Kwasnitschka, Tom (2012) Evolutionary Stages of Submarine Volcanism in the Cape Verdean Archipelago. (PhD/ Doctoral thesis), Christian-Albrechts-Universität , Kiel, 102 pp.

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Abstract

During most of their development, volcanic ocean islands are hidden beneath the sea, either as seamounts that have not or not yet reached the surface, or as guyots that have been eroded and subsided. The Cape Verdean Archipelago offers the unique opportunity to study the development of seamounts under the condition of a quasi stationary oceanic plate over a continuous magmatic source in the upper mantle, meaning that the volca-noes do not automatically subside with increasing crustal age and they are not carried away from their magmatic source due to plate motion. As a result, no linear island chain forms and new volcanoes develop in the direct vicinity of old ones.
This thesis studies the evolutionary stages of Cape Verdean seamounts based on their morphology using multi beam swath bathymetry of RRS CHARLES DARWIN cruise 168 and R/V METEOR cruises 63/2, 79/3 and 80/3. Rock samples from dredging during R/V POSEIDON cruise 320/2 as well as dredging and ROV deployment during and R/V METEOR cruises M79/3 and M80/3 complement this bathymetric data.
Eleven seamounts were studied with respect to their geometry of volcanic linea-ments, distribution of satellite cones along the flanks, size of their volcanic shield, tec-tonic features, mass wasting events and the presence and shape of summit plateaus in-cluding wave cut surfaces. Two seamounts were described for the first time: Sodade Seamount is a young pillow volcano in the western projection of the northern island chain, whereas Tavares Seamount may form the continuation of the southern chain. The distribution and various stages of development of submarine volcanoes reveal that the progression of volcanic activity on the western ends of the island chains is discontinu-ous. The summits of Nola Seamounts, actually a cluster of two intergrown seamounts, were eroded by exposure above sea level during glacial low stands, arguing for an ex-tended period of inactivity. Yet they are surrounded by morphologically young or re-cently active volcanoes and seamounts to the west, south and east. Brava Island and the Ilhéus Secos on the western end of the southern chain are in an erosional stage, yet both Cadamosto Seamount to the south and Fogo Island to the east show recent activity, while Tavares Seamount to the west is likely to have been inactive for an extended peri-od.
Maio Seamount forms a solitary extinct edifice stuck in an early stage of seamount development. Maio Rise and Boavista seamount developed on island flanks and are heavily affected by mass wasting, possibly due to their inclined basement. Cabo Verde and Senghor Seamounts are large solitary edifices to the east of the archipelago. Cabo Verde is more affected by mass wasting, whereas Senghor shows a large degree of verti-cal tectonics possibly connected with central subsidence of a caldera system. It features a flat summit plateau rising as a Somma-type edifice on top of the volcanic shield to 94m bsl. Both Nola East (58m bsl) and Nola West (35m bsl) have more residual summit relief than Senghor, arguing for a shorter period of exposure. All wave cut surfaces are at-tributed to glacial sea level low stands up to -122m during the past 640 ka. Due to the lack of age dates for these plateaus and missing bathymetric coverage of the shelf be-tween Maio and Boavista it is not possible to correlate their vertical position or tempo-rally constrain their formation more precisely.
Detailed studies of incipient volcanic activity were conducted within the Charles Darwin volcanic field (CDVF) of solitary and compound cones on the western flank of Santo Antão. Two structures featuring large crater like central depressions were sur-veyed. Tambor cone is composed of dense, highly fragmented volcaniclastics showing signs of thermal and viscous granulation. Two types of deposits were discriminated based on grain size distributions visible on video and imagery recorded during ROV sur-veys, with maximum clast sizes in the pebble and block range, respectively. The two de-posit types alternate across four units that can only tentatively be correlated due to lat-erally highly varying deposit thickness attributed to a pulsing, debris jet like eruption style. Tangential antithetic joints along the crater wall and a collapse scar on the eastern sector suggest subsidence of the crater floor.
Kolà crater shows a negative relief cut into a group of small cones and a step-wise morphology of the inner crater walls. The lower portion is formed by steep cliffs of a lava flow penetrated by the eruption. Volcanic deposits are highly fragmented. Juvenile components include pebble sized sub-spherical dense lava clasts with concentric align-ments of crystals and quenched rims. Fragments of lava country rock likely originate from the basal lava flow. Coarse crystalline gabbro blocks are extremely well rounded, likely due to abrasion upon fluidization in the vent during the eruption. Laterally discon-tinuous and mutually capping beds of these components suggest a highly effective lat-eral transport mechanism. Four units that can be correlated well around the crater due to significant changes in lithology. Morphology and deposits described at Kolá suggest analogies to subaerial diatremes.
The detailed study of the Tambor and Kolá volcanoes was made possible by the de-velopment of a remote sensing technique based on the photogrammetric reconstruction of outcrop geometries including full color photo mosaicking from high definition video sequences recorded by the ROV Kiel 6000 on site. The models were corrected and georeferenced to a vehicle track synthesized from a Doppler velocity log (DVL) and ultra short baseline (USBL) positioning system in combination with orientation and pressure sensors on the vehicle. This workflow not only improved the qualitative study of depos-its based on 3D models of outcrops, but also facilitated the quantitative measurement of bedding and joint orientations as well as the distribution of grain size fractions visible in the source video. This approach provides a new level of productivity in robotic deep sea exploration.

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