TESTING THE DRIVING FORCES FOR EXHUMATION OF THE GREATER HIMALAYAN SEQUENCE IN NORTHEAST BHUTAN: IMPLICATIONS OF SPLIT-STREAM U-PB ZIRCON GEOCHRONOLOGY

The hinterlands of continental orogens are often composed of highly strained metamorphic rocks that have structural and metamorphic characteristics consistent with lateral crustal flow towards the orogenic front, yet the kinematic processes that exhumed these rocks to the surface remain unclear. In the Himalaya, a thick section of rocks that achieved mid- to lower-crustal depths, the Greater Himalayan Sequence (GHS), is exposed along the strike of the orogen, between the Main Central Thrust (MCT) and the South Tibetan Detachment (STD). In the eastern Himalaya in Bhutan, the top-to-the-south shear zone, the Kahktang Thrust (KT), divides the GHS into upper (GHSu) and lower (GHSl) levels. This project aims to determine the mechanism of exhumation of the GHSu, bounded by the KT and STD, by testing predictions of two end-member models, critical taper and channel flow. The channel-flow model predicts a younging trend in melt crystallization and metamorphic age in the center of the channel, while the critical-taper model predicts a lack of age variation across the GHSu. Nine samples were collected along a transect across the KT and GHSu in eastern Bhutan, and zircons from these samples were analyzed using laser-ablation split-stream ICPMS analysis (LASS) at UC-Santa Barbara. The LASS technique allows the simultaneous collection of U-Pb ratios and trace-element abundances, permitting age data to be connected with different parts of the P-T path. Schist, paragneiss and layer-parallel leucosome from below the KT revealed dates from ca. 840–2400 Ma. Alternatively, a paragneiss collected 4 km above the KT revealed inherited zircon dates of ca. 380–2400 Ma, with five single rim dates between ca. 41–28 Ma. Layer-parallel leucosomes and a cross-cutting pegmatite 800 m above the KT yielded zircon crystallization ages from ca. 22–16 Ma. However, a layer-parallel leucosome 4 km above the KT yields dates as young at ca. 14 Ma. For all samples, the trace-element analyses are similar and suggest that the zircon crystallized under plagioclase-stable (Eu anomaly = -1.5 to -0.1) and garnet-unstable (Lu/Dy = 5 to 20) conditions. These preliminary results reveal a younging age trend toward the middle of the GHSu section, suggesting that channel flow may have exhumed the GHSu in eastern Bhutan.