Structure and tectonics of intermediate-spread oceanic crust drilled at DSDP Holes 504B and 896A, Costa Rica rift
Dilek, Y. (1998). Structure and tectonics of intermediate-spread oceanic crust drilled at DSDP Holes 504B and 896A, Costa Rica rift, in: Cramp, A. et al. Geological evolution of ocean basins: results from the Ocean Drilling Program. Geological Society Special Publication, 131: pp. 179-197. https://dx.doi.org/10.1144/GSL.SP.1998.131.01.12
In: Cramp, A. et al. (1998). Geological evolution of ocean basins: Results from the Ocean Drilling Program. Geological Society Special Publication, 131. Geological Society: London. ISBN 1-86239-003-7. XI, 323 pp., more
In: Hartley, A.J. et al. (Ed.) Geological Society Special Publication. Geological Society of London: Oxford; London; Edinburgh; Boston, Mass.; Carlton, Vic.. ISSN 0305-8719; e-ISSN 2041-4927, more
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| Keywords |
Characteristics > Structure
Drilling
Earth sciences > Geology > Tectonics
Earth structure > Earth crust > Oceanic crust
Structures
Costa Rica [Marine Regions]
Marine/Coastal |
| 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. |
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