Timing of depth-dependent lithosphere stretching on the S.Lofoten riftedmargin offshoremid-Norway: pre-breakup or post-breakup?
N.J. Kusznir, R. Hunsdale and A.M. Roberts
Journal name: Basin Research
Issue: Vol 16, No 2, June 2004 pp. 279 - 296
Info: Article, PDF ( 1.48Mb )
Depth-dependent stretching, inw hich 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 b stretching factors approaching infinity are required within 100 km of the continent-ocean boundary to restore Lower Eocene sediments and flood basalt surfaces (~54Ma) 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 b stretching factors <1.05. Further north on the Lofoten margin, reverse modelling of post-breakup subsidence with a b stretching factor of infinity predicts palaeo-bathymetries of ~1500m to the west of the Utrst Ridge and fails to restore Lower Eocene sediments and flood basalt tops to sea level at ~54Ma. 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 ~54Ma is required compatible with lower- crustal thinning during sea- floor spreading initiation. For the northern Vring margin, breakup lithosphere b stretching factors of ~2.5 are required to restore Lower Eocene sediments and basalts to sea level at deposition, while Palaeocene and Late Cretaceous upper- crustal b stretching factors for the same region are <1.1. The absence of significant Palaeocene and late Cretaceous extension on the southern Lofoten and northern Vring 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.