Leg 206 is the first part of a two-leg strategy to sample a complete crustal section from extrusive lavas, through the sheeted dikes, and into the uppermost gabbros from in situ ocean crust, by drilling a deep hole into basement formed at a superfast spreading rate. Drilling crust generated at a superfast spreading rate will provide comprehensive documentation of one end-member style of mid-ocean-ridge accretion (COSOD II, 1987; Ocean Drilling Program, 1996).
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 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.
To date, legs with drilling fast spreading crust as their primary objective (DSDP Leg 54 and ODP Leg 142) have been mostly unsuccessful and apart from sampling of surficial recent basalt at ridge axes, little is known of the shallow and intermediate depth structure of fast-spreading crust (Table T1). Penetration of ~100 m of fast-spreading basalt has been achieved during several legs with other objectives (Legs 92, 136, 200, and 203). One recent example is the drilling at Site 1224 during Leg 200 in the eastern North Pacific, which sampled the upper 146.5 m of basaltic oceanic crust formed at a fast spreading rate (142 mm/yr full rate) (Stephen, Kasahara, Acton, et al., 2003). Studies of Site 1224 are ongoing but limited to the extrusive basalt flows recovered. A continuous section through the upper oceanic crust and ultimately into mid-crustal gabbros is imperative to calibrate geophysical observations and validate theoretical models of the ocean crustal processes.
Drilling of superfast–spreading rate ocean crust during Leg 206 was undertaken to characterize magmatic accretion processes and the primary and secondary chemical composition, as well as the tectonic, magnetic, and seismic structure of the uppermost oceanic crust. The target depth for Leg 206 was 500–800 m subbasement, which was achieved with 502 m of basement penetration 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 basement lithostratigraphy. Paleomagnetic studies will establish the relative contributions of the major rock types to marine magnetic anomalies, and the position of our site (~5 km) from the center of a magnetic reversal will provide information on crustal cooling rates and the contribution of deep plutonic rocks to surface magnetic anomalies. The reentry hole (Hole 1256D) drilled during Leg 206 provides the first test of the lateral variability of the ocean crust and provides an essential comparison for the 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 will 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 will supplement geological, chemical, structural, magnetic, and physical property observations 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.
A major objective of Leg 206 was to establish a cased reentry hole that is open for future drilling to the total depth penetrated during the leg. Although it was never likely that one drilling leg would be enough to reach the dike–gabbro transition zone, our efforts have provided the groundwork for a second leg solely focused on deepening Hole 1256D. The return visit to this site, by the Integrated Ocean Drilling Program, should be able to penetrate deep enough to determine the geological nature of the geophysically imaged "axial melt lens" believed to be present close to the gabbro–dike transition. Drilling of this boundary in situ will allow the relationships between vigorous hydrothermal circulation, mineralization, dike injection, and the accretion and freezing of the plutonic crust to be investigated.
Leg 206 drilled ocean crust that formed at a superfast spreading rate in the equatorial Pacific ~15 m.y. ago. Our rationale for choosing this particular location and confidence that drilling crust formed at a superfast spreading rate provides the best chance of reaching gabbros in normal oceanic crust in a two-leg drilling strategy is
Purdy et al. (1992) describe an inverse relation between spreading rate and depth to an axial low-velocity zone, interpreted as a melt lens (Fig. F6). Since the Purdy et al. compilation, careful velocity analysis, summarized by Hooft et al. (1996), has refined the conversion from traveltime to depth and data from additional sites have been collected (Carbotte et al., 1997). The fastest-rate spreading centers surveyed with modern MCS reflection, ~140 mm/yr full rate at 14°–18°S on the East Pacific Rise (EPR), show reflectors, interpreted as the axial melt lens, at depths of 940–1260 mbsf (Detrick et al., 1993; Kent et al., 1994, Hooft et al., 1994, 1996). At 9°–16°N on the EPR where spreading rates are 80–110 mm/yr, depths to the melt reflector are mostly 1350–1650 mbsf, where well determined (Kent et al., 1994; Hooft et al., 1996; Carbotte et al., 1997).
Recent interpretation of magnetic anomalies formed at the southern end of the Pacific/Cocos plate boundary identify crust that was formed at full spreading rates of ~200–220 mm/yr from ~20 to 11 Ma (Wilson, 1996) (Fig. F7). The implication from reflection seismic studies of axial low-velocity zones is that crust formed at such superfast spreading rates should have a relatively thin thickness of dikes and the top of the gabbros should occur at shallow depths.
The theoretical basis for expecting an inverse relation between spreading rate and melt lens depth is fairly straightforward. The latent heat released in crystallizing the gabbroic crust must be conducted through the lid of the melt lens to the base of the axial hydrothermal system, which then advects the heat to the ocean. The temperature contrast across the lid is governed by the properties of magma (1100°–1200°C) and thermodynamic properties of seawater (350°–450°C where circulating in large volumes) and will vary only slightly with spreading rate. The heat flux through the lid per unit ridge length will therefore be proportional to the width of the lens and inversely proportional to the lid thickness. For reasons that are not understood, seismic observations show uniform width of the melt lens, independent of spreading rate. With width and temperature contrast not varying, the extra heat supplied by more magma at faster spreading rates must be conducted through a thinner lid (dike layer) to maintain steady state (see Phipps Morgan and Chen [1993] for a more complete discussion). To reach the dike–gabbro transition in normal oceanic crust with minimal drilling, it is therefore best to choose the fastest possible spreading rates. A setting similar to the modern well-surveyed area at 14°–18°S could be expected to reach gabbro at a depth of ~1400 m, based on 1100 m to the axial magma chamber reflector and subsequent burial by an additional 300 m of extrusives (Kent et al., 1994). At faster rates, depths could possibly be hundreds of meters shallower. In contrast, seismic velocity inversions at the axes of the Juan de Fuca Ridge and Valu Fa Ridge in Lau Basin, occur at depths of ~3 km (Purdy et al., 1992) at intermediate spreading rates comparable to those at Site 504.
A possible factor for the good drilling conditions in the 6.9-m.y.-old crust cored at Hole 504B compared with very young fast-spreading crust from Legs 54 and 142 is the equatorial latitude of formation. High equatorial productivity results in high sedimentation rates (>30 m/m.y.) and the rapid burial of the igneous crust when middle levels of the crust are still hot. Conductive reheating of the upper crust due to a thick sediment blanket and restricted seawater access facilitates drilling by accelerating the cementation in the upper basement and increasing the competency of healed fractures.
The fast spreading rates highlighted in Figure F7 occurred near the equator, and rapid initial sedimentation rates of at least 35 m/m.y. have been confirmed at ODP Sites 844 and 851 from Leg 138 and Site 572 from DSDP Leg 85. A sediment cover of ~240 m was estimated for the proposed drill site.
Thermal stresses that resulted in drilling-induced fracturing deep in Hole 504B should be diminished in this older crust (~15 Ma, compared with 6.9 Ma for Hole 504B). Although such fracturing did not prohibit deep penetration in Hole 504B, it did inhibit recovery rates. The predicted lower temperatures at Site 1256 provide some indication that better deep drilling conditions than Hole 504B can be expected.
Full spreading rates for the southern Cocos/Pacific plate boundary (Figs. F7, F8) were ~200–220 mm/yr (Wilson, 1996), 30% to 40% faster than the fastest modern spreading rate. This episode of superfast spreading ended with a reorganization of plate motions at 10.5–11.0 Ma; subsequent plate vectors have been similar to present-day motions. The southern limit of crust formed at the superfast rates is the trace of the Cocos/Nazca/Pacific triple junction, as Nazca/Pacific and Cocos/Nazca spreading rates were not as fast. The older age limit of this spreading episode is hard to determine with the limited mapping and poor magnetic geometry of the Pacific plate. It is at least 18 Ma and could reasonably be 24–25 Ma. The northern limit of this province is entirely gradational, with rates dropping to ~150 mm/yr somewhat north of the Clipperton Fracture Zone. By apparent coincidence, the fastest spreading rates occurred within a few degrees of the equator.
Using the fastest possible spreading rate as a proxy for shallowest occurrence of gabbro still allows a range of possible drilling sites. There is no reason to expect a difference in crustal structure between the Cocos and Pacific plates, but logistics favor a site on the Cocos plate. Transits from a variety of Central American ports would be only 2–4 days, and sediments are ~200 m thinner than on the Pacific plate. It seems prudent to choose an anomaly segment at least 100 km long and a site at least 50 km from the end of the segment. For ages 12–16 Ma (Anomalies 5AA–5B) these criteria are easy to satisfy because the southernmost segment of the Pacific-Cocos Ridge had a length of at least 400 km. For ages 17 Ma (Anomaly 5D) and older there is a fracture zone to avoid, but the length of anomaly segments is at least 150 km.
The only serious drawback to this area for a crustal reference section for fast spreading rates is the low original latitude, for which the determination of magnetic polarity from azimuthally unoriented core samples is nearly impossible. Also, the nearly north-south ridge orientation makes the magnetic inclination insensitive to structural tilting. The polarity problem could be solved with a reliable hard rock orienting device, but development efforts for such a tool have been abandoned. Magnetic logging with either the General Purpose Inclinometry Tool (GPIT) fluxgates that are part of the Formation MicroScanner/Dipole Shear Sonic Imager (FMS/sonic) tool string or, preferably, a separate magnetic tool with functional gyroscopic orientation should also be adequate for polarity determination, as demonstrated in Holes 504B and 896A by wireline logs collected during Leg 148 (Worm et al., 1996).
Alternative sites have other, often more serious, drawbacks. Sites flanking the EPR south of the equator generally have poor accessibility and for the age range 10–25 Ma have a complicated tectonic setting and spreading rates are poorly known. North of the equator, sites are available in the same Cocos-Pacific system, which is better understood and more tectonically stable, but there is a severe trade-off between latitude and spreading rate. A magnetically desirable latitude of 20° would reduce the spreading rate to ~60% of the rate for the sites we propose, which may significantly increase the depth to gabbro. To detect structural rotations about a nearly north-south ridge axis, paleolatitude should probably exceed 25°, which means that no site satisfying this criterion will also offer fast spreading rate and short transit to common ports.