Modeling the impact of melt on seismic properties during mountain building

Abstract

Initiation of partial melting in the mid/lower crust causes a decrease in P wave and S wave velocities; recent studies imply that the relationship between these velocities and melt is not simple. We have developed a modeling approach to assess the combined impact of various melt and solid phase properties on seismic velocities and anisotropy. The modeling is based on crystallographic preferred orientation (CPO) data measured from migmatite samples, allowing quantification of the variation of seismic velocities with varying melt volumes, shapes, orientations, and matrix anisotropy. The results show nonlinear behavior of seismic properties as a result of the interaction of all of these physical properties, which in turn depend on lithology, stress regime, strain rate, preexisting rock fabrics, and pressure-temperature conditions. This nonlinear behavior is evident when applied to a suite of samples from a traverse across a migmatitic shear zone in the Seiland Igneous Province, Northern Norway. Critically, changes in solid phase composition and CPO, and melt shape and orientation with respect to the wave propagation direction can result in huge variations in the same seismic property even if the melt fraction remains the same. A comparison with surface wave interpretations from tectonically active regions highlights the issues in current models used to predict melt percentages or partially molten regions. Interpretation of seismic data to infer melt percentages or extent of melting should, therefore, always be underpinned by robust modeling of the underlying geological parameters combined with examination of multiple seismic properties in order to reduce uncertainty of the interpretation.

Deformation Behavior and Inferred Seismic Properties of Tonalitic Migmatites at the Time of Pre‐melting, Partial Melting, and Post‐Solidification

“…For syn‐melt conditions, the seismic velocities decrease (Table 3). Typically, larger melt fractions lead to greater seismic velocity reduction (Hammond & Humphreys, 2000; Lee et al., 2017). However, in this case study, the metatexite with a lower melt fraction has a greater reduction in seismic velocity (Tables 3 and 4).…”

“…Hence, assessing the properties of the different parts of the migmatite would allow for an improvement of our ability to link seismic characteristics to partial melting and/or melt frozen status. Since different degrees of melting occur in the lower crust (Sawyer, 2014; Vanderhaeghe, 2001), exploring the effect of such different melt fractions within the same rock type is important (Lee et al., 2017). To improve our ability to assess seismic data in the light of absence, presence or former presence of melt, it is important to assess the evolution of seismic properties of a migmatitic unit taking its properties pre‐, syn‐ and post‐melting into account.…”

Linking Magma Storage and Ascent to Eruption Volume and Composition at an Arc Caldera

“…At Toba, Long Valley, and Yellowstone calderas, positive radial anisotropy coincident with an LVZ has been interpreted to indicate the presence of stacked sills containing melt (Jaxybulatov et al, 2014; Jiang et al, 2018). The causes of anisotropy are nuanced, however, and there exist trade‐offs between melt aspect ratio, orientation, and melt segregation for a given level of anisotropy and V S reduction (Hammond & Kendall, 2016; Holtzman & Kendall, 2010; Lee et al, 2017). Despite these trade‐offs, a horizontal, low‐aspect ratio melt lens or melt layer model captures both the anisotropy and shear‐wave anomaly amplitudes at Okmok, whereas high aspect ratio, vertically oriented, or spherical melt lenses are unable to do so (Hammond & Kendall, 2016: See their Figures 2 and 3; Lee et al, 2017: See their Figure 5).…”

Melt organisation and strain partitioning in the lower crust

“…Despite the basic relationships being known, the behaviour of partially molten crust as observed at outcrop is not always easily explained by the models and experiments (Lee et al, 2017;Rosenberg and Handy, 2005), meaning that many aspects of how partially molten crust actually deforms remain unknown. For example, it is unclear why very large volumes of melts are seen to remain approximately in-situ within the crust in the form of migmatites, despite their sometimes immediate proximity to one or several shear zones (e.g.…”