CONSTRAINTS ON THE AGE OF DUCTILE EXTENSION ALONG THE BASAL SOUTH TIBETAN DETACHMENT, ANNAPURNA RANGE, CENTRAL NEPAL

The South Tibetan Detachment system (STDS) is one of the major structural features contributing to the Cenozoic evolution of the Himalayan-Tibetan orogen, despite its predominately extensional kinematics. Individual detachments are found at multiple structural levels and are thought to have been active at different times, frequently with evidence of reactivation. The STDS is often closely associated with leucogranite intrusions, which from detailed field studies have been found to be pre-, syn- and post-kinematic. Himalayan leucogranites are typically rich in accessory minerals such as monazite and zircon, ostensibly making them ideal for (U-Th)/Pb dating. However, they are notorious for containing multiple generations of accessory minerals, including inherited crystals that have (U-Th)/Pb dates only a few million years older than the crystallization age of the leucogranite. Under such circumstances, a sufficiently large number of accessory mineral crystals must be dated in order to identify the subpopulation most likely to reflect the magmatic age of the host leucogranite.

We present preliminary results from a study of leucogranites associated with the Annapurna detachment, the basal and primary strand of the STDS in the Kali Gandaki Valley, central Nepal. Previous workers, relying on sparse U/Pb monazite datasets, have interpreted ductile extension on the Annapurna detachment to have ceased by ca.22.5 Ma (Godin et al., 2001, Tectonics). However, we recently identified a pre-kinematic leucogranite that contains extensional fabric related to slip on the detachment, as well as a post-kinematic leucogranite that cuts across this fabric. Twenty-nine laser ablation (U-Th)/Pb dates of monazites from these leucogranites are concordant and near-concordant, ranging from latest Oligocene to middle Miocene; only the youngest crystals from each leucogranite are likely to indicate the crystallization age of the leucogranite. While this study is still in its early stages, the available data constrain the timing of ductile extension on the Annapurna detachment to between 15.74 Ma ± 0.69 and 19.00 Ma ± 0.59 (uncertainties are quoted at 2σ) – much younger than previously thought.