Applications of Tephrochronology in the Northern Cordilleran Volcanic Province and the Mount Edgecumbe Volcanic Field

The Northern Cordilleran Volcanic Province (NCVP) of northwestern British Columbia and Yukon, and the Mount Edgecumbe Volcanic Field (MEVF) of southeast Alaska, represents a lesser-known stretch of extension-related volcanism in North America that has a poorly understood Holocene eruptive history. This largely stems from the volcanic vents in this region being remote and difficult to access, requiring significant logistical effort, costly transport, and extensive hiking. This thesis explores tephrochronology as a means of more accessible preliminary study of past volcanic activity in these areas. Glass major element geochemistry is used to characterise tephra sequences found in sections, peat cores, and lake sediment cores. In the NCVP, 5 to 9 newly described tephra are found along a ~165 km transect of the Stewart-Cassiar Highway. Two new basaltic tephra likely originate from the Iskut Volcanic Field, with one found only ~5 cm below the surface of the core. A widely distributed phono-trachyte and associated basalt are likely sourced from Mount Edziza, in addition to the Sheep Track pumice and upper Finlay tephra. From the MEVF, distal tephra indicate that eruptive activity continued after the late Pleistocene pulse (14.6–13.1 ka; Praetorius et al. 2016), beginning with basaltic andesite tephra, and continuing an upward increasing silica trend with one andesitic, two dacitic, and multiple rhyolitic tephra. Only rhyolite tephra were erupted after ~11,070–10,240 cal yr B.P. At least two, up to three rhyolitic tephra at Biorka Island are associated with an age of ~4.6 cal kyr B.P., one of which is a coarse bright white ash. Including this white ash, two similar young white rhyolitic ashes are distributed tens of kilometres from Mount Edgecumbe in north, east, and southeast directions. The geochemistry of rhyolite tephra from the MEVF show effectively indistinguishable glass geochemistry, but steps in characterising associated Fe-Ti oxide minerals show differences across spatial distribution. These findings demonstrate that the MEVF has produced explosive eruptions into the early through mid-Holocene, with rhyolite the only composition produced in its most recent activity. Overall, this study reinforces the need to better understand this region of volcanoes, as there is more to their Holocene history than previously thought. Tephrochronology is an effective more accessible first look into these explosive eruptive records. These findings warrant further exploration of proximal sites to tie tephra deposits with source vents and better assess the hazards they present.

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