The thermal and metamorphic evolution of the Northern Highlands Terrane, Scotland
Abstract
The Northern Highlands Terrane (NHT) in Scotland preserves a long record of metamorphism and convergent deformation related to several orogenic events that occurred from Neoproterozoic to Devonian time. Deconvolving the signatures of multiple tectonic events and determining the rates of metamorphism in settings like the NHT are important parts of better understanding the thermal and mechanical processes controlling convergent tectonics. I have used monazite-xenotime thermometry and geochronology, in conjunction with metamorphic petrology and additional accessory phase geochronology, to place constraints on the timing and rates of thermal metamorphism in a variety of structural settings throughout the NHT. Our data show that the ductile thrust nappes of northernmost Scotland preserve a record of Scandian (435-410 Ma) orogenesis. High grade metamorphism in the hinterland Naver nappe likely resulted from the widespread infiltration of granitic magmas at c. 425 Ma, which coincided with peak metamorphism. The timing of metamorphism in the hinterland Scandian thrust nappes is apparently younger than at least some deformation in the foreland Moine thrust zone, suggesting this orogenic wedge experienced large-scale out-of-sequence deformation and metamorphism. In contrast to the Scandian nappes, the Sgurr Beag nappe records primarily Precambrian metamorphism related to the Knoydartian orogeny (780-725 Ma). Additionally, monazite in the Sgurr Beag nappe preserves a record of widespread metasomatism and metamorphism at c. 600 Ma, possibly related to the break-up of Rodinia at that time.
A potentially important heat source in orogenic systems, like those preserved in Scotland, is the thermal energy dissipated during deformation, otherwise known as shear heating. It is important to consider to how shear heating may contribute to metamorphism during orogenesis. This is challenging because there are few, if any, methods of relating observations from typical orogenic systems to magnitudes of shear heating. We have developed a model that is adaptable to a wide range of parameters that can be measured from naturally deformed rocks and places first-order constraints on magnitudes of shear heating. While our models suggest that shear heating is not particularly important in the NHT, in lower initial temperature mylonite zones shear heating could be more significant.
General Audience Abstract
The Northern Highlands Terrane (NHT) in Scotland preserves a long record of metamorphism and convergent deformation related to several orogenic events that occurred from Neoproterozoic to Devonian time. Understanding the record of each of these events and the rates at which metamorphic changes occurred is important for improving our understanding of the processes at work in continental collisions. The work presented in this thesis involves determining the temperatures recorded by metamorphic minerals and the ages of those minerals in order to reconstruct the temperature-time evolution of samples in a variety of positions within the NHT. Our data show that the collision and thermal metamorphism at 435-410 Ma is well preserved in northernmost Scotland. We argue that metamorphism in this area resulted from the widespread intrusion of hot magmas, which coincided in time with peak metamorphism. The timing of metamorphism in the core (hinterland) of this mountain belt is apparently younger than shallower deformation at the edges (foreland) of the mountain belt, suggesting active deformation and metamorphism retreated toward the hinterland during crustal shortening. In another part of the NHT, known as the Sgurr Beag nappe, a much older metamorphic event that occurred at 780-725 Ma is better preserved. In this area, the mineral monazite appears to record evidence of widespread fluid alteration at ~600 Ma, which has not previously been widely recognized in Scotland. A potentially important heat source in the Earth’s crust is shear heating associated with the thermal energy produced during deformation. It is important to consider what contribution shear heating may have made to the preserved metamorphic record in orogenic belts. This is challenging because there are few, if any, methods of relating observations from typical metamorphic rocks to estimated magnitudes of shear heating. We have developed a numerical model that is adaptable to a wide range of realistic natural scenarios and places first-order constraints on potential magnitudes of shear heating. While our models suggest that shear heating is not particularly important in the NHT, in some lower temperature fault zones shear heating could be more significant.
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