Initial Reports of the Deep Sea Drilling Project - Volume 69

This volume covers Leg 68 (Site 501), Leg 69, and Leg 70 (Site 504) of the cruises of the Drilling Vessel Glomar Challenger. These initial reports present the results of drilling and downhole experiments conducted at Sites 501, 504, and 505, which are on the south flank of the Costa Rica Rift. The sites were drilled during Deep Sea Drilling Project Leg 69 and parts of Legs 68 and 70 between July and December of 1979. The purpose of the drilling was to investigate geothermal phenomena in basaltic basement and overlying sediments at two locations in the same segment of an oceanic spreading center—the Costa Rica Rift. Drilling was only part of the scientific program at the Costa Rica Rift. The chemistry of the interstitial waters was extensively investigated to evaluate reactions within the sediments and the influence of the formation of secondary minerals in the basement. The drilling program that is the focus of this volume was undertaken to investigate a region of contrasting geothermal and hydrological regimes in relatively young oceanic crust (3.9-5.9 m.y. old). In the northern part of the region numerous outcrops of basement permit the free circulation of seawater through the upper crust, keeping the basement at relatively low temperatures. This type of thermal regime is common along the relatively young flanks of mid-ocean ridges. One hundred kilometers to the south the ocean basement is almost completely covered by a 250- to 300-meter layer of sediment that caps the crust with an impermeable blanket and prevents the direct exchange between bottom water and basement pore water. Here, all of the heat from the cooling lithosphere must flow conductively through the sediment. The level of flux is about 200 mW/m2, which results in a temperature drop of 50 to 60°C across the sedimentary layer. The relatively high temperatures in the crust promote the alteration of the igneous basaltic rocks and the cementation of fractures. This type of thermal regime, which is found where high sedimentation rates lead to the fast burial of basement topography, is rare. One of the major objectives of the program was to drill into basement in a young high temperature crust.