Structure and Tectonics of Intermediate-Spread Oceanic Crust Drilled at DSDP/ODP Holes 504B and 896A, Costa Rica Rift

Abstract

Abstract Deep Sea Drilling Project/Ocean Drilling Program (DSDP/ODP) Hole 504B, situated on the southern flank of the intermediate-spreading Costa Rica Rift in the eastern equatorial Pacific, penetrates 2.1 km into 5.9 Ma old oceanic crust and provides an in situ reference section for the physical and chemical structure of the upper oceanic lithosphere. Hole 896A, located on a basement high nearly 1 km south of Hole 504B, penetrates 290 m into the upper volcanic sequence, and together with Hole 504B provides an opportunity to examine variations in basement lithostratigraphy and structure. The sea-floor bathymetry in the vicinity of Holes 504B and 896A is defined by east-west trending asymmetric elongated ridges and troughs with wavelengths of 5–6 km and relief of 100–150 m. Hole 504B penetrates two major fault zones at about 800 metres below sea floor (mbsf) in the lower volcanic rocks and at 2111 mbsf in the lower dykes. The fault zone at 800 mbsf coincides with the lithological boundary between the volcanic sequence (layer 2B) above and the transition zone (pillow and massive lava flows, dykes) below, separates contrasting domains of magnetic properties, and marks a south-dipping (outward-facing) normal fault. The fault zone at 2111 mbsf in the bottom of the hole represents a dip-slip fault defined by closely spaced east-northeast striking microfractures with steep dips and steeply plunging lineations. The dyke margins that have been reoriented palaeomagnetically have strikes subparallel to the ridge axis of the Costa Rica Rift and steep dips towards both the north (inward-dipping) and the south (outward-dipping). The main deformation of the dyke complex has been fracturing and veining as suggested by the analyses of core samples and geophysical downhole measurements. Chlorite and/or actinolite veins represent extension fractures and form two orthogonal vein sets. E-W to ESE-WNW striking, steeply dipping extensional veins are commonly parallel to the dyke margins and to the orientation of the Costa Rica Rift axis; N-S to NNE-SSW striking veins are dyke-orthogonal, probably representing thermal contraction cracks. The apparent increase in grain size of the diabasic dyke rocks around 2 kmbsf coincides with an increase in abundance of actinolite at the expense of clinopyroxene and plagioclase, and a sharp decrease in the occurrence of chlorite in the core. These changes in the core mineralogy are accompanied by a steady increase in compressional wave velocity, which reaches a value of 6.8 km s −1 within the sheeted dyke complex nearly 1.4–1.6 km into the basement. The boundary between seismic layers 2 and 3 thus corresponds to changes in physical properties of the rocks, rather than a lithological boundary, and occurs over a finite depth interval within the sheeted dyke complex. The upper oceanic crustal rocks recovered from Hole 896A include massive and pillow lava flows with interlayered breccias. An intensely fractured zone that coincides with sharp changes in acoustic velocity, electrical resistivity and magnetic properties in the volcanic strata occurs at 360 mbsf and marks a fault zone. The thickness of the volcanic strata, which have the same lithostratigraphy and magnetic properties, increases from Hole 896A in the south to Hole 504B in the north, suggesting off-axis ponding of lava(s) in the vicinity of Hole 504B. The sea-floor bathymetry defined by linear hills and intervening troughs in the vicinity of Sites 504 and 896 is interpreted to reflect a basement topography defined by fault-bounded asymmetric volcanic hills that developed within the crustal accretion zone in the spreading environment; it is inferred that the volcanic section drilled into Hole 896A represents one of these abyssal hills in the region