Abstract
We report on newly discovered mud volcanoes located at ~4500 m water depth ~90 km west of the deformation front of the accretionary wedge of the Gulf of Cadiz, and thus outside of their typical geotectonic environment. Seismic data suggest that fluid flow is mediated by a >400-km-long strike-slip fault marking the transcurrent plate boundary between Africa and Eurasia. Geochemical data (Cl, B, Sr, 87 Sr/ 86 Sr, d 18 O, dD) reveal that fluids originate in oceanic crust older than 140 Ma. On their rise to the surface, these fluids receive strong geochemical signals from recrystallization of Upper Jurassic carbonates and clay-mineral dehydration in younger terrigeneous units. At present, reports of mud volcanoes in similar deep-sea settings are rare, but given that the large area of transform-type plate boundaries has been barely investigated, such pathways of fluid discharge may provide an important, yet unappreciated link between the deeply buried oceanic crust and the deep ocean.
Highlights
Fluid seepage and mud volcanism are common at active and passive continental margins (Kopf, 2002); typical driving mechanisms are (1) rapid sedimentation in combination with compaction and tectonic stress, (2) intrusive processes like salt diapirism, (3) dewatering of hydrous minerals, and (4) formation of hydrocarbons. These factors are met in the Gulf of Cadiz, where several kilometer-thick Mesozoic to Holocene sediments accumulated in an accretionary wedge, hosting numerous mud volcanoes (MVs) preferentially at fault intersections (Fig. 1; Magalhães et al, 2012)
Our findings confirm that seismogenic strikeslip faults provide pathways for deep-seated flu
ATI MV Porto MV Bonjardim MV / CRMV CAMV / Mercator MV Present-day seawater fluid compositions. Such a scenario is in compliance with the evidence for crustal alteration in MV fluids in the Gulf of Cadiz (Scholz et al, 2009)
Summary
Fluid seepage and mud volcanism are common at active and passive continental margins (Kopf, 2002); typical driving mechanisms are (1) rapid sedimentation in combination with compaction and tectonic stress, (2) intrusive processes like salt diapirism, (3) dewatering of hydrous minerals, and (4) formation of hydrocarbons. The fluid composition of the proximal MVs is close to the suggested endmember of clay-mineral dewatering, while that of ATI MVs shows a strong imprint of recrystallization of Upper Jurassic carbonates (Fig. 3). The distal, non-ATI MVs plot within the binary mixing field of “clay” and “crust”, suggesting a negligible influence of carbonate recrystallization there This interpretation is in line with stratigraphic evidence for the subsurface extension of Upper Jurassic sediments (Fig. 4). Dehydration, are typical for mineral hydration processes such as the alteration of volcanic ash or oceanic crust (Gieskes and Lawrence, 1981) This trend is hardly visible at ATI MVs, likely due to the strong imprint of carbonate recrystal-. Examples for fluid convection are mainly reported from the eastern Pacific, where pore fluids circulate through interconnected seamounts in young oceanic crust (Fisher et al, 2003)
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