The Devonian basins of west Norway are located in the hanging wall of a regionally extensive west dipping extensional shear zone, the Nordfjord-Sogn Detachment Zone, which separates eclogite-bearing, lower crustal rocks in the footwall from Caledonian allochthonous rocks in the hanging wall. The shear zone is deformed into a series of west plunging, open synforms and antiforms, with the Devonian basins located in the synforms. Previous tectonic interpretations of the Caledonian orogen invoke NW-SE contraction, followed by E-W extension. However, analysis of age relationships between faults and folds below sediments within the Hornelen basin indicates a more complex deformation sequence, including (1) foreland-directed thrusting; (2) early orogen-parallel extension, forming orogen-orthogonal grabens; (3) vertical flattening of the upper crust, forming orthorhombic normal faults; (4) E-W extension contemporaneous with orogen-parallel shortening, resulting in orogen-transverse folds and a steeply dipping conjugate strike-slip fault pattern; and (5) subhorizontal extension along orogen-parallel faults. The sequence of structural relationships shows a gradual change in orogen-orthogonal strain from the initial phase of shortening to final phases of extension, which reflects a relative decrease in forces from the colliding continents and a relative increase in gravitational forces related to crustal thickening. The structural observations show that extension in the upper crust was initiated by orogen-parallel extension rather than shortening. This is in conflict with recent models suggesting that the collapse of the Caledonides was initiated by the northward collision of Avalonia into Euramerica. Alternatively, we suggest that the strain pattern recorded in western Norway is a direct result of orogenic collapse and that both orogen-parallel extension and shortening may be a common feature in continental collisional orogens, as intermediate phases between overlapping orogen-parallel shortening (collision) and collapse (extension).¿ 1997 American Geophysical Union