Geometry of the Dolpo-Mugu Folds: Implications for the Deep Crustal Structure of Central Nepal
Current shortening calculations (700-300 km) across the strike of the Himalayan orogen are significantly less than what has been predicted by plate reconstruction models and other regional studies (1200-800 km). This study investigates a region in the High Himalaya of western Nepal that contains shortening structures not currently accounted for in shortening budgets within the thrust belt. The first-order structures in the study area are a synclinorium and anticlinorium defined by the contact, -- the South Tibetan Detachment (STD) -- between two main tectonostratigraphic units: the Tethyan Himalayan sequence (THS) and the Greater Himalayan Crystallines (GHC). Because the STD is interpreted to have ceased movement ca. 19 Ma, the timing for these folds is bracketed between ~19 Ma and ~11 Ma. A new geologic map is constructed by compiling previous maps and serves as the foundation for structural analysis. These folds can be traced for ~150 km in an east-west direction. Cross sections, structure contour maps, and stereoplots of the STD show an amplitude of ~9km with a shallow plunge to the SE. Three hypotheses for thrust belt architecture below the anticlinorium are considered. These are (A) a duplex, (B) a fault-bend fold and (C) a blind thrust. Each of these models makes predictions of horizontal shortening. The duplex hypothesis predicts ~33 km of horizontal shortening, the fault-bend fold model predicts ~25 km of shortening, and the blind thrust model predicts ~8 km of shortening. Based on geometry and structural position I correlate the Dolpo-Mugu folds with similar folds to the east and west along the strike of the orogen; the Gurla Mandhata Crystalline complex is ~150 km to the west of Dolpo-Mugu, and the Manaslu folds are ~200 km to the east for a cumulative along strike axis of 350 km. These models predict vertical thickening within the GHC in contrast to previous models that show constant thickness in this region of the Himalaya.