DYKE COMPLEX OF THE EAST PACIFIC RISE EXPOSED IN THE WALLS OF HESS DEEP AND THE STRUCTURE OF THE UPPER OCEANIC-CRUST

Detailed observations have been made of the extrusive and intrusive units of the uppermost crust emplaced at the axis of the East Pacific Rise during dives conducted with the submersible Nautile on the walls of Hess Deep. At four widely separated locations, the same sequence was found: an upper, thin (100-200 m) unit of extrusives (pillow lavas) underlain by a mixed zone of variable thickness (50-500 m, averaging 200 m) where extrusives are found and intrusives and massive subhorizontal layers which may represent sills. The mixed zone itself is underlain by a sheeted dyke unit, that may be up to 1200 m thick, where dykes are subvertical (i.e. untilted) and parallel to the East Pacific Rise fabric. This sequence may be a general feature of medium to fast-spreading crust because it is similar to that observed in Hole 504B of the Costa Rica Rift and is consistent with images of the upper oceanic crust derived from seismic experiments on the axis of the East Pacific Rise. Dyke thickness is a good measure of the depth to the roof of the axial magma chamber. The observed sections provide much needed "ground-truth" for East Pacific Rise crust. The thickness of the lava sequence and the overall thickness of Layer 2 (at most 1400 m) appear to have been overestimated in previous studies of the oceanic crust.