Yadong cross structure and South Tibetan Detachment in the east central Himalaya (89°-90°E)

The Yadong cross structure (YCS), occurring at ∼89°30' east longitude in the Himalaya, is the largest across-strike discontinuity in the geologic structure and topography of the High Himalaya between the Himalayan syntaxes. It is manifest by a plan view left offset of the topographic crest of the range and a coincident, apparent, left strike offset of the South Tibetan Detachment System (STDS) of about 70 km. New field mapping indicates that the STDS intersects the west side of the YCS at Zherger La and further suggests that the YCS is the surface expression of a large north-northeast striking west facing lateral ramp in the Himalayan thrust system. The Greater Himalayan allochthon is apparently draped across this lateral ramp, resulting in a north-northeast striking monoclinal flexure of the allochthon manifest in the Chomolhari range. Superimposed steep west-northwest directed normal shear appears to have obliterated earlier northerly directed STDS shear fabric along along the YCS segment of the Tethyan belt/Greater Himalayan belt contact. Recent seismicity suggests that the Greater Himalayan allochthon is still moving southward along the lateral ramp. U-Pb monazite dates on leucogranites in the footwall of the STDS on opposite sides of the YCS are suggestive of a south-to-north decrease in the crystallization age of these granites. The age pattern is consistent with southward "extrusion" of the Greater Himalayan allochthon and suggests the possibility of determining an average slip rate for the STDS by dating granites in the immediate footwall of the STDS that are substantially separated in north-south section. An average slip rate of about 7 mm/yr between 23 and 12 Ma is suggested by the data presented here.