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Unravelling the Shimla klippe: Microstructures and 40Ar/39Ar geochronology 

Jia-Urnn Lee and Marnie Forster

Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

The Himalaya orogen consists generally of four main crustal-scale tectonic sheets that overlie each other. From south to north and ascending through the structural pile these are: the Sub-Himalaya, the Lesser Himalaya, the Greater Himalaya and the Tethyan Himalaya.  Often klippes of the Greater Himalaya are exposed in the underlying Lesser Himalaya, where the surrounding Greater Himalayan sheet has been eroded away. The Shimla klippe (of Greater Himalayan origin) is one such tectonic lens of medium metamorphic grade. It consists of garnet bearing rocks of the Jutogh Group (part of the Greater Himalaya) overlying the low-grade Lesser Himalayan rocks. The klippe was emplaced by the Jutogh Thrust into its current day position. This thrust is considered to be a branch of the Main Central Thrust (MCT), the major ductile shear zone that, in much of the Himalaya, juxtaposes the Greater Himalaya against the Lesser Himalaya. Studying the klippe provides insight into the structural, thermal and geochronological evolution of these tectonic sheets.

 

For the past few decades, rocks in the Shimla klippe have been thought to record Miocene-Pliocene cooling ages, reflecting the period of deformation along the Main Central Thrust. Microstructural analysis shows ongoing garnet growth within the shear zone, and indicates that temperature conditions were sufficient for white mica recrystallisation during (MCT) shear zone activity. Similarly geochemistry of white micas of the Jutogh rocks suggests that they have a similar Si-Al composition as other Himalayan white micas of Oligo-Miocene age. However preliminary 40Ar/39Ar analysis of white micas on overlying Jutogh Group rocks close to the klippe contact yielded unexpected results. Miocene-Pliocene apparent ages related to cooling or deformation along the MCT were expected. However, Mesozoic-Paleozoic (~200-350 Ma) ages were obtained from rocks in this klippe instead. The age disparity between these rocks and the expected age range of the MCT (16-24 Ma) suggests that: a) the klippe may not have been emplaced by the MCT; or b) that the Jutogh Thrust is not a branch of the MCT. This is an ongoing study to investigate the nature of the old ages of the rocks and the nature of the thrust system that underlies the Shimla klippe.


Figure 2. Preliminary 40Ar/39Ar age spectra of Jutogh rocks within the Shimla klippe showing mainly Mesozoic apparent ages in what is expected to be Miocene-age rocks. A thermal event that occurred before ~55 Ma has only partially reset the argon isotopic system, thus preserving old ages. The staircase-shaped spectra are characteric of white micas that have argon gas in different retentivity sites that outgas at different times. York and Arrhenius plots for these samples (not shown here) indicate that sites of higher argon retentivity outgas during higher temperature intervals of the experiment.