LEG 111

Costa Rica Rift

Technical difficulties associated with deep penetration hampered earlier DSDP attempts to document the lithostratigraphy, alteration history, and geophysical properties deep within the oceanic crust and to test the validity of the inferred analogous relationship to ophiolites. Hole 504B, located in 5.9-Ma crust approximately 200 km south of the Costa Rica Rift, was a unique exception, where DSDP Legs 69, 70, and 83 cased through 274.5 m of sediment and cored 1075.5 m of the basement; 571.1 m of pillow lavas and minor flows of oceanic layers 2A and 2B underlain by a 209-m zone of transition into 295 m of sheeted dikes and massive units of layer 2C.

During Leg 111, Hole 504B was deepened by 212.3 m to a total depth of 1,562.3 mbsf (1,287.8 m into basement), recovering slightly altered, phyric to highly phyric, fine- to medium-grained olivine tholeiitic basalts, chemically similar to basalt recovered from the shallower basement DSDP cores. Deep in the hole, the temperature gradient is linear, decreasing from 116ūC/km in the pillow lavas to 61ūC/km in the dikes. The chemical composition of sampled borehole waters is apparently controlled by vertical convection in the borehole and exchange of borehole water with the ocean bottom water that flows downhole into the upper 100-200 m of basement. Hole 504B was also logged with an extensive set of tools and, when calibrated against the properties of the recovered basalt, yielded a nearly continuous geophysical, geochemical, and lithological characterization of the basement, documenting that alteration products are tightly confined to fractures along boundaries between individual extrusive or intrusive events and that the boundary between pillow lavas and dikes is a relic of early listric faulting between pillows over the dikes in the rift valley.

Sediment coring was also conducted at local heat-flow maximum and minimum sites near Hole 504B (Sites 678 and 677, respectively) to enable high-resolution studies of the Plio-Pleistocene biostratigraphy and chemical studies of the pore waters to differentiate advective from diffusive exchange between the ocean-bottom water and the basement through the sediment cover. The profiles of sediment pore-water composition versus depth differ greatly between these sites and indicate that ocean-bottom seawater flows down through the 300-m-thick sediment into basement at the low heat-flow site, whereas significantly altered seawater formed in basement upwells through the 180-m-thick sediment into overlying seawater at the high heat-flow site; rates of flows are estimated at a few mm/yr at both sites. The similarity in pore-water composition from basal alteration products at both sites suggests that the advective flow rates in sediment are negligible compared to those in basement.