TECTONIC EVOLUTION OF THE HIMALAYAN OROGEN AS CONSTRAINED BY ALONG-STRIKE VARIATION OF STRUCTURAL GEOMETRY, EXHUMATION HISTORY, AND FORELAND SEDIMENTATION

I systematically review the essential observations relevant to the along-strike variation of Himalayan geologic framework and its role in Himalayan evolution, exhumation, metamorphism, and foreland sedimentation. A main focus of the review is to elucidate the emplacement history of the high-grade Greater Himalayan Crystalline Complex (GHC) that occupies the core of the orogen. Because the Main Central Thrust (MCT) above and the South Tibet Detachment (STD) below bound the GHC in most parts of the Himalaya, it is critical to determine the relationship between these first-order Himalayan structures in map and cross section views. The exposed map pattern in the central Himalaya indicates that the MCT has a flat-ramp geometry. The thrust flat in the south carries a 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 pre-existing 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 (i.e., Zanskar) or lies within 1-2 km of the MCT frontal trace (i.e., 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 and exhumation history of the entire Himalayan orogen.