Mode of Cenozoic east-west extension in Tibet suggesting a common origin of rifts in Asia during the Indo-Asian collision

Cenozoic rifts in Tibet were traditionally interpreted as a result of topog raphic collapse of the Tibetan Plateau, reaching the maximum elevation that can be supported by its mechanical strength. Recent studies have emphasize d possible similarities between rifting in Tibet and extension in the Basin and Range of the western United States. However, when examined in detail, one finds that spacing of long (>100 km) rifts in Tibet (similar to 100-300 km) is significantly greater than that in the Basin and Range (similar to 20-40 km). From south to north, rift spacing decreases systematically: from 191+/-67 km in the Himalaya south of the Indus-Yalu suture to 146+/-34 km in south Tibet between the Indus-Yalu and Bangong-Nujiang sutures and farth er north to 101+/-31 km in central Tibet between the Bangong-Nujiang and Ji nsha sutures. Instability analysis suggests that the mantle lithosphere mus t have been involved in east-west Tibetan extension. Specifically, the wide ly spaced rifts in The Himalaya and Tibet may have been related to the pres ence of a relatively light crust (density <similar to 2.90 g cm(-3)) and a strong mantle lithosphere (similar to 40 km thick and a factor of 5 stronge r than the upper crust). The observed systematic decrease in rift spacing c an be explained by the known decrease in the crustal thickness in Tibet, fr om similar to 70-80 km in The Himalaya in the south to similar to 50-55 km in central Tibet in the north. A regional comparison of rifts in east Asia suggests that both the involvement of the mantle lithosphere and the age of rift initiation are similar for Tibetan rifts, the Baikal rift, and the Sh anxi graben. This implies that topographic collapse or a convective event i n the mantle cannot be the sole cause for the development of the Tibetan ri fts. A regional boundary condition applied throughout east Asia must be req uired.