WESTERN EXTENSION OF THE HIMALAYAN GEOTHERMAL BELT

The Himalayan Geothermal Belt' (HGB) was first described (Tong and Zhang, 1981) as manifested by at least 600 hot and warm spring systems occurring within a 150 km wide and c. 3000 km long belt stretching from the Pamir Mountains through Tibet into Yunnan. In many locations the geothermal anomaly is high enough for generation of electricity. Thailand has a binary plant producing 300 kWe (from 117oC water). Yangbajang, in Tibet, generates 25 MWe, providing Lhasa with about 40% of its electricity. In Tibet and Yunnan, an additional 7 MWe are generated in 7 small plants.

The westward extension of the HGB is manifested by warm to hot springs clustering along the Main Karakoram Thrust (MKT), Main Mantle Thrust (MMT) and Main Central Thrust (MCT) in the Peshawar Region of Pakistan. The warmest, 68oC Garam Chashma Hot Springs emerge from the post-collisional leucogranites of the Hindukush Range, which yield K-Ar (biotite) age of 20-18 Ma.

Hot-end reservoir temperature estimates computed using various chemical geothermometers ranged from a low of 105oC calculated from chalcedony and 155oC from quartz solubility curves, while the Mg-Li geothermometer indicated the hot-end temperature of 128oC. Much higher estimates of about 260oC were obtained comparing mixing diagrams for a conservative solute, e.g. Cl- with the enthalpy (temperatures).

The source of the elevated heat flow within the HGB has been attributed to “advective sweeps of infiltrated meteoric water from the hot brittle, upper crust” (Hochstein & Regenauer-Lieb, 1988). Using the geothermal gradient of 0.026 °C/m, as reported by Hochstein & Yang (1995) from wells to the east from our study area, permits inference of a 2000-m-deep circulation of meteoric water. Yet, the presence of a regionally dominant compressional tectonic regime (Molnar & Tapponnier, 1977; De Mets et al. 1994; Paul et al. 2001), with intense mylonitization along the thrust faults, raises doubt regarding the feasibility of a simple topography-driven mechanism for such deep “advective sweeps of infiltrated meteoric water.” The topography-driven pressure from the highest ridges to the north of the study area may attain a maximum of 25 MPa as compared to the tectonic lateral stress of 90 MPa. The question remains open as to which of these is prevalent for driving deep groundwater circulation.