Thick Devonian clastic sequences accumulated in fault-bounded basins in W Norway at the close of the Caledonian orogeny and mark the beginning of a phase of crustal extension. Traditionally, the eastern boundary fault to the Devonian basins has been regarded as a thrust but cross-sections based on the use of a branch-line reveal it to be an extension fault. There is clear omission of structural section across the boundary and by matching up metamorphic rocks in the hanging- and foot-walls of the boundary fault, over 40 km of extensional dip slip can be measured at the N end of the fault. The boundary fault is a flat-lying listric fault with rollover anticlines in the hanging-wall. The fault also cuts up and down section along strike to form lateral ramps parallel to the movement direction. These ramps appear as wrench faults in outcrop. The ramp-flat geometry of the fault may have been inherited from a precursor Caledonian thrust fault which reversed its sense of slip in early Middle Devonian times to form the basins. Over 25 km of onlapping sediment accumulated in the largest basin during faulting in 14 Ma, with an average slip rate on the fault of 3 mm yr-1.

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