The Sognefjord transect through the lower to middle Palaeozoic Caledonian mountain belt in southern Norway provides one of the best and most completely documented examples of late collisional tectonics in an Alpine-type orogen. It exposes a 250km long cross-section from the cratonic foreland, in the east, through the heavily deformed continental margin (Baltica), to the remains of the Caledonian ocean (Iapetus) at the Norwegian west coast. Exceptionally detailed and complete structural data are available along the whole transect, together with good stratigraphic, radiometric, petrological, and geophysical control. In this synthesis, the structural data are analysed, in terms of the kinematics and relative age of the different deformation phases, and correlated along the whole transect. The analysis is then used, in conjunction with the other data, to carry out a retrodeformation, reconstructing the crustal geometry at different stages backward in time. The earliest of the present reconstructions (c. 410 Ma) marks the time of formation of the well-known West Norwegian eclogites, in an over-deepened root of Baltica which had developed in the ductile lower crust as a response to extreme crustal shortening. The brittle upper crust took up the shortening by the SE movement of a rigid sheet of Precambrian basement (Jotun complex) above the low-angle Jotunheimen contractional detachment, across a rigid wedge of the Baltic Shield. During the final stages of contraction (c. 410-395 Ma), the upper crust acted as an orogenic lid, against which the root 'collapsed' upwards by sub-vertical shortening and lateral E-W extension. During this process of inverted gravity spreading, the eclogites were carried upwards from 60-70 km to 40 km (exhumation phase 1, rate 2-3 mm a -1) and retrograded within their deforming gneissic matrix. At the end of this phase, the strain field in the upper crust changed from contraction to extension, concomitant with a broad up-doming (base Devonian unconformity) which caused a further 10 km exhumation by 385 Ma (exhumation phase 2, 1 mm a-l). This was followed by the main phase of crustal extension with the development of low-angle normal top-to-W or NW fault and shear zones, of which the Nordfjord-Sogn detachment was the most important (50 km of normal displacement). Exhumation in this phase took place by rapid uplift and erosion of the footwall of the detachment, causing the currently exposed eclogites in outer Sognefjord to rise the remaining 30 km (exhumation phase 3,1.5 mm a-l), to become juxtaposed against Devonian conglomerates on the hanging wall. The reconstructions confirm the general picture of eclogite exhumation in western Norway, and fill out some of the details. However, they do not support the idea that the process was due to extensional orogenic collapse caused by advective or convective lithospheric thinning. Although gravity played a significant role at various stages in the process, the main phase of crustal extension seems to have been mainly related to changes in Devonian plate...

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ABSTRACT. A detail ed investigation h as been carried out on the dynamics of an Alpine valley glacier of r elatively simple shape a nd the results are considered in relation to the development of secondary structures. I ce veloc ity reaches a maximum near the top of a small ice fall (40 m a-I) which also coincides approximately with the eq uilibrium li ne. Flow lines converge in the accumu la tion area but a re roughl y parallel in the ablation a rea. The " regional " strain-rate pattern is ra the r complex. Approxima te longitudinal extension is evident in the accumul ation area and strain-rates reach high va lues at the south margin and in the ice fall (up to 0. 12 a-I). I n the a bla tion area, strain-ra tes are comparatively small and in general indicate longitudinal compression. " Local" deformation rates obtained in th e a rea beneath the ice fall and along a flow line near one of th e margins reveal com plex patterns of deformation within small a reas.T here is no clear relationship between foliati on and strain-rates (and by ana logy stresses), except in the case of longitudinal foli a tion in marginal a r eas which, if actively developing, li es approximately parallel to a direction of maximum sh ear strain-rate . It is more important to consider the rela tionship of this st ructure to stra in history. R esults from this study indicate that, regardless of the initia l o ri entation of th e foliation in r elation to the strain ellipse, it attains approx imate parallelism with the long axis of the ellipse as deformation progresses.It is also shown that many foliati ons originate from pre-existing layered structures such as stratifi cation or crevasse traces. This problem is discussed particularly with reference to a n a rcuate foliation which originates in the ice fa ll and is beli eved to represent tensional veins, subsequently subj ected to compressive strain within and below the ice fall. RESUME. DYllamiqlle et structure du glacier de Gries en Suisse. U ne etude approfondie a ete effectu ee sur la dynamique d'un glacie r d e vallee a lpin de forme relativeme nt simple et les resultats sont consideres en rapport avec le developpement de st ru ctures secondaires. La vitesse de la g lace a tteint son maximum (40 m a-I ) au voisinage d'une petite chute qui coin cide approximativement avec la linge d'equilibre. Les lignes de courant convergent dans la zon e d'accumulation, mais d emeurent en r evanch e a peu pres paralleles dans la zone d 'a blation. La distributio n "regionale" des vitesses de deformation est assez complexe. L es d eformations de traction plus ou moins longitudinales sont evid entes da ns la zon e d'accumulation e t les vitesses de deformati o n atteignent des valeurs elevees sur la rive sud et dans la chute du glacier (jusqu'a 0,12 a -I) . Dans la zone d'ablation, les vitesses de deformation sont comp¥.ativement faibl es et indiquent en general une com pressio n longitudina le . L es vitcsses de deformation "locales" observees dans la zone situee au-dessous de la chut e du glacier, de mem...

Crustal roots formed beneath mountain belts are gravitationally unstable structures, which rebound when the lateral forces that created them cease or decrease significantly relative to gravity. Crustal roots do not rebound as a rigid body, but undergo intensive internal deformation during their rebound and cause intensive deformation within the ductile lower crust. 2-D numerical models are used to investigate the style and intensity of this deformation and the role that the viscosities of the upper crust and mantle lithosphere play in the process of root rebound. Numerical models of root rebound show three main features which may be of general application: first, with a low-viscosity lower crust, the rheology of the mantle lithosphere governs the rate of root rebound; second, the amount of dynamic uplift caused by root rebound depends strongly on the rheologies of both the upper crust and mantle lithosphere; and third, redistribution of the rebounding root mass causes pure and simple shear within the lower crust and produces subhorizontal planar fabrics which may give the lower crust its reflective character on many seismic images.