University of Tasmania
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Complex volcanic architecture produced by basaltic submarine and emergent volcanism in intraplate settings

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posted on 2023-05-28, 10:03 authored by Jodi FoxJodi Fox
The physical volcanology of intraplate basaltic submarine and submarine to subaerial volcanic sequences is poorly understood despite most volcanism on Earth being basaltic and occurring within oceans. Direct observation of active intraplate basaltic submarine volcanism is expensive and logistically difficult. Progress in understanding the products and processes of submarine and submarine to subaerial basaltic intraplate volcanism is dependent on the study of ancient successions. Basaltic submarine volcanic processes are similar regardless of tectonic setting but the environment of eruption and deposition can be highly influential on facies architecture and complexity. New field and geochronological data and interpretations for three important, accessible intraplate basaltic submarine and submarine to subaerial successions are presented in this thesis. The three sites are: (1) Heard Island, southern Indian Ocean, (2) Stanley, Tasmania, Australia and (3) Cape Grim, Tasmania, Australia. Heard Island records the transition from submarine to subaerial intraplate basaltic volcanism on oceanic crust. Stanley and Cape Grim represent excellent examples of intraplate basaltic submarine volcanism on continental crust. Heard Island is a Subantarctic volcanic island located on the Kerguelen Plateau and includes Australia's only active volcano, Mawson Peak. Heard Island is unusual for a volcanic island in that despite its significant height (2813 m), the island has not subsided; the basement marine rocks are exposed just above current sea level. This thesis presents the first detailed geological map of Heard Island drawn together from the integration of unpublished data and reports, modern satellite images, private and public rock collections and new \\(^{40}\\)Ar/\\(^{39}\\)Ar isotopic dates and published information. Heard Island comprises an Eocene limestone basement (Laurens Peninsula Limestone), overlain by the submarine basaltic lavas, sedimentary and volcaniclastic rocks of the Late Miocene-Early Pliocene Drygalski Formation. New exposures of Laurens Peninsula Limestone and the previously undescribed volcanic rocks of the Drygalski Formation on the northern coast of Big Ben are documented. The two main volcanic centres, Big Ben (~324 km\\(^3\\)) and Laurens Peninsula (~8 km\\(^3\\)), have been constructed on these submarine sequences during the last 720 ± 78 kyrs. Laurens Peninsula comprises trachyte and basanite lavas and Big Ben is dominated by basalt and basanite lavas. Small (<250 m high) basaltic cones <15 kyrs old are located around the coast of Heard Island. The construction of Mawson Peak, a basaltic cone overlapping the dissected summit crater of Big Ben, likely commenced at a similar time to the eruption of the small basaltic coastal cones. Mawson Peak has been volcanically active as recently as September 2018. The volcanic rocks at Stanley, Tasmania, Australia, record a relatively short-lived (~ 1 myrs), small volume (~0.2 km\\(^3\\)) episode of submarine volcanism during the Cenozoic (11.1-10.3 Ma) and are the youngest Cenozoic basaltic rocks in Tasmania. Volcaniclastic rocks (Stanley Beds) at Stanley are overlain by the submarine basaltic lavas of the Green Hills Volcanics. The submarine basaltic lavas include pillows, megapillows and massive sheet lavas. The Stanley teschenite forms a distinctive, circular, steep sided geological feature prominent in the landscape. Field mapping and the first \\(^{40}\\)Ar/\\(^{39}\\)Ar isotopic ages for lavas at Stanley have revealed that Green Hills Volcanics and the teschenite were formed at ~11 Ma and the Stanley Beds were deposited shortly before that time. Basaltic volcanism at Stanley post-dates offshore volcanism in the Bass Basin (59-7.9 Ma). Volcanism at Stanley occurred between the episodes of intraplate basaltic volcanism in Victoria, Australia that produced the Older (95-19 Ma) and the Newer (<7.8 Ma-4.5 ka) Volcanic Provinces. Cape Grim in far northwestern Tasmania, Australia, was a site of submarine intraplate basaltic volcanism during the Cenozoic. The succession is exceptionally well preserved and is dominated by pillow lava, massive lava and pillow fragment breccia. The total volume of volcanic products (1.1 km\\(^3\\)) is larger than at Stanley but is relatively small on a global scale. The oldest unit, the Woolnorth Tuff (WT) lies unconformably on the Neoproterozoic Rocky Cape Group. The WT is composed almost entirely of devitrified basaltic glass shards and olivine crystal fragments. The WT is overlain by the Slaughter Bluff Volcanic Breccia (SBVB) at Cape Grim. The SBVB is dominated by diffusely bedded pillow fragment breccia. North of Cape Grim, the Little Trefoil Basalts intrudes the Woolnorth Tuff. To the south of Cape Grim, the WT is overlain and intruded by the Studland Bay Basalts (SBB). The SBB comprises mounds of basaltic pillow lavas followed by a succession of diffusely bedded, matrix dominated pillow fragment breccia and basaltic breccia. Detailed field mapping and sampling have revealed that the environment of deposition of all Cape Grim units was submarine and that they were emplaced in relatively rapid succession. The first \\(^{40}\\)Ar/\\(^{39}\\)Ar dating of the volcanic rocks reveals an Early Miocene (23-24.5 Ma) age for the entire sequence. Northern exposures of the WT record a continuous series of phreatomagmatic explosions over a period of days or months and the ash was deposited into a tidally influenced submarine environment. The mapping project identified basaltic cryptodomes within the SBB (23.73 ± 0.08 Ma). The basaltic cryptodomes are older (24.52 ± 0.12 Ma) than the SBB within which they were formerly assigned. This project has separated the basaltic cryptodomes into a stratigraphic unit named the Cape Grim Basalts. Research conducted as part of this thesis has revealed that intraplate basaltic volcanism that involves the submarine environment as an eruption or depositional setting can produce a spectacularly diverse range of volcanic products. Short-lived episodes of intraplate basaltic volcanism in or adjacent to the submarine environment can produce significant facies complexity within a relatively small spatial footprint. Effusive, explosive and shallow intrusive processes at multiple vents can occur in rapid succession resulting in the marked aggradation of coherent and clastic volcanic facies. These facies include pillows, massive basalts, pillow fragment breccias, basaltic cryptodomes and syn-eruptive, gravity driven, tidally influenced tuff. Basaltic cryptodomes and associated sediment-magma interactions are an under-reported but important part of continental intraplate basaltic submarine successions. The description and interpretation of the basaltic cryptodomes at Cape Grim will facilitate the future identification of basaltic cryptodomes elsewhere and contribute to understandings of their emplacement processes and dynamics.


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