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.

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Partial melts can form as a result of crustal thickening due to orogenesis. Even small melt fractions weaken the crust, so that partially molten volumes should accumulate significant amounts of strain. However, relatively little is known of how strain partitions in partial melts, and how effective the melt expulsion processes from the partially molten crust are. Using examples from the Western Gneiss Region (WGR), Norway, we consider a case of co-existing migmatites and shear zones. Field, image analysis, and microanalytical methods allow (semi)quantification of melt volume, rock mineralogy and mineral chemistry, and microstructures. Integration of these analyses implies effective syn-melt strain partitioning and subsequent freezing of both the shear zone and migmatite texture. We propose a mechanism that allows i) syn-melt strain localisation at an outcrop scale through stress-driven melt organisation, resulting in significant relative competence differences in a partially molten rock volume; and ii) formation of fine-grained rocks at outcrop

As seismic data from the lower crust becomes more readily available, it is important to link seismic properties to the ongoing processes within lower crustal evolution. This includes high temperature, pre‐ and post‐migmatization solid state deformation as well as melt‐present deformation. We selected two tonalitic migmatites with variable former melt content (one metatexite and one diatexite) from the lower crustal Daqingshan area, northern North China Craton to assess the link between seismic properties and rock structure and rheology. Field observation along with microstructural features suggest that the characteristics of hornblende and plagioclase within the residuum of the metatexite can be used to derive information on the pre‐melt deformation. Residuum's plagioclase CPO (crystallographic preferred orientations) is consistent with high temperature dislocation creep as the main deformation mechanism; similarly, hornblende shows a strong CPO related to dislocation creep. During syn‐melt (melt present) conditions, phenocrysts of plagioclase in the metatexite's neosome and K‐feldspar and peritectic hornblende in the diatexite's neosome are present. The rheology of the rock was dominated by melt; hence is inferred to follow Newtonian flow. After melt crystallization deformation is minor but again dominated by dislocation creep. The seismic properties (seismic velocity, anisotropy, Vp/Vs ratio, etc.) for pre‐ and post‐melt have similar values expected values for solid mafic rocks, whilst the syn‐melt seismic velocities are generally lower and Vp/Vs ratios and seismic anisotropies are higher.

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