Transpression and transtension are strike-slip deformations that deviate from simple shear because of a component of, respectively, shortening or extension orthogonal to the deformation zone. These three-dimensional non-coaxial strains develop principally in response to obliquely convergent or divergent relative motions across plate boundary and other crustal deformation zones at various scales. The basic constant-volume strain model with a vertical stretch can be modified to allow for volume change, lateral stretch, an oblique simple shear component, heterogeneous strain and steady-state transpression and transtension. The more sophisticated triclinic models may be more realistic but their mathematical complexity may limit their general application when interpreting geological examples. Most transpression zones generate flattening (k < 1) and transtension zones constrictional (k > 1) finite strains, although exceptions can occur in certain situations. Relative plate motion vectors, instantaneous strain (or stress) axes and finite strain axes are all oblique to one another in transpression and transtension zones. Kinematic partitioning of non-coaxial strike-slip and coaxial strains appears to be a characteristic feature of many such zones, especially where the far-field (plate) displacement direction is markedly oblique (<20 ~ to the plate or deformation zone boundary. Complex foliation, lineation and other structural patterns are also expected in such settings, resulting from switching or progressive rotation of finite strain axes. The variation in style and kinematic linkage of transpressional and transtensional structures at different crustal depths is poorly understood at present but may be of central importance to understanding the relationship between deformation in the lithospheric mantle and crust. Existing analyses of obliquely convergent and divergent zones highlight the importance of kinematic boundary conditions and imply that stress may be of secondary importance in controlling the dynamics of deformation in the crust and lithosphere.

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Crustal thickening during transpressive orogenesis may produce anatectic granites which may then localize deformation leading to transcurrent movement. Granites may be transported from sites of generation through the mid-crust in dyke-like channelways within relatively narrow strike-slip shear zones which link to major fault zones in the upper crust. Extensional jogs within fault systems provide developing sites for the assembly of plutons from magma arriving from below. The model is based upon observations from the Cadomian belt of NW France which exposes sections through middle and upper levels of the late Precambrian crust within different elements of the orogen. The mechanism provides a favourable alternative to diapirism, and explains the common collocation of granites and shear zone/fault systems within orogenic belts.Growing realization that diapirism is not the dominant process by which granite magmas ascend through the crust (e.g. Bateman 1984) requires investigation of alternative mechanisms. Valuable insights may be gained from studies of granitic magmatism in transpressive settings. Transpression will thicken the crust and structurally invert sedimentary basins, which may lead to high-temperature metamorphism and anatexis. The common collocation within transpressional orogens of granitic rocks with crustal-scale strike-slip shear zones and faults (e.g. Hutton 1988; Hutton & Reavy 1992) implies that such tectonic features may act as fundamental controls on the ascent and emplacement of magmas. The St Malo and Mancellian regions of NW France expose middle and upper crustal sections within the late Precambrian, Cadomian orogenic belt and display granitic rocks intimately associated with shear zones and fault systems. We integrate previously published and newly acquired field, geochemical and isotopic data to derive a generalized model for the ascent and emplacement of granite magma within a transpressional orogen.Geological setting. The late Precambrian, Cadomian belt of NW France (Fig. 1) records subduction-related orogenic activity which culminated in the accretion of magmatic arc and marginal basin terranes along a continental margin above a south-dipping subduction zone (D'Lemos et al. 1990 and references therein). In northern Brittany, steep, sinistral strike-slip shear zones and associated structures are interpreted to have formed during regional (polyphase) transpression in an oblique convergent setting (Treloar & Strachan 1990, but see also Brun 1992 and reply). The Fresnaye shear zone ( Fig. 1) separates arc-related terranes to the NW from behind-arc terranes to the southeast. Arc-related terranes are characterized by abundant c. 700-570 Ma calc-alkaline plutonism intrusive into mid-Proterozoic basement (Tregor-La Hague region) and Brioverian supracrustal sequences (St Brieuc region; Fig. l). In contrast, the St Malo and Mancellian regions to the southeast (Fig.

the onset of fluid-assisted, grain size-sensitive diffusional creep in the most highly deformed and altered parts of the fault zone. Phyllonitic fault rocks also occur in older, more deeply exhumed parts of the fault zone, implying that phyllonitization had previously occurred at an earlier stage and that this process is possible over a wide temperature (depth) range within crustal-scale faults. Our data provide an observational basis for recent theoretical and experimental studies which suggest that crustal-scale faults containing interconnected networks of phyllosilicate-bearing fault rocks will be characterized by long-term relative weakness and shallow (-5 km)

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Within the Caledonides of central Sutherland, Scotland, the Neoproterozoic metasedimentary rocks of the Moine Supergroup record NW-directed D 2 ductile thrusting and nappe assembly, accompanied by widespread tight-to-isoclinal folding and amphibolite-facies metamorphism. A series of metagranite sheets which were emplaced and penetratively deformed during D 2 have been dated using SHRIMP U-Pb geochronology. Zircon ages of 424 AE 8 Ma (Vagastie Bridge granite), 420 AE 6 Ma (Klibreck granite) and 429 AE 11 Ma (Strathnaver granite) are interpreted to date emplacement, and hence regional D 2 deformation, during mid-to late Silurian time. Titanite ages of 413 AE 3 Ma (Vagastie Bridge granite) and 416 AE 3 Ma (Klibreck granite) are thought to date post-metamorphic cooling through a blocking temperature of c. 550-500 8C. A mid-to late Silurian age for D 2 deformation supports published models that have viewed the internal ductile thrusts of this part of the orogen as part of the same kinematically linked system of forelandpropagating thrusts as the marginal Moine Thrust Zone. The new data contrast with previous interpretations that have viewed the dominant structures and metamorphic assemblages within the Moine Supergroup as having formed during the early to mid-Ordovician Grampian arc-continent orogeny. The mid-to late Silurian D 2 nappe stacking event in Sutherland is probably a result of the collision of Baltica with the Scottish segment of Laurentia.

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