Multi-mode gravity tectonics during northern North Sea rifting: the Snorre fault block case

Abstract

Continental rifts are characterized by up to 30 km wide rotated fault blocks with stratigraphic dip away from the central rift axis. Although gravity-induced mass movements are well known features of collapsed fault block crests, I here demonstrate the occurrence of polymodal gravity-driven mass transport down the back slope of a first-order rift fault block. I identify (1) early sliding related to syntectonic crestal collapse of second-order rift faults, (2) large-scale bed-parallel sliding of the L-M Jurassic sedimentary package, and (3) the accumulation of two 7 km long, 1–2 km wide and up to 750 m thick volumes of complexly slumped material in the hanging walls of two ramp-forming faults. Early sliding is documented by 100 m of repeated Brent Group stratigraphy in a cored well in the study area (well 34/4-15A). These smaller slides have intact internal stratigraphy but show elevated deformation band densities. The seismic data also show evidence for ca. 2 km of massive translational sliding of the ca. 400 m thick and ca. 300 km² large Jurassic section above a lowermost Jurassic bedding-parallel detachment. This translational slide did not deform much internally, except for ductile folding where it slid over underlying active rift faults. Chaotic seismic facies in fault hanging walls are interpreted as contorted Jurassic beds, formed by multiple slumping and sliding events that stacked mobilized sediments into a 750 m thick column. These complex slump volumes occur where fault displacement is highest along two relayed faults. A model is favoured where the large translational slide ruptured with an opening of space against the fault that was progressively filled with slumped material from the footwall. While the large-scale translational sliding only caused moderate internal subseismic deformation, early sliding and, particularly, the complex slumping caused significant internal deformation. This study shows the importance of carefully searching for and distinguishing between different types of mass movement in rift systems.