Structural and lithological discoveries along the strike of the Himalaya change paradigms and possibilities

Himalayan tectonic models have been largely based on the geology of the central Himalaya, a region featuring three units separated by two sub-parallel faults. A common element of all models has been a “pipe to the surface” in cross-section view. That is, all models involve the emplacement of the Himalayan crystalline core from depth to the surface by motion between and along the two faults during the Early / Middle Miocene. Reconstructing this deformation in the central Himalaya has proved difficult, in large part due to a lack of fault cutoff relationships since there the lower unit, middle crystalline core, and upper unit are dominantly Early Proterozoic, Late Proterozoic, and Phanerozoic, respectively. Our recent studies of the western Himalaya yield progress: there, (1) the two faults bounding the crystalline core merge towards the foreland, ruling out “pipe to the surface” emplacement of the crystalline core, and (2) new detrital and igneous U-Pb zircon data have confirmed inferences that large, correlative Late Proterozoic sequences occur in all three Himalayan units. The lack of a “pipe to the surface” in the western Himalaya allows a hypothesis that dramatic variation in tectonics and exhumation may occur along the arc of the orogen. This hypothesis can be tested by tracing the intersection line of the two faults to the east. Our new structural and geochronological studies indicate that this intersection line outcrops in the central Himalaya about 20 km north of Kathmandu. This result (as well as a re-assessment of the Lesser Himalayan Crystalline Nappe geology) allows the intersection line to be traced eastwards sub-parallel to the arc of the orogen. This geometry precludes the Early / Middle Miocene emplacement of the crystalline core from depth to the surface via motion between and along the two faults throughout the entire Himalaya, leading to a new “tectonic wedging” paradigm for the emplacement of the crystalline core. This paradigm, the lithostratigraphic advances in the western Himalaya, and similar advances in the eastern Himalaya all provide bases for improved reconstructions of Himalayan deformation. The range of possible Himalayan structural evolutions is substantially narrowed, allowing for more detailed investigations of interplay between tectonic and climatic systems in the evolution of this collisional orogen.