RESEARCH ARTICLES CURRENT SCIENCE, VOL. 94, NO. 3, 10 FEBRUARY 2008 338 *For correspondence. (e-mail: dmmaurya@yahoo.com) Evidence of neotectonic reactivation of the Katrol Hill Fault during late Quaternary and its GPR characterization A. K. Patidar 1 , D. M. Maurya 1, *, M. G. Thakkar 2 and L. S. Chamyal 1 1 Department of Geology, M.S. University of Baroda, Vadodara 390 002, India 2 Department of Geology, R.R. Lalan College, Bhuj, Kachchh 370 001, India The Katrol Hill Fault (KHF) is an E–W trending major intrabasinal fault located in the central part of Mainland Kachchh. Several lines of geomorphic evi- dence suggest periodic reactivation of the KHF during the Quaternary period. The present study is based on geomorphic investigations along the entire length of the KHF and detailed analysis of Quaternary faulting observed supplemented with ground penetrating radar (GPR) investigations. The dip of the KHF is observed between 45° and 80° due south within the Mesozoic formations, reducing to 40–45° in the overlying Qua- ternary deposits. Offsetting in Quaternary sediments overlying the KHF indicate three events of faulting during the late Quaternary. Based on the stratigra- phic set-up of Quaternary sediments, it is suggested that Event 1 occurred sometime in the late Pleisto- cene, while the Events 2 and 3 took place during early Holocene and <2 ka respectively. The reverse move- ment indicated by faulting in Quaternary sediments, splaying nature of the fault as revealed by GPR and changes in the geometry of the fault towards the sur- face indicate periodic reactivation of the KHF in com- pressive stress regime. Keywords: Ground penetrating radar, Kachchh, Katrol Hill Fault, Quaternary reactivation, tectonic geomorphology. THE Kachchh rift basin, located at the western extremity of India, opened during the Early Jurassic, and became fully marine in Middle Jurassic which resulted in the deposition of more than 2000–3000 m thick Mesozoic and Cenozoic sediment succession 1,2 . The framework of the present fault-controlled geomorphic configuration of Kachchh is attributed to inversion of the basin in the late Cretaceous 2,3 . In general, the post-rift (inversion phase) geological evolution of the basin is marked by periodic reactivation of various E–W-trending intrabasinal faults like the Island Belt Fault (IBF), Kachhh Mainland Fault (KMF), South Wagad Fault (SWF), Katrol Hill Fault (KHF) and others (Figure 1), which are also responsible for recurrent seismic activity in the region 4 . It is therefore essential to characterize the active faults of the area and their precise subsurface investigations to document the successive tectonic events and related landform develop- ment during the Quaternary. The present study deals with the E–W trending KHF that marks the northern boundary of the Katrol hill range in the central part of the Mainland Kachchh. The Katrol hill range corresponds to the flexure zone to the south of the KHF with the rocky plain of Bhuj to the north. The Mesozoic sequence of Mainland Kachchh is divided into four formations named as the Jhurio (Jhura), Jumara, Jhuran and Bhuj formations in ascending order 1,2 . In the Katrol hill range, the Jumara and Jhuran formations show higher degree of deformation, as evidenced by the E–W trending asymmetrical domal and anticlinal structures, which are truncated over the KHF 1,2 , illustrating the com- pressive stress regime (Figures 1 and 2). The southern limbs of the flexures show inclination of ~5–10°, while the northern limbs are steeply dipping to vertical and terminate against the KHF 1,2 . Several N–S, NNE–SSW, NNW–SSE-trending igneous intrusive dykes cut across these domes and deform the country rocks 1 . The KHF also exhibits lateral offsetting along NNE–SSW and NNW–SSE-trending transverse faults 5,6 . Here, we discuss the tectonic geomorphology and Quaternary sediments along the KHF, delineate its structural characteristics and provide evidence of its reactivation during late Quater- nary (Figure 1). The results of geomorphic, stratigraphic and Ground Penetrating Radar (GPR) studies are synthe- sized to interpret the tectonic behaviour of the KHF in the recent past. Geomorphology of the Katrol Hill Fault The E–W-trending KHF is a major range-bounding fault that divides the Mainland Kachchh into the northern and southern parts. Geomorphologically, the KHF is ex- pressed as an E–W-trending line of north-facing scarps separating the rocky plain comprising sandstones of Bhuj Formation to the north and the rugged terrain of Katrol hill range made up of highly deformed Mesozoic rocks older than the Bhuj Formation. The KHF, therefore, marks a sharp lithotectonic contact between the Bhuj Formation