The role of shear deformation in crustal-scale fluid flow

Matthew J. Carter
University of Minnesota, Department of Earth Sciences Minneapolis, Minnesota, United States of America
[email protected]

Crustal-scale fluid flow is required for detachment system development and evolution, yet the mechanism to transport fluids into the ductile deforming crust is unclear. The northern Snake Range detachment zone is a good location to examine this problem, because the geologic and fluid flow histories are well known. Samples were collected from a 150 m section of quartzite mylonite shear zone. Thin sections from the upper 20-30 m of this zone contain fluid inclusion planes related to late-stage, high-angle fractures. Throughout the shear zone, fluid inclusions and pores are present along quartz grain boundaries related to shear deformation. Raman spectroscopy indicates fluid inclusions have two end member compositions: CO2 and H2O. End-member inclusion mixtures are typical, but pure H2O and CO2 inclusions are more common at the top and bottom of the shear zone, respectively. Isochores calculated from microthermometry data of fluid inclusion planes suggest entrapment conditions were at the brittle-ductile transition. Preliminary NanoSIMS results for pores within shear fabrics have δ¹⁸O values of 45-65 ‰, whereas quartz grain interiors are 15-25 ‰. This data reveals isotopic disequilibrium along grain boundaries. Results suggest two major fluid reservoirs interacted within this mylonite zone – CO2-rich (metamorphic) and H2O-rich (meteoric). Reservoir fluids are circulated and mixed by brittle fracture, whereas shear deformation exclusively transports metamorphic fluids along grain boundaries and pores. Results imply shear deformation has a key role in distributing fluids, and that crustal-scale fluid circulation may be possible by dynamic interactions of brittle and shear related deformation processes.

AAPG Search and Discovery Article #90157©2012 AAPG Foundation 2012 Grants-in-Aid Projects