Age and composition of dikes in Southern Tibet: New constraints on the timing of east-west extension and its relationship to postcollisional volcanism

Controversy exists over whether east-west extension in southern Tibet is re lated to plateau uplift or to the accommodation of plate boundary forces. R elationships between the onset of extension, plateau uplift, and the therma l state of the lithosphere are critical to this debate. We present new data on postcollisional, north-south-trending dikes in southern Tibet. Their ag es range hom 18.3 +/- 2.7 Ma to 13.3 +/- 0.8 Ma, and define the onset of re gional east-west extension in southern Tibet. Dikes are compositionally ind istinguishable from postcollisional lavas in southern Tibet, being either u ltrapotassic, having a source in the subcontinental lithospheric mantle, or calc-alkaline with a dominantly crustal origin. The ultrapotassic dikes de monstrate that east-west extension and subcontinental lithospheric mantle-d erived magmatism were temporally and spatially linked, supporting models th at relate the latest phase of plateau uplift to subcontinental lithospheric mantle thinning. Thus, the onset of extension by 18.3 a 2.7 Ma represents the time at which the potential energy of the plateau exceeded convergent b oundary forces,This places a new age limitation on the attainment of high e levation in southern Tibet, with implications for models that relate Cenozo ic monsoon intensification to plateau uplift.