Timing and magnitude of depth-dependent lithosphere stretching on the southern Lofoten and northern Vøring continental margins offshore mid-Norway: implications for subsidence and hydrocarbon maturation at volcanic rifted margins

Abstract

Subsidence analysis on the southern Lofoten and northern Vøring segments of the Norwegian rifted margin shows depth-dependent stretching of continental margin lithosphere in which lithosphere stretching and thinning at continental break-up at ~ 54 Ma greatly exceeds that of the upper crust within 100 km landward of the COB. For the southern Lofoten margin lithosphere β stretching factors approaching infinity are required at 54 Ma west of the Utrøst Ridge to restore the top Basalt (inner lava flow) reflectors and top Tare formation (54 Ma) to presumed sub-aerial depositional environments, while for the northern Vøring margin lithosphere β values of 2.5 are required. In contrast, upper crustal extension by faulting shows little stretching with β < 1.1 at break-up or immediately preceding break-up in the Paleocene and Late Cretaceous. The presence of lithosphere depth-dependent stretching and the absence of significant Paleocene and Late Cretaceous upper crustal extension imply that depth-dependent stretching of the southern Lofoten and northern Vøring margins occurred during sea-floor spreading initiation rather than during pre-break-up intra-continental rifting. Depth-dependent stretching, where upper-crustal extension is significantly smaller than whole-crustal or whole-lithosphere extension within 75–150 km of the COB, has been observed worldwide for both volcanic and non-volcanic rifted continental margins. Temperature and hydrocarbon maturation modelling show that the inclusion of depth-dependent stretching has an important effect on temperature and hydrocarbon maturation evolution in depth and time. Failure to include the large β factors for the lower crust and lithospheric mantle (below the less stretched upper crust) leads to a serious under-prediction of temperature and hydrocarbon maturation. While the effect of emplacing thick sill intrusions or magmatic underplating at continental break-up has an important effect on predicted temperature and %VR, their effects can be small in comparison with that of depth-dependent stretching.