Abstract
Numerous newly-identified traces of active faults in the Himalayan foothill zone along the HFF around Chandigarh, in Pinjore Dun, along the piedmont zone of the Lower Siwalik hill front and within the Lower Tertiary hill range reveal the pattern of thrust and strike-slip faulting, striking parallel to the principal structural trend (NNW-SSE) of the orogenic belt. The active Chandigarh Fault, Pinjore Garden Fault and Barsar thrust have vertically dislocated, warped and backtilted fluvial and alluvial-fan surfaces made up of Late Pleistocene-Holocene sediments. West- and southwest-facing fault scarplets with heights ranging from 12 to 50 m along these faults suggest continued tectonic movement through Late Pleistocene to recent times. Gentle warping and backtilting of the terraces on the hanging wall sides of the faults indicate fault-bend folding. These active faults are the manifestation of north-dipping imbricated thrust faults branching out from the major fault systems like the Main Boundary Fault (MBF) and Himalayan Frontal Fault (HFF), probably merging down northward into a decollement. The Taksal Fault, striking NNW-SSE, shows prominent right-lateral movement marked by lateral offset of streams and younger Quaternary terraces and occupies a narrow deep linear valley along the fault trace. Right stepping along this fault has resulted in formation of a small pull-apart basin. Fault scarplets facing ENE and WSW are the manifestation of dip-slip movement. This fault is an example of slip-partitioning between the strike-slip and thrust faults, suggesting ongoing oblique convergence of the Indian plate and northward migration of a tectonic sliver. Slip rate along the Taksal Fault has been calculated as 2.8 mm/yr. Preliminary trench investigation at the base of the Chandigarh Fault Scarp has revealed total displacement of 3.5 m along a low angle thrust fault with variable dip of 20° to 46° due northeast, possibly the result of one large magnitude (Mw 7) prehistoric earthquake. Taking into consideration the height of the Pinjore surface (20 to 25 m), tentative age (8.9 ± 1.9 ka), displacement during one event and average angle of fault dip (25°) gives slip rate of about 6.3 ± 2 mm/yr, a rate of horizontal shortening of 5.8 ± 1.8 mm/yr and recurrence of faulting of 555 ± 118 years along the Himalayan Frontal Fault.
Highlights
Active faults are considered to be the source for earthquakes in the seismically-active zones of the world
With the presumptions: 1) that the climatic conditions might have remained the same during Upper Pleistocene and Holocene in both the areas; 2) alluvial fan sedimentation took place in similar fashion in both the Duns by the rivers flowing down the thrust front with a similar provenance of Lower Siwalik hills, and 3) the tectonic scenario along the Himalayan Front was similar, it can be envisaged that the fan aggradation in the Pinjore Dun might have initiated at around 50 ka and lasted up to 10 ka
The present study along the Northwestern Himalayan Frontal Zone around Chandigarh and Pinjore Dun has helped in understanding the ongoing crustal deformation and its relationship with the plate motion along the IndoEurasian plate boundary
Summary
Active faults are considered to be the source for earthquakes in the seismically-active zones of the world The ideal place to investigate active tectonics is along the front of the Himalaya, demarcating the present active tectonic boundary between the Indian and Eurasian plates (Nakata, 1972). Bearing this in mind, emphasis was given toward identification of active faults in the area around Chandigarh and Pinjore Dun (Dun = valley) located southeast of Kangra, which falls within the meisoseismal area of the 1905 earthquake We summarize our preliminary findings from the trench study
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