Major shear zones and autochthonous Dalradian in the north-east Scottish Caledonides
The Buchan area within the Grampian Caledonides of north-east Scotland is a classic locality for the study of low-pressure regional metamorphism. Using detailed geophysical surveys, pits and boreholes to augment sparse outcrop data, we have revealed a system of steep shear belts of Grampian age. When combined with a reassessment of the stratigraphy, the new information leads us to the view that the Banff nappe does not exist. We also argue that the Tay nappe is of limited extent in Buchan, and that the tract of uninverted Southern Highland Group (upper Dalradian) is autochthonous.
Grampian migmatites in the Buchan Block, NE Scotland
Rocks exposed along the Scottish coast between Fraserburgh and Inzie Head contain information critical to understanding the evolution of the Buchan Block, the type locality for low‐P, high‐T regional metamorphism, and its relationship with the rest of the Grampian terrane, one of the major tectonostratigraphic components of the Scottish Caledonides. The ~8 km long section traverses a regional network of shear zones and, at the highest grades around Inzie Head, passes into the core of the Buchan Anticline, a large‐scale open fold that is commonly regarded as a late structure, post‐dating metamorphism. The metasedimentary rocks increase in grade from upper amphibolite to granulite facies and preserve unequivocal evidence for partial melting. The diatexite migmatites around Inzie Head, along with other gneissose units within the Buchan Block, have been regarded as allochthonous Precambrian basement rocks that were thrust into their current position during the Grampian orogenesis. However, field observations show that the onset of in situ partial melting in metapelitic rocks, which was associated with the formation of garnet‐bearing aplites and associated pegmatites, occurred around Fraserburgh, where shear fabrics are absent. Thus, the rocks preserve a continuous metamorphic field gradient that straddles the shear zone network. This observation supports an alternative interpretation that anatexis was the result of mid‐Ordovician (Grampian) metamorphism, rather than an older tectonothermal event, and that the Inzie Head gneisses are autochthonous. Using an average mid‐Dalradian pelite as a plausible representative protolith, phase equilibria modelling satisfactorily reproduces the observed appearance and disappearance of key minerals providing that peritectic garnet produced with the first formed melts (represented by the garnet‐bearing aplites) depleted the source rocks in Mn. The modelled metamorphic field gradient records a temperature increase of at least 150 °C (from ~650 °C near Fraserburgh to in excess of 800 °C at Inzie Head) but is isobaric at pressures of 2.7–2.8 kbar, suggesting the Buchan Anticline developed synchronous with partial melting. The Buchan Anticline is likely an expression of crustal thinning and asthenospheric upwelling, which produced voluminous gabbroic intrusions that supplied the heat for Buchan metamorphism.
Fold nappes as asymmetric mantled gneiss domes and ensialic orogeny
Abstract Descriptions are presented of the development of asymmetric gravity-driven structures rising from wedge-shaped bodies of light fluids overlain by denser fluids. Two programmes of experiments are described: 1. (1) the stabilization of wedges of oil overlain by glucose syrup in the field of gravity 2. (2) the stabilization while spinning in a centrifuge of wedge-shaped bodies of a light stiff silicone putty beneath denser and less viscous silicone putties. The development in these experiments of structures analogous to either basement cored domes or fold nappes typical of many orogenic belts is found to depend mainly on the inclination of the interface between the light and the overlying heavier fluids. Simple dome-like structures rise from this interface if it is horizontal or very shallow, whereas nappe-like structures alone develop if the interface is steeper than about 25° to the horizontal. A range of inclinations exist from which complex combinations of “domes” and “nappes” interfere to produce what would appear to be episodic deformations on small scales. It is argued that these experiments demonstrate that basement cored fold nappes are merely asymmetric mantled gneiss domes and that an Atlantic-type continental margin is, with an increase in the geothermal gradient, analogous to the unstable configuration of fluids at the start of the experiments. The conceptual model of ensialic orogeny due to Ramberg (1967) based on experiments like these is described and its implications discussed.