Seismic reflections from normal faults in the northern North Sea

Publisher Website
Google Scholar
Abstract
Fault-plane reflections from gravity-driven, listric faults in thick sedimentary sequences are well known. However, reflections from normal faults associated with crustal extension are much rarer and less well studied. We discuss a number of examples of such reflections, present on commercial seismic reflection data from the northern North Sea. Some of these examples have previously been described as reflected refractions, but we show that this requires unrealistic velocities within the sedimentary column. Instead, we consider it more likely that these events are true fault-plane reflections. Depth migration and image-ray migration are used to determine the probable geometry of the faults in both cross section and map view. Estimates of the true geological dip remain sensitive to the velocity structure used, particularly that applied to the Triassic half-graben fill. Despite this uncertainty, the depth-migrations show that these faults are composed of two near-linear segments in cross section. The upper parts (in the Jurassic and Upper Triassic section) dip at 40–50°, but the deeper parts (Triassic hangingwall against basement in the footwall) typically dip at 30–35°. Occasionally, reflections indicate that faults continue into the basement with a 30–35° dip. We interpret this fault geometry as originating from two phases of fault movement. Triassic extension caused rotation of sets of initially steep planar faults. Following Late-Triassic-to-Middle-Jurassic thermal subsidence, renewed extension in the Late Jurassic caused the faults to cut up through the overburden at a steeper angle. Without recognition of a two-phase extension history, such faults might be mistakenly interpreted as forming with a listric geometry.