Notes: Simple elastic plate models have been used to calculate the flexure of the lithosphere caused by volcanic loading at the Canary Islands and sediment loading at the Moroccan continental margin. By comparing the calculated flexure to observations based on seismic refraction and free-air gravity anomaly data, constraints have been placed on the long-term (〉106 years) elastic thickness of the lithosphere, Te. The best fit between the calculated and observed flexure in the vicinity of the Canary Islands is for Te= 20 km. This value of Te also explains seismic reflection data in regions that flank the island provided that the lithosphere underlying the Moroccan margin is sufficiently weak (Te 〈 5 km) for sediment loading to contribute little to the island flexure. a backstripping study, in which gravity data are used to constrain the value of Te, supports the suggestion that the lithosphere underlying the Moroccan margin, like its conjugate at the Baltimore Canyon Trough, is weak. Although sediment loading at the Moroccan margin appears therefore to have exerted little influence on the structure of the Canary Islands, there is evidence that island flexure may have influenced the stratigraphic development of the Moroccan margin, especially in the region of the ‘slope anticline’. A Te, of 20 km is about 15 km lower than would be expected on the basis of the thermal age of the oceanic lithosphere that underlies the Canary Islands. Similar low values have been reported from oceanic islands in the Pacific ‘superswell’ region of French Polynesia where they have been attributed to re-heating of the lithosphere by one or more hotspots. A general E-W age progression of the volcanic rocks suggests that the Canary Islands were also generated by a hotspot. They lack, however, the topographic swell and gravity/geoid high which usually accompanies these features. One possibility is that the low Te values are the result of a pre-existing weakness in the oceanic crust. A Te of 20 km is large enough, however, for a significant part of the mantle to still be involved in the support of the island loads. A more likely explanation is thermal weakening by a hotspot which has been localized enough to reduce Te but not to produce a swell or gravity/geoid high. Irrespective of its cause, the low Te suggests that oceanic lithosphere does not necessarily progressively increase its strength with age, so that even 140 Ma old lithosphere is vulnerable and can, in some regions, be significantly weakened by later thermal and mechanical processes.

Notes: We present new images of the lower crust and Moho beneath the Valencia Trough—a young rift basin in the western Mediterranean. These images were obtained from a two-ship, wide-aperture reflection experiment and show several features not distinguishable on previously available conventional single-ship reflection profiles.The Moho, which was previously only seen intermittently, can now be confidently traced throughout the basin. We have constructed a present-day depth-to-Moho map and estimated the degree of crustal thinning for the whole basin. Crustal thinning is at a maximum in the centre of the basin, where β values reach 3.15 ± 0.25. At the margins of the basin the β value decreases to 1.5 ± 0.1.The reflective character of the lower crust and Moho is different beneath different parts of the basin. We have been able to correlate these differences with the amount of stretching. We therefore interpret the variations of the observed lower crustal reflectivity as having been caused by the most recent (Neogene) stretching event that opened the Valencia Trough. Along the Iberian margin there is well-developed lower crustal reflectivity consisting of 1-2s two-way time (TWT) of 1-4 km long, near-horizontal reflectors underlain by a more continuous, although not significantly stronger, reflector interpreted to be the reflection Moho. Offshore, this lower crustal reflective unit thins rapidly, such that it is undetectable 40-50 km from the coastline where the crust has been stretched by a factor of 1.7 ± 0.1. As the lower crustal reflectivity becomes undetectable the reflection Moho becomes a robust, continuous event. Where β exceeds 2.4 ± 0.2, however, the Moho is a weak event and difficult to trace. We infer that either the extension itself or associated melting significantly weakened or even destroyed the lower crustal reflectivity in the centre of the basin and enhanced the Moho where extension was moderate.The Balearic margin is somewhat anomalous in that there appears to have been flexural loading of the crust due to thrusting and folding that occurred at the same time as extension in the Valencia Trough. The lower crust shows evidence of weak, but locally variable lower crustal reflectivity. It is possible that the lower crustal reflectivity was preserved simply because the Moho was flexed downward and so decompression, and hence melting, of the upper mantle was restricted. This suggests that the melting itself rather than the extension is the primary mechanism of lower crustal modification.