Footwall rocks of the northern Snake Range detachment fault (Hampton and Hendry's Creeks) offer exposures of quartzite mylonites (sub-horizontal foliation) that were permeated by surface fluids. An S-C-C' mylonitic fabric is defined by dynamically recrystallized quartz and mica. Electron backscatter diffraction analyses indicate a strong preferred orientation of quartz that is overprinted by two sets of sub-vertical, ESE and NNE striking fractures. Analyses of sets of three perpendicular thin sections indicate that fluid inclusions (FIs) are arranged according to macroscopic fracture patterns. FIs associated with NNE and ESE-striking fractures coevally trapped unmixed CO2 and H2O-rich fluids at conditions near the critical CO2-H2O solvus, giving minimum trapping conditions of T = 175-200 °C and ~100 MPa H2O-rich FIs trapped along ESE-trending microcracks in single crystals of quartz may have been trapped at conditions as low as 150 °C and 50 MPa indicating the latest microfracturing and annealing of quartz in an overall extensional system. Results suggest that the upper crust was thin (4-8 km) during FI trapping and had an elevated geotherm (>50 °C/km). Footwall rocks that have been exhumed through the brittle-ductile transition in such extensional systems experience both brittle and crystal-plastic deformation that may allow for circulation of meteoric fluids and grain-scale fluid-rock interactions.
Bibliographical noteFunding Information:
We gratefully acknowledge funding from National Science Foundation grant EAR-0838541 and Swiss grant SNF-200020-126973/1 to CT and MJC. MJC also acknowledges support from an AAPG student grant award and from a travel and analytical grant from the Department of Earth Sciences at the University of Minnesota . We are grateful to Nick Seaton for his assistance with EBSD data acquisition and for assistance in fracture measurements by students on the 2011 University of Minnesota-Twin Cities Advanced Field Geology course as well as field assistant Sean Sitek. Constructive and helpful reviews by V. Famin and M. Stipp were greatly appreciated.
© 2015 Elsevier Ltd.
- Brittle-ductile transition
- Core complex
- Fluid inclusions
- Snake Range