The influence of melting and melt drainage on crustal rheology during orogenesis

Partial melting significantly weakens crustal rocks by introducing a low-viscosity liquid phase. However, near-concomitant melt drainage can remove this weak phase, potentially reversing the rheological effects such that the strength of a specific lithology depends on when the prograde pressure-temperature path intersects a melting reaction, how much melt is produced, and how long this melt is retained before it is lost. Phase equilibria and mixed rheology modeling of typical metapelite and metagreywacke compositions indicate that these rocks undergo continuous but pulsed melt production during prograde metamorphism. Depending on whether melt removal is continuous or episodic, and assuming geological strain rates, the lithologies can retain a very low strength less than 1 MPa or transiently strengthen to ∼5 MPa following melt loss. Lithologies undergoing episodic melt loss can therefore cycle between being relatively weak and relatively strong components within a composite crustal section. Melt production, retention, and weakening in the middle to lower crust as a whole is more sustained during heating and melt production, consistent with geodynamic inferences of weak, melt-bearing lower crust. However, the long-term consequence of melting and melt loss is a 50–400% increase in the strength of residual lithologies. The strengthening is more pronounced in metapelite than metagreywacke and is achieved through a combination of dehydration and the removal of the weak mica framework coupled to increased proportions of strong feldspars and garnet. Despite prolonged weakness, melting and melt loss therefore ultimately result in a dry and elastic lower crust