ATTENUATION DIFFERENCES IN LAYER 2A IN INTERMEDIATE-SPREADING AND SLOW-SPREADING OCEANIC-CRUST

In situ seismic attenuation Q(-1) logs are derived from borehole veloc ity profiles and reveal sharp boundaries between morphologies of the e xtrusive Volcanic layers in intermediate-and slow-spreading oceanic cr ust. Q(-1) logs are calculated from the scattering attenuation associa ted with vertical velocity heterogeneity in Ocean Drilling Program Hol es 504B and 896A and in Hole 395A, located in 5.9-7.3 Ma crust on the Pacific and Atlantic plates, respectively. Our results strongly tie cr ustal properties to seismic measurables and observed geological struct ures. we find that the scattering attenuation can be used to identify the extrusive volcanic sequence because it is closely related to chang es in the degree of vertical heterogeneity. We interpret a distinct de crease in the Q(-1) log at the transition below the extrusive volcanic layer to correspond with the seismic layer 2A/2B boundary. The bounda ry is located at 465 m depth below the sea floor in both Hole 395A and 504B, although this is likely to be a coincidence of the sediment thi ckness at these sites. Layer 2A is estimated to be approximately 150 m thick in Hole 504B and > 300 m thick in Hole 395A. Cyclic sequences o f high-porosity pillows and low-porosity massive units in the uppermos t 100 m of volcanics in Hole 395A result in large velocity heterogenei ties which cause > 5 times more attenuation in this layer than in Hole 504B. In Hole 896A, by contrast, fewer pillows, more massive flows, a nd a greater volume of carbonate veins decrease the velocity heterogen eity and attenuation significantly over only 1 km distance from Hole 5 04B. We conclude that the attenuation in the extrusive volcanics of th e ocean crust is largely controlled by Variation in local heterogeneit y and morphology as well as by subsequent hydrothermal alteration. The observed differences in Q(-1) profiles and layer 2A thickness at thes e sites may be attributed to variations in the volume and duration of volcanic activity at mid-ocean spreading centers for these Pacific and Atlantic ridge segments. (C) 1997 Elsevier Science B.V.