none 10.1144/jgs2024-117 Geological Society of London Geological Society of London 1881 178918664 44798 2026071920322700568 10.1144 2026-08-30T15:48:55Z 2025-07-04T06:12:40Z 0 Journal of the Geological Society JGS 0016-7649 2041-479X 08 18 2025 11 03 2025 182 6 Paola Tartarotti Dipartimento di Scienze della Terra, Università degli Studi di Milano Statale, via Mangiagalli 34, 20133 Milano, Italy https://orcid.org/0000-0002-2236-2702 Laura Crispini DISTAV – Dipartimento di Scienze della Terra, Ambiente e Vita, Universita’ di Genova, Corso Europa 26, 16132 Genova, Italy https://orcid.org/0000-0001-5770-8569 Marilena Moroni Dipartimento di Scienze della Terra, Università degli Studi di Milano Statale, via Mangiagalli 34, 20133 Milano, Italy https://orcid.org/0000-0003-1443-1808 Alessandro Proscia Dipartimento di Scienze della Terra, Università degli Studi di Milano Statale, via Mangiagalli 34, 20133 Milano, Italy Melissa Tondo Dipartimento di Scienze della Terra, Università degli Studi di Milano Statale, via Mangiagalli 34, 20133 Milano, Italy Dipartimento di Scienze Chimiche e Geologiche, Università degli Studi di Modena e Reggio Emilia UNIMORE, via G. Campi 103, 41125 Modena, Italy https://orcid.org/0009-0008-4511-1448 The lavas–dykes transition in oceanic crust represents a critical boundary between two portions of the crust with different lithology, structure, porosity, permeability and other physical properties. We refined the structural features of the lavas–dykes transition in ODP/IODP Hole 1256D and DSDP/ODP Hole 504B drilled in in situ oceanic crust created at superfast- and intermediate-spreading rate in the Pacific Ocean. Both holes, although showing some differences in the thickness of lithological sections, have a comparable lavas–dykes transition zone characterized by a wide diffusion of veins, breccias, vein network and cataclasites, possibly with development of pseudotachylytes. Fracturing started with fissuring of basalt already in the ridge axis domain due to thermal contraction of lavas. Fissures triggered the circulation of seawater from above and hydrothermal fluids from below, enhancing alteration of the crust and the mechanical response of the basaltic crust near the lavas–dykes boundary. The high concentration of permeable structures including cataclasites in the lavas–dykes transition in the two holes favour interpreting this crustal interval as a (large-scale?) mechanically weak zone, possibly a fault zone at least for the intermediate-spreading ridge of Hole 504B. The lack of clear kinematic indicators prevents a conclusive interpretation. Other processes that may have led to fracturing include fluid overpressure and dyking. These processes are not mutually exclusive: intense fluid circulation concentrated in the transition zone is responsible for alteration inducing a dramatic weakening of the rock; conversely, rock fissuring due to thermal cracks, dyking and eventually faulting developed permeable structures that may channel the circulation of fluids. Cataclasites occurring at a shallower depth in Hole 1256D and at deeper depth in Hole 504B with respect to those described so far suggest that the extension of the lavas–dykes boundary should be revised by taking into consideration not only the change in lithology but also the structural features. In addition, our finding of chlorite and amphibole at shallower depths with respect to those described so far suggests that the alteration boundary across the lavas–dykes transition is thicker than previously thought, allowing the upflow of high-temperature hydrothermal fluids to shallower levels. 08 22 2025 11 03 2025 jgs2024-117 10.1144/jgs2024-117 1 10.1144/crossmark-policy www.lyellcollection.org true 2024-06-07 2025-05-30 2025-06-02 2025-08-22 Università degli Studi di Milano http://dx.doi.org/10.13039/100012352 PSR2021 https://www.geolsoc.org.uk/publications/lyell-collection/user-license-1-1 https://www.geolsoc.org.uk/publications/lyell-collection/user-license-1-2 10.1144/jgs2024-117 https://www.lyellcollection.org/doi/10.1144/jgs2024-117 https://www.lyellcollection.org/doi/pdf/10.1144/jgs2024-117 https://pubs.geoscienceworld.org/jgs/article/182/6/jgs2024-117/659366/structural-control-on-fluid-circulation-and 10.2973/dsdp.proc.83.103.1985 10.1144/GSL.SP.1990.054.01.04 10.1016/0264-3707(91)90035-D 10.1029/93JB02953 10.2973/odp.proc.sr.137140.025.1995 Initial Reports of the Deep Sea Drilling Project Alt J.C. 217 1985 Alt, J.C., Laverne, C., Muehlenbachs, K. 1985. 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