Formation of Hot Water Springs in Sri Lanka

As a wonderful gift of nature, several hot water springs occur in Sri Lanka, though it is not situated within a very active tectonic zone of the earth. The present research was conducted to model the formation of hot water springs in Sri Lanka based on geological and tectonic settings by using geological, geophysical and remote sensing techniques. It is found that the major boundary between Highland Complex and Vijayan Complex show multiple thrust planes and shear zones showing imbrecated lithological slices of both Highland Complex and Vijayan Complex. Other than the ductile structures, brittle fractures, joints and fault planes extend as mega lineament for several kilometers. These lineaments produce good secondary aquifer conditions with adequate porosity. The surface mapping and geophysical evidence of the lineaments show that they are very deep and interconnected. Thus it is proposed that structurally controlled hot water springs in Sri Lanka are associated with deep geologically weak zones of fracture or fault systems within the thrust boundary which shows some neo-tectonic activities.


Introduction
Several hot water springs are located mostly in the eastern part of Sri Lanka. Generally, hot water springs in the world are commonly associated with volcanic terrain. But hot water springs in Sri Lanka do not have any relation to volcanic activities, since the island is located in an area free of volcanic activity. Therefore, the formation of hot water springs in Sri Lanka is an interesting phenomenon to investigate based on the geological setting of the area.
First records and pioneering work on hot water springs in Sri Lanka were made by Parson (1907) and Danial (1908). Hot water springs at Mahapelassa off Embilipitiya, Kanniyai off Trincomalee and Mahaoya are well known to people. These thermal springs have their out flow temperature ranging between 34°C and 56°C and lie close to the boundary between Highland-Vijayan lithologic Complexes ( Figure  1) consisting mainly of crystalline metamorphic rocks (Cooray, 1984). The hottest springs lie at Kapuralla (56°Q, followed by Mahaoya (55°Q; Mahapelassa (44-46°C) and Marangala-Wahawa close to Padiyatalawa (42-45°C); Nelumwewa which was known earlier as Madawewa is now under a lake, and records 45°C in mud samples (Fonseka 1994). Recently two hot-springs were reported at Weherapokuna within the Ulhitiya national park and Adampane (Cooray 1984, Dissanayake and Jayasena 1988, Fonseka 1994). Table 1 lists the currently known hot water springs in Sri Lanka.  Tectonically active or inactive deform zones; 2.
Environments associated with volcanic activity; and 3. Environments associated with radio active zones. In all these environments, the common feature is the relatively high geothermal gradient, which is a key factor for the formation of hot water springs.

This Proterozoic rock terrain represents a small portion of the Gondwana super continent. Sri Lanka was juxtapositioned with India, Antarctica, Madagascar and Mozambique in
Gondwana super-continent. Evolution of Gondwana super continent and reconstruction has been subjected to much debate. However, it has been explained by using recent paleomagnetic and geologic data (Yoshida and Vitanage 1993). Jurassic and Cambro-Ordovician paleomagnetic pole position deduced from rocks of Sri Lanka and Antarctica conform approximately when Sri Lanka moved and rotated to juxtapose with Lutzow-Holm Bay.
Geophysical, geochemical and remote sensing studies were carried out in an integrated attempt to understand the geological and tectonic setting of the areas of hot water springs in Sri Lanka. Based on the results of these investigations, a conceptual geological model is developed to explain the formation of hot water springs of Sri Lanka. 2) Occurrence of more hornblende rich gneisses.

Geological setting of Highland Complex-Vijayan Complex boundary
3) Increasing of migmatization.

Figure 2. Geological map of Mahapelassa area showing boundary zone between HC and VC ( Source GSMB)
All these evidence show that the boundary between Highland Complex and Vijayan Complex of Sri Lankan litho-tectonic subdivision is a tectonically active minor boundary and is a geologically weak zone.
Therefore, a fairly high geothermal gradient can be expected in this zone when compared with similar tectonic areas in the world e.g.

Geochemical parameters of hot waters
According to geochemical parameters (

Model
Most of previous interpretations of formation of hot water springs in Sri Lanka have indicated a relationship with the HC-VC boundary but they mostly considered heat from shallow intrusive igneous activities only. Also most of them (Fonseka 1994

Conclusions
Neo-tectonic activities along multiple thrust boundary between HC and VC are the reasons for formation of hot water springs in Sri Lanka.
The water of these springs are meteoritic waters i.e. surface water percolates through deep fractures and return after being heated by geothermal energy at depth. According to geochemical parameters and geological and geographical setting, hot water springs in Sri Lanka can be divided into three groups as follows: Group 1 : Mahapelassa. According to the proposed model, there are possibilities for more locations where hot water springs may occur in Sri Lanka, specially in the HC-VC boundary zone, mostly along the mega lineaments. Hot water springs in Sri Lanka do not show any relation to volcanic activities. Based on this model it is recommended to carry out more explorations along major fractures or lineaments in these areas.