Relationships between right-lateral shear along the Karakoram fault and metamorphism, magmatism, exhumation and uplift: evidence from the K2-Gasherbrum-Pangong ranges, north Pakistan and Ladakh

The Karakoram fault is a major NW–SE-aligned dextral strike-slip fault that has accommodated minor extrusion of thickened Tibetan crust since the Mid-Miocene. Controversy exists as to whether the fault is lithospheric in scale, resulting in synkinematic metamorphism and granite melting, or whether it is an upper crustal fault cutting through earlier metamorphic and granitic rocks. Structural, metamorphic and geochronological data from the Karakoram fault show that: (1) metamorphism and granite genesis occurred prior to strike-slip shearing, meaning that U-Pb zircon and monazite ages from deformed granites cannot be used as proxies for dating strike-slip shearing, only as a maximum age constraint on initiation of ductile shearing; (2) sheared Miocene leucogranites along the fault are equivalent to the Baltoro granite batholith, offset 40–150 km along the fault, and show a strain gradient from high ductile strain to undeformed away from the fault; (3) ⁴⁰Ar/³⁹Ar and fission-track ages of K2 orthogneisses and Gasherbrum diorite show that rocks adjacent to the fault cooled during Cretaceous to mid-Tertiary time, and were not reset during shearing, indicating that shear heating was not important; (4) the Gasherbrum diorite, adjacent to the fault, has never been at depths greater than 6 km since the early Cretaceous and less than 3 km of material has been eroded off since then. We conclude that the Karakoram fault has had no significant influence on regional metamorphism or melting, at least at levels exposed today, and is restricted to the upper–middle crust.