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doi:10.2204/iodp.pr.345.2014

Introduction

The accretion of new ocean crust at mid-ocean-ridge spreading centers is one of the fundamental processes of Earth evolution. Understanding the nature of the crust and mantle and the underlying geologic processes that form them is an ongoing fundamental justification for drilling in the ocean basins (Hess, 1960; see Ocean Drilling Program [ODP] science plan at www.odplegacy.org/program_admin/long_range.html and Integrated Ocean Drilling Program [IODP] Initial Science Plan at www.iodp.org/isp). At mid-ocean-ridge spreading centers, magmas are delivered to the base of the crust and pass through a lower crustal filter on their way to higher level magma chambers and eventual eruption as mid-ocean-ridge basalt (MORB) (see review by Klein, 2007). These melts evolve through complex processes of crystallization and deformation in the lower plutonic crust, most of which take place within the first million years of lithospheric evolution (see Coogan, 2013, for a broad overview). Our knowledge of these lower crustal processes has been strongly influenced by ophiolite studies (e.g., Pallister and Hopson, 1981; Nicolas, 1989), but only direct observation of modern ocean crust can inform a definitive understanding.

Two drilling approaches have been used to directly observe modern plutonic ocean crust: total penetration, whereby holes are initiated in the upper lavas with eventual penetration into the plutonic crust (ODP Site 1256), and off-set drilling, in which holes are directly initiated in tectonic exposures of the plutonic crust, thus bypassing the upper crust entirely (e.g., ODP Sites 735 and 894 and IODP Site U1309D). The concept of offset drilling is that by drilling relatively short holes that start at a variety of crustal levels, a composite section could be constructed that equals or even improves upon the results that could be obtained through a total crustal penetration. This approach is particularly effective for areas such as the Hess Deep Rift where use of complementary technology (remotely operated vehicles and submersibles) allows for regional-scale characterization of the crust, providing a comprehensive geological framework on which to hang the off-set sections.

The structure of the ocean crust, and in particular the nature of the plutonic sequence, varies significantly for mid-ocean ridges formed at different spreading rates (Fig. F1). The classic view of a layered, continuous ocean crust, the so-called Penrose model (Fig. F1A), is expected to be representative of fast-spreading crust (Anonymous, 1972). The structure of slow-spreading crust varies with position along the ridge, with segment centers having a thicker crustal component than segment ends (Fig. F1B, F1C) (Dick, 1989; Cannat et al., 1995). Ultraslow-spreading crust also shows significant variation in the thickness and rock type distribution with depth (Dick et al., 2003). To date, ocean drilling has recovered some significant sections of the ocean crust, both by total penetration holes (i.e., Deep Sea Drilling Program [DSDP] Hole 504B and ODP Hole 1256D) and offset drilling legs with numerous shallow to deep holes (e.g., ODP Legs 118, 147, 153, 176, and 209 and IODP Expedition 304/305) (Fig. F1).

The principal objective of IODP Expedition 345 was to sample the young primitive plutonic crust that formed at the fast-spreading East Pacific Rise (EPR), filling in a major lithologic gap in our sampling of the lower oceanic crust (blue rectangle in Fig. F1A). The expedition took advantage of exposed primitive gabbroic rocks at the Hess Deep Rift. Current knowledge of Hess Deep crustal sections (see “Previous research at the Hess Deep Rift”), in combination with geophysical studies of the EPR, drilling of the plutonic crust of slow-spread crust, and studies of ophiolite complexes, allows for the formulation of a series of specific questions that may be tested by drilling at Hess Deep.

Rationale for Hess Deep drilling

The Hess Deep Rift was selected for Expedition 345 as it is the best studied tectonic window into fast-spreading crust. An unprecedented geological framework for fast-spreading crust is available based on extensive investigation of contiguous sections of the mid- to upper crust along the Northern and Southern Escarpments and sections of the mid- to lower crust and uppermost mantle along the rift valley floor (Francheteau et al., 1990; Karson et al., 1992, 2002; MacLeod et al., unpubl. data, 2008), and Leg 147 (Gillis, Mével, Allan, et al., 1993) (see “Previous research at the Hess Deep Rift”). These geological relationships, coupled with investigations of recovered samples, provide a comprehensive framework for the igneous, metamorphic, and deformation processes active at the fast-spreading EPR.

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