Fault crest collapse and gravity sliding in the late Jurassic northern North Sea rift

Abstract

Active rifting generates a basin-and-range style rift topography with rotated fault blocks and associated hanging-wall basins. During this process, fault block crests become prone to gravity collapse, particularly where weak sediments or sedimentary rocks are involved. The Mort landslide studied here is a large-scale slide formed by gravity collapse of a major Jurassic North Sea rift fault block at the time when local rift topography approached 1.5–2 km. From seismic data, this 24 km3 large marine slide is seen to stretch ∼15 km along the major Snorre rift fault, making it one of the largest mapped North Sea rift landslides. It is characterized by a single basal detachment (the Mort detachment) on which Triassic-Jurassic sediments moved up to 2–2.5 km. Well-developed detachment corrugations at the 100-m to kilometer scale reveal dip-slip sliding toward 100–110° (ESE), parallel to the slip direction of the underlying Snorre rift fault. The slide is broken up by normal faults that detach onto the low-angle Mort detachment, but in general, stratigraphic layering is well preserved. It shows a complex pattern where folds within the slide are upright with wavelengths up to 2 km, attesting to the soft nature of the slide. A prominent set of folds are ESE-plunging and mimics the corrugated geometry of the underlying Mort detachment. Other folds trend subparallel to the slide and probably formed as a result of differential movements during sliding. The toe zone shows evidence of layer back-rotation and thrusting, resting on the upper Jurassic Heather Formation. Postdated by uppermost Jurassic claystones of the Draupne Formation, the slide must have formed toward the end of the rift maximum around 155 Ma, at a time when the surface had reached its maximum relief of 1.5–2 km. The implications of such a gravitational collapse are exhumation of older (e.g., reservoir) rocks on fault block crests and possibly a change in local fluid migration pathways from local basins to those crests. Internal deformation must however be considered when evaluating their reservoir or fluid flow properties.