Abstract
Partial melt in the deforming mid- or lower continental crust causes a strength decrease and drives formation of lithological heterogeneities. However, mechanisms of formation of syn-melt deformation zones and strain partitioning in partially molten rock remain poorly understood. We use field and microstructural observations to unravel the evolution of a partial melt shear zone, Seiland Igneous Province, northern Norway. The Øksfjord shear zone (ØSZ) is one of several paragneiss shear zones present within gabbros of the Seiland Igneous Province, formed by syn-intrusive deep crustal shearing during lithospheric extension related to continental rifting. Microstructures from the ØSZ show evidence for different deformation conditions. The first phase was active pre-melt and involved deformation at high subsolidus temperatures. This was followed by syn-melt deformation of the shear zone causing a relative strength increase towards the shear zone centre upon crystallization. The third phase nucleated two parallel shear zones at the edges of the ØSZ; melt textures are absent and microstructures indicate deformation at lower temperatures and higher stresses. In effect, melt migration towards the shear zone centre ultimately led to strengthening of the shear zone core, with post-crystallization deformation focusing along shear zone margins where significant heterogeneities are present.
Supplementary material: Figures illustrating grain boundary dihedral angle data, EBSD maps, grain size distribution and additional pole figures is available at https://doi.org/10.6084/m9.figshare.c.4819494
Experimental studies of partially molten rock show that there is dramatic strength drop when partial melt forms a connected network at c. 7% melt volume (e.g. Rosenberg and Handy 2005). This strength decrease is propagated as melt volume increases and deformation partitions between the solid rock and liquid melt (Vanderhaeghe 2009). Partial melting is common in the middle to lower continental crust owing to high temperatures, decompression and/or the influence of volatiles promoting pervasive melting (Sawyer 1994; Brown 2001; Vanderhaeghe 2009). Partial melt adds to the heterogeneous nature of these rocks (e.g. grain size, mineralogy, microstructure, etc.), and such lithological heterogeneities are important factors in controlling strain partitioning on all scales (Fossen and Cavalcante 2017). Rheological relationships have been well constrained from experiments; however, experiments do not always explain observed partial melt at outcrop scale in the field or at crustal scale from the geophysical response (Brown et al. 1995; Rosenberg and Handy 2005; Karato 2010; Lee et al. 2017). For example, if melt localizes strain, it is unclear why very large volumes of melt remain in situ within the crust (crystallizing in the form of migmatites), despite their sometimes immediate proximity to one or several shear zones that should act as conduits for melt escape (Labrousse et al. 2004; Lee et al. 2018). Rushmer (2001) showed that a significant volume change during melting can lead to melt migration and extraction from a system, leading to strain hardening. However, if only small volume changes are involved, melt can remain trapped along grain boundaries, resulting in prolonged weakening.
It is important to consider how shear zones evolve through time and what role partial melt plays in their evolution. The active deformation mechanisms and strain localization in partial melt shear zones vary during their evolution from phases of melt-free to syn-melt and post-melt deformation. Strain localization is influenced by many parameters within shear zones; for example, pre-existing fractures, weak layers or structures (Passchier 1982; Austrheim and Boundy 1994; Pennacchioni and Cesare 1997), margins of a lithological heterogeneity such as paired shear zones (Pennacchioni and Mancktelow 2007) an