Fluid inclusions in post Late Devonian brittle faults in the Sunnfjord region, West-Norway: application to P-T estimates and geothermal gradient reconstructions

Abstract

Previous structural analysis of post L. Devonian faults in the Sunnfjord region outlined the presence of four populations of faults (Braathen,1998). In the present study, analysis of fluid inclusions hosted by minerals sealing populations 1,2 and 4 faults are used as a first approach to estimate pres- sures and temperatures of major faulting episodes. Population 1 faults from Leknes contain primary fluid inclusions with saline aqueous solutions contai- ning 5.8 wt% NaC1. Population 2 faults from Våge also contain aqueous inclu- sions with 11.5-12.5 wt% NaC1 whereas population 4 faults from Atløy contain as much as 21.0 wt% NaC1 with possible traces of FeC1. Fluid inclusions in population 1 and 2 faults formed at temperatures between 300o and 4000C at 3-5 kbar pressure and population 4 formed at pressures <3kbar and temperatures between 134 and 300oC. Given these data, the geothermal gradient for L.Devonian to E.Carboniferous faulting when population 1 and 2 faults formed, was 20-25oC\/ km.