Successful M.Sc. Defence – Alexandra Gray (uOttawa)
Thesis Title: Geology of the Monowai Rift Zone and Louisville Segment of the Tonga-Kermadec Arc: Regional Controls on Arc Magmatism and Hydrothermal Activity
Supervisor: Dr. Mark Hannington
iMAGE-CREATE affiliate Alexandra Gray has successfully defended her M.Sc. thesis at the University of Ottawa. Alexandra’s defense took place over zoom on April 5, 2022, in which she gave her public lecture followed shortly by her thesis defence on her work on the Monowai-Louisville segment of the Tonga-Kermadec arc. An abstract of Alexandra’s work follows.
Alexandra Gray Biography:
Alexandra Gray completed her undergraduate degree at the University of New Brunswick with a BSc Honours in Earth Sciences and a minor in Music in 2019. Following this, she began her masters at the University of Ottawa under the supervision of Dr. Mark Hannington. While completing this degree she spent 3 months working at the GEOMAR-Helmholtz Centre for Ocean Research in Kiel, Germany. While there she was able to work alongside and learn from members of the Marine Resources Group. She also had the opportunity during her master’s degree to spend 3 weeks aboard the German Maria S. Merian research vessel for the Leg 1 of the MSM88 cruise. Following her successful thesis defence in April 2022, Alexandra began work at the junior exploration company Red Pine Exploration.
The Tonga-Kermadec arc in the SW Pacific comprises a chain of more than 90 volcanic complexes. A continuous 400-km long chain of volcanic activity along the central portion of the Tonga arc has become the focus of intensive research, extending previous studies that have focused on the southern Kermadec chain. Earlier interpretations of the Tonga arc have focused on a perceived lack of volcanism between ~21°S and ~27°S, adjacent to a bend in the trench caused by the collision of the subducting Louisville Seamount Chain (LSC). During swath mapping in 2002, it was revealed that this portion of the arc, including the Louisville and Monowai segments, is in fact one of the most volcanically active parts of the Tonga-Kermadec system. At this location, a combination of oblique convergence of the Pacific Plate and southward compression due to the collision of the LSC has resulted in left-lateral strike-slip faulting and rifting of the arc crust. This has produced a series of left-stepping arc transverse graben and horst structures that localize the voluminous volcanic activity.
For this study, a new 1:250,000 scale geological map of the Louisville and Monowai segments has been constructed as a framework for a quantitative analysis of arc volcanism and the eruptive history of these segments. Two types of volcanoes dominate the arc front: deep caldera systems (collapse structures formed due to the evacuation of magma) within the arc rifts, and smaller volcanic cones between the rifts. The cone volcanoes tend to have small summit craters (<10 km3) whereas the large caldera volcanoes have major depressions of up to 50 km3. The cones are relatively undeformed, whereas the larger calderas are affected by multiple stages of collapse, asymmetric subsidence, and distortion caused by regional stresses. Surveys of the crater walls of the cone volcanoes show a predominance of volcaniclastic deposits, whereas the caldera volcanoes contain a high proportion of coherent lava flows. The caldera volcanoes also show a prevalence of basaltic melts compared to the more andesitic and dacitic cones. The largest caldera volcano is the Monowai volcanic complex (25°53’S) occupying a deep depression (Monowai Rift Graben) that crosses the arc front. The volcanic complex consists of a large caldera (12 km wide, 1600 m deep) and an adjacent stratovolcano (Monowai Cone) rising nearly to sea level. We suggest that the different types of volcanoes along the Louisville and Monowai segments reflect the influence of deep structures within the arc crust that have localized strike slip and normal faulting.
We extend congratulations to Alexandra for her successful defense and wish her further success within her future endeavors.
Photos of Alex from her expedition aboard the German Maria S. Merian Research Vessel (Leg 1 of the MSM88 Cruise)