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Exploring Magmatic Evolution by Linking Field Data to a Magma Dynamics and Forward Geophysical Model at the Three Sisters Volcanic Complex, OR, USA

Annika Dechert1, Gabe Eggers2, Josef Dufek1, Hélène Le Mével3, Nathan Andersen4

  • Affiliations:  1 Department of Earth Sciences, University of Oregon, Eugene, USA; 2 Department of Earth and Environmental Science, Wesleyan University, Middletown, USA; 3 Earth and Planets Laboratory, Carnegie Institution for Science, Washington DC, USA; 4 U.S. Geological Survey, Cascades Volcano Observatory, Vancouver, USA 

  • Presentation type: Talk

  • Presentation time: Friday 14:15 - 14:30, Room R280

  • Programme No: 1.5.1

  • Theme 1 > Session 5

Abstract

The Three Sisters volcanic complex in the central Oregon Cascades is considered a "very high threat" by the U.S. Geological Survey, with eruptions of South Sister as recently as 2,000 years ago and inflation just west of South Sister since the mid-1990s. Geodetic models suggest the source of inflation is 5--7 km deep, and most models require some component of magmatic intrusion, perhaps into an existing magma reservoir. We conducted a Bouguer gravity survey to image the hypothesized shallow magma system and potentially discriminate between a predominantly magmatic versus hydrothermal deformation source. To further explore the evolution of the Three Sisters magma system to its current state of activity, we compare these gravity data to model scenarios using a coupled magma dynamics and forward gravity modeling approach. Evolution of the Three Sisters magmatic system is modeled via a 3D multiphase numerical model with additions of stochastic intrusions. The thermal phase equilibria of the intrusions produce magma bodies with a density contrast to the background crust, creating a model gravity anomaly at the surface of the simulation that is compared to the Bouguer measurements. We explore a range of primitive magma fluxes as well as strictly intrusive scenarios and those that consider eruptions. We seek the range of plausible average mantle flux rates, volumes, and depths of magma required to produce the current gravity signal. Additionally, we consider forward resistivity and seismic velocity models for the magmatic histories that best fit the gravity data to highlight opportunities for future geophysical studies.