DETERMINING THE RATES AND TIMING OF EXHUMATION IN THE HIMALAYAN OROGEN: CONSTRAINTS FROM AR-AR DATING OF DETRITAL MICAS FROM THE FORELAND BASIN SEDIMENTARY RECORD, PAKISTAN

Information on the early stages of an orogen's exhumation history is often obscured in the mountain belt itself by later metamorphism, tectonism or erosion. Hence the sedimentary record, of material eroded from the mountain belt prior to later metamorphism and preserved in basins in front of the orogen, can provide invaluable insights to this early history.

Comparison of orogen-derived detrital white mica ages with the host sediment depositional age (determined by magnetostratigraphy) provides a method to identify periods of rapid exhumation in the source region. Individual micas are dated using the ⁴⁰Ar-³⁹Ar isotopic method. These ages represent the timing of cooling through the white mica closure temperature (~350ºC) during exhumation in the orogen. Times when the detrital mica age and the sediment depositional age are similar implies rapid orogenic exhumation at this time.

We applied this technique to the Kamlial Formation of the Potwar Plateau, Pakistan. These are Himalayan foreland basin sediments magnetostratigraphically dated at 18-14 Ma. Our current work on this formation shows that in sediments aged 17-18 Ma, the youngest micas are 23 Ma, implying an averaged cooling rate of >50ºC/Ma for this period. Sediments aged 15-16 Ma contain micas dated 16-17 Ma, implying extremely rapid exhumation at this time. Sediments deposited at 14 Ma have a distinctly different mica population: all micas are aged >30 Ma and more than 80% of grains are aged between 200-450 Ma (compared to <15% in the older samples). Comparison with data acquired by the same technique from co-eval sediments in India (N. White et al., J. Geol. Soc. London, 2001) provides an indication of the degree of variation in exhumation patterns along strike in the orogen. Potential causes for the period of rapid exhumation around 16 Ma, and the anomalous mica ages in the 14 Ma sandstone sample, will be discussed in the light of Himalayan tectonics and variations in sediment depositional facies.