Attenuation differences in Layer 2A in intermediate- and slow-spreading oceanic crust

Author(s): Goldberg, D.; Sun, Y. F.
Author Affiliation(s): Primary:
Lamont-Doherty Earth Observatory, Borehole Research Group, Palisades, NY, United States
Volume Title: Earth and Planetary Science Letters
Source: Earth and Planetary Science Letters, 150(3-4), p.221-231. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0012-821X CODEN: EPSLA2
Note: In English. Lamont-Doherty Earth Obs., Contrib. No. 5671. 66 refs.; illus.
Summary: In situ seismic attenuation Q-1 logs are derived from borehole velocity profiles and reveal sharp boundaries between morphologies of the extrusive volcanic layers in intermediate- and slow-spreading oceanic crust. Q-1 logs are calculated from the scattering attenuation associated with vertical velocity heterogeneity in Ocean Drilling Program Holes 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 crustal properties to seismic measurables and observed geological structures: we find that the scattering attenuation can be used to identify the extrusive volcanic sequence because it is closely related to changes in the degree of vertical heterogeneity. We interpret a distinct decrease in the Q-1 log at the transition below the extrusive volcanic layer to correspond with the seismic layer 2A/2B boundary. The boundary 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 thickness 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 of high-porosity pillows and low-porosity massive units in the uppermost 100 m of volcanics in Hole 395A result in large velocity heterogeneities which cause >5 times more attenuation in this layer than in Hole 504B. In Hole 896A, by contrast, fewer pillows, more massive flows, and a greater volume of carbonate veins decrease the velocity heterogeneity and attenuation significantly over only 1 km distance from Hole 504B. We conclude that the attenuation in the extrusive volcanics of the ocean crust is largely controlled by variation in local heterogeneity and morphology as well as by subsequent hydrothermal alteration. The observed differences in Q-1 profiles and layer 2A thickness at these 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. Abstract Copyright (1997) Elsevier, B.V.
Year of Publication: 1997
Research Program: DSDP Deep Sea Drilling Project
IPOD International Phase of Ocean Drilling
ODP Ocean Drilling Program
Key Words: 18 Geophysics, Solid-Earth; 20 Geophysics, Applied; Acoustical logging; Atlantic Ocean; Attenuation; Crust; DSDP Site 395; DSDP Site 504; Deep Sea Drilling Project; Equatorial Pacific; Heterogeneity; IPOD; Leg 109; Leg 111; Leg 137; Leg 140; Leg 148; Leg 45; Leg 69; Leg 70; Leg 78B; Leg 83; Leg 92; North Atlantic; ODP Site 896; Ocean Drilling Program; Oceanic crust; Pacific Ocean; Pacific Plate; Plate tectonics; Q; Sea-floor spreading; Seismic logging; Spreading centers; Velocity structure; Well-logging
Coordinates: N011335 N011338 W0834348 W0834357
N224521 N224521 W0460454 W0460454
Record ID: 1997076853
Copyright Information: GeoRef, Copyright 2017 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands