Exhumation and attainment of steady state in the Himalaya: insights from the detrital sediment record

The sediment record of material eroded from an orogen can make a unique contribution to determining the source regions exhumation. This study of the Himalayan foreland basin sediments at Jawalamukhi, India (Meigs et al 1995) contributes to the documentation of attainment of steady-state in the orogen in two ways: The difference between a detrital minerals cooling age and its host sediments depositional age, is termed the lag time. In a sediment record, steady-state exhumation produces peak ages that young with time and have a constant lag time (Garver et al 1999). We Ar-Ar dated detrital micas from the sediment succession, magnetostratigraphically dated between 13-4 Ma. From 13 Ma, until the High Himalayan signature was cut off by thrusting by 6 Ma, the youngest ages remain constant and lag times increase up section, indicating that the High Himalayan region that sourced these micas was not in exhumational steady state. Critical wedge theory states that an orogen attains steady-state width when accretionary influx balances erosion (Dahlen and Suppe 1988). We use petrography, Sm-Nd whole rock signatures and Ar-Ar dating of detrital micas to date a period of frontal accretion in the Himalaya. Prior to 7 Ma, low-grade metamorphic and sedimentary material dominated the detritus, with subordinate contribution from the High Himalaya. By 6 Ma, Himalayan-aged micas, kyanite, staurolite and sillimanite are cut out, whilst basaltic fragments make a significant contribution to detritus. By 5 Ma, the mica population becomes dominated by previously unrecorded Precambrian grains, granitoid material is significant and ɛ_Nd values become more negative. These data indicate major exhumation of the Lesser Himalaya at this time. Conglomerate facies from 8.7 Ma could suggest earlier erosion of the Lesser Himalaya (Brozovic and Burbank 2000) or more proximal erosion of the already exhumed Haimantas (White et al 2002). ɛ_Nd values on these clasts, currently being analysed, will resolve the issue.