Caledonian and related events in Scotland

The Scottish Caledonides. sited near the triple-junction between Laurentia, Amazonia and Baltica, is divided into at least five discrete blocks, each with a history incompatible with that of the block now lying adjacent to it . With the exception of the Hebridian margin, with its extensional Torridon ian basins and Cambrian passive margin sequence, all blocks have undergone terrane-scale movements. The Moine, Central Highland Division and the bulk (if not all) of the Grampian Group have shared a common regional metamorphi sm, involving thickening and uplift, at c, 800 Ma and c. 450-480 Ma. This i s incompatible with their being in the extensional regime that appears to c haracterise much of Neoproterozoic Laurentia. They, along with the polymeta morphic Dalradian block, now replace a region of passive margin and an unkn own width of attendant Iapetus oceanic crust. These metamorphic blocks are grossly out-of-place. The Midland Valley is a severely contracted block of ancient crust, once fr inged by extensive oceanic basins to the N and S. An Ordovician-Devonian ar c was founded on this older craton, and supplied sediment to basins on eith er side of it. This arc, during its Lower Palaeozoic life, matured, finally to yield relatively quartz-rich sediment, but was re-activated during the Devonian. An arc, similar to that of the Midland Valley, also supplied sedi ment to the Southern Uplands, Metamorphic debris in the Southern Uplands ha d a provenance in either this arc-basement or in a basement somewhere along the orogen. No metamorphic detritus in the Highland Border Complex has yet been demonstrated to have a Dalradian source. Much of Scotland was assembled in a strike-slip regime. Evidence for strike -slip tectonics can be seen from the Late Proterozoic through to the Devoni an. In periods of transtension, basins opened to accumulate sediment; in pe riods of transpression, those sediments were compressed and uplifted to yie ld sediments to successor basins. In the Neoproterozoic, during the phase o f transpression, the basins were buried and metamorphosed. but during the P alaeozoic the basins were at a much higher level and escaped metamorphism. A substantial volume of the Neo pro terozoic-Palaeozoic sediment that accum ulated in Scotland was derived from two orogens, both of which were sited s ome distance away. During the Neoproterozoic, the Grenville orogen was the main source, and in later (Devonian) time sediment accumulated in Scotland from the major, Late Palaeozoic continent-continent collision of Greenland- Scandinavia. These two external sources were augmented by a substantial con tribution of sediment supplied from the Midland Valley are or its lateral e quivalent and by mild uplifts within the Scottish basements.