Date of Award
Master of Science
William Kean, Barry Cameron
AMS, Loch Borralan, Moine Thrust Zone, Pluton, Scotland, Thrust Fault
The Loch Borralan pluton was emplaced within the Assynt Region of the Moine Thrust zone during the Scandian event (ca. 435-425 Ma) of the Caledonian Orogeny (478-425 Ma). It consists of two major magma suites, the syenitic early suite (431.1 ± 1.2 Ma), and the quartz syenitic later suite (429.2 ± 0.5 Ma). The region is characterized by a series of in-sequence thrust faults that strike NE-SW and dip approximately 20± to the SE, including (from lower to upper): the Sole Thrust, the Borralan Thrust (hypothesized, but not exposed), the Ben More Thrust, and the Moine Thrust. A series of imbricate thrusts between the Sole and Borralan Thrusts juxtapose repeated Cambrian and Ordovician strata. The Loch Borralan pluton intruded between the Sole and Ben More thrust faults, and may be bounded below by a hypothesized Borralan Thrust fault. Based on the overlap in pluton crystallization age and orogenic activity, the combination of macroscopic field lineation and foliation measurements, anisotropy of magnetic susceptibility (AMS) lineation and foliations measurements, mineral shape preferred orientation (SPO) analysis, and petrographic deformation microstructure analysis will be used to determine if the pluton expresses deformation features and fabrics corresponding to thrust fault tectonics.
The dominant magnetic mineralogy as determined by thermomagnetic data and hysteresis plots was determined to be magnetite and titanomagnetite. Some paramagnetic components were seen in the early suite, and can be attributed to biotite. Both minerals contributed to the AMS signal that was used to interpret pluton fabrics.
The pluton contains S>L (foliation stronger than lineation) fabrics throughout, defined by alignment of alkali feldspar grains. The foliation strike of both the early and late suites are subparallel to the thrust faults, providing evidence that the fabrics are related to deformation.
The early suite is only well exposed in the southeast at the top of the Borralan thrust sheet, and proximal to the bottom of the Ben More Thrust fault. Foliations strike approximately 030 and dip 20° SE. Mineral lineations were not readily seen in the early suite. AMS results show mean principal susceptibilities (K1>K2>K3) parallel to field measurements.
Mineral foliations in the late suite have a similar strike to early suite foliations, but dip roughly 50-60± both to the NW and SE. AMS foliations parallel mineral foliations, and strike generally NE-SW. AMS foliation dips are more variable, spanning the range of possible dip angles. The variance in foliation dips are likely caused by a composite magnetic fabric resulting from thrusting combined with thrust parallel flattening. Lineations in the late suite are scarce, but generally plunge shallowly to the NE and SW indicating horizontal extrusion parallel to the strike of the thrust faults. Similarly, most late suite AMS lineations plunge shallowly perpendicular to thrust transport direction corroborating the field measurements. Some late suite AMS lineations plunge parallel to thrust transport direction, suggesting there are also components of simple shear related to thrusting, with partitioning varying throughout the late suite.
The foliation dip degree variation between the two suites could be the result of the spatial relationships and/or timing. The pluton is interpreted to have been emplaced syn-kinematically, deformed via thrusting related simple shear in the early suite, and with a component of flattening and lateral extrusion added in the late suite.
Shape preferred orientation (SPO) measurements were conducted using the intercept method of image analysis to relate mineral fabrics to AMS measurements. SPO are generally weak, and have low shape ratio values. Mineral orientation parallels magnetic mineral fabric within each thin section, which supports AMS measurements as representative of mineral fabric.
Petrographic microstructural thin section analysis was performed to analyze deformation mechanisms to deformation conditions. Feldspars exhibited both crystal plastic (ductile) and brittle microstructures. Examples of crystal plastic microstructures seen in feldspars include: perthite and myrmekite, undulose extinction, grain boundary bulging. Feldspars also showed quartz filled fractures (brittle deformation), which occasionally formed conjugate sets relative to AMS principal susceptibilities. Quartz was primarily seen only in the northwest extent of the pluton. Quartz also had both crystal plastic and brittle microstructures. Crystal plastic microstructures seen in quartz include: dynamic recrystallization, undulose extinction (recrystallized and primary grains), and grain boundary bulging. Examples of brittle microstructures in quartz are intragranular fractures. Deformation temperature conditions range from high grade to low grade. The presence of structures such as perthite and myrmekite textures infer deformation temperatures of around 600±C, and on the low end brittle deformation in either quartz or feldspars suggest sub 300±C. The range of deformation temperatures suggest that the pluton was emplaced synkinematically and deformation continued throughout its cooling history.
The strong agreement between field and AMS measurements combined with evidence for high temperature deformation conditions suggests the pluton intruded syn-tectonically. The majority of deformation is seen in the early suite, and southeastern late suite. These locations are closest to the Ben More thrust, suggesting that movement along this thrust caused the majority of deformation. Based on AMS orientations, deformation in the early suite is accommodated as thrust motion related simple shear. Late suite deformation has a combination of thrusting simple shear, general flattening, and lateral extrusion. Since the pluton was likely roofed by thrust faults restricting upward flow, space for the magma was created through lateral extrusion.
Calhoun, Justin, "Fabric and Microstructural Analysis of the Loch Borralan Pluton, Northwest Highlands, Scotland" (2014). Theses and Dissertations. 396.