HR: 0800h
AN: T41B-0573    [Abstracts]
TI: Constraints on Coupled Microprocesses, Fluid Flow, and Rheological Evolution in a Syn- metamorphic Shear Zone
AU: * Marsh, J H
EM: jeff.marsh@umit.maine.edu
AF: University of Maine, Dept. of Earth Sciences, 5790 Bryand Glob. Sci. Center, Orono, ME 04469, United States
AU: Johnson, S E
AF: University of Maine, Dept. of Earth Sciences, 5790 Bryand Glob. Sci. Center, Orono, ME 04469, United States
AU: Koons, P O
AF: University of Maine, Dept. of Earth Sciences, 5790 Bryand Glob. Sci. Center, Orono, ME 04469, United States
AU: Yates, M G
AF: University of Maine, Dept. of Earth Sciences, 5790 Bryand Glob. Sci. Center, Orono, ME 04469, United States
AB: Microstructural and petrologic data gathered across the sheared margin of the Lincoln Syenite in south-central Maine, USA are used here to evaluate processes involved in shear-zone initiation and development. These data suggest that heterogeneous plastic strain and fluid infiltration enhanced metamorphic reactions, and determined the degree to which textural and chemical reorganization were achieved. Within the study area, a transition from a coarse-grained, granular, igneous rock (Cpx+Opx+Ksp+Bt) to a recrystallized and strongly foliated rock (Act+Ksp+Bt+Qz) is preserved both texturally and petrologically across intermediate domains. Systematic variations in biotite composition track the progress of the bulk reaction, and show highest OH/(F+Cl) ratios in the most strongly foliated domain, where recrystallization is complete. Reaction progress calculations for each of the progressively sheared and recrystallized domains are used to constrain the volume of fluids which moved through the system. Calculated values are incorporated into numerical models that investigate the evolution of permeability and the distribution of stresses within the developing structural fabric, possibly leading to pumping or channeling of fluids during progressive shearing. The effect of fluids on deformation mechanics and metamorphic reactions are of primary importance in understanding rheological changes in crustal rocks during deformation, and are the focus of this study. The textural and mineralogical changes discussed above should have significant implications for the mechanical behavior of the transformed volume. The fine-grained, interconnected product phases, biotite, actinolite, and quartz, within the developing matrix foliation comprise weakened domains between feldspar megacrysts along which strain may be localized. Numerical modeling and experimental studies indicate that these types of processes can cause partitioning of higher strain-rates into biotite-rich, foliated zones, and result in a drastic weakening of the bulk rock. Thus, we suggest that the bulk effective viscosity of the Lincoln Syenite was decreased substantially along its southeastern margin through strain-enhanced reactions and coalescence of elongate biotite and actinolite grains and grain aggregates.
DE: 5114 Permeability and porosity
DE: 8012 High strain deformation zones
DE: 8030 Microstructures
DE: 8045 Role of fluids
DE: 8159 Rheology: crust and lithosphere (8031)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting