Coupling of deformation and reactions during mid-crustal shear zone development: an in situ frictional-viscous transition

A well preserved strain and reaction gradient records the progressive transformation of a megacrystic Kfs+Cpx+Opx+Bt1±Qtz syenitic pluton to a strongly sheared Kfs+Act+Bt2+Ab+Qtz tectonite within the exhumed Norumbega Fault System, Maine, USA. Detailed microstructural analysis indicates that fracturing and localized fluid infiltration initiated the deconstruction of the existing K‐feldspar and two‐pyroxene load‐bearing framework, and that feedback among metamorphic reactions, fabric development and enhanced permeability during progressive shearing led to the development of an interconnected, biotite‐ and actinolite‐rich foliation. The activation of dislocation creep in biotite and quartz, and dissolution–precipitation creep in actinolite and feldspar, with increasing strain ultimately resulted in a transition from dominantly frictional to dominantly viscous deformation processes. Petrological data show that various scales of geochemical disequilibrium exist across the strain and reaction gradient, and that reaction progress was limited by slow chemical diffusion during the early stages of deformation. Petrological modelling results indicate that the existing plutonic assemblage was metastable at mid‐crustal conditions, and that fluid infiltration and deformation allowed the product assemblage to advance towards chemical equilibration. Comparison of the observed microstructures and deformation mechanisms with experimental and numerical modelling results suggest that the development of an interconnected biotite‐dominated fabric probably caused a major (up to three fold) reduction in bulk rock strength and localization of strain into the foliated margin.