CRUSTAL STRUCTURE, GRAVITY-ANOMALIES AND FLEXURE OF THE LITHOSPHERE IN THE VICINITY OF THE CANARY-ISLANDS

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 (>10(6) years) elastic thickness of the lithosphere, T-e. The best fit between the calculated and observed flexure in the vicinity of the Canary Islands is for T-e = 20 km. This value of T-e also explains seismic reflection data in regions that flank the island provided that the lithosphere underlying the Moroccan margin is sufficiently weak (T-e <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 T-e, 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 T-e 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 T-e values are the result of a pre-existing weakness in the oceanic crust. A T-e 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 T-e but not to produce a swell or gravity/geoid high. Irrespective of its cause, the low T-e 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.