Coesite-bearing eclogites in several deep crustal metamorphic assemblages now exposed in extensionally-collapsed orogens indicate the tectonic denudation of more than 90km of crustal rocks and pre-collapsed crustal thicknesses of at least 120km. For mountain ranges and orogenic plateaux up to 5 km in elevation and average crustal densities of about 2.8, crustal thickness cannot exceed about 80 km unless pre-shortening crustal/ lithosphere thickness ratios were less than 0.135 or some way can be found to preferentially thicken the lithospheric mantle. This problem can be avoided and very thick orogenic crusts built up if granulite facies rocks transform to denser eclogite facies during shortening, where the petrographic Moho is continuously depressed below a density/seismic velocity Moho buffered at about 70 km and mountains at about 3 km. Advective thinning of the lithosphere combined with the resultant heating and eclogite to sillimanite-granulite/amphibolite transformation causes surface uplift of about 2 km, a rapid change in isostatic compensation level, and a switch from a shortening to an extensional/collapse regime. We have developed a simple numerical model based upon field observations in southwestern Norway in which coherent regional-scale transformation of lower crustal rocks to eclogite facies during lithospheric shortening is followed by heating, transformation of eclogite to amphibolite and granulite, extension, and crustal thinning by coaxial then non-coaxial mechanisms. The model also explains strong lower crustal layering (eclogite and other lenses in horizontallyextended amphibolites), regionally horizontal gneissic fabrics, rapid return from orogenic to 'normal' crustal thickness with minor erosion, the lateral and vertical juxtaposition of low-grade and high-grade rocks and rapid marine transgression shortly after orogeny.Young orogens have maximum regional average elevations of about 5 km; their crustal thicknesses, determined directly from seismic reflection/refraction studies or indirectly from gravity anomalies, do not exceed about 70 km (Meissner 1986). The pre-plate tectonic view was that the surface elevation (e) of a mountain belt in perfect isostatic balance is related to its compensating root (r) and crustal thickness (Cz) by the relative densities of crust (Pc) and mantle (Pro), the floating iceberg principle in which the mantle was considered to behave as a fluid (Stokes 1849; Airy 1855; Heiskanen & Vening Meinesz 1958). The plate tectonic view is that the crust forms part of the lithospheric boundary conduction layer (thickness lz) with a mantle density of Pm" The whole l z column of crust and mantle is compensated to the asthenospheric mantle of density Pa" The crust 'floats', whereas the mantle portion 'sinks', the lithosphere; therefore, the level at which the surface sits relative to the oceanic ridges depends upon l~ and CJl~. Also, the cooler upper portion of the boundary layer acts as a strong flexural beam to support loads, which allows a departure from pure zerostrength A...

The South Mayo Trough, a broad synclinorium containing a 10‐km sequence of Early Ordovician turbidites passing up into shallow water sandstones, is interpreted as a forearc basin with an ophiolitic basement that formed a backstop to an accretionary prism on the northern edge of a north‐facing arc. The arc collided with the Laurentian margin in Early Llanvirn times to deform the Dalradian Supergroup rifted margin clastic sequence. Prior to the late Llandovery, sinistral transcurrent faulting transposed the Connemara Dalradian terrane to its present position on the “outboard” ocean ward side of the South Mayo Trough. The South Mayo Trough was fed principally by arc and ophiolite‐derived elastics during the Arenig with an increasing metamorphic component during the Llanvirn. The Late Ordovician transpressional “docking” of Connemara provided the southerly source for high‐grade metamorphic clasts in the Deny veeny Formation.

The origin of the Newer Granites is long-standing problem. In the Caledonian orthotectonic zone the intrusions span the period of late orogenic convergence and uplift, but attempts to relate them as a group to late Iapetan subduction have been unsuccessful. A range of rock types is represented, mainly with I-type affinities, and granodiorite is the most voluminous. In contrast, granitic intrusions south of the Moniaive shear zone in Scotland and also in the north of England have significant S-type characteristics, span the trace of the Iapetus suture and have ages in the range 400-390 Ma, significantly younger than intrusions to the north. We refer to these younger granitic intrusions, along with others of similar character along-strike to the southwest, as the Trans-Suture Suite. We explore the link between the Trans-Suture Suite and recently recognized orogen-wide sinistral transtension in the Early Devonian period. Importantly, the Trans-Suture Suite intrusions are accompanied by an intense suite of lamprophyre dykes, the origin of which is to be sought in extension, decompression and heating of enriched Avalonian sub-continental lithosphere. In some instances the granite intrusions carry clots of lamprophyric origin and the Criffel body is particularly important in being continuously zoned from an I-type with lamprophyric enclaves to an S-type interior. We propose that generation of these lamprophyres during transtension advected heat into the base of the crust to produce the S-type component of the Trans-Suture Suite. Modelling presented shows that generation of voluminous S-type magmas requires the coincidence of several factors: hydrated sub-continental lithospheric mantle preserved during 'soft' collision under the Trans-Suture Suite zone; thermal relaxation to remove any subduction refrigeration; crust composed of juvenile volcanogenic material; and Devonian transtension. Our models suggest that if hydration pre-dated transtension then only small granitic bodies could be produced, unless the zone of lamprophyre generation extends beyond the rift zone. The emplacement of the Trans-Suture Suite intrusions overlapped the Acadian deformation period that succeeded the transtensional episode during which the granite magmas were generated.

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The previous conflict between stratigraphical and geochronological evidence for the age of the Grampian orogeny in Scotland and Ireland has now been largely resolved. Dalradian deposition continued on the Laurentian margin through late Proterozoic into Ordovician time. The Grampian orogeny was a brief, arc-accretion event that took place around the Arenig-Llanvirn boundary, at about 470 Ma, an extension of the Taconic orogeny of the Appalachians. Exhumation of the orogen was rapid; Rb-Sr mica ages that have been taken to indicate pre-Ordovician orogeny and slow cooling are considered to be problematical.