North Atlantic Neoproterozoic tectonism - constraints from the Shetlands

Project Investigators

Kathryn Cutts (PhD Candidate)

Associate Professor Martin Hand

Project Collaborator

Doctor Chris Clark (Curtin University)

Project Details

In recent years there has been considerable debate regarding the significance of the Neoproterozoic tectonic record in the Scottish Highlands. While there is little doubt that Neoproterozoic (c. 870-750 Ma) tectonism involved high-temperature metamorphism and magmatism, debate has centred on the timing of events and their style. Particularly on whether the events represent crustal extension or compression. The importance of resolving this debate lies in reconciling models for the breakup of Laurentian and Baltican Rodinian elements within the Neoproterozoic geological record in the North Atlantic.

In the Scottish Highlands Neoproterozoic tectonism is recorded in the Moine Supergroup which was deposited between 1000-870 Ma. The expression of Neoproterozoic tectonism has to a large extent been obscured by intense deformation and regional metamorphism associated with Caledonian Orogeny (470-430 Ma), which records the closure of the Iapetus Ocean between Laurentia and Baltica. As a consequence the Neoproterozoic record is cryptic, with evidence of metamorphic style drawn largely from modelling of mineral paragenesis or inferences based on the geochemical composition of Neoproterozoic magmatic rocks.

While the vast bulk of attention has been focused on the Scottish Highlands, further north, the Shetland region has been largely ignored. The Shetlands contain arguably the best preserved tectonic section of the western Caledonian Orogeny, whose elements can be directly correlated with those of the Scottish Highlands. In the western Shetlands, Palaeoproterozoic rocks belonging to the Lewisian Complex underlie the extension of the Moine Thrust, which marks the western edge of the Caledonian Orogeny. Structurally above the Moine Thrust are psammites and garnet-bearing metapelites belonging to the Morar Group of the Moine Supergroup. Further east, these are broadly overlain by equivalents of the Glenfinnan Group of the Moine Supergroup. In turn the Glenfinnan Group is overlain by equivalents of the Grampian Group (c. 700-500 Ma). At the top of the structural section in the eastern Shetlands a spectacular ophiolite complex represents oceanic crust obducted onto the continental margin during the Caledonian Orogeny.

Importantly in terms of the debate surrounding the record of Neoproterozoic tectonism in the North Atlantic, the Shetlands contain good exposures of the key Morar and Glenfinnan groups, which show the Neoproterozoic metamorphic and magmatic assemblages in the Scottish Highlands. As with the Scottish Highlands, the effects of the Caledonian Orogeny are severe, with the majority of all the major fold and thrust structures, and the majority of the mesoscopic structural elements reflecting Caledonian tectonism. However recent mapping by Associate Professor Martin Hand and others has identified a region within the Caledonian structural assemblage that appears to preserve largely intact, an earlier high-T structural and metamorphic history. Within this region, migmatitic metasedimentary and meta-igneous rocks contain coarse-grained mineral assemblages and structural fabrics that are ideally suited to assessing the event, timing its thermobarometric evolution. On the basis of regional considerations these apparently pre-Caledonian tectonic elements are logically Neoproterozoic in age. And unlike in the Scottish highlands, the apparent lack of a Caledonian overprint means that the structural geometry of Neoproterozoic events can be assessed. Therefore a unique opportunity exists to assess the tectonic record of what may be the best preserved Neoproterozoic rocks in the North Atlantic region.

This project has several goals:

• to assess the age of the apparently pre-Caledonian tectonism;
• to determine the thermobarometric evolution of the pre-Caledonian metamorphism and
• to determine the structural geometries associated with the pre-Caledonian tectonism .

This project provides the opportunity of studying the metamorphic geology in a region that has had little previous study. Results obtained from this study may also help to resolve a geological debate concerning the nature of Neoproterozoic metamorphism of rocks in the North Atlantic region, possibly requiring the re-evaluation of simple breakup models for Rodinia.

The field work component of this project was conducted in July, 2006 and May 2007.