The Dalradian rocks of Shetland and their implications for the plate tectonics of the northern Iapetus

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Synopsis The Dalradian succession in Shetland is shown to have experienced an extensional tectonic and metamorphic episode prior to the Caledonian orogeny. Sedimentary deposition ended with a major basic and ultrabasic volcanic episode following a long period of extensional basin deposition involving both basic volcanics and turbidites. The ending of deposition was marked by the emplacement of a spinifex-textured komatiite indicating instantaneous rifting of the crust and the underlying Laurentian plate, and the formation of a Laurentian passive continental margin, probably during the opening of Iapetus. The entire contents of the Dalradian depositional basin were rotated bodily 90° to face the rift and to form a 12 km wide vertical belt in the middle of Shetland. A continued rotation in the same sense formed a 25 km wide recumbent inverted limb to form a mega-monocline. The formation of this monocline is considered to have occurred during the extensional phase accompanying rifting because the absence of subsidiary buckle folds of any and every size precludes compressional folding. The metamorphism of the rocks shows a gradual increase from bottom to top of the stratigraphic succession, predating the formation of the monocline, and probably owes its existence to crustal thinning and heating from below the deposition basin during extension. Much of the tectonic and metamorphic history shown by the Dalradian of Shetland may have been overprinted and masked in Scotland by the Caledonian orogeny. In the Dalradian of Shetland the Caledonian orogeny is represented only by the emplacement of the Shetland Ophiolite.