Browsing by Author "Rojas, Flavia"

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    Fault intersection-related stress rotation controls magma emplacement at the Nevados de Chill´ an Volcanic Complex
    (2025) Espinosa Leal, Javier; Browning, John; Cembrano, José; Mitchell, Thomas; Rojas, Flavia; Moorkamp, Max; Griffith, W. Ashley; Meredith, Philip
    It has been suggested that fracture and fault intersections promote enhanced transport of fluids in the brittle crust by forming zones of increased permeability. However, the underlying mechanisms that control the emplacement of magma at fault intersections remain poorly understood. To better understand the relation between magma emplacement, volcano development and fault zone intersections, we examine the Nevados de Chillán Volcanic Complex (NChVC, 36.8°S) in the Southern Andean Volcanic Zone. The complex is thought to be located atop the intersection between two sets of NE-right lateral strike-slip faults and a seismically active regional scale NW-oriented inherited structure, also interpreted as a regional fault zone. We collected data on the orientation and frequency of tens of dykes and thousands of fractures, at the volcano scale, from representative outcrops using three-dimensional digital image correlation techniques, with images taken from Unmanned Aerial Vehicles (UAVs). We use these data to generate a conceptual model of the response of the different fracture sets to regional loads and the potential consequence in terms of magma emplacement. In our conceptual model, N-S to NW-SE striking fractures become reactivated by fault intersection-related local stress field rotations. This, in turn, favors NW-SE aligned magma emplacement, and the evolution of NW-SE aligned volcanoes. Our findings provide a mechanical explanation for rotated magma emplacement pathways, which do not necessarily require a transient stress state imposed by unlocking the megathrust.
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    Textural evidence of fragmentation and densification processes in a fossilised shallow conduit on the flank of Nevados de Chillán Volcanic Complex
    (2024) Rojas, Flavia; Browning, John; Tuffen, Hugh; Cembrano, José; Espinosa-Leal, Javier; Unwin, Holly E.; Mitchell, Thomas M.; Hofer-Apostolidis, Karin; Meredith, Philip G.
    Eruptive style transitions are common in silicic volcanoes and an improved understanding of transitional controls is necessary for hazard forecasting. Examples of hybrid eruptions where both explosive and effusive eruptive behaviours occur simultaneously have led to a re-examination of models used to understand these complex and poorly understood processes. Exposed fossilised conduits record evidence of magmatic processes and provide the opportunity to examine structures and textures related to these transitions. Here we present a conceptual model of the evolution of a narrow (2.5 m wide) conduit located on the SW flank of the Nevados de Chillán Volcanic Complex, Chile. This conduit records evidence of fragmentation and densification processes through intercalated and juxtaposed banded, porous and dense domains. To understand how the products of each eruptive style relate and evolve during conduit formation, we combined qualitative textural analyses at different scales (outcrop, optical microscope and electron microscope), pore size and shape measurements using ImageJ, connected porosity measurements made using a helium pycnometer and total water content measurements using Fourier transform infrared spectroscopy. The results allow us to identify five principal phases of the conduit evolution: (I) an explosive phase where the conduit is filled with pyroclastic material, evidenced in the pyroclastic deposit preserved at the conduit wall, (II) a cyclic process of fragmentation and densification within the conduit that generates intercalation of the porous and dense domains, and leads to a hybrid explosive-effusive phase, (III) the formation of a dense magma plug that eventually seals the conduit and deforms vesicles and bands, (IV) the compaction of the pyroclastic domain due to the ascent of the plug, driving porosity reduction (to as little as 3% in the densest bands), with micro-folds and glassy fiamme, and (V) a final phase of post-sintering vesicle relaxation, yielding regular, mainly rounded, shapes. We compare our results with other exposed and examined conduits to propose a model of conduit evolution during small-volume, short-lived silicic eruptions.