Models for the tectonothermal evolution of the eastern Dalradian of Scotland

Abstract

The preserved array of pressures in the eastern Dalradian indicates that considerable syn-to post-metamorphic differential uplift has occurred. This inferred differential uplift suggests that Buchan sillimanite zone rocks originally lay at higher structural levels than presently adjacent cooler kyanite zone rocks to the west. A number of features are believed to coincide with the western margin of the sillirnanite zone. These are a maximum in temperature, sharp thermal features, a high strain zone, and a train of metabasites. These features are explained by invoking synmetamorphic movement between the Buchan sillimanite zone and the kyanite zone to its west, involving some horizontal component of movement. It is suggested that the lateral, now eroded, equivalents of the Buchan area once provided part of the required tectonic thickening for other parts of the Dalradian. Areas surrounding the Buchan area suffered tectonic burial followed by metamorphism during uplift relative to the Buchan area.

The Dalradian rocks of the north-east Grampian Highlands of Scotland

“…Harte and Hudson, 1979;Hudson, 1985;Baker, 1987), it has been possible to deduce a sequence of structural and metamorphic events to account for all of the features described above. The development of andalusite, characteristic of high-temperature, lowpressure metamorphism, clearly extended westwards from the Buchan area, across the position of the Portsoy-Duchray Hill Lineament as is shown by the relics of andalusite, for example in the Auchindoun area.…”

“…Ashcroft et al (1984) suggested that it was subvertical shear movements along the Portsoy-Duchray Hill Lineament after the emplacement of the basic magmas, with relative uplift to the west, that brought up the higher grade rocks. However, Baker (1987) and Beddoe-Stephens (1990) refined this to suggest that westerly or north-westerly directed thrusting across the lineament emplaced a thick upper Dalradian sequence of the Buchan Block above older rocks to the west, which were hence subjected to increased overburden pressure and near-isothermal compression. Although this explanation has been generally accepted, there is little stratigraphical evidence for overthrusting and the Dalradian stratigraphy seems to young consistently from west to east across the Portsoy-Duchray Hill Lineament with no repetition (Fettes et al, 1991).…”

“…Major stratigraphical and structural discontinuities occur across the shear-zone and marked differences in metamorphic history on opposite sides indicate major westward overthrusting during the regional D3 event (Baker, 1987;Beddoe-Stephens, 1990). …”

“…The main tectonic activity was focussed in mid-Ordovician time at around 470 Ma when thrusting to the west-north-west and associated deformation, metamorphism and fluid flow, occurred roughly coeval with intrusion of basic and ultrabasic bodies (Baker, 1987;Oliver, 2002).…”

“…More-detailed studies of the metamorphism have been undertaken by a number of workers (Chinner, 1966;Ashworth, 1975Ashworth, , 1976Hudson, 1980Hudson, , 1985Hudson and Harte, 1985;Baker, 1985Baker, , 1987Beddoe-Stephens, 1990;Dempster et al, 1995).…”