Focusing Fluids in Faults: Evidence From Stable Isotopic Studies of Dated Clay‐Rich Fault Gouge of the Alberta Rockies

Abstract

AbstractIsotopic studies of Canadian Rocky Mountain thrust faults preserve the timing and identity of orogenic fluids and their fault zone pathways. Using previously dated samples, we measure the O‐ and H‐isotopic compositions of fault gouge. These nearly 100% neomineralized gouges and their associated damage zones act as primary orogenic fluid pathways. As such, they provide a specific and local look into the nature of the Late Jurassic to Early Eocene orogenic plumbing system in the Alberta Rockies. Considering clay polytype stability and regional temperature conditions, we obtain a range of geofluid isotopic compositions during Jurassic‐Eocene thrust faulting: δ18Ofluid ranged from ∼−3.3 to 9.2 ± 3.2‰; δDfluid ranged from −119 to −46 ± 13‰ VSMOW. The range of O‐ and H‐isotopic compositions reflects mixing of fluid sources, including the pervasive presence of surface‐sourced fluids (up to ∼90%). The interpreted prevalence of a surface fluid source in fault rocks is in agreement with regional isotopic trends previously observed in undated veins of fractured host rock. Our results confirm that thrust faults of the Alberta Rocky Mountains acted as major fluid‐focusing conduits during orogenic activity. We further show that these faults incorporated both deeply sourced and surface‐sourced fluids into zones of enhanced and dynamic permeability, heterogeneously distributing fluids along fault planes across the fold‐thrust belt, promoting the growth of fault‐zone weakening clay minerals.