Phases of deformation from cross-cutting structural relationships in external thrust sheets: insights from small-scale structures in the Ramgarh thrust sheet, Darjiling Himalaya, West Bengal

摘要

Cross-cutting structural relationships are used in Precambrian terrains as a routine tool to work out different phases of deformation even in the absence of chronological data. Such studies assume that the different phases are separated by large time-gaps. While such an assumption may be eventually supported by chronological data in Precambrian terrains, extending this approach to fold-and-thrust belt settings like the Himalayas is likely to result in wrong structural interpretations, as emplacement of each thrust sheet can result in several sets of structures that may be interpreted as 'phases' of structures based on cross-cutting relationships. Therefore, small-scale structures in thrust belts like the Himalaya need to be associated with well-constrained individual deformation events and not merely be attributed to 'phases' of deformation. We illustrate this by looking at small-scale structures from the frontal Ramgarh thrust sheet in the Gorubathan area of the Darjiling Himalaya from which six 'phases of deformation' can be identified from cross-cutting relationships. Given that these structures are observed in an external thrust sheet near the mountain front formed during a single phase of Himalayan orogeny, it is extremely difficult to visualize six time-separated phases of deformation affecting the rocks in the Ramgarh thrust sheet. We describe the geometry of the small-scale structures in the Ramgarh thrust sheet from Gorubathan area and interpret them to be Himalayan in origin (pending confirmation from chronological data) and as related to the emplacement of the Ramgarh thrust sheet rather than multiple phases of deformation.