LEG 136
Hawaiian Arch

During Leg 136, a seafloor site was prepared for future experiments to develop the Ocean Seismograph Network (OSN). Distribution of land-based seismic stations are inadequate to study the Earth with uniform resolution and the primary objective of ODP was to drill and case a reentry hole into basement to provide a site for the future emplacement and testing of a broad-band ocean seismometer, the first site of the OSN. This initial task was completed with the drilling of Hole 843, approximately 225 km southwest of Oahu. The hole was drilled through 242 m of sediment and 70 m into the basaltic basement. The long-term (5-10 year) goal of the OSN is to establish a global network of 15 to 20 permanent seismic observatories in the deep ocean. Such a network would 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 sources and propagation mechanisms.

Other issues were also addressed at Site 843. During Leg 128 in the Japan Sea and Leg 131 to the Nankai accretionary prism, long-term monitoring instruments in dedicated boreholes were established. During Leg 136, a new type of instrument was successfully tested, a reentry cone plug designed to seal boreholes for long-term temperature monitoring and fluid sampling. The assembly is deployed and retrieved by JOIDES Resolution, but fluids and data are accessible by ROV, submersible, or wireline reentry vehicle, establishing the ODP boreholes as natural laboratories.

Coring of the sedimentary and basaltic sequences at Sites 843 and 842 provided geological data important for the study of the Pacific Basin. These sediments and basalts are analogs of the material through which Hawaiian lavas first erupted. The upper 15 m of ash-rich, red clay sediments at Site 842 spans approximately 3.5 m.y. of deposition. Distinct ash layers between 1 and 3 Ma, probably the result of explosive volcanism related to the volcanic centers on the islands of Maui and Oahu, possibly mark the emergence of volcanic centers from the sea. In contrast to the normally nonexplosive style of Hawaiian eruption, activity near the sea surface is marked by violent explosions and production of abundant ash. Ash layers in middle Miocene-late Eocene clay- rich sediments, deposited when Site 842 was too far south to receive significant input from the Hawaiian hot spot, may record as yet unknown widespread central Pacific volcanic events.