ORIGINAL RESEARCH PAPER Lateral continuity of basement seismic reflections in 15 Ma ultrafast-spreading crust at ODP Site 1256 Sreeja Nag • Stephen A. Swift Received: 4 March 2010 / Accepted: 14 March 2011 / Published online: 17 April 2011 Ó Springer Science+Business Media B.V. 2011 Abstract The Ocean Drilling Program (ODP) initiated drilling at Site 1256D in the Guatemala Basin, about 1,000 km off the East Pacific Rise to penetrate plutonic rocks, anticipated to be relatively shallow in this region, formed at an ultra-fast spreading rate. IODP Expedition E312 successfully drilled into gabbros at *1,150 m in basement. Multi-channel seismic traces show weak later- ally coherent sub-basement reflections at borehole depths. Synthetic reflectivity seismograms were computed using a Ricker wavelet and impedance profiles from borehole sonic logs. These seismograms show significant sub-basement amplitude peaks. A zero-offset vertical seismic profile, shot on E312, was processed to investigate the authenticity of these reflections and their relationship to borehole geology. A dual scheme of the median filtering and F–K dip filtering was used. Tests with synthetic seismograms indicate the approach is effective at reasonable SNR levels. Downgoing energy is clearly identified but negligible upgoing energy is visible over random noise. These results indicate that lava flows and igneous contacts in upper ocean crust have sig- nificant topography on lateral scales less than the Fresnel Zone (*300 m) due to igneous and tectonic processes. Keywords East pacific rise  Vertical seismic profiling Introduction Though a large number of seismic experiments have been performed along and near the axis of the East Pacific Rise (EPR) (Detrick et al. 1987; Harding et al. 1993; Kent et al. 1990), relatively few have investigated the structure of mature ( [ 5 Ma) fast spreading crust in settings where crustal evolution can be studied with fewer geologic complexities. One exception is the study of Hallenborg et al. (2003), which presents results of a 15 Ma off-axis region in the Guatemala Basin, Eastern Pacific that was drilled at Site 1256 during ODP Leg 206 and IODP Expeditions 309 and 312. Multi-channel seismic (MCS) reflection and refraction data collected using the R/V Maurice Ewing in March–April 1999 were used to gen- erate profiles along the flowlines and isochrons. Hallen- borg et al. 2003 found a large number of bright reflectors, typically 1–5 km in lateral extent, something not seen near the axis of the EPR (Ranero et al. 1997). On the EPR, multichannel seismic profiles (Ranero et al. 1997) found sub-horizontal reflectors at *0.5–0.8 s within the basement, which is the expected depth of the seismic layer 2/3 transition, and *2 km long reflective segments deeper in the crust. The origin of the reflectors at Site 1256 is unclear, but Hallenborg et al. speculate that they maybe formed by off-axis faulting, off-axis magmatism (Benn et al. 1988) and fracture-induced hydrothermal alteration. Figure 1 shows the final, processed reflection profile from the Hallenborg et al. survey that crossed Hole 1256D (Fig. 3). The top-view of the area under study is shown in Fig. 2. Site 1256 was chosen to penetrate thin crust formed at an ultra-fast spreading rate because the depth to the top of the relict magma chamber would be minimized (Wilson et al. 2003). S. Nag (&) Engineering Systems Division, Massachusetts Institute of Technology, Cambridge, MA 02139, USA e-mail: sreeja_n@mit.edu S. Nag  S. A. Swift Department of Geology and Geophysics, MS#24, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA e-mail: sswift@whoi.edu 123 Mar Geophys Res (2011) 32:429–439 DOI 10.1007/s11001-011-9122-4