Using thermochronology to test the validity of two balanced cross sections, Karnali River, far-western Nepal

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A balanced cross section provides a viable subsurface geometry; however, multiple subsurface cross section geometries may be possible to match the observed surface geology. Low-temperature thermochronologic data and thermokinematic models can be used to test these multiple subsurface geometries. In this study, I use muscovite 40Ar/39Ar, apatite and zircon fission track, and zircon (U-Th)/He ages in conjunction with two balanced cross sections along the Karnali River in far western Nepal to determine if the cross sections are viable. The balanced cross sections are reconstructed to determine the kinematic sequence and then sequentially deformed in ~10 km increments with flexural loading and erosional isostatic unloading applied to each step. Ages are assigned to each 10 km increment of displacement and then used to create a 2-D thermokinematic model, which predicts the cooling ages along the section. By varying the location of the décollement ramp, shortening rates, and radiogenic heat production, the model is modified to better match the existing observed age data. Comparison of the modeled cooling ages to the thermochronologic data indicates that although the cross sections are valid and balanced, the subsurface fault geometry does not reproduce the existing observed cooling ages over the young uplift imparted by the active ramp in the décollement below the Lesser Himalayan duplex. Thus, a new balanced cross section is needed with a different location and geometry of the ramp to produce the existing cooling ages.

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