Origin and evolution of 'intracratonic' thermal fluids from central-western peninsular India

The chemical and isotopic composition of several thermal springs and associ ated gas phases in a large sector of central-western peninsular India has b een investigated. Such springs have meteoric isotopic signature and emerge, after very well developed convective circulation at depth, along important tectonic structures such as the Son-Narmada-Tapti rift zone and the West C oast Fault. Chemical components in both gas and liquid phases and geothermo metric estimations suggest that such springs are not related to the presenc e of any active hydrothermal systems at shallow depth in any of the studied areas. The hottest convective water emerges at Tattapani at near boiling p oint for water at atmospheric pressure ( > 90 degrees C) in association wit h an N-2-rich gas phase of clear meteoric signature. Since such fluids do n ot carry any corrosive components, they could be conveniently exploited for industrial purposes, such as drying processes. From a tectonic point of vi ew, the presence of thermal emergences scattered in a wide area along geolo gically well defined structures, which also generate frequent moderate eart hquakes, suggests that such structures are active. Although the isotopic co mposition of thermal springs points to a meteoric origin, their feeding aqu ifers are not topographically driven as in most active Alpine orogenic belt s. The relative high quantity of total helium in the associated gas phase s uggests also that they are really deep, old, long circulating waters. We pr opose for such waters the term 'intracratonic thermal waters' since the iso topic signature of He in the gas phase does not show any release of primord ial He-3 in any of the areas of spring emergence. Based on the quite low He -3/He-4 ratio in the gas phase we suggest also that, in spite of its morpho logical shape, the Narmada-Son-Tapti rift zone cutting the Indian subcontin ent in two is more related to paleo-suture rather than to a mid-continental rift system. (C) 2000 Elsevier Science B.V. All rights reserved.