14. SOME ASPECTS OF PLATE TECTONICS IN THE ARABIAN SEA Robert B. Whitmarsh, National Institute of Oceanography, Wormley, Godalming, Surrey, United Kingdom INTRODUCTION The arguments presented by McKenzie and Sclater (1971), that the ridge was built up over a transform fault The sediments and underway data collected during Leg during the early Tertiary pause in sea-floor spreading, are 23, besides contributing to knowledge of the oceanic circumstantial. Their argument may be paraphrased as paleo-environment, also contribute to three specific aspects follows. Identical sea-floor spreading magnetic anomalies, of the plate tectonics history of the Arabian Sea. These are attributable to the Heirtzler et al. (1968) time-scale of re- the origin of the Laccadive-Chagos Ridge, the identification versals, can be recognized both in the northwest Arabian Sea of magnetic anomalies and fracture zones in the Arabian (around 15°N, 65° E) and south of India (around 10°S, Sea, and the origin of the sea floor between the Owen 75°E). These anomalies cannot be definitely identified Fracture Zone and Arabia. These particular subjects are in-between these two areas. The anomalies have an treated separately below. east-west trend and therefore a transform fault or faults must have existed to explain the almost 3000-km ORIGIN OF THE LACCADIVE-CHAGOS RIDGE right-handed offset of the anomalies. One right-handed The Laccadive-Chagos Ridge is a major linear feature on transform fault of unknown offset and trend (named the floor of the Indian Ocean. The ridge extends a distance Anahita) can be postulated in the Arabian Sea. This of 2200 km in a north-south direction, from the Laccadive transform fault may be parallel to the Laccadive-Chagos Islands at 14°N to the Chagos Islands at 6°S. The ridge is Ridge; if so, it supports the hypothesis that the slightly arcuate being concave to the west. A considerable Laccadive-Chagos Ridge was also a transform fault, a fault length of the crest of the ridge is composed of shoals, which must have had a very large offset. McKenzie and banks, coral reefs, and atolls at depths less than 1500 Sclater conclude their argument by pointing out the meters, and broadly these shoal areas are grouped as the existence of two more right-handed fracture zones south of Laccadive, Maldive, and Chagos islands. Between the shoals India (named Indira and Indrani). The strike of the Indira the water depth is commonly around 2000 meters and Fracture Zone, due north, is poorly determined but roughly exceptionally reaches 4000 meters between the Maldive and parallels the Laccadive-Chagos Ridge. The strike of the Chagos islands. Site 219 was drilled in one such gap Indrani Fracture Zone is indeterminate. between Minicoy Island and the Laccadives. To the west of It seems to the present writer that one major condition the ridge there is a relatively flat broad area of sea floor at that should be satisfied by the axis of the Laccadive-Chagos about 4000 meters depth, around 500 meters above the Ridge, if it was a transform fault, is that it should be adjacent abyssal plain, and it was on this platform that Site "parallel" to the trace of the Owen Fracture Zone (i.e., the 220 was drilled. South of where it merges with the Indian ridge and the Owen Fracture Zone should be on small continental slope, to about 3°N, the east flank of the ridge circles about the same pole) since the latter was certainly an is bounded by abyssal plain sediments at 2500 meters active transform in the Early Tertiary (see Whitmarsh et al., depth. These sediments are 1.5 to 2.5 km thick east of the this volume). Clearly the ridge is not "parallel" to the Owen Laccadive Islands (Eremenko, 1966). South of the equator Fracture Zone. Further, careful study of a number of a scarp is developed, which drops down to at least 4500 bathymetric and seismic reflection profiles obtained over meters, with a parallel ridge and trough (the Chagos the west flank of the Maldives reveals that there are a Trench) flanking the Chagos mass. number of parallel north northeast sea-floor or subbottom The ridge has been variously explained, as a transition scarps (Figure 1). A similarly trending deep trough exists at zone between the Indian continent and the ocean (Narain 7°N, 69°45'E in crust which is demonstrably of Eocene age et al.,1968); as a fracture zone (McKenzie and Sclater, (see next section). This trend intersects the north-south to 1971); and as a hot-spot ridge (Francis and Shor, 1966; 350° axis of the Laccadive-Chagos Ridge at this lattitude. It Morgan, 1972). Narain et al. believe the ridge is a is very likely that these scarps and the trough represent transitional region simply because the crustal thickness small fracture zones formed during the Early Tertiary determined by seismic refraction shooting (Francis and sea-floor spreading. Thus, it appears that the Laccadive- Chagos Ridge was not a transform fault in the Early Shor, 1966) is intermediate between that normally found Tertiary. under continents and oceans. This argument is invalid because similar intermediate crustal thicknesses have been McKenzie and Sclater did not discuss the possibility that observed under a chain of volcanic islands in the central the large net offset of anomalies between the Arabian Sea Pacific (Hill, 1969; Furumoto et al., 1971). Also, normal and south of India could as well have been produced by a oceanic crustal structure and recognizable sea-floor spread- small number of fracture zones each with an offset of only ing anomalies were found just east of Chagos Bank (Francis some hundreds of kilometers. This explanation is favored and Shor, 1966; McKenzie and Sclater, 1971). Thus, it by the writer and is expanded in the next section. seems impossible that the ridge represents a west to east Thus, only the explanation of Morgan (1972) for the transition between ocean and continent. Laccadive-Chagos Ridge remains. His hypothesis is that 527 R. B. WHITMARSH the Deccan Traps-Reunion line and a change in the rate of decrease corresponding (1) to the beginning and end of the static or very slow-spreading phase postulated by McKenzie and Sclater and (2) to the spreading ridge crossing the hot-spot, if the ridge was actively spreading at the time. All known ages are plotted in Figure 2. This figure shows both a systematic decrease in age of volcanism as well as a marked change in the rate of age decrease along the ridge at Deccan Traps Site 219 40 Central Indian Ocean Mauritius Chages Bank Maldives 0 π 2000 KILOMETERS 4000 Figure 2. Plot of isotope ages and age estimates of the periods of volcanism along the Deccan Trap-Reunion hot-spot ridge. Deccan Trap age from Wellman and McElhinny (1970); Site 219 age is a minimum since igneous basement was not reached (see Chapter 3); the 7°n 72°E date at 1000 km is a minimum for the time when the spreading ridge crossed the hot-spot deduced from the Figure 1. Sea-floor scarps deduced from collected sound- oldest sediments overlying basalt at Site 238 at the foot ings and from seismic reflection profiles obtained by of the Chagos Bank (Fisher et al, 1971); dates for Glomar Challenger, Conrad, and Oceanographer. Each Mauritius and Reunion from McDougall and Chamalaun long dash denotes one observation of a scarp with the (1969). square or line (soundings or reflection profile, respectively) denoting the side on which the scarp face the point where the hot-spot passed under the spreading lies. Short dashes represent interpolations between ridge. Thus, the evidence supports a hot-spot origin for the observations. Oceanographer data courtesy of N.O.A.A., ridge. It also appears that the hot-spot crossed a spreading Atlantic Oceanographic and Meterological Laboratories, plate boundary before 32m.y. ago. The data are too Miami; Conrad data from cruise CONRAD-9, courtesy of scattered to resolve variations due to any period of the Lamont-Doherty Geological Observatory. nonspreading in the Early Tertiary. A further line of evidence is provided by the subsidence currently a hot-spot exists beneath the island of Reunion history of Site 219. This site was under very shallow water situated on the African plate. In the past the Madagascar in the Paleocene, and the sea floor began to sink in the plate has moved north-east, relative to the mantle, and Early Eocene. Total subsidence was of the order of 2100 consequently the Reunion-Nazareth Bank line of volcanic meters, but there is no indication of when sinking ceased, seamounts and islands was formed. During some period or whether it still continues today. It can also be inferred prior to the formation of the Central Indian Ocean ridge, from the coral atolls and reefs that the Chagos Bank, the and due to the northward drift of the Indian plate, the Maldives, and a large part of the Laccadives were also close hot-spot lay successively beneath peninsular India (Deccan to sea level, if not subaerial, before the initiation of coral Traps), the Laccadives, the Maldives, and Chagos Bank, thus reef growth, and that the upward growth of the reefs has forming the Laccadive-Chagos Ridge. Francis and Shor kept pace with subsequent subsidence. Thus there is good (1966) favored the migration of volcanity in the opposite evidence that most of the foundation of the Laccadive- direction, but since their hypothesis does not take into Chagos Ridge was very close to sea-level at some stage. It is account the Reunion-Nazareth Bank line, it is not pursued rare in the oceans that fracture zones have islands along further here.