Application of Gravity and Radon Studies to Delineate the Concealed Section of the Khisor Thrust

Abstract

Radon concentration, a geochemical parameter of naturally occurring noble gas and potential relative gravity field variation, has been studied in the current investigations. These investigations were carried out along and in proximity to the Khisor Thrust in the Trans Indus ranges marking the southern boundary of Bannu basin in Pakistan. The studies were aimed to demark the concealed section of the thrust fault beyond termination of its surface signature. The consistent increasing pattern of radon variation while approaching the approximate location of fault and decrease while moving farther from it suggests the existence of a fault to the subsurface, which should be verified through gravity studies. Variation of radon concentration between 2.5 and 4.1 kBq/m3 was observed on or near the fault zone in exposed and concealed segments, whereas the lower bound variation of 1.1–2 kBq/m3 was encountered on points moving away from fault zone. This variation of radon concentration promisingly supported the idea of the existence of a fault zone beyond the cessation of surface signature. The argument was further validated through the relative gravity variation studies following the radon concentration. The Bouguer anomaly was calculated along all 16 profiles cross-cutting the fault zone. The regional gravity mapped along 231 points of investigation showed a variation between − 90 and 8 mGal. The residual gravity highlighting the effects of upper crust disturbance because of under-thrusting has shown anomalous values between − 11 and 18 mGal. The results when contoured have shown a good congruence with those received from radon concentration contrasts. Conclusively, it can be perceived that these two nondestructive and less laborious techniques work in composite usefully to delineate the unseen earthquake potential source.