Oceanic crustal structure at ODP Site 1256 from seismic wide-angle tomography and down-hole logging

Our view on the structure of oceanic crust is largely based the interpretation of seismic refraction and wide-angle experiments, revealing that the upper basaltic crust (layer 2) is a region of strong velocity gradients. In contrast, the lower gabbroic crust (layer 3) is relatively homogeneous, although it generally displays a gentle increase in velocity with depth. Furthermore, the upper crust has been sub-divided into layer 2A, composed of extruded basalts, and layer 2B, formed by basaltic sheeted dikes. Site 1256, drilled during the Ocean Drilling Program (ODP) into the upper crust and later extended into the uppermost gabbroic crust during the Integrated Ocean Drilling Program (IODP), is among the deepest drill sites sampling intact oceanic crust. It is the only site world-wide that crossed the entire basaltic upper crust, reaching plutonic rocks at ~1.35 km below the top of the basement, recovering 150 m of dominantly gabbroic rocks at the base of the hole. Three campaigns of down-hole logging at hole 1256D provided a unique set of high-resolution sonic-log velocities of seismic layer 2 and from the uppermost top of seismic layer 3. However, Hole 1256D was drilled at a site with rather limited seismic data coverage, especially lacking seismic refraction and wide-angle profiling. During a seismic survey of the RRS JAMES COOK in the Guatemala Basin in December of 2022, a seismic profile with 12 Ocean-Bottom-Seismometers spaced at 7 km intervals, receiving signals from a tuned airgun array of 4500 cubic-inches shot at 150 m spacing was collected. The data provide excellent seismic records to derive a detailed sound-velocity model of the oceanic crust at the drill site from tomographic travel time inversion of first arrivals (Pg, Pn) and a prominent wide-angle reflection from the crust-mantle boundary (PmP) or seismic Moho. The results show that the seismic structure along the 115 km long line is extremely homogeneous. The velocity-depth profile from tomography further provides an excellent low-frequency match of the down-hole logging observations, supporting that modern seismic data are a powerful remote sensing tool to study the oceanic crust and lithosphere. An interesting observation is that the thickness of the oceanic crust at Site 1256 is extremely thin at only 4.6 to 5.1 km, compared to a global average thickness of about 6 km. This appears to be a regional feature supported by another seismic profile about 150 km north-eastwards. The thin crust agrees with a weak seismic event at ~6.8 s two-way travel time (twtt), i.e., ~1.6 s twtt below basement obtained from re-processing 6-km-long streamer data from the ODP pre-site survey at Site 1256.