Displacement along the Karakoram fault, NW Himalaya, estimated from LA-ICP-MS U–Pb dating of offset geologic markers

Abstract: The magnitude of fault offset, a key parameter of fault geometry and kinematics, provides critical information on the role of the Karakoram fault (KKF) in accommodating deformation of the Tibetan crust. Geologic markers used for estimating the magnitude of offset along the KKF include: (1) geomorphologic features, (2) correlative stratigraphic sequences, (3) ophiolite melange belts, (4) igneous bodies, and (5) regional fault systems. The debated offsets of the KKF range from 40km to as much as 1000km. Conflicting offsets may result from a lack of available offset correlations based on quantitative measurements. The Kunsha granite and the Ayilari granite provide another set of potential markers for estimating the offset along the KKF. U–Pb zircon ages and textural observations provide the basis for correlating these granites. Zircon U–Pb ages show that both the Kunsha granite and the South Ayilari granite crystallized around 50Ma ago, whereas the North Ayilari granite formed mostly around 20Ma ago. Reconstruction of batholiths on two sides of the Namru–Menshi basin, as well as the Yalung–Zangbo suture and the trace of the South Kailas thrust, show the offset along the KKF to be about 52±2km. The chronologic data appear to preclude a 100km offset, based on the correlation between the North Ayilari granite and the Kunsha granite. Our data further disproves the conjecture that the North Ayilari granite is synkinematic granite of the KKF, and thus contradicts a 280–400km ophiolite melange belt offset, since it is based on the an incorrect fault initiation time. We calculate a slip rate of 4.5±0.1mm/yr since 12Ma, which implies that the average long-term slip rate is low. This does not support the lateral extrusion model, which predicts a high slip rate. It does support the concept that the collision between India and Eurasia proceeded by distributed deformation rather than lateral extrusion along faults that bound a rigid Tibet. [Copyright &y& Elsevier]