Geological mapping of the Menez Gwen segment at 37°50′N on the Mid-Atlantic Ridge: Implications for accretion mechanisms and associated hydrothermal activity at slow-spreading mid-ocean ridges

Highlights • Systematic analysis of ship bathymetry enables segment scale geological mapping. • The Menez Gwen segment experiences magmatic periods every 300 to 500 ka. • Periods of enhanced magmatic activity are a regional phenomenon. • Hydrothermalism at Menez Gwen accompanies a waning, intense magmatic period. • Faulting and along-axis permeability variations focus hydrothermal venting. Abstract Slow-spreading mid-ocean ridges have the potential to form large seafloor massive sulphide (SMS) deposits. Current exploration for SMS deposits commonly targets associated active hydrothermal venting on the ridge axis, which makes the discovery of inactive vent sites and SMS deposits in the off-axis regions unlikely. Geological maps of the seafloor, which help understand the timing and location of SMS formation, usually focus on individual hydrothermal vent sites and their immediate surroundings, and are often too small to aid in SMS exploration. This study uses ship-based multibeam echosounder (MBES) data and a systematic classification scheme to produce a segment-scale geological map. When combined with spreading rate, this allows us to not only reconstruct the segment's spreading history, but also reveals important processes that localize hydrothermal venting. Geological mapping around two known hydrothermal vent sites on the Menez Gwen segment at 37°50′N on the slow-spreading Mid-Atlantic Ridge showed that hydrothermal venting accompanies the tectonic break-up of a large, cooling magmatic body. Venting is focussed by faulting and resulting permeability changes. The large magmatic body is associated with an axial volcano that formed as a last stage of a period with intense magmatic accretion. Such magmatic accretion periods occur every 300 to 500 ka at the Menez Gwen segment, with increasing intensity over the past 3.5 Ma years. The most recent, most intense magmatic period appears to be a regional phenomenon, also affecting the neighbouring Lucky Strike and Rifted Hills segments. Understanding the accretional setting and the spatial and temporal constraints of hydrothermal venting enables us to develop criteria in MBES data to aid exploration for inactive SMS deposits

Detachment tectonics at Mid-Atlantic Ridge 26°N

Spreading processes associated with slow-spreading ridges are a complex interplay of volcanic accretion and tectonic dismemberment of the oceanic crust, resulting in an irregular seafloor morphology made up of blocks created by episodes of intense volcanic activity or tectonic deformation. These blocks undergo highly variable evolution, such as tilts or dissection by renewed tectonic extension, depending on their positions with respect to the spreading axis, core complexes, detachment or transform faults. Here, we use near-seafloor magnetic and bathymetric data and seismic profiles collected over the TAG Segment of the Mid-Atlantic Ridge to constrain the tectonic evolution of these blocks. Our study reveals that the presence and evolution of oceanic core complexes play a key role in triggering block movements. The deep subvertical detachment fault roots on the plate boundary, marked by a thermal anomaly and transient magma bodies. Thermal and magmatic variations control the structure and morphology of the seafloor above the subhorizontal detachment surface, occasionally leading to relocating the detachment

Structural control, evolution, and accumulation rates of massive sulfides in the TAG hydrothermal field

The Trans‐Atlantic Geotraverse (TAG) hydrothermal field on the Mid‐Atlantic Ridge is one of the best‐studied hydrothermal systems to date. However, high‐resolution bathymetric data obtained in 2016 by an autonomous underwater vehicle (AUV) reveal new information about the distribution of active and inactive hydrothermal deposits, and their relation to structural features. The discovery of previously undocumented inactive vent sites contributes to a better understanding of the accumulation rates and the resource potential of seafloor massive sulfide deposits at slow‐spreading ridges. The interpretation of ship‐based and high‐resolution AUV‐based data sets allowed for the determination of the main tectonic stress regimes that have a first‐order control on the location and distribution of past and present hydrothermal activity. The data reveal the importance of cross‐cutting lineament populations and temporal variations in the prevalent stress regime. A dozen sulfide mounds contribute to a substantial accumulation of hydrothermal material (~29 Mt). The accumulation rate of ~1,500 t/yr is comparable to those of other modern seafloor vent fields. However, our observations suggest that the TAG segment is different from many other slow‐spreading ridge segments in its tectonic complexity, which confines sulfide formation into a relatively small area and is responsible for the longevity of the hydrothermal system and substantial mineral accumulation. The inactive and weakly active mounds contain almost 10 times the amount of material as the active high‐temperature mound, providing an important indication of the global resource potential for inactive seafloor massive sulfide deposits

AUV Abyss workflow: autonomous deep sea exploration for ocean research

Autonomous underwater vehicles (AUVs) equipped with multibeam echosounders (MBES) are essential for collecting high-resolution bathymetric data in the deep sea. Navigation of AUVs and accuracy of acquired MBES data is challenging, especially in deep water or rough terrain. Here, we present the AUV Abyss operational workflow that uses mission planning together with a long baseline (LBL) positioning network, and systematic post-processing of the MBES data using feature matching. The workflow enables autonomous exploration even in difficult terrain, makes ultrashort baseline navigation during the AUV survey obsolete and with this, increases the efficiency of ship time. It provides an efficient workflow for multi-survey mapping campaigns to produce high-resolution, large-coverage seafloor maps. Automated documentation of post-processing steps enhances the archiving of produced results, facilitates knowledge transfer, adaptation to other systems and management of large datasets. Comprehensive documentation allows developing routines that provide a first step towards automatization of AUV operations and MBES data processing

Structural controls on hydrothermal systems on slow- and intermediate-spreading mid-ocean ridges

This thesis presents workflows and methodologies for geological mapping of the ocean floor at different scales ranging from ship-based surveys to collection and interpretation of high-resolution data using autonomous underwater vehicles. Systematic approaches for interpretation of common remote sensing data of MORs are developed with the aim of assessing the geology and the key geological controls on hydrothermal systems. Examples are presented from 1) a regional-scale study of the Rodriguez Triple Junction in the Indian Ocean, 2) multi-scale mapping around hydrothermal occurrences in an area of the central Indian Ocean, and 3) site-specific mapping around 32 hydrothermal occurrences in the Indian Ocean. Geological mapping reveals larger SMS deposits occur >8 km from the MOR axis, where long-lasting hydrothermal circulation can be sustained by relatively stable crust and deeply rooted faults. Six distinct settings that host hydrothermal venting are identified in association with specific types of MOR crust: 1) off-axis volcanic fields, 2) axial shield volcanoes, 3) rift valleys, 4) tectonic massifs, 5) oceanic core complexes, and 6) shear-zone settings. This refines previous classifications of the settings of MOR hydrothermal vents as either ‘magmatic’ or ‘tectonic’ with additional interpretations of favourable geological formations and structures that enable targeted exploration for SMS deposits, where only remote sensing data are available.Diese Dissertation präsentiert Arbeitsabläufe und Methoden für das geologische Kartieren des Meeresbodens über Maßstabsgrenzen hinweg, von schiffsbasierten Untersuchungen bis hin zur Sammlung und Interpretation von hoch-auflösenden Daten mittels autonomer Unterwasserfahrzeuge. Der systematische Ansatz bei der Interpretation typischer Fernerkundungsdaten von MOR ermöglicht es, die Geologie und Schlüsselprozesse an bekannten Hydrothermalvorkommen zu erfassen. Dies wird anhand von Beispielen gezeigt, wie 1) der Studie der „Rodriguez Triple Junction“ in regionalem Maßstab, 2) einer maßstabsübergreifenden Kartierung an ausgewählten Hydrothermalvorkommen im zentralen Indischen Ozean, sowie 3) der Analyse von insgesamt 32 Hydrothermalvorkommen im Indischen Ozean im lokalen Maßstab. Größere Hydrothermalvorkommen befinden sich in mehr als 8 km Entfernung zur MOR-Achse, wo relativ stabile Ozeankruste und tiefgreifende Störungszonen eine langfristige hydrothermale Zirkulation begünstigen. Insgesamt konnten sechs verschiedene Arten von Ozeankruste definiert werden, die hydrothermale Zirkulation begünstigen: 1) das abseits der Rückenachse gelegene vulkanische Feld, 2) der axiale Schildvulkan, 3) der zentrale MOR-Graben, 4) das tektonische Massiv, 5) der ozeanische Kernkomplex und 6) die Scherzonen-Umgebung. Diese Einteilung verfeinert die bisher geltende Unterscheidung von „magmatischen“ und „tektonischen“ Vorkommen und ergänzt die Klassifizierung um konkrete lithologische Begebenheiten, Grenzwerte und Strukturen

Structural controls on hydrothermal systems on slow- and intermediate-spreading mid-ocean ridges

This thesis presents workflows and methodologies for geological mapping of the ocean floor at different scales ranging from ship-based surveys to collection and interpretation of high-resolution data using autonomous underwater vehicles. Systematic approaches for interpretation of common remote sensing data of MORs are developed with the aim of assessing the geology and the key geological controls on hydrothermal systems. Examples are presented from 1) a regional-scale study of the Rodriguez Triple Junction in the Indian Ocean, 2) multi-scale mapping around hydrothermal occurrences in an area of the central Indian Ocean, and 3) site-specific mapping around 32 hydrothermal occurrences in the Indian Ocean. Geological mapping reveals larger SMS deposits occur >8 km from the MOR axis, where long-lasting hydrothermal circulation can be sustained by relatively stable crust and deeply rooted faults. Six distinct settings that host hydrothermal venting are identified in association with specific types of MOR crust: 1) off-axis volcanic fields, 2) axial shield volcanoes, 3) rift valleys, 4) tectonic massifs, 5) oceanic core complexes, and 6) shear-zone settings. This refines previous classifications of the settings of MOR hydrothermal vents as either ‘magmatic’ or ‘tectonic’ with additional interpretations of favourable geological formations and structures that enable targeted exploration for SMS deposits, where only remote sensing data are available.Diese Dissertation präsentiert Arbeitsabläufe und Methoden für das geologische Kartieren des Meeresbodens über Maßstabsgrenzen hinweg, von schiffsbasierten Untersuchungen bis hin zur Sammlung und Interpretation von hoch-auflösenden Daten mittels autonomer Unterwasserfahrzeuge. Der systematische Ansatz bei der Interpretation typischer Fernerkundungsdaten von MOR ermöglicht es, die Geologie und Schlüsselprozesse an bekannten Hydrothermalvorkommen zu erfassen. Dies wird anhand von Beispielen gezeigt, wie 1) der Studie der „Rodriguez Triple Junction“ in regionalem Maßstab, 2) einer maßstabsübergreifenden Kartierung an ausgewählten Hydrothermalvorkommen im zentralen Indischen Ozean, sowie 3) der Analyse von insgesamt 32 Hydrothermalvorkommen im Indischen Ozean im lokalen Maßstab. Größere Hydrothermalvorkommen befinden sich in mehr als 8 km Entfernung zur MOR-Achse, wo relativ stabile Ozeankruste und tiefgreifende Störungszonen eine langfristige hydrothermale Zirkulation begünstigen. Insgesamt konnten sechs verschiedene Arten von Ozeankruste definiert werden, die hydrothermale Zirkulation begünstigen: 1) das abseits der Rückenachse gelegene vulkanische Feld, 2) der axiale Schildvulkan, 3) der zentrale MOR-Graben, 4) das tektonische Massiv, 5) der ozeanische Kernkomplex und 6) die Scherzonen-Umgebung. Diese Einteilung verfeinert die bisher geltende Unterscheidung von „magmatischen“ und „tektonischen“ Vorkommen und ergänzt die Klassifizierung um konkrete lithologische Begebenheiten, Grenzwerte und Strukturen