Holes U1415B and U1415C: Expedition 345

Integrated Ocean Drilling Program (IODP) Hess Deep Expedition 345 was designed to sample lower crustal primitive gabbroic rock that formed at the fast-spreading East Pacific Rise (EPR) in order to test competing models of magmatic accretion and the intensity of hydrothermal cooling at depth. The Hess Deep Rift was selected to exploit tectonic exposures of young EPR plutonic crust, building upon results from Ocean Drilling Program Leg 147 as well as more recent submersible, remotely operated vehicle, and near-bottom surveys. The primary goal was to acquire the observations required to test end-member crustal accretion models that were in large part based on relationships from ophiolites, in combination with mid-ocean ridge geophysical studies. This goal was achieved with the recovery of primitive layered olivine gabbro and troctolite with many unexpected mineralogical and textural relationships, such as the abundance of orthopyroxene and the preservation of delicate skeletal olivine textures. Core recovered at Site U1415 originated at a stratigraphic level at least 2 km beneath the sheeted dike–plutonic transition and thus represents intervals of the lower half to one third of the EPR plutonic crust. The orientation of the magmatic fabrics and magnetic inclinations of the core suggest that Site U1415 is composed of a series of blocks (30 to >65 m vertical thickness) formed by mass wasting. Sampling three or four blocks of relatively fresh rock proved advantageous, as it facilitated observations of two distinct types of layering and troctolite units with varying grain size, lithologic associations, and textures. The results of the IODP Hess Deep Expedition 345 provide a reference section for primitive fast-spreading lower crust that did not exist before. The mineralogical and textural observations show that in several respects the Oman ophiolite is not the ideal model for fast-spreading ocean crust and call into question some aspects of both of the end-member accretion models that were to be tested. This highlights the necessity of ocean drilling to address questions related to the origin and evolution of the lower ocean crust.

A series of near-bottom surveys were conducted during Integrated Ocean Drilling Program (IODP) Expedition 345 to characterize the seafloor and shallow subsurface of the Site U1415 area. The purpose of these surveys was to observe and characterize the nature and local slope of the seafloor and the thickness of the subsurface material covering more competent basement, with the ultimate goal of siting drill holes. The surveys were either exploratory, to characterize the area surrounding the proposed drill sites and to assess the viability of additional areas, or targeted, to characterize a specific hole location. All of the surveys involved visual observations; five of these surveys also collected acoustic subbottom profiling data using a 3.5 kHz near-bottom pinger.