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doi:10.2204/iodp.proc.335.202.2015

Introduction

Understanding in situ properties of oceanic crust has been an important goal of marine geology and geophysics. Seismic studies, in particular, have provided crucial insights into structure and intrinsic properties of oceanic crust (e.g., Carbotte et al., 2008; Harding et al., 1989; Tolstoy et al., 2008; Vera and Diebold, 1994). Although seismic velocity profiles in oceanic crust are well understood (e.g., Spudich and Orcutt, 1980; White et al., 1992), the anisotropy of seismic waves has received less attention. Observations of seismic anisotropy may provide useful insights into intrinsic properties of crust and mantle, such as preferred orientation of mineral fabric and structural features, and the distribution of in situ stress and strain (e.g., Russo and Silver, 1994; Savage, 1999; Schoenberg and Sayers, 1995; Silver and Chan, 1988). In the upper crust, seismic anisotropy is usually attributed to microfracturing and large-scale fractures (Rasolofosaon et al., 2000; Stephen, 1985), whereas in the low crust and mantle, it is related to the crystal preferred orientation or fabric of anisotropic minerals (Ko and Jung, 2015; Rasolofosaon et al., 2000).

Several active-source seismic studies of P and S body waves have reported 1% to 5% azimuthal anisotropy in the upper crust, attributed to near-vertical water-filled cracks in Layer 2 (Dunn and Toomey, 2001; Stephen, 1985; White and Whitmarsh, 1984). No apparent spreading-rate dependence of the anisotropy magnitude was observed, but it was reported to decrease with depth due to gradual crack closing in the upper 2 km of the crust (Dunn and Toomey, 2001). Observed anisotropy direction varied from roughly ridge-parallel (Dunn and Toomey, 2001) to ridge-oblique by 20°–60° (Stephen, 1985; White and Whitmarsh, 1984). Due to limited spatial resolution, these seismic studies could constrain anisotropy directions only within a 10° azimuthal range, at best. An alternative technique with potentially higher azimuthal resolution is provided by dipole sonic logging tools, which record cross-dipole shear wave fields around boreholes using orthogonal source and receiver pairs (Esmersoy et al., 1994; Iturrino et al., 2005). These data are usually acquired continuously with depth and can be analyzed for the presence of shear wave splitting in anisotropic formations. Dipole logging tools have been used routinely in the Ocean Drilling Program (ODP) and the Integrated Ocean Drilling Program (IODP) in recent years and allow for anisotropy measurements of oceanic crust of different ages when recorded at different geographic sites and under different tectonic settings.

Borehole studies of shear wave anisotropy in oceanic crust to date are sparse. Iturrino et al. (2005) analyzed sonic logs from Deep Sea Drilling Project Hole 395A near the Kane Fracture Zone on the Mid-Atlantic Ridge and ODP Hole 735B near the Atlantis Fracture Zone on the Southwest Indian Ridge (slow to intermediate crustal spreading rates). The results revealed a complex pattern of varying degree of anisotropy with depth from 1% to 15%, with the mean direction of the fast shear wave velocity (V_S) oriented oblique to (Hole 395A) and perpendicular to (Hole 735B) nearby ridge segments, but varying widely about the mean V_S at different depths. The authors attributed this result to a combination of intrinsic effects from fracturing, foliation, and porosity heterogeneity, as well as to stresses potentially inducing changes in the local borehole environment. Resolving the sources of anisotropy in the oceanic crust at both locations proved to be challenging. However, those sites are located in the vicinity of an active fracture zone, which may considerably complicate both the stress field and the tectonic setting near the site. In this study, we analyze shear wave anisotropy in another deep hole, ODP/IODP Hole 1256D, located on the undeformed flank of the East Pacific Rise.

Hole 1256D is one of the few deep holes that penetrate through the upper oceanic crust and offers a unique opportunity to study anisotropy as a function of crustal depth. The site is located on the Cocos plate on the eastern flank of the East Pacific Rise (Fig. F1), which formed ~15 m.y. ago at a fast spreading rate (>200 mm/y full rate) (see the “Expedition 335 summary” chapter [Expedition 335 Scientists, 2012]). Drilled during four expeditions (ODP Leg 206 and IODP Expeditions 309, 312, and 335) to a total depth of ~1522 meters below seafloor (msbf), Hole 1256D intersects roughly 250 m of sediment, 700 m of lava, 350 m of sheeted dikes, and then encounters gabbro at ~1410 mbsf (Fig. F2). Sonic logging data were recorded during three of the four expeditions, but they did not record data over the full depth interval due to obstructions in sections of the hole during operations. In this report, we present anisotropy analysis and results over the longest logged interval of Hole 1256D using the Dipole Sonic Shear Imager (DSI; 300–1215 mbsf; acquired during Expedition 309), which recorded the highest data quality and allows for the best estimations of shear wave anisotropy.

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