The formation of non-volcanic rifted margins by the progressive extension of the lithosphere:The example of the West Iberian margin

Non-volcanic margins such as the West Iberian margin exhibit certain characteristics, such as a deficit of synrift igneous rock, a zone of exhumed subcontinental mantle in the continent-ocean transition and an apparent extension discrepancy. These observations can be explained as a consequence of the progressive extension of the lithosphere above relatively cool mantle. The evolving rheological stratification of the lithosphere controls the style of extension at different lithospheric levels at different times; extension is probably heterogeneous at all stages, with lower crustal and upper mantle boudinage controlling the patterns of thinning and mantle upwelling early in the rift history, and complete crustal embrittlement and mantle serpentinization controlling the formation of late-stage detachment faults. Extension in the brittle crust is via multiple phases of faulting, with a general focusing of extension towards the incipient ocean. The lack of melt is explained by a combination of heterogeneous extension of the lower lithosphere and a cool subcontinental geotherm. The extension discrepancy may in places be controlled by depth-dependent stretching of the crust through lower crustal boudinage, but may also simply be the result of incomplete recognition of the entire polyphase faulting history. The latter seems to be the case for West Iberia. Evidence for all these process can be found at the West Iberian rifted margins as well as those preserved and partially exposed in the Alps. © The Geological Society of London 2007.