Ocean Crustal Seismic Layer 2C

Abstract

Normal oceanic crust is chiefly defined by a seismic structure consisting of layers 2 and 3. Houtz and Ewing (1976, 10.1029/JB081i014p02490) further subdivided layer 2 into layers 2A, 2B, and 2C. Though it has been associated with the dike sections sampled in Holes 504B and 1256D, layer 2C is often not observed; whether it is a distinguishable feature of the seismic structure is a question of importance to interpretations of the seismic structure and depends particularly on the defining characteristics of layers 2C and 3. A review and analysis of existing data reveal that the defining characteristic of layer 3 is a gradient of 0.1 ± 0.1/s, with a velocity at the transition to layer 3 of 6.85 ± 0.20 km/s. Layer 2C is observed in about half of seismic profiles examined. It can be recognized as a transition of the average velocity gradient from >1 above to <1 but more than 0.2/s below. The characteristic gradient is 0.64 ± 0.22/s, and the velocity at the transition to layer 2C is 6.06 ± 0.26 km/s, a value in excellent agreement with the layer 2C velocities reported by Houtz and Ewing. Model travel time curves calculated from sonic logs and seismic profiles from Sites 504 and 1256 show that layer 2C would have been observed in airgun-sonobuoy profiles at these sites, where layer 2C is composed almost entirely of dikes, whereas layer 2B is composed chiefly of lavas.