Cataclastic strain from external thrust sheets in fold-thrust belts: Insights from the frontal Indian Himalaya

Abstract

Pure strain is an important component of the total displacement vector in fold-thrust belts like the Himalayan orogenic belt. We describe a methodology to compute cataclastic strain from external thrust sheets using the Bootstrapped Modified Normalized Fry Method and validate it in two different structural settings in the frontal Himalaya. In the frontal imbricate zone of the Dharan salient in the Darjiling Himalaya we measured the highest strain from a central imbricate thrust (T3) and found that strain decreased away from the fault zones within individual thrust sheets. Modelling of our results indicated that there was significant layer-perpendicular flattening in the fault zones while layer parallel shortening related strain dominated the thrust sheets. Also, fault parallel shear decreased away from the fault zones. In contrast, layer-perpendicular flattening was largely absent in the fault zone associated with the Main Frontal thrust (MFT) and the MFT sheet in the Mohand Range of the Dehradun recess. Here the strain distribution pattern was consistent with a trishear fault propagation monocline which is our preferred model for the structure of the MFT sheet in the recess. Modelling of our results suggested that fault parallel shear decreased away from the MFT fault zone like in the thrust sheets from the Dharan salient and that the fault propagation folding was accomplished by flexural slip folding. We contend that our methodology can be effectively used to quantify and study the pure strain part of the total displacement vector in external thrust sheets from fold-thrust belts worldwide.