CONSTRAINTS ON UNROOFING RATES IN THE HIGH HIMALAYA, EASTERN NEPAL

Thermobarometric data for samples across the Main Central thrust zone in eastern Nepal show an inversion in temperature but not in pressure. These data have been interpreted to represent a portion of the paleogeotherm at the time of Main Central thrust deformation. A 40Ar/39Ar age on hornblende (closure temperature (Tc) = 500 +/- 50-degrees-C) constrains the timing of this deformation to be approximately 21 +/- 0.2 Ma. The 40Ar/39Ar ages of other minerals (muscovite, Tc = 350-degrees-C, age (t) = 12.0 +/- 0.2 Ma; K-feldspar, Tc = 220-degrees-C, t = 8.0 +/- 0.2 Ma) from the same location further constrain the cooling history of this region. Together the geochronologic and thermobarometric data yield an average unroofing rate of 1.2 +/- 0.6 mm/yr for the High Himalaya of eastern Nepal. Simple thermal models show that these geochronologic and thermobarometric data are consistent with a wide range of different initial geotherms, applied boundary conditions and magnitude of radiogenic heat production. The variation through time of the unroofing rates can only be poorly constrained, however. The unroofing histories were found to be largely insensitive to the details of the assumed initial geotherm, fairly sensitive to the magnitude of radiogenic heat production, and extremely sensitive to the nature of the boundary conditions applied below the fault zone. This study underscores the difficulty in constraining uplift histories on the basis of cooling rates even when thermobarometric data are available to supplement geochronologic constraints on the cooling history of the region.