Structural Styles associated with Fault-Propagation Folds in Salt-Influenced Rift Basins: Halten Terrace, Offshore Norway

Normal fault growth is typically associated with the development of fault-propagation folds; especially in evolving salt-influenced rift basins, whereby the presence of a ductile, evaporite-rich unit in a brittle succession may decouple folded supra-salt (cover) and faulted sub-salt (basement) strata. Cover structures and the underlying fault blocks may form hydrocarbon traps, although imaging can be poor. This study examines the along-strike variability and controls on structural styles associated with a salt-influenced fault-related fold underlain and controlled by a series of down- stepping fault blocks.

A 2000 km², high quality, 3D seismic reflection survey of Hallen Terrace, offshore Norway, has been used to understand how normal fault growth in the presence of salt controls fault-linkage, fold development, basin physiography and syn-rift stratigraphic response. Moreover, we relate fold geometry, and cover-restricted fault strike variations to salt thickness, sub-salt fault throw and polarity. Hallen Terrace forms an ideal setting as the pre-rift Triassic salt lacks the effects of extensive diapirism and is well imaged. This permits detailed mapping of salt thickness variations and deconvolution of the spatial relationship between supra- and sub-salt faults and folds.

Quantitative analysis and 3D mapping of fault geometry shows salt strongly influences fault-linkage i.e. the propagation of sub-salt faults to higher stratigraphic levels and development of fold amplitude along-strike. We have identified a series of <15 km long, <400 ms throw, N-S- to NE-SW-striking basement-restricted faults below an 18 km long, >15 km wide, NW-facing, S-plunging monocline. At the southern extent of the fault-fold system, cover-restricted faults show similar strikes and polarity to their W-dipping, NE-SW striking sub-salt counterparts and are soft- linked. In the centre, throw is focused along W-dipping, N-S hard-linked faults to the east, and fold amplitude increases with greater salt thicknesses atop ·steeper sub-salt topography. Northwards, total throw decreases and greater fault polarity and strike disparity occurs between sub- and supra-salt faults associated with cover gravity- gliding and folding over increasingly flat, wider spaced, lesser-throw fault blocks.

We illustrate how basement-involved deformation can influence the fold structural style at different stratigraphic levels producing an extensive range of structural geometries and trapping styles. These are controlled by sub- and supra-salt fault coupling, salt thickness and fault throw.