YADONG CROSS STRUCTURE AND SOUTH TIBETAN DETACHMENT IN THE EAST CENTRAL HIMALAYA (89-DEGREES-90-DEGREES-E)

The Yadong cross structure (YCS), occurring at similar to 89 degrees 3 0' east longitude in the Himalaya, is the largest across-strike discon tinuity in the geologic structure and topography of the High Himalaya between the Himalayan syntaxes. It is manifest by a plan view left off set of the topographic crest of the range and a coincident, apparent, left strike offset of the South Tibetan Detachment System (STDS) of ab out 70 km New field mapping indicates that the STDS intersects the wes t side of the YCS at Zherger La and further suggests that the YCS is t he surface expression of a large north-northeast striking west facing lateral ramp in the Himalayan thrust system. The Greater Himalayan all ochthon 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 n ormal shear appears to have obliterated earlier northerly directed STD S shear fabric along along the YCS segment of the Tethyan belt/Greater Himalayan belt contact. Recent seismicity suggests that the Greater H imalayan allochthon is still moving southward along the lateral ramp. U-Pb monazite dates on leucogranites in the footwall of the STDS on op posite sides of the YCS are suggestive of a south-to-north decrease in the crystallization age of these granites. The age pattern is consist ent with southward ''extrusion'' of the Greater Himalayan allochthon a nd suggests the possibility of determining an average slip rate for th e STDS by dating granites in the immediate footwall of the STDS that a re substantially separated in north-south section. An average slip rat e of about 7 mm/yr between 23 and 12 Ma is suggested by the data prese nted here.