Timing of depth-dependent lithosphere stretching on the S. Lofoten rifted margin offshore mid-Norway: pre-breakup or post-breakup? N. J. Kusznir, n R. Hunsdale w 1 and A. M. Roberts z n Department of Earth & Ocean Sciences, University of Liverpool, Liverpool, UK wTechnology Division, Statoil ASA, Forushagen, 4035 Stavanger, Norway zBadley Geoscience Ltd, Hundleby, Spilsby, Lincolnshire, UK ABSTRACT Depth-dependent stretching, in which whole-crustal and whole-lithosphere extension is signi¢cantly 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- £oor spreading initiation and early sea- £oor 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 in¢nity are required within 100 km of the continent^ocean boundary to restore Lower Eocene sediments and £ood basalt surfaces ( 54 Ma) to interpreted sub-aerial depositional environments at sea level as indicated by well data. For the same region, the upper crust shows no signi¢cant Palaeocene and Late Cretaceous faulting preceding breakup with upper-crustal b stretching factors o1.05. Further north on the Lofoten margin, reverse modelling of post-breakup subsidence with a b stretching factor of in¢nity predicts palaeo-bathymetries of 1500 m to the west of the Utrst Ridge and fails to restore Lower Eocene sediments and £ood basalt tops to sea level at  54 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- £oor spreading initiation. For the northernV ring 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 o 1.1.The absence of signi¢cant Palaeocene and late Cretaceous extension on the southern Lofoten and northernV ring margins prior to continental breakup supports the hypothesis that depth-dependent stretching of rifted margin lithosphere occurs during sea- £oor spreading initiation or early sea- £oor spreading rather than during pre-breakup rifting. INTRODUCTION Depth-dependent stretching of continental lithosphere has been observed at many rifted continental margins (Roberts et al., 1997; Driscoll & Karner, 1998; Davis & Kusznir, 2004). Stretching estimates, independently de- termined from upper-crustal faulting, whole-crustal thinning and post-rift lithosphere thermal subsidence (Fig. 1a), show that extension increases with depth within  150km of the continent^ocean boundary (COB) such that whole-crustal and whole-lithosphere extension are signi¢cantlygreater than upper-crustal extension.Further towards the continent, stretching and thinning estimates of the upper crust, whole crust and lithosphere converge as the b stretching factors decrease. Total continental margin extension may be determined by laterally integrat- ing the thinning factor (1  1/b) across the margin (Davis & Kusznir, 2004). Margin extension is summarised in Fig. 1b for Goban Spur margin o¡shore UK, South China Sea margin, Exmouth Plateau margin o¡shore W Australia, V ring Basin margin o¡shore Norway and Vulcan Basin margin o¡shore NWAustralia (Roberts et al.,1997; Driscoll & Karner, 1998; Baxter et al., 1999; Davis & Kusznir, 2004). Extension of the whole crust and lithosphere exceeds that of the upper crust in all cases and occurs at both volcanic and non-volcanic margins. Depth-dependent lithosphere stretching appears to be a fundamental property of all rifted continental margins and provides an important constraint in determining the geodynamic processes responsible for rifted margin formation. An important Correspondence: N. J. Kusznir, Department of Earth & Ocean Sciences, University of Liverpool, Liverpool L69 3BX, UK. E-mail: N.Kusznir@liverpool.ac.uk 1 Previously at Phillips Petroleum Company,Tananger, Norway. Basin Research (2004) 16, 279–296, doi: 10.1111/j.1365-2117.2004.00233.x r 2004 Blackwell Publishing Ltd 279