Project Summary

In the past decade we have collected a tremendous amount of high-quality seismic reflection data from fast-spreading ridges, and now have a comparable dataset for the intermediate-spreading Juan de Fuca Ridge. At the same time investigators have used manned submersibles and remotely operated vehicles to investigate cross-sections of intact intermediate-spreading and fast-spreading crust exposed at tectonic windows; these investigations have mapped geologic units for tens of kilometers, corresponding temporally to crust generated over 0.5-1.0 m.y. Missing from these studies is a direct link between geologic and seismic structure. We propose to rectify this situation by conducting seismic experiments adjacent to the Hess Deep and Blanco Fracture Zone tectonic windows in order to investigate the geologic nature of observed seismic boundaries at crust generated at both intermediate-spreading and fast-spreading ridges. These two regions provide a good contrast because the exposed cross sections were generated at differing spreading rates and the rock units have significantly different thicknesses (thinner at fast-spreading crust exposed at Hess Deep, thicker at intermediate-spreading crust exposed at Blanco). If correlations can be made between seismic structure and outcrop geology, then seismic techniques provide the means of rapidly mapping crustal structure over large regions where no tectonic windows are present. This will allow important constraints on diverse processes such as volcanic construction, dike intrusion, and faulting.

At each study area we will collect 13-16 scarp-parallel and 4-7 scarp-perpendicular profiles. The profiles closest to the scarp will be used to tie the seismic stratigraphy with the observed outcrop geology. The remaining profiles will map lateral variability in along-axis and cross-axis directions. Two MCS profiles in each study area will be extended to the adjacent rise axis in order to directly link the crust in the detailed survey areas with that forming at the fast-spreading East Pacific Rise and intermediate-spreading Juan de Fuca Ridge. Wide-angle data will be collected along these profiles in order to constrain the deeper velocity structure of each region, both near and ~10 km from the scarp edge. At the Hess Deep site OBSs will also be positioned where the dike-gabbro boundary is exposed. Primary questions that should be addressed by these seismic experiments include: 1) Is there a correlation between seismic and geologic structure at Hess Deep? 2) Is there a correlation between seismic and geologic structure at the Blanco Transform? 3) If there is a correlation, are the same units correlated at fast-spreading and intermediate-spreading crust? 4) Is the seismic structure at Hess Deep typical of fast-spreading crust? 5) Is the seismic structure at Blanco typical of intermediate-spreading crust? 6) What is the evolution of the shallow velocity structure from zero-age rise-axis crust to the 1-2 Ma crust in the two study areas?