BACKGROUND AND OBJECTIVES OF THE OCEAN SEISMOGRAPHIC NETWORK, AND LEG 136 DRILLING RESULTS

A. Dziewonski, R. Wilkens, J. Firth, and Shipboard Scientific party

The primary objective of Ocean Drilling Program Leg 136 was to prepare a seafloor site for future experiments needed to develop the Ocean Seismographic Network (OSN). OSN-1 (ODP Hole 843B) has been established on the Hawaiian Arch. approximately 225 km south-southwest of the island of Oahu (Fig. 1). The hole was drilled through approximately 243 m of sediment and 70 m into the basaltic basement. It will provide a site for borehole seismometer experiments that include noise measurements, recording of data from teleseismic events for comparison with an existing, high-quality station on Oahu, and testing of new broad-band sensors and other instrumentation for long term deployment. The 5-10-yr goal of the OSN is to establish a global network of 15-20 permanent seismic observatories in the deep ocean, with the expectation that these facilities will be shared with other fields of geoscience requiring long-term observations on the ocean floor. OSN will revolutionize studies of global Earth structure, upper mantle dynamics and lithosphere evolution, earthquake source mechanisms, oceanic Crustal structure, tsunami warning and monitoring, and deep ocean noise propagation and generation. A significant part of this report will be devoted to providing the reader with the background and objectives of the OSN.

Other issues were also addressed at Site 843. Two previous cruises, Leg 128 in the Japan Sea, and Leg 131 to the Nankai accretionary prism, have left long-term monitoring instruments in dedicated boreholes. A new and different type of instrument related to using ODP boreholes for long-time observations was tested during Leg 136: a reentry cone plug designed to seal boreholes for long-term temperature monitoring and fluid sampling. The experiments conducted during these three legs reflect a growing sentiment within the ocean drilling community that oceanic boreholes will play an increasingly important role as resources for future establishment of deep-water observatories.

Coring of the sedimentary and basaltic sequences at Site 843 and its nearby companion, Site 842, provided geological data that are important for several avenues of study. The sediments and basalts at the sites are analogs of the material through which Hawaiian lavas first erupted. Analysis of the chemistry and physical properties of this material will allow determination of the extent to which they contaminate Hawaiian magmas and will shed light on the role that the sediments underlying the volcanic edifices play in the mechanical behavior of Hawaiian volcanoes. Volcanic ash blown downwind from the Hawaiian Islands reflects the history and chemical evolution of the volcanic centers of the island chain. Paleomagnetic and biostratigraphic dating define the past 3.5-Ma history of ash and red clay sedimentation. Preliminary paleontologic dating: (to be followed by isotopic studies) indicates a crustal age of approximately 95 Ma, the first basement age recovered in the region. Penetration 70 m into basement is the deepest in this part of the Pacific. Shipboard analyses indicate that the altered, well-cemented basalts that were recovered include a wide range of MORB compositions, from normal to enriched varieties.