The timing and significance of mid-crustal shearing and exhumation of amphibolite facies rocks along the Great Glen Fault Zone, Scotland

The Rosemarkie Inlier lies on the NW side of the Great Glen Fault Zone (GGFZ) and is composed of foliated and lineated Lewisian gneisses and Moine metasedimentary rocks. The mylonitic foliation strikes NE–SW (parallel to the GGFZ), dips steeply SE and contains a gently-moderately plunging mineral lineation. Microstructural and quartz c-axis fabric analyses indicate that oblique sinistral shearing occurred under amphibolite facies conditions. LA-ICPMS analyses on monazite rims in the gneisses yield ²⁰⁶Pb/²³⁸U ages of 401.8  ± 4.8 Ma (including 2σ uncertainty and a propagated additional 1% external uncertainty). Similar deformation and recrystallization temperatures indicated by quartz fabrics (610°C) and monazite–xenotime thermometry (616  ± 25°C), respectively, in the gneisses suggests that ductile sinistral shearing was ongoing at c. 402 Ma. The c. 402 Ma rim age is the youngest monazite age recorded in the Northern Highland Terrane (NHT) and indicates that sinistral shearing at mid-crustal levels was ongoing along the GGFZ in Lower Devonian (Emsian. 407–393 Ma) times when the thrust sheets of the NHT to the NW had already been exhumed. The Rosemarkie basement rocks are unconformably overlain by Middle Devonian (Eifelian, 393–387 Ma) sedimentary rocks, indicating time-averaged exhumation rates of c. 1.75 mm a^–1 between 402 and ~390 Ma, assuming a geothermal gradient of 30°C km^–1.