Microstructural and seismic properties of partial melt shear zones

The coexistence of partial melt with high strain shear zones is a common feature of many continental deformation zones. Partial melt is known to cause a strength decrease but the exact mechanisms and relative timing of the formation is still debated. This thesis provides a field, microstructural and seismic study of syn-melt shear zones for former and actively melting mid to lower crust. Field and microstructural data was gathered from two field areas, Seiland Igneous Province (SIP), Norway, and Western Gneiss Region (WGR), Norway, with evidence of syn-melt deformation. Both the SIP and WGR showed evidence to indicate partial melt forms an interconnected network even at low melt volumes, thus maximising the weakening effect of partial melt. However, the response to syn-melt deformation does not produce a consistent microstructural signature. Melt migration in the SIP ultimately led to strengthening of the shear zone core, with post-crystallisation deformation focused along shear zone margins where significant heterogeneities are present. In contrast, pervasive open-system melting in the WGR resulted in progressive strain localisation through stress-driven melt organisation and deformation-assisted channelized melt flow, forming fine-grained shear zones with a mylonitic appearance, but lacking subsequent shearing in the solid-state. Seismic modelling assesses the impact of melt and solid phase properties (melt volume, shape, orientation, and matrix anisotropy) on seismic velocities and anisotropy allowing a comparison of former melt zones with areas of present day partial melting. Seismic properties are non-linear as a result of the variation of these physical properties, which in turn depend on lithology, stress regime, strain rate, pre-existing fabric, and pressure-temperature conditions. Interpretation of seismic data to infer melt percentages or extent of melting should be underpinned by robust modelling of the underlying geological parameters combined with examination of multiple seismic properties in order to reduce uncertainty of the interpretation and geodynamic models.