A discrete exotic terrane refers to a large crustal fragment that can be recognised by its distinct sedimentary, igneous, metamorphic and structural history compared with that of its eventual neighbours, and has been juxtaposed into position by major strike–slip faults.
Nine discrete exotic terranes make up the Basement in the British Isles. These consist primarily of Precambrian metamorphosed rocks but also contain some unmetamorphosed sedimentary units from the Lower Palaeozoic (e.g. in north-west Scotland). In general, the oldest Basement rocks in the southern British Isles are considerably younger than those in the northern British Isles. (Section 5.1–Section 5.2)
In addition to the nine Basement terranes, the geological history of the British Isles can be interpreted in terms of a series of five distinct orogenic and overlying covering units. These are the Precambrian and Lower Palaeozoic Basement, the Caledonian Orogenic Belt, the Older Cover, the Variscan Orogenic Belt and the Younger Cover. Although each of these units has a distinct geological history, as inferred from its lithology and tectonic structures, they do not correlate with distinct geological periods, and are therefore referred to as lithotectonic units.
The Caledonian Orogenic Belt consists of the high-grade metamorphic Caledonides to the north of the Highland Boundary Fault, and the low-grade to non-metamorphic Caledonides from the Southern Uplands southwards. In both areas, regional outcrop patterns and fold axes follow the same NE–SW Caledonian structural trend. In the southern British Isles, the Caledonides lie unconformably over the Basement rocks, whereas in north-west Scotland, the Moine Thrust Zone, which is tectonic in origin, separates the Precambrian Basement from the overlying Caledonides. (Section 5.3)
The Older Cover consists of Devonian and Carboniferous strata that either overlie the underlying Caledonian Orogenic Belt at an angular unconformity, or are faulted up against it. The most significant faults are the NE–SW trending Highland Boundary Fault and the Southern Uplands Fault, which bound the Midland Valley Terrane. (Section 5.4)
The Variscan Orogenic Belt consists of intensely deformed Upper Palaeozoic rocks and occurs in the far south-west of the southern British Isles. In the south-west of England, the Variscan is intruded by a series of large granitic bodies. (Section 5.5)
The Younger Cover consists of a succession of weakly folded strata of post-Carboniferous age that rests unconformably on the older lithotectonic units. (Section 5.6)