TIMING OF SLIP ON THE SOUTH TIBETAN DETACHMENT SYSTEM IN THE MARSYANDI VALLEY, ANNAPURNA RANGE, CENTRAL NEPAL

To understand the geologic history of an orogen, it is imperative to constrain the nature of key deformational features. For example, the South Tibetan Detachment system (STDS) likely played a significant role in the Cenozoic tectonic evolution of the Himalaya, but it is incompletely understood. The STDS is a collection of normal-sense, Oligocene-Miocene-aged structures that are inferred to mark a significant decoupling horizon between N-S-shortening, highly metamorphosed footwall rocks and E-W-extending, weakly to unmetamorphosed hanging wall sediments. However, the STDS is complex: individual detachments appear to have been diachronous and outcrop at multiple structural levels.

The excellent exposures of the STDS in the Annapurna Range allow for detailed studies to be carried out on individual detachments. Two normal-sense structures associated with the STDS outcrop in the Marsyandi drainage, the basal of which is the Chame detachment, while a structurally higher detachment has been correlated with the Phu detachment. Zircon U/Pb dates from variably deformed leucogranites suggest the majority of high-temperature slip on the Chame detachment ceased by ca. 16 Ma, although quartz and feldspar microstructures in a non-mylonitic granite suggest slip was ongoing. Rocks in the footwalls of both structures contain multiple minerals suitable for thermochronologic studies, allowing us to evaluate the total duration of normal-sense slip.

We present ⁴⁰Ar/³⁹Ar muscovite and biotite, and (U-Th)/He zircon and apatite results from a transect across the Chame detachment into the footwall of the overlying detachment. 1D thermal-kinematic models using Pecube-D suggest the footwalls of these structures initially experienced rapid exhumation (~1.5 km/Myr), which we attribute to tectonic exhumation via normal-sense slip. Exhumation rates decrease, likely as slip ceased, to ~0.25-0.75 km/Myr at ca. 15 Ma for the majority of samples, while rocks in the immediate footwall of both detachments record the decrease at ca. 12 Ma. Our results suggest both detachments were synchronous, and any difference in timing of cessation of slip on the two detachments is outside of the resolution of our results. Finally, thermal-kinematic models and topographic analysis suggest potential normal-sense slip over the last ca. 2 Ma at a similar structural level to the overlying detachment.