Eocene dextral strike‐slip and extensional faulting in the Bridge River Terrane, southwest British Columbia

Abstract

The early Mesozoic Bridge River Terrane of oceanic affinity lies in the eastern Coast Mountains of southwestern British Columbia and is one of several terranes located near the Intermontane Superterrane‐Insular Superterrane boundary. Eocene strike‐slip faulting and related extensional faulting and plutonism are largely responsible for the present upper crustal configuration of the Bridge River Terrane. This paper presents U‐Pb geochronology and a detailed structural and geochronological analysis of Eocene deformation of the Bridge River Terrane near Lillooet, British Columbia. Evidence for Eocene magmatism and deformation within the Bridge River Terrane includes 48.5–46.5 Ma U‐Pb zircon ages from four deformed granitic rocks, two of which are from the Mission Ridge pluton. All are cut by the Mission Ridge normal fault, which juxtaposes low‐grade Bridge River Group in the hanging wall against medium‐grade Bridge River Schist and associated intrusions in the footwall. The Mission Ridge fault is a low‐angle east‐dipping brittle normal fault with at least 10 km of downdip displacement and a probable dextral strike‐slip component. In the Bridge River Schist, in the footwall of the Mission Ridge fault, penetrative ductile fabrics in northwest‐trending, steeply to shallowly‐dipping mylonites with northwest‐southeast trending subhorizontal stretching lineations record a dextral sense of shear. Development of these fabrics predates normal movement on the Mission Ridge fault and is attributed to dextral movement on the Yalakom fault system within the middle crust. Two Middle Eocene syntectonic foliated intrusions, dated by U‐Pb zircon methods, have the same kinematic signature as the previously described mylonites; these constrain timing of dextral movement on the Yalakom fault system to be earlier than, contemporaneous with, and in part younger than the intrusion of the circa 47 Ma Mission Ridge pluton. The Mission Ridge fault is correlated with the Petch Creek fault, the probable extension of the Ross Lake fault near the United States border. They were broken and offset by about 100 km of dextral movement on the north‐trending Fraser fault. The Petch Creek fault hanging wall consists of the Hozameen Group, which, like the Bridge River Group, is an assemblage of low‐grade oceanic rocks. The footwall of the Petch Creek fault, the Custer Gneiss, is higher grade than the Bridge River Schist in the footwall of the Mission Ridge fault but has the same lithological and kinematic signature. Mylonitic fabrics in both the Bridge River Schist and Custer Gneiss have subhorizontal stretching lineations that were generated prior to normal movement on the Mission Ridge‐Petch Creek fault system and are attributed to earlier dextral motion along the Yalakom fault system. The clear offset of these geologic features constrains about 100 km of movement on the Fraser fault to be younger than 46.5 Ma. The Fraser fault is cut by 34 Ma phases of the Chilliwack batholith.