Ultra-large rift basins, which may represent palaeo-propagating rift tips ahead of continental rupture, provide an opportunity to study the processes that cause continental lithosphere thinning and rupture at an intermediate stage. One such rift basin is the Faroe-Shetland Basin (FSB) on the north-east Atlantic margin. To determine the mode and timing of thinning of the FSB, we have quantified apparent upper crustal b-factors (stretching factors) from fault heaves and apparent whole-lithosphere b-factors by flexural backstripping and decompaction. These observations are compared with models of rift basin formation to determine the mode and timing of thinning of the FSB. We find that the Late Jurassic to Late Palaeocene (pre-Atlantic) history of the FSB can be explained by a Jurassic to Cretaceous depth-uniform lithosphere thinning event with a b-factor of 1.3 followed by a Late Palaeocene transient regional uplift of 450-550 m. However, post-Palaeocene subsidence in the FSB of more than 1.9 km indicates that a Palaeocene rift with a b-factor of more than 1.4 occurred, but there is only minor Palaeocene or post-Palaeocene faulting (upper crustal b-factors of less than 1.1). The subsidence is too localized within the FSB to be caused by a regional mantle anomaly. To resolve the b-factor discrepancy, we propose that the lithospheric mantle and lower crust experienced a greater degree of thinning than the upper crust. Syn-breakup volcanism within the FSB suggests that depth-dependent thinning was synchronous with continental breakup at the adjacent Faroes and Møre margins. We suggest that depth-dependent continental lithospheric thinning can result from small-scale convection that thins the lithosphere along multiple offset axes prior to continental rupture, leaving a failed breakup basin once seafloor spreading begins. This study provides insight into the structure and formation of a generic global class of ultra-large rift basins formed by failed continental breakup.

Fault displacement data were measured over four orders of magnitude for a fault set cross-cutting Upper Jurassic rocks exposed along the Dorset coast. Fault data were subdivided into three data-sets, based on data source and field character. Distribution analysis showed that these faults conform to a power law. The scaling relationship is, however, not constant over the entire displacement range of the faults. Faults with displacement >2m are characterized by a negative power law having an exponent of ≈0.96, while faults with displacement <1m are related by a negative exponent of ≈0.7. The change at the small scale is interpreted as being a product of the influence of lithological heterogeneity on fracture initiation and growth.

Abstract. High-resolution seismic reflection profiles using a Chirp source image a north-south extensional fault set, which cuts rocks of Upper Jurassic age, cropping out on the seafloor of Weymouth Bay, Dorset, England, United Kingdom. The same fault set cuts rocks of similar age along the adjacent coast, and field mapping can be compared directly with the Chirp profiles. Survey lines were shot perpendicular to the fault strike to produce dip sections from which displacements could be measured. One hundred and fifty-three faults were picked on a 15 km line, yielding a fault density of-10 km -•, similar to that measured in the coastal section.Chirp-resolved fault displacements as small as 0.5 m and a maximum fault displacement of 221 m could be inferred from the data. Distribution analysis of offshore fault data indicated that fault displacement is power law with a well constrained exponent of-0.9. This value is consistent with the power law exponent estimate for fault displacement, over the scale range 2-8 m, onshore. Thus Chirp near-shore seismic reflection profiles can inf111 a data gap for fault size-frequency relationships that commonly occurs when combining data from outcrops/cores and conventional seismic reflection profiles.

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