RETROGRADE FORMATION OF CHLORITE, EVOLVING FLUID COMPOSITION, AND EMBRITTLEMENT OF THE SIMPLON LOW-ANGLE NORMAL FAULT ZONE, SWITZERLAND

Previous work proposed that localization of subvertical simple shear during footwall uplift in the Simplon footwall is linked to evolving shear-zone fluid composition. Specifically, a relative increase in the carbonic component of the fluid within ductily deforming, quartzo-feldspathic host rocks resulted in a shift in fluid characteristics from a wetting to a nonwetting state. It is hypothesized that nonwetting, carbonic fluids dynamically influence pore-space geometry in a manner that reduces transmissivity. In the footwall, localized bending strain and exhumation may cause transiently high pore fluid pressure resulting in an environment that is conducive to brittle failure. This path of evolution from wetting to nonwetting, fluids is documented by fluid inclusion observations from kinematically referenced fluid inclusion arrays. Carbonic fluids are only found in association with brittle structures that clearly crosscut ductile footwall fabrics. Important questions are related to this process: (1) During ductile deformation at mid-crustal conditions, where were the initial, wetting fluids derived?; and (2) By what mechanisms were these fluids driven to more carbonic, nonwetting compositions? By combining textural, fluid inclusion, isotopic analysis, and mass balance considerations it may be possible to fully address both of these important questions. Of particular interest is the hypothesis that shear-zone fluids were driven to nonwetting, carbonic compositions by the retrograde formation of chlorite. The formation of chlorite is known to occur within the same temperature and pressure ranges derived for the earliest phase of brittle failure in the footwall (450°-600°C and 400-700 Mpa). Also, for every mole of chlorite formed, 4 moles of water are consumed, which equates to an increase of 0.016 modal abundance of chlorite to shift the bulk fraction of X_CO2 from 1% to 5% in a closed system. Preliminary stable isotope data from fault related fluid inclusions indicate that the water has low dD and d¹⁸O values between –64 to –28‰ and –14 and –9‰, respectively. d¹³C-CO₂ values between -9.7 and -2.6 indicate that the carbonic phase is from a mixed source. Although the results from the isotopic analyses are inconclusive, other lines of evidence are consistent with chlorite formation influencing fluid composition.