Maldive Islands, is up to 2 km and its width in the upper part is 75–150 km. Along the northern part of the eastern coast there is a steep trench with a depth of 2.5–3 km. The ridge is bounded by broad accumulative plains that suggest efficient sedimentary supply from coral shallows and ridge slopes.

In the south, the ridge has an asymmetrical structure (Fig. 3). Its east slope is steeper, up to 10°, as compared with the western slope (0.5°). In the Chagos archipelago area, near the eastern ridge foot, there is the Chagos trench. Note that the southern part of the ridge is rotated in the south-west direction. Its relief takes the orientation of the structures of the Central Indian spreading ridge (Kanaev, 1979).

According to seismic data, the thickness of carbonate sediments (Vp=3 km/s) at the Chagos-Laccadive Ridge crest does not exceed 600 m and at the ridge foot 1000 m. The crustal thickness between the Maldive and Laccadive Islands is about 15 km and includes a layer with Vp=5 km/s and a thickness of 3 km, and a layer with Vp=6.8 km/s and a thickness of 12 km. A refractor with Vp=8 km/s lies below. The ridge is characterized by a weak variable magnetic field up to 300 nT. The ridge also is weakly expressed in the gravity field. The free-air anomalies over its foot are −20 to −60 mGal and those in the mountains and islands area amount to 50 mGal (Kanaev, 1979).

A number of hypotheses were suggested regarding the origin of the Ninety-East Ridge. McKenzie and Sclater (1971) suggested that the Ninety-East Ridge was uplifted as a result of the interaction of the Indian and Australian plates during the Late Eocene–Oligocene change from their northward motion to the northeast direction. However, the lack of considerable free-air anomalies over the ridge assumes isostasy. This fact lessens the validity of this hypothesis (Bowin, 1973).

In Luyendyk (1997) it is pointed out that the Ninety-East Ridge was a remnant fragment of the continental crust. Deep-sea drilling data obtained by Glomar Challenger (Davies et al., 1974) showed that the ridge is not composed of continental rocks but of tholeiitic basalts. According to micropaleontological data (Sites 254, 253, 214, 216) basalt ages increase from the south (37 Ma) to the north (75 Ma). These ages are close to the ages obtained by magnetic data for the Central Indian Basin segment to the west. The analysis of the sediment facies and also the bathymetric data showed that the ridge was formed in shallow water. As the first approximation, the ridge basement subsidence follows the theoretical curve of the cooling model (Sclater et al., 1974, Parsons & Sclater, 1977).

Sclater and Fisher (1974) suggested the idea that the Ninety-East Ridge originated at the junction of a “leaky” transform fault with the western part of the ancient spreading axis of the South-East Indian Ridge. According to paleomagnetic and magnetic field data, after the beginning of the Gondwana break-up (∼190 Ma ago) India separated from Australia and Antarctica in the Early Cretaceous and moved to the north from the spreading axis along the transform fault. During about 80–40 Ma basaltic melting occurred at the junction of the spreading ridge with the transform fault (∼50° S) that caused the formation of the quasilinear ridge structure. From 32 Ma onward the Indian plate began to move to the northeast together with the Ninety-East Ridge. This was caused by the activity of the eastern part of the South-East Indian Ridge.

The other hypothesis of the Ninety-East Ridge genesis is its formation from a deep mantle source such as a plume—hot spot hypothesis (Morgan, 1972, Peirce, 1978, Duncan, 1978, Morgan, 1981, Curray et al., 1982). The volcanics obtained by deep-sea drilling from the Ninety-East Ridge are not similar to mid-ocean ridge depleted basalts, but are close to tholeiitic basalts of the transitional T–MORB type. Petrochemical analyses of the basalts shows that they are enriched in isotopes of incompatible elements such as Sr, Nb, Rb, Zr, Y, radioactive P and others (Peirce et al., 1989). In particular, for these basalts the 87Sr/86Sr ratio varies from 0.7044 to 0.7056, which considerably more than in basalts from spreading ridges (0.7022–0.7029). Such enriched basalts are typical for hot spot oceanic islands, for example, Hawaii, Galapagos.

To the northeast from Reunion Island at the eastern slope of the Mascarene Ridge (Nazareth Bank) JOIDES Resolution has drilled two Sites (705 and 706; Fig. 1). At Site 706 plagioclase basalts were recovered. Their age by paleontological data is 36 Ma. Directly at the Maldive Ridge five Sites (712, 713, 714, 716 and 716) were drilled. Besides, one more hole (Site 219) is located in the eastern part of this ridge (Whitmarsh et al., 1974). However, the basement was reached only at Sites 713 and 715, where olivine tholeiites (Site 713) and subalkaline olivine basalts (Site 715) were collected with isotopic ages of 49.6 and 57.5 Ma, respectively (Fisk and Howard, 1990). Hence, the lava age also increases to the North. It should be noted, that volcanics collected at the Mascarene and Maldive Ridges are enriched in isotopes of incompatible elements, Nb and Zr in particular (Duncan, 1991).

We have performed an analysis of the available geological–geophysical data and first of all, recent deep-sea drilling and geothermal data to make a choice between “leaky” transform fault and hot spot hypotheses of the origin of both ridges, to determine the rate of their growth and the applicability of the cooling model (Parsons and Sclater, 1977), their relationship to respective lithospheric plates. All the data from deep-sea drilling sites in the ridge area, heat flow data obtained by R/V Akademik Mstislav Keldysh, R/V Pegas, magnetic data by R/V Morskoi Geophysik, (Russian Academy of Sciences) and Indian Ocean heat flow data were used in the present study (Anderson et al., 1977, Sychev et al., 1987, Cochran et al., 1988, Verzhbitsky, 1996).

Section snippets

The analysis of geological—geophysical data in the area of the Ninety-East and Chagos-Laccadive Ridges

The sublongitudinal profile through the Ninety-East Ridge, with the locations of the deep-sea drilling sites is shown in Fig. 4. The absolute age of the basalts obtained during deep-sea drilling was measured by the K-Ar and 40Ar/39Ar methods (Duncan, 1978, Duncan, 1991).

Paleomagnetic data show that from Cretaceous till Early Oligocene the Indian plate moved to the north at about 8–10 cm/a (Duncan, 1978). In order to find the age changing along the Ninety-East Ridge we carried out model

Discriminating the hypotheses of the Ninety-East and Chagos-Laccadive ridges genesis

As stated earlier, an analysis of the deep-sea drilling, geothermal and other geological-geophysical data demonstrates the oceanic origin of the Ninety-East and Chagos-Laccadive Ridges, id est, in accordance with the cooling model of oceanic lithosphere evolution (Parsons and Sclater, 1977). There are two rather plausible hypotheses of ridge genesis related to the Indian plate displacement to the north from the Late Cretaceous to the Oligocene. The first hypothesis suggests that of a stationary

Conclusions

The analysis of the deep-sea drilling, geothermal and other geological-geophysical data leads to the following conclusions:

  • 1.

    The average rate of the Ninety-East and the Chagos-Laccadive Ridges formation calculated by the deep-sea drilling data is 9 cm/a. It shows that the genesis of these ridges is related to the northwards displacement of the Indian Plate in the Cretaceous–Paleogene time with an average velocity of 9 cm/a.

  • 2.

    The variations of the basaltic ages along the Ninety-East and

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