Graduate Thesis Or Dissertation
 

14-SchnurS-Dissertation-TristanAgeSummaries.pdf

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  • The spatial distribution and geologic histories of submarine volcanoes provide insight into submarine eruptive behavior, deep earth processes and plate tectonics. This dissertation examines the evolution of individual submarine volcanic edifices as well as linear trails of seamounts at three spatial and temporal scales. In order to understand constructive and destructive processes at submarine arc volcanoes on time scales of decades I present a study of the eruptive behavior and geology of NW Rota-1 seamount between 2004 and 2014. I use remotely operated vehicle (ROV) dive observations to develop geologic maps of the summit area (< 0.5 km2) in 2009 and 2010, before and after a significant landslide event. I combine these maps with repeat bathymetric surveys and hydrophone recordings to understand how changes in eruptive style affect the geology of the summit area and ultimately control the frequency and magnitude of landslides. At larger spatial and temporal scales, I consider the Tristan da Cunha island group (5,000 km2), which has a geologic history spanning less than 2 million years. Tristan da Cunha is one of the assumed hotspot locations for the Walvis Ridge, the highest-volume hotspot trail in the South Atlantic and one of the longest-lived primary hotspot trails in the world. However, Tristan da Cunha is much younger than surrounding seamounts and does not fit into the age progression of the Walvis Ridge. Based on 26 new 40Ar/39Ar ages I provide an updated maximum age for the island group of 1400.7 ± 8.1 ka and a median age of only 319.5 ± 17.8 ka, confirming that Tristan da Cunha is young and therefore an age outlier relative to nearby seamounts. Upper mantle convection models published by others suggest that plume material is being carried towards Tristan from the true plume center in the vicinity of Gough Island, located about 400 km away, which is often selected as a hotspot location for the Walvis Ridge. I therefore reject Tristan da Cunha as one of the Walvis hotspots and hypothesize instead that the true location of the second eruptive center in the Walvis system is located to the southwest, near Blue Seamount, the youngest seamount in the trail. At the largest scales I consider the 70-million-year history of the 1800 km long Young Walvis Ridge Guyot Province, a diffuse region of volcanism marking the young end of the Walvis Ridge hotspot trail. The Walvis Ridge is important as a key constraint on African plate motion but it is poorly understood due to its broad, bifurcated morphology, which differentiates it from other primary hotspot trails. I present 69 new high-quality 40Ar/39Ar ages for 30 previously-undated seamounts in the Guyot Province to show that this diffuse region of volcanism preserves an age progression, but that the spatial distribution of the seamounts does not always trace plate motion. The placement of seamounts at the young end of the trail is likely controlled by fracture zones extending from the nearby Mid-Atlantic Ridge, while the complex distribution of seamounts in the Guyot Province is a function of lithospheric thickness. The dual eruptive centers simultaneously generate volcanism on younger and older lithosphere, with melts under younger lithosphere rising to the surface directly from the underlying plume and melts under older lithosphere being forced to use fracture zones as conduits. Strikingly, all seamounts formed over the past ~30 million years are fracture zone-oriented, irrespective of lithospheric thickness. This appears to confirm that the Walvis plume is indeed waning, as suggested by a decrease in volume of volcanism along the trail. The diffuse nature of plume expression is likely a result of prevalent mantle flow patterns and proximity to the Mid-Atlantic Ridge, resulting in the drawback of material towards the ridge and transport of material to the northwest, dispersing plume material over a broad area. These three studies represent different perspectives on submarine volcanism and reflect the variety of spatial and temporal scales on which we study submarine volcanoes and seamount trails.
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