Seismic wide-angle constraints on crustal thickness and structure at Ocean Drilling Program Site 1256: How typical are its features for oceanic crust?

Ocean Drilling Program Site 1256 was drilled in the Guatemala Basin, eastern Pacific Ocean, sampling superfast-spreading crust. It is one of the deepest drill holes sampling intact oceanic crust and the only site that has penetrated gabbroic rocks away from a tectonic window. Two gabbroic units were sampled at 1157 m and 1283 m below the basement. We collected seismic refraction and wide-angle reflection data across the drill site, and the resulting tomography models show that the first encountered gabbro does not mark the top of the seismic boundary between the upper (layer 2) and the lower (layer 3) crust, which we observe 500−600 m deeper. We propose that the drilled gabbroic rocks may represent either shallow intrusions or depth variations of the magma lens, marking the upper limit of a layer 2−layer 3 transition zone. Seismic tomography and wide-angle migration of mantle reflections reveal rather thin crust of 5 ± 0.2 km (i.e., ∼1.5 s two-way traveltime), being 1 km thinner than normal oceanic crust. The crustal deficit occurs solely within the lower crust. The observed thin crust distinctly differs from typical fast-spreading crust and may indicate the occurrence of a depleted mantle source. Yet, our preferred interpretation is that at superfast spreading rates of >200 mm/yr, the melt transport through the mantle is too slow to provide enough melts to form 6 km of oceanic crust.