WHEN DID THE ROOF COLLAPSE - LATE MIOCENE NORTH-SOUTH EXTENSION IN THE HIGH HIMALAYA REVEALED BY TH-PB MONAZITE DATING OF THE KHULA-KANGRI GRANITE

Th-Pb ion microprobe measurements made on 12 monazite grains from the Khula Kangri granite, Tibet-Bhutan frontier, are interpreted to indica te that crystallization occurred at 12.5 +/- 0.4 Ma. The leucogranite is cut by the Gonto La detachment, part of the Southern Tibet detachme nt system that has allowed upper-level, north-directed extension of th e Himalayan orogen. Significant orogen-normal extension in southern Ti bet appears to have continued 8-10 m.y. later than previously recogniz ed. This is the first reported crystallization age for a leucogranite east of the Yadong cross structure, an apparent 70 km offset of the hi gh Himalaya and Southern Tibet detachment. West of the Yadong cross st ructure, reliable ages for high Himalaya events (major Main Central th rust slip, granite generation and emplacement, attainment of critical topography, and major detachment extension) group between ca. 24 and 1 9 Ma. We interpret the west-to-east change across the Yadong cross str ucture to be due to either (1) an abrupt, similar to 10 m.y. younging of principal high Himalayan events or (2) a deeper (thus younger) expo sed part of the footwall of the southern Tibet detachment. Near Khula Kangri, the Southern Tibet detachment is cut by the highly oblique Yad ong-Gulu rift; a manifestation of Tibet plateau east-west extension. I ntegrated estimates of magnitude, and rate, of detachment displacement suggest that the observed postcrystallization north-directed extensio n lasted for 1-3 m.y., after which time the Yadong-Gulu rift formed. T his interpretation is consistent with initiation of east-west extensio n of Tibet at ca. 8 Ma.