TECTONICS AND SEDIMENTATION IN THE HANGINGWALL OF A MAJOR EXTENSIONALDETACHMENT - THE DEVONIAN KVAMSHESTEN BASIN, WESTERN NORWAY

The Middle Devonian Kvamshesten Basin in western Norway is a late-orog enic basin situated in the hangingwall of the regional extensional Nor dfjord-Sogn Detachment Zone. The basin is folded into a syncline with the axis subparallel to the ductile lineations in the detachment zone. The structural and stratigraphic development of the Kvamshesten Basin indicates that the basin history is more complex than hitherto recogn ized. The parallelism stated by previous workers between mylonitic lin eation below the basin and intrabasinal fold axes is only partly refle cted in the configuration of sedimentary units and in the time-relatio ns between deposits on opposing basin margins. The basin shows a prono unced asymmetry in the organization and timing of sedimentary facies u nits. The present northern basin margin was characterized by bypass or erosion at the earliest stage of basin formation, but was subsequentl y onlapped and eventually overlain by fanglomerates and sandstones org anized in well-defined coarsening-upwards successions. The oldest and thickest depositional units are situated along the present southern ba sin margin. This as well as onlap relations towards basement at low st ratigraphic level indicates a significant component of southwards tilt of the basin floor during the earliest stages of deposition. The infe rred south-eastwards tilt was most likely produced by north-westwards extension during early stages of basin formation. Synsedimentary intra basinal faults show that at high stratigraphic levels, the basin was e xtending in an E-W as well as a N-S direction. Thus, the basin records an anticlockwise rotation of the syndepositional strain field. In add ition, our observations indicate that shortening normal to the extensi on direction cannot have been both syndepositional and continuous, as suggested by previous authors. Through most of its history, the basin was controlled by a listric, ramp-flat low-angle fault that developed into a scoop shape or was flanked by transfer faults. The basin-contro lling fault was rooted in the extensional mylonite zone. Sedimentation was accompanied by formation of a NE- to N-trending extensional rollo ver fold pair, evidenced by thickness variations in the marginal fan c omplexes, onlap relations towards basement and the fanning wedge geome try displayed by the Devonian strata. Further E-W extension was accomp anied by N-S shortening, resulting in extension-parallel folds and thr usts that mainly post-date the preserved basin stratigraphy. During sh ortening, conjugate extensional faults were rotated to steeper dips on the flanks of a basin-wide syncline and re-activated as strike-slip f aults. The present scoop-shaped, low-angle Dalsfjord fault cross-cut t he folded basin and juxtaposed it against the extensional mylonites in the footwall of the Nordfjord-Sogn detachment. Much of this juxtaposi tion may post-date sedimentation in the preserved parts of the basin. Basinal asymmetry as well as variations in this asymmetry on a regiona l scale may be explained by the Kvamshesten and other Devonian basins in western Norway developing in a strain regime affected by large-scal e sinistral strike-slip subparallel to the Caledonian orogen.