Exhumation history of the Karakoram fault zone mylonites: New constraints from microstructures, fluid inclusions, and 40Ar-39Ar analyses

Abstract

The Karakoram fault zone is a dextral strike-slip fault bounded by the Pangong and Tangtse strands on its NE and SW flanks, respectively. In the Tangtse shear zone, the microstructures of mylonitic leucogranite exhibit superposition of high-temperature deformation followed by low-temperature deformation. The mylonites show fluid immiscibility, containing brine and carbonic inclusions. The occurrence of carbonic- and brine-rich inclusions in the oscillatory-zoned plagioclase indicates that they were trapped during the formation of the leucogranite. Eventually, these fluids recorded a near-isobaric drop in temperature down to 40 Ar- 39 Ar biotite ages indicate that the area cooled down to 400–350 °C over 10.34–9.48 Ma, and this period also coincides with a major phase of fluid infiltration and trapping of secondary reequilibrated carbonic and saline-aqueous inclusions. The 10.34–9.80 Ma period recorded a low-temperature deformation at greenschist conditions, when the involved fluid evolved following a near-isobaric path at ∼2 kbar. Subsequently, between 9.80 Ma and 9.48 Ma, the sudden drop in pressure (1.75–0.5 kbar) caused by mylonites produced reequilibrated fluid inclusion textures. These observations suggest that the Karakoram fault zone rocks show a single progressive deformation event with bimodal fluid evolution, in which the carbonic- and brine-rich inclusions were available prior to high-temperature deformation during the initiation of the Karakoram fault zone. The trapping of secondary inclusions between 10.34 Ma and 9.48 Ma with pressure decrease of ∼2–0.5 kbar yields an average uplift rate of 1 mm yr −1 for the Karakoram fault zone.