BALANCED CROSS SECTIONS, STRUCTURAL EVOLUTION AND SHORTENING, NW HIMALAYAN FOLD-THRUST BELT

Abstract

The Sub-Himalaya Zone (SHZ) in the northwestern Himalayas represents a foreland basin containing a sequence of Tertiary sedimentary rocks. It is separated from the Indo- Gangetic plains in the south by the Main Frontal Thrust (MFT), and the Lesser Himalaya Zone (LHZ) in the north bythe Main Boundary Thrust (MBT). A sequence of Precambrian to Early-Palaeozoic sedimentary rocks constitutes the LHZ. The rocks of the SHZ and the LHZ display structural features typical of fold-thrust belts (FTBs), which are characteristic of thin-skinned tectonic set up. In the area of study, the Kangra and Dehra Dun recesses flank the Nahan salient on either side. The rocks within the salient and recesses have undergone thrusting and thrust-related folding above a gently dipping detachment. Five balanced cross sections, one in the Kangra recess (Jawalamukhi section), three in the Nahan salient (Subathu, Morni and Nahan sections) and one in the Dehra Dun recess (Dehra Dun section), have been constructed primarily using surface structural data and geological map. Well logs and seismic reflection profiles, where available, have been used as additional constraints. The structural geometry in the Jawalamukhi section is largely controlled by three buried thrusts, which splay from the basal detachment. The MFT and other thrusts towards foreland splay from an upper detachment traced by upper flats of two buried thrusts. Towards the hinterland, the fault-bend fold in the hangingwall of a buried thrust has been breached by a break-back sequence of out-of-sequence thrusts, one of which is the MBT. The overall structural evolution in this sector can be explained by "synchronous thrusting" model, in which in-sequence initiation of thrusts at depth was followed by motion on all the thrusts leading to out-of-sequence thrusting at higher structural levels. The minimum total horizontal shortening in the section is 93.8 km. The shortening partitioned within the FTB is of the orderof67km(41%). The three balanced cross sections across the Nahan salient show broadly similar structural geometry. The structural geometries are rather simple with relatively widely spaced ramps and related folds in the foreland. Towards hinterland structural geometry becomes complex with low ramp spacing, interference of axial surfaces of fault-related folds, folded thrust trajectories and exposed detachments. The middle part of the belt is characterized by linked thrusts that describe approximately leading imbricate fan or hinterland-dipping duplex in different sections. The structure in the LHZ, i.e., in the hinterland part of the belt, is dominated by a large number of horses. The structural evolution in the Nahan salient can be best explained in terms of forward-breaking in-sequence thrusting, followed by reactivation of pre-existing ramps leading to out-of-sequence thrusting in an approximately break-back style. The Dehra Dun section incorporates only the SHZ. In this section, two ramp anticlines, related to two buried thrusts, are separated by a flat intermontane valley. In accord with fold-thrust belts from other parts of the world, the estimated values of shortening along with the structural geometry in this fold-thrust belt also vary considerably both longitudinally and transversely. This is also in conformity with the critically tapered wedge model that suggests that adjacent segments in a fold-thrust belt may have very contrasting structural geometry and evolutionary history, depending onthe wedge taper.