The Billefjorden fault is a 2.0- to 4.0-km-wide zone of parallel and branching faults trending N4°W. Individual fault strands dip 39° to 68°. The Balliolbreen fault, the principal strand, is an east-dipping reverse fault which displaces Hecla Hoek metamorphic rocks on the east against Devonian Old Red Sandstone on the west. Large-scale mapping shows the Devonian rocks to be complexly folded and faulted and unconformably overlain by Carboniferous strata. The pre-Carboniferous folds are tight and overturned adjacent to the fault zone but become open and upright westward. Fold axes and thrusts, with separations not exceeding a few hundred metres, have sinuous patterns and trends ranging from N60°E to N45°W and do not intersect the fault zone with the consistent trend characteristic of strike-slip faulting. The gentle dip of individual fault strands as well as the pattern of folds and thrusts, suggests east-west crustal shortening. The Old Red Sandstone thins rapidly eastward in the area of the Billefjorden fault zone; thus large displacement is not required to explain the absence of Old Red Sandstone on Ny Friesland east of the fault. The over-all straight trace of the fault zone could be explained by reverse faulting along a pre–Old Red Sandstone strike-slip fault in Hecla Hoek rocks. These results suggest that the Billefjorden fault zone had post–Old Red Sandstone reverse-slip rather than strike-slip, and no support is found for earlier suggestions of 200 to 1,000 km of Late Devonian left-slip on the Billefjorden fault zone.

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