U–Pb SHRIMP and 40Ar/ 39Ar ages constrain the deformation history of the Karakoram fault zone (KFZ), SW Tibet

Abstract

The ~ 800-km-long dextral Karakoram fault zone (KFZ) bounds SW Tibet and, despite a number of recent studies, its spatial and temporal evolution and consequent role in continental deformation remains under debate. In order to better constrain fault evolution we present U–Pb Sensitive High Resolution Ion MicroProbe (SHRIMP) and 40Ar/ 39Ar analyses of granitic samples from the southern portion of the Karakoram Fault. Zircons from the ductile shear zone in the Namru region of the Ayilari granite batholith record a crystallization age compatible with Gangdese magmatism belt (also known as the Trans-Himalayan magmatic Belt, THB) followed by a medium temperature cooling event between about 12.8–11.5 Ma. And zircons from the ductile shear zone in the Zhaxigang region of the Ayilari granite batholith record a crystallization age of 18.7 Ma. The Ayilari granite batholith near Namru appears to have experienced neither the 32 Ma nor 25–22 Ma magmatic events reported by previous studies. Biotite 40Ar/ 39Ar ages provide evidence that the 12.8–11.5 Ma cooling event coincides with the initiation of the KFZ cutting through the Ayilari granite, both at Namru and Zhaxigang. Consequently, we conclude that the previously reported 32 Ma, 25–22 Ma and 22–14 Ma events bear no relationship to the activity along the Karakoram fault, but resulted from either local metamorphic events caused by regional tectonics (such as activity on the south Kailas thrust fault), or from magmatism around Shiquanhe (also known as Gar). This study supports the conclusion that the KFZ, at least in the Zhaxigang–Namru region, propagated along strike to the Gar–Menshi area as a thin-skinned structure at around 13 Ma.