Timing of depth-dependent lithosphere stretching on the S. Lofoten rifted margin offshore mid-Norway: pre-breakup or post-breakup?

Depth-dependent stretching, in which whole-crustal and whole-lithosphere extension is significantly greater than upper-crustal extension, has been observed at both non-volcanic and volcanic rifted continental margins. A key question is whether depth-dependent stretching occurs during pre-breakup rifting or during sea-floor spreading initiation and early sea-floor spreading. Analysis of post-breakup thermal subsidence and upper-crustal faulting show that depth-dependent lithosphere stretching occurs on the outer part of the Norwegian volcanic rifted margin. For the southern Lofoten margin, large breakup lithosphere beta stretching factors approaching infinity are required within 100 km of the continent-ocean boundary to restore Lower Eocene sediments and flood basalt surfaces (similar to54 Ma) to interpreted sub-aerial depositional environments at sea level as indicated by well data. For the same region, the upper crust shows no significant Palaeocene and Late Cretaceous faulting preceding breakup with upper-crustal beta stretching factors <1.05. Further north on the Lofoten margin, reverse modelling of post-breakup subsidence with a beta stretching factor of infinity predicts palaeo-bathymetries of similar to1500 m to the west of the Utrost Ridge and fails to restore Lower Eocene sediments and flood basalt tops to sea level at similar to54 Ma. If these horizons were deposited in a sub-aerial depositional environment, as indicated by well data to the south, an additional subsidence event younger than 54 Ma is required compatible with lower-crustal thinning during sea-floor spreading initiation. For the northern Voring margin, breakup lithosphere beta stretching factors of similar to2.5 are required to restore Lower Eocene sediments and basalts to sea level at deposition, while Palaeocene and Late Cretaceous upper-crustal beta stretching factors for the same region are < 1.1. The absence of significant Palaeocene and late Cretaceous extension on the southern Lofoten and northern Voring margins prior to continental breakup supports the hypothesis that depth-dependent stretching of rifted margin lithosphere occurs during sea-floor spreading initiation or early sea-floor spreading rather than during pre-breakup rifting.