Phase equilibria modeling constraints on channel flow of metasedimentary crust beneath the southeastern Tibetan Plateau

Abstract

Lower crust of the southeastern Tibetan Plateau includes 100 s of km²-scale zones of low resistivity at 20–40 km-depths indicating the presence of melt ± fluid ± graphite. These zones coincide with interconnected zones of low shear wave velocity. Data from central and southeastern Tibet support the occurrence of metasedimentary felsic granulites in the lower crust along a backarc-like geothermal gradient. Phase equilibria modeling of average metasedimentary pelite and greywacke bulk compositions constrained by these geophysical data explored the impact of partial melting on shear wave velocity for a range of fluid content. Low shear wave velocities beneath southeastern Tibet can be reproduced with melt volumes up to 8 vol%, consistent with incipient melt weakening across the melt connectivity transition at 25 km-depths. Observed shear wave velocities at 35 to 50 km-depths, however, are consistent with melt volumes below the threshold for melt weakening. The results are consistent with limited to no regional scale channel flow wherever metasedimentary rocks are present in the deep crust of southeastern Tibet.