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doi:10.2204/iodp.sp.345.2012

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; Ocean Drilling Program [ODP] science plan [www.odplegacy.org/program_admin/long_range.html]; IODP Initial Science Plan [www.iodp.org/isp/]). The ocean crust forms by solidification of basaltic melts produced by decompression melting in the mantle and subsequent modification during delivery to the lithosphere (see review by Klein, 2007). These basaltic melts evolve through complex processes of crystallization and mechanical deformation, most of which take place within the first million years or so of lithospheric evolution (see Coogan, in press, for a broad overview)

Studies of ophiolites (Anonymous, 1972; Nicolas, 1989) provide a framework for understanding the accretion and evolution of ocean crust and, as in the case of this expedition, provide nearly all of the extant observations of the type of crust under study. Ophiolite studies, however, suffer from a lack or uncertainty of geologic context, including such fundamental parameters as spreading rate, orientation of structures with respect to the ridge and stratigraphic level, and tectonic setting. Ophiolite studies provide a useful overall framework for understanding the genesis and architecture of the ocean crust, but only direct observation of modern ocean crust can inform a definitive understanding.

The state of ocean drilling into the igneous basement is summarized in Figure F1, where sites are organized by crustal depth and spreading rate. Two sites in particular, Sites 504 and 1256, have achieved significant penetration, generating a wealth of information regarding the igneous, thermal, and hydrologic state of the upper ocean crust, formed intermediate and fast spreading rates, respectively. Although the transition into the upper plutonic crust was drilled at Site 1256, the lower plutonic crust has not yet been sampled in an intact hole.

A complementary approach to sample the lower crust is to take advantage of “tectonic windows,” where lower crust and upper mantle exposures are unroofed by tectonic processes (Offset Drilling Working Group, 1994), thus bypassing the upper crust entirely. The offset drilling approach has had varying success in terms of hole penetration (Fig. F1) but has yielded significant scientific return at Hess Deep (Gillis, Mével, Allan, et al., 1993), the Southwest Indian Ridge (Robinson, Von Herzen, et al., 1989; Dick, Natland, Miller, et al., 1999), and the Mid-Atlantic Ridge (Cannat, Karson, Miller, et al., 1995; Kelemen, Kikawa, Miller, et al., 2004; Blackman, Ildefonse, John, Ohara, Miller, MacLeod, and the Expedition 305/306 Scientists, 2006).

The principal objective of Expedition 345 is to sample the lower levels of young plutonic crust that formed at the fast-spreading East Pacific Rise (EPR), filling in a major lithologic gap (dotted oval in Fig. F1). The expedition will take advantage of the offset drilling approach by initiating holes directly into exposures of relatively deep lower crustal rocks at the Hess Deep Rift. Expedition 345 will build on results of previous drilling into the shallow-level plutonic rocks (Leg 147) and extensive regional mapping and sampling surveys, providing a comprehensive geological framework for a composite crustal section. Current knowledge of the Hess Deep crustal sections, in combination with geophysical studies of the EPR, drilling of the plutonic foundation of slow-spreading 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.

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