DEFORMATION MECHANISMS IN THE SOUTH KALIJHORA THRUST AND THRUST SHEET IN THE DARJILING HIMALAYAN FOLD-AND-THRUST BELT, WEST BENGAL, INDIA

摘要

The South Kalijhora thrust (SKT) is a footwall imbricate of the Main Boundary Thrust (MBT) and is well-exposed along a ramp in a section of the Tista River valley in the Darjiling Himalayan fold-and-thrust belt. The SKT transports Lower Siwalik sandstones over Middle Siwalik sandstones. The SKT fault zone lithology, however, consists of an unsorted conglomerate unit of unknown stratigraphic age; this study postulates that the conglomerate unit is basal Siwalik. Near the SKT fault zone, elastico-frictional deformation is the dominant deformation mechanism in the Lower Siwalik sandstones of the SKT sheet. However, quasiplastic deformation of quartz grains from the sandstone is observed from the hinge zones of the folds in the sheet near the SKT fault zone and no recovery microstructures are observed indicating that dislocation glide is the only quasiplastic deformation mechanism operative. Near the MBT, however, the sandstones from both the fold hinges and the limbs exhibit strong quasi-plastic dislocation creep that is overprinted by elastico-frictional fracturing. In the fault zone, the fine-grained matrix of the conglomerate unit exhibit schistosity and folding whereas the clasts exhibit shear fracturing. Lignite boulders in the SKT fault zone, however, exhibit flow around other clasts.