Abstract

Th-Pb ion microprobe measurements made on 12 monazite grains from the Khula Kangri granite, Tibet-Bhutan frontier, are interpreted to indicate that crystallization occurred at 12.5 ± 0.4 Ma. The leucogranite is cut by the Gonto La detachment, part of the Southern Tibet detachment system that has allowed upper-level, north-directed extension of the Himalayan orogen. Significant orogen-normal extension in southern Tibet appears to have continued 8–10 m.y. later than previously recognized. This is the first reported crystallization age for a leucogranite east of the Yadong cross structure, an apparent 70 km offset of the high Himalaya and Southern Tibet detachment. West of the Yadong cross structure, reliable ages for high Himalaya events (major Main Central thrust slip, granite generation and emplacement, attainment of critical topography, and major detachment extension) group between ca. 24 and 19 Ma. We interpret the west-to-east change across the Yadong cross structure to be due to either (1) an abrupt, ∼10 m.y. younging of principal high Himalayan events or (2) a deeper (thus younger) exposed part of the footwall of the southern Tibet detachment. Near Khula Kangri, the Southern Tibet detachment is cut by the highly oblique Yadong-Gulu rift; a manifestation of Tibet plateau east-west extension. Integrated estimates of magnitude, and rate, of detachment displacement suggest that the observed postcrystallization north-directed extension lasted for 1–3 m.y., after which time the Yadong-Gulu rift formed. This interpretation is consistent with initiation of east-west extension of Tibet at ca. 8 Ma.

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