Deep submarine volcanoes in two geodynamic settings (Canary basin and Gulf of Cadiz)morphology and shallow structure

Resumen

This doctoral thesis involves morphostructural and morphometric studies of diverse hot vent and cold seep seafloor features in two different settings: the Canary Basin and the Gulf of Cadiz, both along the Atlantic margin. The study areas include forty volcanic hydrothermal mounds (M00 to M45) in the central area of the Canary Basin at water depths of between 4800 and 5200 m, nineteen newly inferred mud volcanoes (MV01 to MV19) as well as fifty-eight previously recorded mud volcanoes from various fluid expulsion fields in the Gulf of Cadiz (MV20 to MV77), with special focus on five mud volcanoes on the Moroccan Margin located at water depths of between 300 and 1800 m (Henriet, Subvent, Chueca, Demetrio de Armas, and Puerto Real). The main datasets were obtained as part of the SUBVENT project and comprise principally bathymetric and backscatter mosaics from multibeam echosounder systems and both high resolution parametric and seismic profiles from parametric echosounders, as well as seismic systems. Several bathymetric mosaics from other research projects have also been used in addition to sediment core and water samples that support the main geophysical datasets. The morphostructural studies are aimed at the detailed characterisation of the various seafloor edifices in both study areas, based on their morphological features and subsurface structures in order to determine their relationships with recent geological, sedimentary, and oceanographic processes and establish the dominant mechanisms controlling their morphological evolution. These seafloor structures are frequently affected by morphological signs of fluid venting (craters, mud outflows), mass movements (slide scars and mass transport or mud lobe deposits), bottom currents (contourite moats and plastered and mounded contourite drifts), and tectonics (bulge-like structures clearly deformed by faulting). Some of these seafloor edifices have been discovered recently and are reported for the first time in this thesis. The main aims of the morphometric studies were the comprehensive and systematic classification of the total population of seafloor edifices in each study area (forty volcanic mounds and fifty-eight mud volcanoes), into different morphometric types based on the main size, slope and shape variables, according to the bathymetric mosaics, in addition to principal component analysis (PCA) and clustering statistical methods. The volcanic mounds have been categorised into five edifice types (Morphostructural Types 1 to 5), with distinctions being made between the diverse domes and volcanoes, while the mud volcanoes are classified into four morphometric groups (Groups 1 to 4), generating a better understanding of the shaping processes controlling the edifice morphologies. In the Canary Basin, the dominant processes behind the formation and morphological evolution of the forty mounds are the different intrusive and extrusive processes related to magmatic and hydrothermal systems, recent tectonic activity and flank instabilities, predominantly affecting the volcanoes, while deep bottom currents play a minor role in shaping these edifices. The classification of mounds has enabled distinctions to be made between domes (Morphostructural Types 1 to 3) and volcanoes (Morphostructural Types 4 and 5), which present linear correlation between their height and slope variables, but not between height and basal size. The evolutionary model takes into account both the size and shape complexity of these seafloor edifices considering that growth occurs in two distinct ways: volcanoes predominantly grow upwards, becoming large cones; while domes preferentially increase in volume through enlargement of the basal area. Furthermore, these studies have allowed us to identify and characterise a new type of geomorphic feature: the intrusive domes. These have only been reported from fossil and shallow environments but might extend to other oceanic areas. In the Gulf of Cadiz, the main key factors controlling the morphological features of the mud volcanoes are mud extrusion and further fluid seepage, and the interaction of bottom currents with the seafloor and the associated sediment deposits. The five recently discovered mud volcanoes present constraining morpho-sedimentary structures and seep activity: Henriet, Subvent, and Chueca MVs are in an active phase of mud flow, with morphological characteristics related to recent mud volcano formation and seepage; while Demetrio de Armas and Puerto Real MVs are in a latent phase, with reduced activity and diffuse fluid flow. The morphometric classification of the overall population of mud volcanoes distinguishes between the small, smooth conical MVs constituting Group 1; the intermediate, steep, irregular MVs in Group 2; the very high, steep, circular MVs with composite summits comprising Group 3; and the smallest, smooth, oval and irregular MVs of Group 4. The wide variability of mud volcanoes is related to different geological and oceanographic conditions, considered in the evolutionary model based on different growth stages: conical edifices are characterised by a continuous volume-height relationship, inverse to shape, following two distinct pathways: they increase their size becoming large cones and decreasing their conicality (enlarging their plan shape); or they significantly increase their size, maintaining their conicality and becoming the highest cones. Large irregular edifices have not been documented. Instead, the smallest MV edifices are the most irregular. The comprehensive study of the morphostructural characteristics and morphometric variables, in addition to the geological and oceanographic shaping processes in the different geodynamic settings, has allowed us to establish the main differences in the origin and formation of the various types of hot and cold vent seafloor edifices, as well as their development and morphological evolution. Intrusive domes are morphologically quite uniform being less affected by modifying processes in contrast to the extrusive hydrothermal volcanoes and mud volcanoes, which are commonly affected by both, erosive and depositional processes.