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

Abstract

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.

Practical Assessment of Quartz Crystallographic Preferred Orientation Strength

“…The distributions investigated include different types of crossed girdles with variable opening angles or rotations (see Figure SI-2 for examples of each distribution, Supporting Information). Type 2 crossed girdle (Figure 1B; Law et al, 2011Law et al, , 2021Law et al, , 2004Lister, 1977) distributions were constructed using randomly generated numbers between 360 and 180 to define an azimuthal distribution that covered half a sphere (as projected in a lower hemisphere plot). Those data were effectively assigned a default plunge of 0°, placing them along the primitive.…”

“…Type 1 crossed girdle distributions (Figure 1A; Law et al, 2021Law et al, , 2013Lister, 1977;Thigpen et al, 2010) were approximated by creating small circles with opposite opening directions through similar methods outlined above. The central septum was then calculated to intersect the small circles by generating a single girdle with a limited distribution based on the rake of the small circles in a vertical N-S plane.…”

“…Due to the nature of quartz deformation, crossed girdle distributions with opening angles of ~55°-90°are common in the literature (see Law, 2014;Law et al, 2004Law et al, , 2011Law et al, , 2013Law et al, , 2021Morgan and Law, 2004, and many other). As shown in Figure 5, even small 5°changes in opening angle between 55°and 90°can significantly affect the B, I, and D calculated for a distribution, whereas for the same range the J PF is essentially invariant.…”

“…Comparison of the spatial trends of the various strength parameters indicates that while the opening angle of crossed girdle distributions may affect B, I, and D, it may not be obvious in the overall patterns; the CPO strength profiles above the Moine thrust (Figure 5;Law et al, 2021) appear similar for B, I, D, and J PF . There are some notable exceptions, however, where changes in opening angle coincide with decoupling between the patterns of B, I, D, and J PF .…”

“…Figure 8 -Distribution strength metrics plotted against opening angle for published data from the A) Moine thrust hanging all(Law et al, 2021) and B) Khimti khola(Larson, 2018). The Pearson correlation coefficient (R2) for linear fits through each of the different metrics are reported.…”