Exploring fluid flow in the Gulf of Cadiz: from mud volcanoes to transform faults

Abstract

Cold seeps have been identified globally spanning diverse geologic settings and represent important seafloor manifestations of material transfer between geosphere and hydrosphere. The material extrusions give rise to seafloor topographic features that are indicative of focused release of overpressure, fluids, and, in the case of mud volcanoes, sediments. As emitted fluids have experienced various diagenetic processes at depth, their expulsion may play a significant role in global biogeochemical cycles. To gain insights into fluid circulation within cold seep systems, it is imperative to acquire a comprehensive understanding of the fluids’ origin and the geochemical processes that exert an influence on their composition. In this dissertation, the plate boundary region between Africa and Eurasia is studied to identify cold seeps of various origin. The sites investigated span from the Gulf of Cadiz in the west, geodynamically dominated by transform faulting, to the Hellenic subduction zone to the East, a mature subduction system with reverse faulting. Seeping features that can be directly observed at or near the seabed in these regions are manifold, and mud volcanoes (MVs) are the most prominent among them. The first and second manuscripts in this study investigate several MVs in the Gulf of Cadiz, aiming to enhance the understanding of MVs’ morphological characteristics, deep-seated fluid migrations, and salinity patterns. A presentation of new discoveries about mud volcanism through the Gulf of Cadiz accretionary prisms is reported in the first manuscript, covering their morphologies, physical properties and lithological changes in mud breccias, and fluid compositions. The six MVs investigated (already known MVs Yuma, Ginsburg and Meknes and newly discovered MVs R2, D2 and Funky Monkey) are located along different strike-slip fault segments. While two of them are textbook examples of inactive mud domes, the other four MVs attest recent activity in different evolution stages. At active seepage sites, we traced back the origin of venting fluids to clay mineral dehydration and identified pronounced crustal influence and less intense pore water freshening present in the deeper MVs located close to major strike-slip faults (Funky Monkey MV). The second manuscript presents in turn a deeper understanding of fluid circulation within “the MV system” to investigate the relationship between evolving surface characteristics and dynamic subsurface conditions. Two MVs situated in the Gulf of Cadiz (Yuma, Ginsburg) and one MV in the Mediterranean Ridge (Milano) have been studied, which show striking differences in fluid composition between their summit and moat/rim. Through the utilization of fluid geochemistry, pore water modeling, and high-resolution seismic data, this study has identified not only the widely-known central conduits within the MV system, expelling freshening fluids due to clay mineral dehydration, but also the rim-related fluid pathways actively emitting saline fluids resulting from the leaching of evaporites. The chemical and fluid fluxes of 5 MVs have been analyzed and related to the MVs life-cycle (depletion and quiescence periods). The results indicate the involvement of additional (shallow) fluid sources during a later evolutionary stage, particularly when the moat is being developed. Based on the calculated flow rates at the moat area, in the case of the Ginsburg MV, this newly described rim-related fluid circulation significantly contributes to fluid cycling in MVs, especially during long intervals of inactivity. Although this research is confined to the Gulf of Cadiz and the Mediterranean Ridge, the rim-related fluid circulation may have global implications. This enhances our understanding of the coverage and timespan of quiescent fluid seeping and its impact on the global fluid and methane budgets, which indicates the need for further efforts to improve the estimates including not only the summit areas of MVs but also the peripheric systems of these complexes. To gain a more comprehensive insight into fluid seepage in the Gulf of Cadiz, the third manuscript shifts the focus from MVs to transform faults. Its objective is to define the contribution of oceanic transform faults as important settings for the circulation of deep-sourced fluid flow and their influence on global mass balances. The fault fluids are sampled from two different pull-apart basins on the dextral strike-slip faults, as known as the South West Iberian Margin (SWIM) faults defining the Africa-Eurasia plate boundary in the Gulf of Cadiz. Chemical and isotopic composition analysis has been conducted on fault fluids and compared with those from nearby tectonic-controlled cold seep sites including MVs and pockmarks. In contrast to the prevalent freshening signals (Cl depletion) observed in the MV fluids, the geochemical results show a missing signal from clay mineral dehydration in the fault fluids, suggesting an almost absent connection with terrigenous sediments. The geochemical composition of fault fluids is, instead, characterized by ⅰ) considerably high Na and Cl concentrations attributed to intensely interaction with Triassic evaporites at depth; and ⅱ) strong enrichment of less radiogenic Sr, compared to present-day seawater values. When related to pure Triassic evaporitic fluids, the relatively radiogenic 87Sr/86Sr ratios in the fault fluids hint a potential contribution from continental crustal fluids, with modeling indicating up to 22%. This provides insights into the impact of transform faults in focusing crustal signals from the depths, thereby advancing our understanding of the deep hydrologic cycle within transform-type plate boundaries. This thesis offers insights into fluid flow dynamics at seep sites in the Gulf of Cadiz plus a comparison to analogue processes further east across the Mediterranean Ridge. Through the investigation of fluids in mud volcanoes and fault systems, this study reveals: i) their relationship and different mechanisms in upward-channeling deep fluids; ii) the signature of fluids and the potential origin from crustal basement; and iii) the significant role these fluid systems play in influencing fluid cycling and global element budgets.