Shearing on the Great Glen Fault: Kinematic and Microstructural Evidence Preserved at Different Crustal Levels

The NE-SW trending Great Glen Fault (GGF) is one of mainland Scotland's most significant crustal-scale faults, although our understanding of its early kinematics is in question. Previous studies generally agree that the GGF was initiated as a Silurian sinistral strike-slip fault displacing c. 425 Ma isotopically dated granitic plutons. Stewart et al. (2001) argued that dikes fed by these plutons were sinistrally sheared by the GGF while in the sub-magmatic state, suggesting continuous strike-slip motion on the GGF by 425 Ma. Strike-slip offset post-dating overlying Devonian sedimentary basins is likely only a few tens of kilometers, requiring substantial (100s of kms) Silurian-aged strike-slip movement on the GGF in most plate reconstruction models for the Caledonian mountain belt, now exposed in East Greenland, Scandinavia, and Scotland. In contrast, a recent study (Searle 2021) has argued that motion on the GGF may instead have initiated in the Upper Paleozoic and that off-set is therefore minimal, bringing current restoration models into question.

Several papers report widespread field and microstructural evidence from crystalline bedrock and overlying Devonian sedimentary rocks for brittle upper-crustal shearing on the GGF. However, evidence for high-temperature crystal plastic shearing at deeper crustal levels on the GGF, potentially of Silurian to Early Devonian age, is limited. During summer 2022, suites of oriented and plastically deformed metasedimentary rock samples were collected from the NW side (Moine/Lewisian gneisses and quartzites), center (Moine quartzites), and SE side (Dalradian quartzites) of the GGF. Additional samples included plutonic rocks from locations adjacent to the GGF and the associated Strathconnon fault that were believed to have been intruded during strike-slip motion, but after regional metamorphism and deformation in the surrounding Moine rocks. Microstructures and quartz c-axis fabrics from samples on the NW side and in the center of the GGF indicate a NW side up to the SW sense of displacement about NE to E plunging slip vectors, and these results are compatible with oblique sinistral motion on the GGF below the brittle-ductile transition zone during Silurian - Early Devonian times. However, radiometric dating is needed to prove the absolute timing of this shearing. In contrast, on the SE side of the GGF, NW side up or NW side down senses of shearing are indicated at different locations. Brittle fracturing is observed in all collected samples, overprinting the earlier high-temperature (300 - 650 °C) crystal fabrics and microstructures developed below the brittle-ductile transition zone. No convincing microstructural evidence for sub-magmatic shearing during pluton emplacement was found in the samples collected. However, the local presence of high-low temperature (c. 650 - 300 °C) solid-state deformation microstructures in both quartz and feldspar grains in these 430 - 425 Ma plutons suggests that the plutons were deforming internally in response to far-field stresses generated by shearing on the adjacent GGF and Strathconnon fault during cooling to background regional temperatures.