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. / Master of Science / The Great Glen Fault (GGF) is one of mainland Scotland's most significant large-scale faults, although our understanding of its early motion is debated. Most geologists agree that the GGF began displacing existing rocks during the Silurian (c. 444 - 419 Ma), including igneous bodies, known as plutons, of approximately the same age (c. 425 Ma). Stewart et al. (2001) argued that during shearing, dikes fed by these plutons were deformed before cooling to background temperatures, which may suggest that the GGF was continuously undergoing lateral strike-slip motion by 425 Ma and that post-Silurian offset was likely only a few tens of kilometers. Most plate reconstruction models for the Caledonian mountain belt, now exposed in East Greenland, Scandinavia, and Scotland, assume that significant lateral motion and shearing occurred on the GGF during the Silurian. However, new research has suggested that the GGF was initiated several million years later, bringing current restoration models into question. Several published papers have reported widespread evidence for upper-crustal brittle shearing of crystalline bedrock and overlying Early Devonian (c. 420 - 359 Ma) sedimentary basins within the GGF. However, evidence for lower-crustal shearing during the same time frame, resulting in plastic deformation, is limited. To address this knowledge gap, I collected suites of oriented bedrock samples and 430 - 425 Ma plutonic rocks from locations adjacent to the GGF and associated Strathconnon Fault believed to have been intruded during strike-slip motion. Samples from the NW side and center of the GGF suggest oblique left-lateral motion within the fault zone, with the rocks on the NW side of the GGF moving upward relative to the SE side, compatible with current generally accepted models for the Silurian-Early Devonian age on the GGF; however, these results must be verified with radiometric dating to constrain the absolute timing of shearing. On the SE side of the GGF, vertical offset is variable at different locations. Brittle upper-crustal shearing is observed in all samples, which overprints early high-temperature (300 - 650 °C) deformation. Early lower-crustal shearing on the GGF is recorded by these deformation indicators and was followed by uplift and fracturing within the GGF of these initially lower-crust rocks. The local presence of solid-state deformation microstructures in the plutons suggest internal deformation due to shearing on the adjacent Great Glen and Strathconnon Faults during their cooling to regional background temperatures.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/115144 |
| Date | 22 May 2023 |
| Creators | Becker, Cassandra |
| Contributors | Geosciences, Law, Richard D., Caddick, Mark J., Spotila, James A. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf, application/pdf |
| Coverage | Scotland |
| Rights | Creative Commons Attribution 4.0 International, http://creativecommons.org/licenses/by/4.0/ |
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