Prolonged Slip on the South Tibetan Detachment Constrains Tectonic Models for Synorogenic Extension in the Central Himalaya

The South Tibetan detachment system (STDS) is one of the most important deformational features in the Himalayan orogen; yet its evolution in space and time remain incompletely understood. Here, we present the results of a new study of the primary, basal strand of the STDS in the Annapurna Himalaya of central Nepal: the Annapurna detachment. The original discovery outcrop of this structure in the Kali Gandaki valley reveals that multiple leucogranite bodies are variably deformed by ductile slip on the detachment. New laser-ablation (U-Th)/Pb dating of complex monazite suites from these bodies indicates that leucogranites in this outcrop intruded over a period extending from at least 22.76 ± 0.30–14.95 ± 0.78 Ma. Field relationships and microstructures within studied samples show that ductile slip on the Annapurna detachment was active—at least episodically—throughout this period and also continued into the more recent past. Based on cooling history models for the outcrop constrained by ⁴⁰Ar/³⁹Ar and (U-Th)/He data, ductile slip likely continued until at least 12 Ma. These results are at odds with previous inferences that slip on the STDS in central Nepal had ceased by ca. 22 Ma and call into question the popular idea that there was an abrupt geodynamic transition from predominantly N-S-directed extension to predominantly E-W-directed extension in the central Himalaya in the early Miocene.