Cenozoic tectonic evolution of the Himalayan orogen as constrained by along-strike variation of structural geometry, exhumation history, and foreland sedimentation
Despite a long research history over the past 150 years, the geometry, kinematics, and dynamic evolution of the Himalayan orogen remain poorly understood. This is mainly due to continued emphasis on the two-dimensionality of the Himalayan orogenic architecture and extrapolation of geologic relationships from a few well-studied but small areas to the rest of the orogen. Confusion and misconception are also widespread in the Himalayan literature in terms of the geographic, stratigraphic, and structural divisions. To clarify these issues and to provide a new platform for those who are interested in studying the geologic development of this spectacular mountain belt, I systematically review the essential observations relevant to the along-strike variation of the Himalayan geologic framework and its role in Cenozoic Himalayan exhumation, metamorphism and foreland sedimentation. A main focus of my synthesis is to elucidate the emplacement history of the high-grade Greater Himalayan Crystalline Complex (GHC) that occupies the core of the orogen. Because the north-dipping Main Central Thrust (MCT) above and South Tibet Detachment (STD) below bound the GHC in most parts of the Himalaya, it is critical to determine the relationship between them in map and cross-section views. The exposed map pattern in the central Himalaya (i.e., Nepal) indicates that the MCT has a flat-ramp geometry. The thrust flat in the south carries a 2-15-km-thick slab of the GHC over the Lesser Himalayan Sequence (LHS) and creates a large hanging-wall fault-bend fold continuing > 100 km south of the MCT ramp zone. In the western Himalayan orogen at the longitude ̃ 77°E, the MCT exhibits a major lateral ramp (the Mandi ramp). West of this ramp, the MCT places the low-grade Tethyan Himalayan Sequence (THS) over the low-grade LHS, whereas east of the ramp, the MCT places the high-grade GHC over the low-grade LHS. This along-strike change in stratigraphic juxtaposition and metamorphic grade across the MCT indicates a westward decrease in its slip magnitude, possibly a result of a westward decrease in total crustal shortening along the Himalayan orogen. Everywhere exposed, the STD follows roughly the same stratigraphic horizon at the base of the THS, exhibiting a long (> 100 km) hanging-wall flat. This relationship suggests that the STD may have initiated along a preexisting lithologic contact or the subhorizontal brittle-ductile transition zone in the middle crust. Although the STD has the THS in its hanging wall everywhere in the Himalayan orogen, no THS footwall cutoffs have been identified. This has made slip estimates of the STD exceedingly difficult. The southernmost trace of the STD either merges with the MCT (e.g., in Zanskar) or lies within 1-2 km of the MCT frontal trace (e.g., in Bhutan), suggesting that the MCT may join the STD in their up-dip directions to the south. This geometry, largely neglected by the existing models, has important implications for the deformation, exhumation, and sedimentation history of the entire Himalayan orogen.