Mylonitic rocks associated with the south-dipping detachment fault of the Yagan–Onch Hayrhan metamorphic core complex in southernmost Mongolia indicate subhorizontal south-southeast–directed extension in the Early Cretaceous; synkinematic biotites give ⁴⁰Ar/³⁹Ar ages of 129 to 126 Ma. The Yagan–Onch Hayrhan core complex demonstrates that late Mesozoic localized high-strain extension, recently recognized in other parts of eastern Asia, also occurred in Mongolia. The presence of Mesozoic metamorphism at Onch Hayrhan, previously presumed to be Precambrian, brings into question the existence of the South Gobi microcontinent.