With Legs 153, 147 and 118, scientific ocean drilling has tested drilling conditions in each of the three principal tectonic environments where windows into the lower ocean crust and mantle exist. These are, respectively, rift valley walls, the amagmatic tips of propagating rifts and the crests of transverse ridges flanking fracture zones. Each of these environments is crucial to a successful offset drilling strategy as they provide unique opportunities to study different aspects and features of the ocean crust. Propagating rifts provide the only logistically realistic opportunity to drill the lower ocean crust and mantle created at fast spreading ridges; rift valley walls provide our best opportunity to directly study the tectonic processes creating seafloor topography at slow spreading ocean ridges and to explore the isochronal structure and heterogeneity of the lower crust and mantle along individual spreading segments. In contrast, the crests of transverse ridges provide an opportunity to study the temporal variability of the lower ocean crust and mantle along lithospheric flow lines, as well as sampling the fabric and composition of the ocean crust where it is least influenced by the tectonic processes creating the rift mountains. Thus, all of these environments are important potential targets for offset-section drilling.
Whereas drilling on Leg 118 demonstrated that the crests of transverse ridges can be easily drilled to depths of 500 m, drilling on the rift valley wall at the MARK area of the Mid-Atlantic Ridge during Leg 153 and on the intra-rift high at Hess Deep in a tectonic block of old EPR lower crust and mantle during Leg 147 was much more difficult. These legs, however, did demonstrate the utility of properly located and spaced holes of 30 to 200 m in characterizing portions of the lower crust and mantle. Thus, despite not having drilled 500-m holes, these legs provided a high scientific return. Nonetheless, the objective of directly characterizing the ocean crust on a seismically meaningful scale remains a principal objective of offset drilling. A revised offset-drilling strategy, based on what we now know we can do, should also be designed to lead to accomplishing this objective even in relatively hostile environments.
Recognizing the high priority of long vertical sections to the lithosphere community, any strategy for offset drilling is likely to include proceeding at the first opportunity to deepen Hole 735B. We must recognize, however, that such an opportunity is not known to exist at fast spreading ridges, and that rift valley walls offer other first order scientific opportunities that the community wants to pursue. Thus, we offer several parallel strategies that can lead to successfully reaching our objectives in this environment. The first strategy is to learn to drill in hard places. The second is to learn how to find easy places to drill. The latter includes locating "sweet" spots in otherwise hostile environments. This might be accomplished either serendipitously during drilling a suite of single bit holes designed to explore the lateral variability of the crust, or might use geophysical techniques, such as on bottom seismology, as described elsewhere in this report.
We take the point of view here that there are logistical and technological strategies that ultimately will improve our ability to find and drill sections of lower crust and mantle to depths of 500 m or more in places like Hess Deep and MARK. A necessary first step in developing these strategies is an engineering leg, as discussed elsewhere in this report in detail. The combination of existing drilling technologies, which have not been tested in these harsh seafloor environments, together with incremental improvements to previously used equipment are likely to significantly improve the ability to drill gabbro and peridotite in tectonically stressed environments. An engineering leg should proceed in a region where drilling conditions have previously been characterized, and are representative of those we wish to overcome. The location should also be in a region that has the best available site survey information. For the immediate future, offset drilling in the more hostile environments of rift valley walls and propagating rifts should proceed with the understanding that we are more likely to drill arrays of numerous shallow holes. This will allow characterization of the lateral variability of the crust and shallow mantle in these environments, lead to the improvement of drilling techniques in serpentinite and gabbro, and at the same time may lead to the discovery of suitable locations for siting deep holes.
As our ability to drill in more hostile environments improves, we also recognize that a deep hole at any given location must not only be based on site specific survey information and the existence of a favorable location for guide base placement, but must include demonstration that the formation itself can be drilled. Although remote sensing techniques may enhance our ability to locate such a spot, they also require a successful pilot hole in a matrix of holes documenting the local lateral variability of the crust. These pilot holes should be as deep as possible using a single bit (to bit destruction).
The different strategies discussed above are shown diagrammatically in Figure 5. If this revised approach to offset drilling is adopted by the JOIDES community, it would be possible to insert any of the three legs in the top row into the 1997 schedule. The engineering leg would be an expensive one, comparable in cost to Leg 147 or 153, but the other two would not. A leg planned as an array of shallow single-bit holes would be similar to what actually happened on Leg 153, but no money would need to be spent on guide bases and casing systems. Offset drilling proposals that might be pursued by this strategy, exist in the JOIDES system, but may need revision with this particular approach in mind. The timescale of progressing to deeper holes at either hard or easy locations cannot be predicted, because these drilling legs would not be scheduled until the outcome of the earlier cruises was known.
Finally, it is important to stress that the prime objective of the lithosphere community in its offset drilling strategy is to obtain long vertical sections of the total column of the oceanic crust down to the mantle. Understanding lateral heterogeneity is a secondary priority. Thus, while a drilling leg consisting of an array of shallow holes is inherently of scientific interest, its most important function may be to pave the way toward one of the deeper holes to which the lithosphere community has consistently aspired.
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