HYDROCHEMICAL CHARACTERISTICS OF GROUND WATER OF THE HIMALAYAN FOLD-AND-THRUST BELT IN NORTHERN PAKISTAN

A total of 71 samples were analyzed from springs and deep and shallow water wells as a part of a larger program of investigations of ground water in a basin under compressive tectonic stress. The study site covers Peshawar Basin and its surroundings in the Himalayan foreland of Pakistan. The earthquake focal mechanism solutions and moment-tensor solutions point to a dominant compressive stress regime resulting from the Cenozoic India-Eurasia collision. The Himalayan seismicity has been mainly explained in terms of movement along the detachment plane in a compressive stress regime due to the under-thrusting of the Indian plate resulting in stresses as high as 90 MPa. Most of the spring water are typically characterized by Ca>Mg>(Na+K), with bicarbonate as the dominant anion, suggesting young and fresh recharge, however, two samples have surprisingly (Na+K)>Ca>Mg, with sulfate for the dominant anion. The hot spring from Garam Chishma also yielded very high values of SiO2, B and Li, indicating origin from deeper ground water circulation. The surface temperature of this hot spring is 67 °C while the source reservoir temperature of 113 °C was calculated using Mg-corrected Na-K-Ca geothermometer which is supplemented by 105 °C temperature yielded by Quartz (maximum steam loss) geothermometer. Similarly, most of the samples from shallow wells are predominantly Ca-bicarbonatic, with a few exceptions, where none of the cation is dominant. Large group of samples from deep wells are also dominated by (Na+K)>Ca>Mg, with sulfate for the dominant anion. Most significantly, water samples from one shallow well and three deep wells, all located in an immediate vicinity of a major thrust zones such as Main Boundary Thrust (MBT) and Main Mantle Thrust (MMT), demonstrate clear imprints of admixture of oil-brines. Our preliminary hypothesis for the origin of the oil-brine components in shallow aquifers is that the major fault lines are “leaking” fluids from deeper parts of the basin under compressive tectonic stress. Thus, the brines are “squeezed out” and ascend along the fault plains from deeply confined and laterally compressed parts of the basin.