195-12 Phase Equilibria Modeling Constraints on Lower Crustal Pipe Flow Beneath the Southeastern Tibetan Plateau

Session: Evolution of Orogenic Belts Through Time: Insights from Sedimentation, Deformation, Magmatism, and Metamorphism (Posters)

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Abstract:

Magnetotelluric imaging across the lower crust of the southeastern Tibetan Plateau reveals 100s of km² elliptical zones of low resistivity at 20-40 km depths, indicating the presence of melt ± fluid ± graphite. These zones coincide with low shear wave velocities. 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 metagreywacke bulk compositions constrained by these geophysical data explored the impact of partial melting on seismic 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.% for metagreywacke and less than 7 vol.% for metapelite, consistent with weakening across the melt connectivity transition at 25 km-depths for greywacke but not for pelite. Observed shear wave velocities at 35 km- and 50 km-depths coincide with melt volumes below the threshold for melt weakening.  Results support only local and limited occurrence of active melt-weakened lower crust. This behavior is likely dependent on the local vs. regional distribution of fertile metasedimentary rocks like metagreywacke at depth. The results preclude the occurrence of regional-scale channel flow via melt weakening in southeastern Tibet, since observed absolute shear wave velocities do not coincide with melt volumes above the threshold for weakening.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10955

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