Evaluating the effects of stress-driven segregation, strain and reaction history, and intrinsic rock properties on melt transport and rock rheology in the naturally deformed lithosphere
Abstract
The segregation, migration, and extraction of melt - and the emplacement and assembly of the melts as plutonic systems - are major controls on mass and heat transfer in the lithosphere. The distribution of partial melts at the grain scale, and partially molten rocks at larger spatial scales, exerts a profound influence on rock rheology, and is of significance for melt segregation, dynamic weakening, and strain localization at a variety of lithospheric levels. Evaluating the rheological effects of melt in the lithosphere requires insight into the relative effects of stress-driven segregation, strain and reaction history, and intrinsic rock properties of naturally deformed lithospheric sections. Melt segregation and distribution are dynamically linked at a variety of spatial scales to relative motion between the melt and solid phase in deforming partially molten rocks, which gives rise to an evolving melt topology and porosity-permeability structure. The extraction of melt from grain boundaries requires connectivity into a channelized migration network or through structural fabrics that allow for the horizontal and vertical transfer of melt in the crust, compelling examples of which have been demonstrated in migmatite-granite complexes in the crust, dike and vein networks in the crust and mantle, and for reactive melt migration pathways in the upper mantle. Numerical models and experimental rock deformation studies have provided important insights into the mechanisms of melt segregation, geometric characteristics of channelized melt migration networks, and the rheological consequences of melt mobilization. However, field-based and microstructural investigations of exhumed lithospheric sections remain critical for evaluating relationships between deformation and melt flow processes at geologically relevant scales, and under natural deformation conditions. For example, field-based studies in the Twin Sisters ultramafic complex (Washington State) document melt migration geometries that differ from patterns predicted by numerical and experimental studies of stress-driven melt segregation. Dunite melt bands in low strain regions of the Twin Sisters complex typically form high angle conjugate geometries, but in highly deformed portions of the host peridotites their geometries systematically become more subparallel. Structural and textural observations suggest that melt flow was contemporaneous with deformation and therefore the organization of the reactive melt flow network was dynamically linked to the magnitude of viscous strain and localization phenomena in the host peridotites. These observations underscore the importance of field, microstructural, and textural studies in tectonic systems that experienced the sequential or coeval effects of melt extraction, migration or accumulation (e.g., migmatite-granite complexes). The permeability of melt flow networks, the distribution of melt and melt connectivity in the orogenic crust, and the timing of partial melting relative to deformation, all affect the efficiency of melt transfer in the lithosphere. The rheological evolution of partially molten rocks further significantly affects the ability of the melt-rich crust to mobilize into zones of active deformation, such as during the formation of migmatite domes (e.g., the Naxos dome, Greece).
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.T12D..02K
- Keywords:
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- 8110 TECTONOPHYSICS / Continental tectonics: general;
- 8120 TECTONOPHYSICS / Dynamics of lithosphere and mantle: general;
- 8159 TECTONOPHYSICS / Rheology: crust and lithosphere;
- 8178 TECTONOPHYSICS / Tectonics and magmatism