Three‐dimensional upper crustal heterogeneity and anisotropy around Hole 504B from seismic tomography

Abstract

Abstract. We have used over 16,000 compressional wave travel times recorded on 11 ocean bottom seismometers to determine the magnitude of P-wave heterogeneity and anisotropy in the upper 2 km of the oceanic crust in an -• 150 km 2 area around Hole 504B, the deepest hole drilled into the oceanic crust. Our best fitting one-dimensional, isotropic upper crustal velocity model for the 150 km 2 area around the drill site is similar to velocity models at Hole 504B determined using downhole sonic logs, vertical seismic profiles, oblique seismic experiments and conventional refraction profiling. There is no evidence seismic layer 2 is anomalously thin at Hole 504B, suggesting that the occurrence of the seismic layer 2/3 boundary within the upper sheeted dike section, which has been documented in Hole 504B, is a general feature of this 5.9 Ma crust. On the scale of a few kilometers and averaged over seismic wavelengths of a few hundred meters the heterogeneity in upper crustal P wave velocity around Hole 504B is comparatively small. Within the 571-m thick extrusive section maximum P wave velocity differences are -300-400 rn s '• (a 6%-8% velocity anomaly). These velocity differences decrease to <100-200 rn s -• (<2%-3% of the average velocity) within the sheeted dike section > 1 km into the crust. The highest upper crustal velocities are found beneath basement ridges west and south of Hole 504B, while the lowest upper crustal velocities occur beneath the flanking basement troughs. There is an azimuthal dependence to the travel time residuals (slow direction N-S) which is consistent with 2%-4% crack-induced anisotropy in the upper crust. The magnitude of upper crustal heterogeneity observed near Hole 504B is significantly less than that found in "zero-age" crust at the Mid-Atlantic Ridge but comparable to that observed off-axis in crust formed at faster spreading ridges.

Tracking stress and hydrothermal activity along the Eastern Lau Spreading Center using seismic anisotropy

“…Activesource seismic studies are not usually designed with the intent to measure three-dimensional variations in anisotropy, but the results thus far are encouraging. Several studies have examined one-dimensional depth-dependent anisotropy in oceanic crust, including measurements along mid-ocean ridge crests (Caress et al, 1992;MacDonald et al, 1994;Sohn et al, 1997;Barclay et al, 1998;Dunn and Toomey, 2001;Barclay and Wilcock, 2004;Tong et al, 2004;Seher et al, 2010), in older oceanic crust (>1 Ma) (Stephen, 1981(Stephen, , 1985White and Whitmarsh, 1984;Shearer and Orcutt, 1986;Detrick et al, 1998), and in back-arc basins (Hirata et al, 1992). Few prior studies have attempted to extract lateral variations in crustal anisotropy at a spreading center (e.g., Sohn et al, 1997;Dunn and Toomey, 2001;Weekly et al, 2014).…”

Site 1256

“…Layer 2/3 boundary is present in the depth range 1.2-1.5 km subbasement. Velocity model of Detrick et al (1998) for Site 504, also based on ocean-bottom hydrophone refraction, is shown for comparison. Apparent differences are dominated by differences in inversion techniques, but differences at 1.3-1.7 km may be barely above uncertainty.…”

On the boundary between seismic layers 2 and 3: A stress change?

“…The textbook explanation relating layer 3 velocities to the presence of gabbro was ruled out as the location of the boundary was proposed to be located either within or at the top of the sheeted-dike complex of the only really deep hole ever drilled into modern oceanic crust, Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) Hole 504B (Cann, Langseth, et al, 1983;Anderson, Honnorez, et al, 1985;Becker, Sakai, et al, 1988;Dick, Erzinger, et al, 1992;Alt, Kinoshita, et al, 1993). This result was obtained by combining a series of constraints on the velocity structure of the upper crust (Alt, Kinoshita, et al, 1993;Hobart et al, 1985;Collins et al, 1989;Swift et al, 1998a;Detrick et al, 1998). As a consequence, the boundary was found to stand at least 400 m, possibly up to 800 m, above yet unsampled gabbroic rocks previously believed to correspond to layer 3 seismic velocities.…”