STRUCTURAL RELATIONSHIP BETWEEN THE MAIN CENTRAL THRUST AND SOUTH TIBET DETACHMENT IN THE NW INDIA HIMALAYA AND ITS IMPLICATIONS FOR A NEW MODEL FOR THE HIMALAYAN DEVELOPMENT

Determining the evolution of the Himalayan orogen has important implications for our understanding of continent-continent collision processes. Past efforts have been concentrated in the central Himalaya where most of the current knowledge and models have been developed. Due to the lack of cross-cutting relationships between the two dominant Himalayan structures - the Main Central Thrust (MCT) and the South Tibet Detachment (STD) - we are currently unable to distinguish many of the Himalayan evolutionary models. However, exposures of the Greater Himalayan Crystallines (GHC) - i.e. the rocks between the MCT and STD - are discontinuous in the NW India Himalaya, so this region may provide the key field relationships for this problem. The STD has been traced to the uppermost Kulu Valley, but to the immediate west Tethyan Himalaya Sequence (THS) rocks, the rocks structurally above the STD, outcrop continuously from the MCT to the Indus-Tsangpo Suture. We conducted field mapping across the Kulu Valley, tracing the STD westwards by focusing on lithology, deformation fabrics, metamorphic grade, and marker beds. From the uppermost Kulu Valley, the STD wraps to the south along the western margin of the upper Kulu Valley, where it dips gently west. Further south, the STD is folded in a southwest-verging overturned anticline. In the overturned southern limb of this fold, the STD extends to the southeast and merges with the MCT; to the south of this STD-MCT branch line, the MCT hanging wall consists of THS rocks. This geometry suggests that the GHC were emplaced as a thrust horse, bounded below by the MCT and above by the STD. As this geometry is not predicted by current models, we have integrated our mapping and analytical results with existing data to develop a new model for the Himalayan development. In the model, the GHC are a thrust horse composed of India plate supracrustal rocks, and erosion and topographic gradients do not have localized mechanical roles.