Himalayan Geology (Journal), Dehradun...........

Abstract: The Pinjor dun is an intermontane valley within the Sub Himalaya of Panjab lying between the rivers Satluj and Ghaggar. The dun is bounded to south by the Frontal Siwalik Range (FSR) and to north by the Inner Siwalik Range (ISR). The FSR is made of an anticlinal ridge, trending northwest - southeast, of folded upper Siwalik strata. The southern margin of the FSR is marked by an abrupt topographic and tectonic break against the Panjab alluvial plains along the Himalayan Frontal Thrust (HFT). The HFT was initiated, south of the Nalagarh Thrust (NT), representing southward propagation of the decollement referred as Main Himalayan Thrust (MHT) sometime between 0.5 Ma and 0.1 Ma. The Chandigarh anticline was formed as a fault-bend fold over the HFT. The anticline is asymmetric, the forelimb on the southern flank has long terraced streams and mature compared to the back limb on the northern flank with deeply incised short streams. The drainage pattern, incision of the landform and geometry of the structure indicates the presence of a blind back thrust and a fold scarp on the northern margin of the Chandigarh ridge. South of the HFT, a 10-15 km wide Piedmont Zone (PZ) has been formed with coalescing alluvial fans. The PZ has rolling topography with streams incised by a few meters and active deformation features. The Pinjor dun is interpreted to represent the continuation of the back limb of the anticline into the flat synformal depression of a piggy - back type basin. The dun is covered by post- Siwalik Dun formation consisting of sand, silt, mudstone and gravel of poorly to unconsolidated and horizontally bedded nature. The sediments were deposited on alluvial fans sourced from the hinterland Siwaliks and the Lesser Himalaya. The OSL ages of the Dun formation range more than 90 Ka to 20 Ka. Earthquakes breaking the surface have been reported along the HFT scarps in Holocene sediments near Chandigarh on the basis of palaeoseismological studies. The Chandigarh anticline and its ridge topography appear to have grown as a result of coseismic cumulative slip during late Quaternary-Holocene generated by repeated large earthquakes, with assumed average 5-10 m dip-slip and 1000 years recurrence interval.