Quantifying crustal flow in Tibet with magnetotelluric data

Several classes of geodynamic models have been proposed to explain the evolution of the Himalayan orogen and the Tibetan plateau. Models invoking flow in a weak lower crustal layer have been successful in explaining a number of geological and geophysical observations. Geophysical observations indicate that the weak layer at mid- to lower crustal depths in Tibet may be the result of partial melting. Laboratory studies indicate an order reduction in strength for melt fractions of 5–10%. By relating these laboratory studies to magnetotelluric observations, which are sensitive to the presence of partial melts, it is possible to estimate the flow parameters associated with the channel flow model.Calculations show that in the Central Tibetan Plateau, if a layer consists of partially molten felsic rocks, then conductances in the range 7000–27,000 S will produce flow velocities of the order 1 cm/a. Beneath the southern part of the Lhasa block and the Qiangtang terrane magnetotelluric studies indicate conductances of up to 20,000 S. These conductances suggest effective viscosities of 2.5 × 1018 to 3 × 1020 Pa s, corresponding to flow velocities between 0.02 and 4.5 cm/a. Together with higher pressure gradients near the margins of the plateau these flow parameters clearly support the validity of the channel flow model in these parts of Tibet.Lower crustal conductances (3000 S) are reported beneath the northern Lhasa block and deformation by channel flow in this region is unlikely. Assuming a felsic rock composition, the minimum effective viscosity of the crust beneath the northern Lhasa block is estimated as 3 × 1020 Pa s. The corresponding flow velocities do not exceed 0.04 cm/a unless higher temperatures than suggested by seismic observations are assumed.