THE CRUSTAL STRUCTURE OF THE CANARY BASIN - ACCRETION PROCESSES AT SLOW-SPREADING CENTERS

Multichannel seismic reflection and gravity data define the structure of Mesozoic ocean crust of the Canary Basin, formed at slow spreading rates. Single and multichannel seismics show a transition from smooth to rough basement topography from Jurassic to Cretaceous crust and a c oeval change in crustal structure. Internal reflectivity of the rough basement area comprises upper, upper middle or whole crust cutting dis crete dipping reflections. Lower-crustal reflectivity is almost absent and reflections from the crust-mantle transition are short and discon tinuous or absent for several kilometers. In contrast, crust in the sm ooth basement area is characterized by sparse lower crustal events and common reflections from the crust-mantle boundary. The crustal struct ure of fracture zones in the rough basement area is associated with de pressions in the basement top and in most cases with thin crust. In th e smooth basement area, fracture zones exhibit neither a clear topogra phic expression nor crustal thinning. We interpret these characteristi cs as indicative of an increase in extensional tectonic activity and d ecrease in magmatic activity at the spreading ridge associated with a general decrease of spreading rate from Jurassic to Cretaceous times. In addition, the crust imaged across the path of the Cape Verde Hot Sp ot in the Canary Basin exhibits a widespread lower crustal reflectivit y, very smooth topography and apparently thick crust. Our data documen t significant changes in the structure of crust formed at slow spreadi ng rates which we attribute to thermal changes in the lithosphere due either to variations in spreading rate or to the presence of a hot spo t beneath the Mesozoic Mid-Atlantic Ridge.