CROSS-SECTIONS OF THE WESTERN HIMALAYAN FOOTHILLS:PROBLEMS IN RESTORATION AND IMPLICATIONS FOR CRUSTAL SHORTENING

A number of geological cross\|sections from the western Himalayan foothills are based on field,seismic reflection,and well data of the Oil & Natural Gas Corporation India.These cross\|sections have been described by two contradictory interpretations.The first interpretation that depicts a prominent basal decollement above the pre\|Tertiary sequence is used for palinspastic reconstruction,using line length balancing,and determination of total shortening in the area.The second interpretation reveals a large variation in the thickness of individual stratigraphic horizons and a number of flower structures.These flower structures never cut the Himalayan thrusts but terminate below them.Away from the thrusts,the faults cut the younger Siwalik molasse (Middle Miocene—Pleistocene)and even reach to the surface as strike slip faults.Apart from the presence of strike slip faults,the other constraints for restoration of the section is oblique ramp structure at the Kangra Reentrant (or Punjab Reentrant).The oblique ramp was formed as a normal fault during the tensional regime in the region and later reactivated as thrust fault during the Tertiary Himalayan orogeny.These features reveal that the assumption of plane strain deformation is not valid for the region.Furthermore,the amount of extension during pre\|Tertiary rifting and layer parallel shortening during the Tertiary compressional phase is still unknown hence balancing of the sections is problematic.