Tectonic Transport Directions, Shear Senses and Deformation Temperatures Indicated by Quartz c-Axis Fabrics and Microstructures in a NW-SE Transect across the Moine and Sgurr Beag Thrust Sheets, Caledonian Orogen of Northern Scotland
Moine metasedimentary rocks of northern Scotland are characterized by arcuate map patterns of mineral lineations that swing progressively clockwise from orogen-perpendicular E-trending lineations in greenschist facies mylonites above the Moine thrust on the foreland edge of the Caledonian Orogen, to S-trending lineations at higher structural levels and metamorphic grades in the hinterland. Quartz c-axis fabrics measured on a west to east coast transect demonstrate that the lineations developed parallel to the maximum principal extension direction and therefore track the local tectonic transport direction. Microstructures and c-axis fabrics document a progressive change from top to the N shearing in the hinterland to top to the W shearing on the foreland edge. Field relationships indicate that the domain of top to the N shearing was at least 55 km wide before later horizontal shortening on km-scale W-vergent folds that detach on the underlying Moine thrust. Previously published data from the Moine thrust mylonites demonstrate that top to the W shearing had largely ceased by 430 Ma, while preliminary isotopic age data suggest top to the N shearing occurred at ~470–450 Ma. In addition, data from the east coast end of our transect indicate normal-sense top down-SE shearing at close to peak temperatures at ~420 Ma that may be related to the closing stages of Scandian deformation, metamorphism and cooling/exhumation.
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This project was supported by National Science Foundation grants EAR 0538010, EAR 0711207 and EAR 1220138 to R.D.L.
The following are available at https://www.mdpi.com/article/10.3390/geosciences11100411/s1, Supplementary File S1: Location, lineation and foliation orientation, quartz c-axis fabric opening angle, deformation temperature and shear sense data for all samples, plus publication versus field sample numbers, Supplementary File S2: quartz recrystallization regime and recrystallized grain size data for all samples, Supplementary File S3: summary of 40Ar/39Ar analytical methods employed for sample MT-10, Supplementary File S4: 40Ar/39Ar data for sample MT-10.
The above materials are also available for download as the additional file listed at the end of this record.
Law, Richard D.; Thigpen, J. Ryan; Mazza, Sarah E.; Mako, Calvin A.; Krabbendam, Maarten; Spencer, Brandon M.; Ashley, Kyle T.; Strachan, Robin A.; and Davis, Ella F., "Tectonic Transport Directions, Shear Senses and Deformation Temperatures Indicated by Quartz c-Axis Fabrics and Microstructures in a NW-SE Transect across the Moine and Sgurr Beag Thrust Sheets, Caledonian Orogen of Northern Scotland" (2021). Earth and Environmental Sciences Faculty Publications. 24.