Thermal history of a pebble in the Indus Molasse at the margin of a Himalayan metamorphic core complex

Abstract

In this paper we report on data from a pebble within an upright fold in the Indus Molasse. The pebble was analysed microstructurally, white mica was separated, irradiated, and subject to40Ar/39Ar geochronology during a step-heating procedure in vacuo. Because of the care with which this procedure was followed, the apparent age spectra that resulted was a near perfect replica of what might be expected as the result of diffusional loss of argon from a fractal diffusion network. The earlier reported phenomenon of fractal feathering in the Arrhenius plot is observed in this sample. If the Arrhenius data are analysed so that the Fundamental Asymmetry Principle is obeyed, the inferred diffusion parameters are close to those obtained from high-pressure experiments. These data are then analysed using r/ r0 plots. Two distinct diffusion domain sizes could be recognised, and a pattern of gas release theoretically consistent with fractal feathering of the smaller domain size. The results can be taken to indicate that the pebble was eroded from the Ladakh Batholith some time after 16.5 Ma, during its extensional exhumation, and incorporated in the molasse. The upright folding during inversion of the core complex took place at a later time, i.e. mid-Miocene or younger.