The formation and evolution of the ocean crust is one of the principal components of the plate tectonic cycle that modulates the surface of our planet as well as buffers the chemistry of the oceans and the upper mantle. Despite the importance of this system, the ocean crust remains relatively poorly surveyed and the mechanisms responsible for the construction of the crust and the extent of chemical exchanges with the oceans remain poorly understood. Although much has been learned from ophiolites (ancient slices of oceanic crust now cropping out on land), many of the best-preserved complexes (e.g., Troodos, Cyprus, and Semail, Oman) formed in suprasubduction zone settings and are not appropriate analogs for crust constructed in the major ocean basins. Marine surveys of ocean crust are generally restricted to sampling erupted lavas except at fracture zones or rare tectonic windows (e.g., Hess Deep) where faulting has exposed deeper portions of the crust. Dredging, manned submersible and remotely operated vehicle observations, and shallow-penetration drill holes have had limited success in describing the lower oceanic crust. In crust that has endured tectonic unroofing, it is often difficult to disentangle ridge constructional processes from the effects of exposure. Deep drilling in situ basement far from fracture zones remains the only effective means to characterize the nature and variability of undisturbed ocean crust.
Axial magma chambers must play a central role in the construction of the ocean crust and the fractionation of lavas erupted on the ocean floor, and they provide the heat source for vigorous black smoker-type hydrothermal circulation. However, the location and geometry of magma chambers remains controversial, as does the significance of shallow axial low-velocity zones imaged at fast and intermediate spreading-rate crust and interpreted to be "melt lenses." Is the crust constructed from the subsidence of frozen magma solidified in the axial melt lens such that all the lower crust passes through this zone and is chemically homogeneous? Or are the melt lenses pockets of late-stage melt that, when crystallized, form zones of relatively fractionated gabbros with similar chemistry to the more evolved lavas and dikes that they underlie? What are the chemical links between the upper gabbros and the overlying dikes and lavas? It might be possible to answer some of these questions in tectonic windows, but it is difficult to place the samples collected from these tectonic windows into a coherent geological context. By drilling a complete section of the upper crust through the extrusive lavas and dikes and into the uppermost gabbros, the relationships between the plutonic processes and the erupted lavas can be deciphered and the contribution of axial magma chambers to hydrothermal budgets assessed.
That deep drilling of the ocean crust is imperative to answer many fundamental questions has been acknowledged since the inception of scientific ocean drilling, but despite more than 30 years of deep-sea drilling, our sampling of the oceanic basement remains cursory and highly incomplete (Figs. F1, F2). The sheeted dike/gabbro boundary has never been sampled in situ, and Hole 504B remains the only hole to penetrate a complete stratigraphy of extrusive lavas and the transition zone into the sheeted dike complex.
Leg 206 is the first part of a two-leg strategy to sample a complete upper crustal section of in situ ocean basement from extrusive lavas, through the sheeted dikes, and into the uppermost gabbros. The key to planning Leg 206 was to find a site where the extrusive lavas and dikes overlying the gabbros could be expected to be relatively thin, thus increasing the likelihood of penetrating through the complete upper crustal section. As discussed below, this requirement led to crust created by superfast spreading because the thickness of the lavas and sheeted dikes decreases as spreading rate increases.
A recent reconsideration of magnetic anomalies formed at the southern end of the Pacific/Cocos plate boundary has identified crust formed at a full spreading rate of ~220 mm/yr from 20 to 11 Ma (Wilson, 1996) (Fig. F3). This is significantly faster than the present fastest spreading rate (~145 mm/yr) for crust forming at ~30°S on the EPR. From this region, created by superfast spreading, a single drill site was selected on young, 15-m.y.-old ocean crust. At this site, initially designated proposed Site GUATB-03C and now known as Site 1256, a reentry cone was to be installed along with a wide-diameter casing that was to extend to basement, both of which were accomplished during Leg 206. Deep drilling at this site during Leg 206 and subsequent expeditions will characterize one end-member style of mid-ocean-ridge accretion.
The rationale for drilling crust formed at a superfast spreading rate is straightforward, and this strategy provides the best chance of reaching the upper gabbros at the shallowest depth and, hence, fewest drilling days. There is an inverse relationship between the depth of axial low-velocity zones imaged by seismic experiments, interpreted to be melt lenses, and spreading rate (Purdy et al., 1992) (Fig. F4). Even allowing for an additional thickness of lavas that flowed from the ridge axis to cover the immediate flanks, the uppermost gabbros should be at relatively shallow depths in superfast-spreading crust. The predicted depth to gabbros at Site 504 on the south flank of the intermediate spreading-rate Costa Rica Rift is >2.5 km, whereas the depth to the axial low-velocity zones at typical fast spreading-rate (~80-150 mm/yr) crust on the EPR is ~1-2 km. The estimated depth to an axial melt lens for ocean crust formed at a superfast spreading rate is ~800-1000 m, and the anticipated depth to gabbros for Site 1256 is ~1100-1300 m, allowing for a reasonable thickness (~300 m) of near-axial lava flows.
Although perhaps only 20% of the global ridge axis is separating at fast spreading rates (>80 mm/yr full rate), ~50% of the present-day ocean crust and ~30% of the total Earth's surface was produced by this pace of the ocean spreading. At least in terms of seismic structure (Raitt, 1963; Menard, 1964), crust formed at fast spreading rates is relatively simple and uniform. Hence, the successful deep sampling of such crust in a single location can reasonably be extrapolated to describe a significant portion of the Earth's surface.
In addition to the shallow depth to gabbros predicted from formation at a superfast spreading rate, Site 1256 in the Guatemala Basin has a number of specific characteristics that indicate this site provides an excellent opportunity to sample, in two legs, a complete section of upper oceanic crust. Site 1256 formed at an equatorial latitude (Fig. F5), and high equatorial productivity resulted in high sedimentation rates (>30 m/m.y.) and rapid burial of the young basement. The thick sediment blanket was needed to enable the installation of a reentry cone with 20-in casing, which was to act as the foundation for deployment of a second 16-in-diameter casing string into the uppermost basement.
At 15 Ma, Site 1256 is significantly older than the crust at Hole 504B (6.9 Ma), and lower temperatures are anticipated at midlevels of the crust. As such, high basement temperatures that can preclude drilling operations should not be reached until gabbroic rocks have been penetrated.
Logistically, Site 1256 has a number of advantages. It is ~3.5 days steaming from the Panama Canal, and the short transit time allows for maximum time on site during a 2-month drilling leg. As transfer between the Pacific and Atlantic Oceans is an oft-transited route because of the demands of scientific drilling, close proximity to the Panama Canal should allow timely rescheduling of return visits to the site.
Current plans for the next phase of scientific ocean drilling include the operation of a riser vessel, eventually capable of supporting a 4000-m riser, which will enhance well control and the clearing of cuttings from deep holes through the use of heavy muds in a closed loop. Should Site 1256 become the locus of multiple legs of deep drilling aimed at eventually sampling a full section of oceanic crust, the ~3500-m depth of the site will be well within the design specifications of the riser vessel required for such a deep penetration.
The major objective of Leg 206 was to establish a cased reentry hole that is open for the full depth penetrated during this leg. This site will provide an important legacy for the Ocean Drilling Program as well as continuity with the Integrated Ocean Drilling Program through the installation of the infrastructure required for drilling a complete upper crustal section of in situ ocean crust.
Drilling of superfast spreading-rate ocean crust during Leg 206 allows us to characterize the nature of magmatic accretion and the primary and secondary chemical composition as well as the tectonic and seismic structure of the uppermost oceanic crust. The target depth for Leg 206 was 500-800 m subbasement, which was reached in Hole 1256D. Cores from Leg 206 will provide an essential link to relate geology to remote geophysical observations (seismics and magnetics) and ground-truth the relationship between seismic stratigraphy and lithostratigraphy. Paleomagnetic studies can establish the relative contributions of the major lithologic units to marine magnetic anomalies, and the position of our site (~5 km from a magnetic reversal) provides information on crustal cooling rates and eventually the contribution of deep plutonic rocks to surface magnetic anomalies. The holes drilled during Leg 206 provide a first test of the lateral variability of the ocean crust and enable comparison with models of crustal accretion, hydrothermal alteration, and the secondary mineral/metamorphic stratigraphy, principally developed from ODP Hole 504B. This will refine models for the vertical and temporal evolution of ocean crust, including the recognition and description of zones of hydrothermal and magmatic chemical exchange. Physical property measurements of cores recovered from fast-spreading ocean crust yield information on the porosity, permeability, and stress regime as well as the gradients of these properties with depth. A full suite of wireline logs supplements geological, chemical, structural, and magnetic observations and physical property studies on the core. The careful integration of borehole observations with measurements of the recovered core is imperative for the quantitative estimation of chemical exchange fluxes between the ocean crust and oceans.
Although it was not impossible, the total depth of penetration at Site 1256 during Leg 206 was never likely to reach the dike-gabbro transition zone. However, our efforts have provided the groundwork for a second leg to return to the site and investigate the geological nature of the geophysically imaged "axial melt lens" believed to be close to the gabbro-dike transition. Drilling of this boundary in situ will allow the relationships between the accretion and freezing of the plutonic crust, vigorous hydrothermal circulation, mineralization, and dike injection to be investigated.
Site 1256 (6°44.2´N, 91°56.1´W) lies in 3635 m of water in the Guatemala Basin on Cocos plate crust formed ~15 m.y. ago on the eastern flank of the EPR (Figs. F3, F6). The depth of the site is close to that predicted from bathymetry models of plate cooling (e.g., Parsons and Sclater, 1977). The site sits astride the magnetic Anomaly 5Bn-5Br magnetic polarity transition (Fig. F7). This crust accreted at a superfast spreading rate (~220 mm/yr full rate) (Wilson, 1996) and lies ~1150 km east of the present crest of the EPR and ~530 km north of Cocos Ridge. The site formed on a ridge segment at least 400 km in length, ~100 km north of the ridge-ridge-ridge triple junction between the Cocos, Pacific, and Nazca plates (Fig. F5). This location was initially at an equatorial latitude within the equatorial high-productivity zone and endured high sedimentation rates (>30 m/m.y.) (e.g., Farrell et al., 1995). The sediment thickness in the region is between 200 and 300 m and was estimated from site survey reflection seismic profiling to be ~250 m at Site 1256.
Site 1256 sits atop a region of smooth basement topography (<10 m relief) and has a seismic structure reminiscent of typical Pacific off-axis seafloor. Upper Layer 2 velocities are 4.5-5 km/s, and the Layer 2-3 transition is between ~1200 and 1500 m subbasement. The total crustal thickness at Site 1256 is estimated at ~5-5.5 km. Northeast of Site 1256 (15-20 km), a trail of ~500-m-high circular seamounts rise a few hundred meters above the sediment blanket (Figs. F8, F9). Basement topography is more pronounced southwest of Site 1256 and in the grid 2 area (Fig. F8), where subparallel narrow ridges (1-2 km) and wider troughs (4-5 km) display ~100 m of basement relief.