New U-Th/Pb constraints on timing of shearing and long-term slip-rate on the Karakorum fault

Abstract

Zircons and monazites from 6 samples of the

North Ayilari dextral shear zone (NAsz), part of the

Karakorum fault zone (KFZ), have been dated with

the U-Th-Pb method, using both ID-TIMS and SIMS

techniques. The ages reveal (1) inheritance from

several events spanning a long period between the

late Archean and the Jurassic; (2) an Eocene-

Oligocene magmatic event ( ~35–32 Ma); (3) an

Oligo-Miocene magmatic event ( ~25–22 Ma), at least

partly synkinematic to the right-lateral deformation;

and (4) a period of metamorphism metasomatism

( ~22–14 Ma) interpreted as thermal and fluid

advection in the shear zone. The Labhar Kangri

granite located ~375 km farther Southeast along the

KFZ is dated at 21.1 ± 0.3 Ma. Such occurrence of

several Oligo-Miocene granites along the KFZ, some

of which show evidence for synkinematic

emplacement, suggests that the fault zone played an

important role in the genesis and /or collection of

crustal melts. We discuss several scenarios for the

onset and propagation of the KFZ, and offset estimates

based on the main sutures zones. Our preferred

scenario is an Oligo-Miocene initiation of the fault

close to the NA range, and propagation along most of

its length prior to ~19 Ma. In its southern half, the

averaged long-term fault-rate of the KFZ is greater

than 8 to 10 mm/a, in good agreement with some

shorter-term estimates based on the Indus river course,

or Quaternary moraines and geodesy. Our results show the KFZ cannot be considered as a small transient fault

but played a major role in the collision history.