Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

A. R. Crawford ¹

Affiliations
1 Department of Geophysics and Geochemistry, Institute of Advanced Studies, Australian National University, Canberra, AU

Abstract

New palaeomagnetic data clarify the manner of break-up of Gondwanaland and show that India was adjacent to Africa and Antarctica, but not Australia, during the Phanerozoic. This does not conflict with possible contiguity of parts of India and Australia before Gondwanaland came into existence.

Relative movement of the pole and a Gondwanaland which is known to have existed at least from the Cambrian to the time of break-up in the late Mesozoic implies non-synchroneity of glaciations geographically distant from one another, the products of which have also been scattered by drift. Precambrian tillites are unlikely to be synchronous; glaciations merely follow the poles.

A difference is established between the Indian peninsula. the Indian shield and Gondwanic India. The development of India has been in five stages; before Gondwanaland existed, during its life-span, during association with Madagascar while separate from Africa, during movement as Gondwanic India, and since collision and fusion with Asia.

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A. R. Crawford ¹

Affiliations
1 Research School of Earth Sciences, Australian National University, Canberra, AU

Abstract

The mount Abu Granite of south-western Rajasthan and related 'Erinpura type' granites in Gujarat give Rb-Sr total rock ages of about 735 m.y. They are therefore of the same age as the Malani Igneous Suite of western and south-western Rajasthan and not older as previously believed. Though possibly updated by re-heating related to emplacement of that Suite, it seems more probable that the two were emplaced at about the same time. This could have taken place in a tectonic environment created by sinistral shear of what are now northern and southern Peninsular India along the Narmada-Son lineament, with bending of the southern end of a rectilinear Aravalli orogenic belt by major fault drag.

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A. R. Crawford ¹

Affiliations
1 Department of Geology, University of Canterbury, Christchuch, NZ

Abstract

An extension of the Narmada-Son lineament of peninsular India can be traced across northern Madagascar, the lineament having been continuous in Gondwanaland. The Indian section, now a narrow seismicrift, appears to be a deep fault. It influenced Precambrian and Phanerozoic sedimentation and is the locus for post-Deccan Trap carbonatite and fluorite occurrences. The Malagasy section shows comparable unusual rocks. Postulated sinistral movement along the Indian section of about 450 km culminating at 750 Ma BP was associated with volcanism, plutonism and base metal mineralisation. The structure may not then have been in existence in Madagascar.

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A. R. Crawford ¹

Affiliations
1 Department of Geology, University of Canterbury, Christchurch, NZ

Abstract

Peculiar aspects of the essentially southerly drainage of Burma relate to Late Cenozoic uplift, since establishment of its major rivers, of a horst 70km wide trending NNW-SSE which extends from the Patkai (Naga) Hills of the Fold Belt to the Shan Plateau. The lineaments extend into the Plateau as fracture zones. This trend affects much of Burma, dominating the Plateau and influencing locally direction of rivers in the Central Lowland. The jadeite deposits of northern Burma occur where the Burman Volcanic Belt crosses the horst. The Bawdwin and other base metal deposits are concentrated near the crossing of the horst lineaments with others, gently arcuate, eastwest in Burma and curving northwards into China The horst appears to be associated with dornal uplift which affected northernmost Burma and possibly the easternmost Himalaya.

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A. R. Crawford ¹

Affiliations
1 Department of Geology, University of Canterbury, Christchurch, NZ

Abstract

The mantle convection pattern under India as calculated by H-S Liu shows only two points of upwelling, which correspond with the Lonar crater and the Girnar circular complex. This suggests that Lonar is more likely to be endogenic rather than of impact origin. The Ramgarh crater, having no such correspondence, is more likely to be an impact. The pattern also shows upwelling in the vicinity of the Taftan volcano of southeastern Iran.

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A. R. Crawford ¹

Affiliations
1 Department of Geophysics and Geochemistry, Australian National University, Canberra, AU

Abstract

Many new Rb-Sr age determination analyses are reported for the crystalline complex of southern Peninsular India. These are mostly total-rock ages, often isochrons.

Data are still insufficient for a reliable geochronology. Rocks of apparent ages ranging from over 3000 m.y. to 720 m.y. have been dated, and a lower Palaeozoic event reflected in mineral ages of about 500 m.y., known to have affected all Ceylon and much of the east coast of India, has been found as far north as Coimbatore.

The oldest rocks have been found in Kerala, the Nilgiri Hills and southern Mysore. One age of 2700 m.y. has been found in Kerala. The craton of Mysore-Hyderabad has an age of at least 2585 ± 40 m.y., which is the apparent age of the Peninsular Gneiss over a wide area. The age of the Dharwar System remains uncertain, although the lavas near Chitradurga give an isochron at 2345 ± 60 m.y. The Chitradurga Granite has an age between 2450 and 2400 m.y. The Closepet Granite presents difficulties arising from its poor definition, but contains components between 2400 and 2000 m.y. Rocks of about 2100 m.y. occur in Kerala and western Tamizhagam, and could be present in Mysore, where an event at this time is recorded by biotite. The Chamundi Hill Granite of Mysore city, and a granite from the Ramanathapuram District of Tamizhagam give 790 ± 60 and 720 m.y. respectively, suggesting the possibility of widespread if sporadic intrusion in the southern half of the region at about this time. There is no real evidence yet of any major reflection of the Vijayan retrogressive metamorphism of Ceylon at 1140 m.y., except possibly in the intrusion of the Sivamalai soda-syenite. However, the known mineral ages of about 1690, 1650 and 1150 m.y. along the west coast suggest repeated marginal mobility of the Mysore-Hyderabad craton otherwise stable since about 2000 m.y. though intruded repeatedly by several dyke suites not yet dated.

Much further sampling is needed, as well as the combination of several methods of dating. To facilitate this, modern geological and tectonic maps of medium scale are very desirable.

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