Authors:

Abstract

Although the geothermal systems are common in volcanically or tectonically active areas, ten geothermal springs are present in the Precambrian basement of Sri Lanka, where such activities are unknown. Thermal springs of Sri Lanka are surface manifestations due to heating, and plume rises through the mixing zone of deep circulating meteoric water. The heating of this deep circulating water in an average geothermal gradient of 30°C/km is increased by a relatively young dolerite dike, considered the heat source. To better elucidate the controls on fluid flow in the geothermal systems, understanding the underground geological structures is important to use this energy explored and extracted efficiently. To get a better scientific understanding of geology, structure, and hydrogeology, and to compile and propose possible geo-structural models associated with thermal springs in northeastern Sri Lanka, the present study has been carried out by geological mapping, and compile present data using computer-based GIS software and structural 3D modeling.

The study revealed that the underground geological structure of the Marangala thermal spring is a synform structure with rock contacts and East-West extending major joint set through which the percolating water flows through the dolerite dyke underground. The structure controlling the Nelum Wewa thermal spring is an antiform and a shear zone extending through the spring and nearly perpendicular to the antiform axis. The underground structure controlling the Mahaoya and Kapuralla thermal springs is a mega scale, which covers approximately 3,000 km² area of Vijayan complex of Sri Lanka. It has several ductile, and brittle structures contained in one mega-scale partially overturned synform. These results suggest that Marangala, Maha Oya, Kapuralla, and Nelum Wewa geothermal springs are associated with antiform and synform ductile structures. Brittle structures associated are shear zones and major joint sets. The probable heat source is a relatively young dolerite dyke. This detailed study of the thermal springs contributes to understanding the geological evolution of Sri Lanka and discovering potential energy sources.

Keywords: Thermal springs, Structural model, Fault zone, Shear zone, Synform, Fault breccia