Abstract
The Chagos-Laccadive Ridge and its northern extension, the Lakshmi Ridge (CLLR), trending parallel with the coastline in the deep eastern Arabian Sea, is a continental fragment (with crustal thickness > 20 km). Sea-floor spreading-type magnetic anomalies are absent and crustal thickness is about 16 km in the region east of CLLR. West of the ridge, magnetic anomalies, from 5 to 28 on the magnetic time scale, are present and the crustal thickness is 11.5 km. The magnetic anomalies, crustal thickness, and considerations of land geology suggest that most of the sedimentary basins in the region east of the ridge were initiated during the rifting stage in the Late Cretaceous, whereas those in the western region evolved during sea-floor spreading since 64 m.y. ago. The NNW-SS and to some extent northeast-southwest and ENE-WSW basement trends, as well as associated horsts, grabens, and growth faults in the eastern region, formed as a result of reactivation of the ancient Precambrian trends observed on the Indian shield during and after rifting, and have determined the shapes, extents, and tectonic styles of the sedimentary basins there. The acoustic structure of sediments suggests that a basal sedimentary layer with a velocity of 4.0 to 4.3 km/sec is present in the region east of CLLR, but is absent west of it. This sediment layer, believed to be composed of clastics, volcaniclastics, and limestone, was probably deposited during the rifting stage. The seismic layers and the velocity structure (1.9 to 3.5 km/sec) of the overlying sediments are similar both eas and west of the CLLR and suggest similar influences on sedimentary evolution in both the eastern and western regions during sea-floor spreading. However, sea-level changes during the Cenozoic in conjunction with tectonics resulted in several unconformities in the shelf sedimentary sequences. By about Oligocene and Miocene times, with the closure of Tethys Sea and the uplift of Himalayas, terrigenous sediments from the Himalayas became important for the northern margin and initiated the Indus deep-sea fan. End_of_Article - Last_Page 611------------
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