Deformation in the Kangra Reentrant, Himachal Pradesh of NW-Sub Himalaya of India: A Paradox

In the Sub-Himalayan belt of India, the largest reentrant is located in the Kangra area of Himachal Pradesh is ~80 km wide, anomalously wider than its counterparts which are extremely narrow (<10 km). The underlying rigid basement of the Indian Plate and overlying lithologies do not present any information on the existence of a weak layer as seen in similar wide Fold Thrust Belts (FTBs) of Zagros mountain belt in Iran and Himalaya mountain belt in India. The overall topographic slope α and dip of basement β that contribute to a narrow wedge taper (α + β) are also similar to some of the FTBs that are present over weak layers. Therefore there exists a paradoxical scenario where the width of the FTB does not fit in with the available rheological information. In the present chapter we demonstrate that the litho-tectonic setting and basin architecture are the primary controls for the present width of the FTB in the Kangra reentrant. Assuming the basement to be extremely rigid, the thickness of incompetent clay rich Paleogene-Neogene section (4–8 km) and high pore pressure appear to be the main drivers that facilitate the forward propagation of the wedge. This has given rise to an outward projecting FTB in the Kangra Reentrant. A comparison with similar FTBs from across different settings is also presented to demonstrate the analogies across such FTBs, especially the ones from the delta-deepwater regime with basement in incompetent shales, which act as extremely efficient basement. These inferences drawn here are further supported by the surface topographic attributes, structural geometry and wedge mechanics. The inferences are further favoured by the poor coupling between the basement and cover strata and will not allow potential accumulation of shear stresses within the rocks of the Kangra area as demonstrated by the shallow and low seismicity of the region. This is particularly true for the region south of the Jwalamukhi Thrust (JMT). However, further north of the JMT there are potential seismogenic zones along thrust faults such as Palampur Thrust (PT) which are actively accumulating shear stresses. These could potentially threaten the area with a high magnitude seismic event, similar to the 1905 event. However, the chances of the rupture propagating towards the Himalayan Frontal Thrust (HFT) are limited.