BASALTIC BRECCIAS IN THE UPPER OCEANIC CRUST, HOLE 504B (COSTA RICA RIFT, PACIFIC OCEAN)
AbstractDSDP/ODP Hole 504B is located in the Panama Basin, eastern Pacific ocean, 200 km south of the Costa Rica Rift, which is a 171-km-long east-west trending segment of the Cocos-Nazca Spreading Center. Hole 504B penetrates 2.1 km into 5.9 m.y. old oceanic crust consisting, from the top to the bottom, of pillow lavas (571.5 m) covered by 274.5 m of sediments; a transition zone (209 m), and a lower zone composed of diabasic dikes (1056 m). Basaltic breccia is a significant component of the volcanic section, down to the upper part of the sheeted dike complex. Six main types of breccias were recognized through core and petrographic observations. Only jigsaw-puzzle breccias can be interpreted as tectonic. Although overall pore-fluid pressures slightly lower than hydrostatic were measured at Hole 504B, jigsaw-puzzle breccias may have formed under local conditions of suprahydrostatic fluid pressure, mostly beneath impermeable barriers such as massive basaltic flows. High fluid pressure conditions were favoured by a structural setting allowing the action of a fault-valve. Highly fractured zones identified by core observations and geophysical logs at depths of about 400-550 mbsf and 800-1100 mbsf, respectively, likely correspond to faults, creating the conditions for a fault-valve mechanism. Breccias of the stockwork zone were produced by focused fluid flow associated with a discharge hydrothermal cycle. All the other breccias recognized in the studied section are interpreted as deriving from either in situ fragmentation of basalts or sedimentation of basaltic debris.
How to Cite
Tartarotti, P., & Aligi Pasquaré, F. (2003). BASALTIC BRECCIAS IN THE UPPER OCEANIC CRUST, HOLE 504B (COSTA RICA RIFT, PACIFIC OCEAN). Ofioliti, 28(1), 59-67. https://doi.org/10.4454/ofioliti.v28i1.190