A SEISMIC STUDY OF LITHOSPHERIC FLEXURE IN THE VICINITY OF TENERIFE, CANARY-ISLANDS

Seismic data have been used to determine the crustal and upper mantle structure of Tenerife, Canary Islands, a volcanic island of Tertiary a ge located on >140 Ma oceanic crust. Reflection data show that oceanic basement dips gently towards the island, forming a flexural moat whic h is infilled by 2-3 km of well stratified material. The moat is chara cterised by a major angular unconformity, which we attribute to volcan ic loading of pre-existing oceanic crust and overlying sediments and t he subsequent infilling of the flexure by material that was derived, a t least in part, from the islands. Refraction data show that the flexe d oceanic crust has a mean thickness of 6.41 +/- 0.42 km and upper and lower crustal velocities of 4.8-5.3 km s(-1) and 6.7-7.3 km s(-1) res pectively. The flexure, which has been verified by gravity modelling, can be explained by a model in which Tenerife and adjacent islands hav e loaded a lithosphere with a long-term (>10(6) yr) elastic thickness of approximately 20 km. Seismic and gravity data suggest that up to 1. 5 x 10(5) km(3) of magmatic material has been added to the surface of the flexed oceanic crust which, assuming an age of 6-16 Ma for the shi eld building stage on Tenerife, implies a magma generation rate of abo ut 0.006 to 0.02 km(3) a(-1). This rate is similar to estimates from o ther African oceanic islands (e.g., Reunion and Cape Verdes), but is s ignificantly less than that which has been calculated at Hawaii. There is no evidence in either the seismic or gravity data that any signifi cant amount of magmatic material has ''underplated'' the flexed oceani c crust. The crustal and upper mantle structure at Tenerife therefore differs from other oceanic islands such as Hawaii and Marquesas where >4 km of underplated material have been reported.