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Geological Society, London, Special Publications; 1990; v. 54; p. 41-50;
DOI: 10.1144/GSL.SP.1990.054.01.04
© 1990 Geological Society of London

Fracture and Faulting

Fracture evolution in the upper ocean crust: evidence from DSDP hole 504B

Susan M. Agar

Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK
Geological Sciences Department, Northwestern University, Evanston, Illinois 60208, USA

Detailed structural studies of ocean crust specimens from DSDP borehole 504B have been integrated with a well defined alteration history to yield fracture evolution paths during movement away from the mid-ocean ridge. Fracturing was the dominant deformation process in the upper ocean crust of 504B. A broad range of fracture types exists. The type and orientation of most early fractures were related to lithology. Late fracture orientations and intensities were less dependent on lithology and may have been more closely related to tectonic stress conditions. Prolonged deformation histories are evident in the transition zone where non-coaxial deformation overprints successive generations of dilational veins. Changes in the orientation of the local stress field between successive generations of fractures occurred between and within lithological units. This suggests the presence of numerous mechanical discontinuities in the upper ocean crust, particularly in the transition zone. Changes in fracture porosity between alteration stages were often large and, together with changes in the characteristics of fractures, would have influenced local thermal gradients. The upper ocean crust rheology would have been extremely time dependent as a result. Zones of weakness were created by clay alteration products in basalts and were located in the most permeable horizons. Movement on these zones was promoted by elevated pore-fluid pressures with major failure most likely during major upwelling associated with a hydrothermal system. Subsequent precipitation of high temperature minerals from circulating fluids may have strengthened permeable horizons causing migration of detachment surfaces to another level.





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