Exchange Stories

About Flavia's research project

In the Southern Andean volcanic arc, volcanic chains are related to different structural domains controlled by the plate boundary scale stress field (Lara et al., 2006). Observations show that these volcanic systems are spatially and genetically related to fault intersections (areas where faults cross) rather than disconnected subsidiary faults within a given system (e.g. Veloso et al., 2020). Areas of fracture intersection have been associated with increased fluid permeability up to two orders of magnitude compared to a single fracture (Stanton-Yonge et al., 2023), which may offer a possible interpretation of how and why volcanoes are spatially related to fault intersections. One fundamental unresolved issue is why some intersecting fault systems are associated with these systems and many others do not show an apparent correlation between enhanced fluid flow within the same arc system. This suggests that there are likely other first-order influence controls. For example, punctuated local variation in the stress state close to volumes of rock within or surrounding fault zone intersections. In this project, we propose to address the question: What is the role of fault intersections on local stress state variations at different scales? For this, we are studying the Puyehue-Cordón Caulle and Nevados de Chillan active Andean volcanic complexes as they offer unique exposure to the problem identified. During my 3-week visit to The Ohio State University, Dr Ashley Griffith introduced the theoretical basis and applications of the 3D Boundary Element Methods (BEM), which is particularly effective in simulating mechanical interaction between geometrically complex faults like our study case (Crider and Pollard, 1998; Griffith and Cooke, 2004; Muller et al., 2005; Stanton-Yonge et al., 2016).  We designed our approach to the study cases and now we continue working and collaborating with the research team on our research question.

 

For our first case study, the Puyehue-Cordón Caulle Volcanic Complex (PCCVC), we have already collected over 200 structural measurements of faults in the field, from seven outcrops at different distances from the fault intersections. From this data, we performed stress inversions using the Multiple Inverse Method (MIM) and strain inversions using Faultkin. We are currently analysing and comparing the inversions of the different zones based on their relationship with the fault intersections. One of our findings until now is that in the Cochamó structural site, located close to an intersection between the Liquine-Ofqui fault system and an E-W lineament, there is more than one stress state solution. Fig. 1 shows the stress and strain inversions for Cochamó structural site. The MIM solution (upper plots) represents the orientation of the principal stress axes derived from the heterogeneous fault-slip data set by generating a combinatorial number of groups extracted from the whole data set (Yamaji et al., 2005, Perez-Flores et al., 2016). The orientations of the calculated stress axes for each generated group are plotted onto separate stereograms (σ1 and σ3) and represented as a tadpole coloured according to the calculated stress ratio (φ = (σ2 − σ3)/(σ1 − σ3)), thus groups of tadpoles with similar colours and similar orientations indicate a compatible stress field solution (Perez-Flores et al., 2016). In this case, we can observe the first blue-coloured group that represents a solution with a vertical σ1 and a semi-circle distributed σ3, this distribution is because σ3 is very similar to σ2, which is associated with an extension in all directions that might be occurring due to the intersection of these structures. We can also observe a second solution in the yellowish-coloured group, with a horizontal σ3 and distributed σ1, representing a different stress state recorded in the same place. Further analysis will be done to make progress on our understanding of the influence of these stress variations due to fault intersections in geofluid migrations in the crust. We plan to submit this work during the following year.

 

For our second case study, the Nevados de Chillán Volcanic Complex, we submitted a paper to the Journal of Volcanology and Geothermal Research (Espinosa-Leal et al., part of Espinosa-Leal PhD project) which was recently accepted for publication. This work is entitled “Fault intersection-related stress rotation controls magma emplacement at the Nevados de Chillán Volcanic Complex” and recompiles data of tens of dykes and thousands of fractures from representative outcrops around the complex. We use these data to generate a conceptual model of the response of the different fracture sets to regional and the potential consequences in terms of magma emplacement (Fig. 2). We propose that N-S to NW-SE striking fractures become reactivated by fault intersection-related local stress field rotations. This favours NW-SE aligned magma emplacement, and the evolution of NW-SE aligned volcanoes. 

 

Now, we continue working from a modelling approach using BEM. This will allow us to study the fault intersection behaviour under tectonically realistic stress boundary conditions imposed by plate interaction at the subduction margin in different stages of the subduction seismic cycle. We plan to submit this work during the following year.